Modern Concept Science and Technology 10

196 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 197Factors on which the pressure at a point in liquid dependsThe pressure due to a liquid depends up on three factors, viz. depth of the liquid, density of the liquid and acceleration due to gravity at that place.i) Liquid pressure is directly proportional to its height. ( i.e. p ∝ h)FACTS WITH REASONSDams are made thicker at the bottom, why?Water is collected in dam. The water pressure increases with the depth. More pressure results more pushing force at the bottom of a dam. So, the dams are made thicker at the bottom to withstand the higher pressure of water.ii) Pressure in a liquid is directly proportional to its density. ( i.e. p ∝ d)FACTS WITH REASONSIf out of two identical bottles, one is filled with water and another is filled with oil, then the bottom of the bottle filled with water experiences more pressure, why?A liquid with more density exerts more pressure at the bottom of the container. The density of water is higher than that of oil. So, if out of two identical bottles, one is filled with water and another is filled with oil, then the bottom of the bottle filled with water experiences more pressure. iii) Pressure in a liquid is directly proportional to acceleration due to gravity. ( i.e. p ∝ g)FACTS WITH REASONSWhen a tank filled with water is carried from Terai region to the Himalayan region then the pressure at its bottom decreases, why?The acceleration due to gravity decreases with the increasing altitude from the sea level. When a water tank filled with water is carried from Terai region to the Himalayan region, the value of acceleration due to gravity becomes less. This causes a decrease in the pressure at the bottom of tank. Transmission of Liquid Pressure: Pascal's LawIn 1653, Pascal formulated a very useful law concerning transmission of liquid pressure. Pascal's law states that \"liquid transmits pressure equally and perpendicularly in all directions when pressure is applied at a point on a liquid kept in a closed container\".Verification of Pascal's lawVerification of Pascal's law can be done with a simple experiment. Take a polythene bag and fill with water. With the help of a needle make a number of holes all around the bag. Now, squeeze the bag as shown in the figure. It can be seen that water comes out through all the holes simultaneously with the same pressure.We can verify Pascal's law in another way, too. Take a spherical vessel completely filled with water and fitted with four water tight pistons having the same cross-sectional area. When a piston is pressed, then all other pistons experience equal pressure and move outward equally. It shows that liquid pressure transmits equally in all directions.Polythene bagWaterTransmission of pressureQSR PSpherical glass vessel

198 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 199Applications of Pascal's lawPascal's law has a number of practical applications in our daily life. Some of them are hydraulic press, hydraulic jack, hydraulic brakes, etc. In these devices, there is multiplication of force on the basis of Pascal's law.Hydraulic MachinesHydraulic machines are the devices which multiply force on the basis of Pascal's law. There are different types of hydraulic machines like, hydraulic jack, hydraulic press, hydraulic brake, hydraulic lift, etc. While working, hydraulic machines use following properties of liquids:i. Liquids are incompressible.ii. Liquid transmits pressure equally and perpendicularly in all directions.ConstructionHydraulic machines consist of cylinders of different crosssectional area. These cylinders are connected with a pipe and filled with a liquid. Let there be two pistons of cross-sectional area A1 and A2.Multiplication of forceSuppose a force (F1) is applied on the small piston of the cross-sectional area (A1). This force exerts an external pressure on the enclosed liquid which is given by the formula, P1 = F1A1. This pressure transmits equally in all directions. The pressure (P1) exerts a force (F2) on the large piston of the cross-sectional area (A2) which is given by the formula, P2 = F2A2.From Pascal's law, P1 = P2or,F1A1 = F2A2 or,F2A2 = F1A1or,Large force on piston BLarge cross-sectional area = Small force on piston ASmall cross-sectional areaor,F2 = A2A1 × F1When A2 > A1, F2 > F1, it means that the large piston experiences many times bigger force than the force applied on the small piston. So, a hydraulic machine is also called a force multiplier.Hydraulic machineThe principle of a hydraulic machine states, \"A large force is developed on a larger piston when a small effort is applied on the smaller piston.\"MEMORY TIPSLiquid used in hydraulic machine should be incompressible and non-combustible.MEMORY TIPS

198 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 199FACTS WITH REASONSHydraulic machine is a force multiplier.A hydraulic machine is called a force multiplier because even if a small amount of force is applied to a small piston, a huge amount of force is produced in another large piston. i.e. P1 = P2 i.e., F1 / A1 = F2 / A2or, F1 × A2 = F2 × A1Since, A2 > A1, thus, F2 > F1 Hence, a hydraulic machine is a force multiplier.Different Types of Hydraulic Machinesi) Hydraulic jackA hydraulic jack is a device which is used to raise a heavy load such as a truck or bus. It is based on Pascal’s law. The pumping piston, lifting piston, lever and oil reservoir are the main parts of a hydraulic jack. The top valve, releasing valve and bottom valve are important valves in the hydraulic jack. The pumping piston has a small cross-sectional area and the lifting piston has a large cross-sectional area. So, we can use small force to lift a huge load. When the lever is pushed down, the top valve will be closed and the bottom valve will be open. The bottom valve helps in the one-way flow of liquid between the pumping piston and the lifting piston (right to left). As a result, the force used in the pumping piston will be multiplied and equal pressure with a large force will be transferred to the lifting piston. Then the load will rise a bit. Now, the lever will be pulled up. The top valve will open, the bottom valve will be closed and oil from the reservoir tank will go to the pumping piston. The top valve helps in the one-way flow of liquid between the oil reservoir and the pumping piston (left to right). Now the process should be repeated many times until the load is completely lifted.ii) Hydraulic brakesHydraulic brakes are those devices which are constructed based on Pascal's law and used as braking systems in vehicles. A special type of oil is used in hydraulic brakes because that oil cannot be compressed and does not burn. As a result, this enclosed oil can pass pressure equally in every brake shoe to stop the vehicle.

200 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 201The brake pedal, piston, master cylinder, brake oil, wheel cylinder and brake shoes are major parts of the hydraulic brake. It works based on Pascal’s law. As the brake pedal is pressed, the piston of the master cylinder increases pressure inside the tube containing brake oil. The brake oil transmits pressure equally in all directions. Through the brake oil, pressure spreads to the wheel cylinder. As the wheel cylinder has a large cross-section area, it magnifies force. Now, a large amount of force is applied to the brake shoes and wheel rim. Finally, the friction between the brake shoes and wheel rim slows down the wheel and the vehicle stops.iii) Hydraulic pressA hydraulic press is a device which is used to press different materials between the large piston and rigid support above it. A hydraulic press is an application of Pascal's law. It is used for compressing cotton bales, extracting oil from oil seeds, punching holes in metals, giving specific shapes to metal sheets, etc.A small piston, a large piston and a rigid support over a large piston are important parts of the hydraulic press. Little force is applied to the small piston. The small amount of force exerted in a small piston will be increased and a very heavy load can be lifted in a large piston. The lifted load will be pressed against a rigid support, which is fixed above it. iv) Hydraulic liftA hydraulic lift is a device which is constructed based on Pascal's law to raise a load easily. Hydraulic lift has a large piston, a small piston and an oil reservoir. The force is applied to the small piston. The compressed air will push the oil. Then the force will be multiplied by the machine and transferred to the large piston. The large piston will be used to lift the load. It is widely used in garages, hydraulic beds in dentists etc. Foot pedalLiquid Piston PipelineTo otherwheelBrakeshoeHingeReturn springBrake-shoeWheel cylinderMaster cylinderHydraulic brakescotton balla1a2F1Hydraulic Press

200 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 201v. Hydraulic excavatorA hydraulic excavator is a device whose digging arm with a hoe is based on Pascal’s law. It is used to dig canals, roads etc. Hydraulic cylinders and pistons are major parts in the hand of the excavator. The force used by the driver on the small piston will be transferred to the arm of the excavator by hydraulic fluid present in the flexible pipes. The arm of the excavator will move down and the backhoe will dig the ground. Solved Numerical 8.1Answer the following questions on the basis of the given figure.i) Which equipment is shown in the figure? On the basis of which law does it work?ii) According to the figure, calculate the pressure exerted at A.iii) Calculate the effort (F2) on piston B.Solution:i) The equipment shown in the given figure is a hydraulic machine. It works on the basis of Pascal's law.ii) Given,Force on piston A (F1) = 240 NArea of piston A (A1) = 20 cm2 = 20 × 10-4 m2 [  100 cm = 1m]We know, P1 = F1A1 = 24020 × 10– 4 = 1.2 × 105 Pa∴ Pressure at A ( P1) = 1.2 × 105 Paiii) Effort on piston B (F2) = ?Area of piston B (A2) = 0.4 m²We know, F2 × A1=F1× A2 or, F2 = F1A2A1 = 240 × 0.420 × 10–4 = 4.8 ×104 N∴ Effort on piston B (F2) = 4.8 ×104 NSolved Numerical 8.2The ratio of the area of cross-section of the two pistons in a hydraulic machine is 1:15. Calculate the force that should be applied on the narrow piston to overcome a force of 1200 N on the larger cross-section piston.Solution: Given,Ratio of the cross-section of the two pistons, A1: A2 = 1: 15Fig: excavator0.4 m2240N F2 = ?A B20 cm2

202 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 203Force on the larger cross-section piston (F2) = 1200 NForce required on the narrow piston (F1) = ?From Pascal's law, F1A1 = F2A2or, F1 = F2A1A2 = 1200 × 115 = 80NThe force that should be applied on the narrow piston is 80 N.UpthrustWhen we push a wooden cork in water, then the cork immediately rises up to the surface. In another example, it is easy to pull a bucket full of water when it is inside the well water. From these two examples, it is clear that the objects appear to be lighter inside water. It is due to the upward force exerted by the water. The same feeling can be experienced in other liquids, too. So, every liquid exerts an upward force on the object immersed in it. This upward force is called upthrust. Thus, the resultant upward force acting on an object when partially or wholly immersed in a liquid is called upthrust.Causes of upthrustWhen an object is immersed in water, it receives pressure from the liquid. The lower surface of the object receives more liquid pressure because the height of the liquid column is higher. So, the lower face of the object is pushed with greater force. The upper surface of the object receives less liquid pressure because the height of the liquid column is less. So, the upper face of the object will receive less force. So, since the force from the bottom is greater, the net resultant force acts upward. This resultant force acting upward in an object produced by a difference in liquid pressure is calledupthrust. The object will be pushed up. Hence upthrust exists. ACTIVITY 2To prove that a solid weighs less in water and more in air.1. Take a stone and tie it to a string as shown in the given figure.2. Fix another end of the string with a spring balance.3. Measure the weight of the stone in air and in water. Do you find a difference in weight?Apparent loss in weight of an object in liquid is equal to the upthrust, i.e. Upthrust = weight in air – weight in liquidIn the figure shown above, upthrust = 8 N – 6 N = 2 N.8N 6NThe tendency of a liquid to exert an upward force on an object placed in it is called buoyancy and the resultant upward force of a liquid is called buoyant force or upthrust.MEMORY TIPSUpthrustSmall forces on the topWater Large forces on the bottomWeight of boxUpthrust is applicable both in liquid and gas. The combined word of liquid and gas is called fluid.MEMORY TIPS

202 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 203Factors Affecting upthrustFollowing factors affect upthrust.i) Density of the liquid Upthrust is directly proportional to the density of the fluid in which an object is immersed. That is, Upthrust ∝ density of the fluid. For example, the cork dips more easily in oil than in water. Similarly, an iron nail floats in mercury but sinks in water. This is because mercury has more density and exerts more upthrust.Upthrust (U) ∝ density of the liquid (d)ACTIVITY 3To prove that an egg sinks in pure water and floats in salty water.1. Take two drinking glasses and fill with water almost to the top. 2. Gently drop an egg in each glass. They sink right to the bottom.3. Take out the eggs from the glasses and add about four teaspoons of common salt in a glass and stir. 4. Gently drop an egg into the salt solution. Observation: The egg floats in salt solution.Explanation: Density of the salt solution is more than that of pure water. Therefore, the salt solution exerts more upthrust on the egg and it starts to float.EggEggSalt Water solutionPure WaterFACTS WITH REASONSIt is easier to swim in the sea water, why?Upthrust is directly proportional to the density of the liquid. Sea water has a greater density than river water. Thus, upthrust experienced by a swimmer in sea water is comparatively more than in river water. So, it is much easier to swim in the sea water.The weight of an object in air is more than that in water, why?Upthrust is directly proportional to the density of the fluid. Density of air is less than that of water. The upthrust due to air is negligible in comparison to the upthrust due to water. So, the weight of an object in air is more than that in water.ii) Volume of the fluid displacedUpthrust is directly proportional to the volume of the fluid displaced by the object. That is, Upthrust (U) ∝ volume of the fluid displaced (V)It means, the larger the volume of a body submerged in a fluid, the greater is the upthrust.FACTS WITH REASONSIt is easy to immerse a small empty airtight plastic bottle into water than a bigger empty plastic bottle, why?Upthrust is directly proportional to the volume of liquid displaced by the body. A big airtight plastic bottle displaces more volume of water than a small plastic bottle. It creates more upthrust on the bigger plastic bottle. So, it is easy to immerse a small empty airtight plastic bottle into water than a bigger empty plastic bottle.Dead Sea float: The Dead Sea is a salt lake. It is more than 400 m below the sea level. Density of pure water is 1000 kg/m3 but the density of Dead Sea water is 1170 kg/m3. So, it is very easy to float in Dead Sea.MEMORY TIPS

204 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 205iii) Acceleration due to gravityThe upthrust is directly proportional to the acceleration due to gravity. i.e., Upthrust (U) ∝ acceleration due to gravity (g)Archimedes' PrincipleExperimental study of the upthrust on a body was first carried out by Greek Scientist Archimedes. The conclusion drawn from his experiment is known as Archimedes' principle. Archimedes' principle states, \"When a body is wholly or partially immersed in a liquid, it experiences an upthrust which is equal to the weight of the liquid displaced by it\".If the volume of the body inside the liquid is 'V' and the density of liquid is 'd', then the upthrust is given by Upthrust (U) = Weight of the liquid displaced = mg = VdgExperimental Verification of Archimedes' PrincipleMaterials required: Eureka can, spring balance, digital balance, water, beaker and wooden block. Procedure: Take a eureka can filled with water and put a beaker at the mouth of the hose as shown in the diagram. Then measure the weight of the wooden block in air and then put it in water to measure its weight in water. Now calculate the difference. The lost weight of the object is equal to upthrust. [upthrust = weight of an object in the air – the weight of the object in water]While putting the wooden block in water, it displaces some water which is collected in the beaker. Measure its weight in the digital balance. When compared, upthrust and the weight of water displaced by it will be equal. Therefore, an object immersed in water experienced upthrust which is exactly equal to the weight of the liquid displaced. It means Archimedes’ principle is true. \"Eureka\" comes from the Ancient Greek word. Its meaning is \"I have found (it)\".MEMORY TIPSSpring balanceStoneWaterTop pan balanceBeakerUrekacanW2W1Verification of Archimedes' principleUpthrust is equal to the apparent loss of weight of the submerged object.MEMORY TIPSWeight of an object in air > in water > in salt solution MEMORY TIPS

204 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 205Solved Numerical 8.3Answer the following questions from the given figure.i. How much is the upthrust exerted on the object immersed in water?ii. Calculate mass of the water displaced.Solution:i. Given, weight of the object in air (W1) = 60 N Weight of the object in water (W2) = 50 N Now, upthrust on the object (U) = W1 - W2 = 60 - 50 = 10 Nii. According to Archimedes' principle, Weight of the liquid displaced = upthrust Mass of water displaced × g = 10 ['g' is the acceleration due to gravity] ∴ Mass of the water displaced = 109.8 = 1.02 kgFACTS WITH REASONSHow do we know the purity of the ornaments?Each metal has its own density. While making ornament if any impurity is mixed in it, the ornament has either more or less density and volume compared to the pure metal. As a result, the ornament displaces liquid either more or less than the volume of pure metal of the same weight.Principle of FlotationA wooden cork floats in water but a piece of stone sinks. It is because the wooden cork has less density than water but the stone has more. Sinking and floating do not depend upon density of the liquid only. But they also depend upon the weight of the liquid displaced. According to law of floatation, \"a floating body displaces liquid equal to its weight.\" Forces Acting on an Immersed Body The weight of the object (gravity) and the upthrust are two major forces that act on an object immersed wholly or partially in the liquid. The weight of the object acts downward toward the centre of the earth. Whereas, opposite to weight, the upthrust always acts upward in the object. These two forces are acting just opposite of each other. The movement of the body is in the direction of the resultant force. There are three possible conditions: i. When the weight of the body is greater than upthrust (i.e. W> U) : The downward force on the body becomes more and the resultant force acts downward. As a result, the body sinks in the liquid.Density of a body > Density of liquidii. When the weight of the body is equal to the upthrust (i.e. W = U) : The downward force on the body is balanced by the upward force. Thus, the resultant force on the body StoneWater50N 60N

206 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 207becomes zero. In this condition, the body floats freely under the liquid surface. In this condition, the weight of the displaced liquid is equal to the weight of the body. This condition is satisfied when the density of the body is equal to the density of the liquid.Density of a body = Density of liquidiii. When the weight of the body is less than the upthrust (i.e. W < U) : The upward force on the body becomes more and the resultant fore acts upward. In this condition, the body floats partially immersed. This condition is satisfied when density of the body is less than the density of the liquid.Density of a body < Density of liquidupthrust (U)weight (W)upthrust (U)weight (W)upthrust (U)weight (W)a) b) c)a. An object sinks in water if its weight is greater than the upthrust. b. An object floats in water if its weight is equal to the upthrust. c. An object floats and most part remains above the water if its weight is less than the upthrust. Verification of the Law of FlotationMaterials required: Eureka can, spring balance, digital balance, water, beaker and wooden block. Procedure: Take a eureka can filled with water and put a beaker at the mouth of the hose as shown in the diagram. Then measure the weight of the wooden block in the air.While putting the wooden block in water, it displaces some water which is collected in the beaker. Measure its weight in the digital balance. When compared, the weight of the wooden block in the air is equal to the weight of water displaced by it. Therefore, an object immersed in water which is entirely floating displaced the liquid equal to its weight. Therefore, the weight of the wooden block = upthrust = weight of the liquid displaced. It means the law of floatation is true.

206 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 207Some Applications of the Principle of Floatationi) Rising of a hot-air balloonA hot air balloon consists of a large bag called envelope, with a wicker basket suspended underneath. In a hot air balloon, the air inside the envelope is heated with the help of a high pressure stove. The density of hot air in the balloon becomes less than the cold air outside it. When hot air is filled in a balloon, then it's volume will increase. It can displace more air from the surrounding. Then the weight of the air displaced by the balloon becomes more than its own weight (i.e upthrust > weight). The net upward force acting on the balloon is more than its own weight, and hence the balloon rises up.To go down the temperature of a hot air balloon should be decreased. The flame of the gas stove should be decreased. Then the density of the air inside the balloon will increase and the volume will decrease. It pushes less volume of air from the surroundings. The weight of air pushed by it will be lesser than its weight. It receives less upthrust and then sinks. ii) Floating and sinking a submarineSubmarine is a watercraft which can be made to float or sink in water. Floating and sinking of a submarine is controlled by upthrust on it. a) When water is admitted in the ballast tanks of a submarine, its weight becomes more than the upthrust and the submarine sinks in water. b) When water is ejected out from the ballast tanks of a submarine, its weight becomes less than the upthrust and the submarine floats on the surface of water.iii) An iron ship keeps floating but an iron nail sinks in waterA ship made from iron has a concave shape. It is capable of displacing more volume of water. In this condition, the weight of water displaced is equal to the weight of the ship. So, a ship made from iron floats in water but a solid piece of iron sinks in water.iv) Floating icebergIce has less density than water. A big piece of ice can displace water equal to its weight. So, ice floats on sea water with a small portion exposed.v) Life beltA life belt is an air-filled rubber tube. When we wear it, it acts as our own body part. It increases our volume but decreases average density. So, it helps us to push comparatively more water than before. If we use it we can displace more water than our weight and keep floating. Hot air balloonSubmarineFloating icebergLife belt

208 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 209vi) While swimmingWhile swimming, a good swimmer can displace water nearly equal to his weight. Because of this, a swimmer can swim easily in water. If a person is unable to displace water equal to his weight, he/she cannot swim and drowns in water.vii) HydrometerHydrometer is an instrument which is used to measure relative density of the liquid. It has a heavy bulb filled with mercury or lead shot and narrow and long stem with calibration. It works with the principle of law of floatation. Its bulb sinks more in low density liquid and sinks less in high density liquid. Due to heavy bulb and light stem, the hydrometer remains vertically upright in the liquid. Lactometer is a special type of hydrometer which is used to measure purity of milk.viii) Helium-filled balloonHelium-filled balloons are usually used as weather balloons. Helium is a very less reactive, stable and lighter gas. It occupies a much larger volume than the regular air. Its density is less than the regular air. So, when it is filled in the balloon, the balloon becomes much bigger, lighter and less dense. So, it can easily displace air more than its own weight from the surroundings. So, it receives more air and rises. When it goes up at a certain height it will stay there without going up because it cannot displace air equal to its weight in that place due to thinner air in the surroundings. ix) Flotation of fishFish is an aquatic animal with lots of air bladders in them. If they fill the air in the air bladder, they become bigger and less dense. Their volume increases. So, they can displace more weight of water from the surroundings than before. As a result, they receive more upthrust and can rise. If they remove air from the air bladder, they become a bit smaller and denser. Their volume decreases. So, they cannot displace more water from the surroundings than before. So, they receive less upthrust and can go down. FACTS WITH REASONSA balloon filled with hydrogen gas rises in the air but the balloon in which we blow air from the mouth falls to the ground. Hydrogen is the lightest gas, occupies more volume and has less density than the same mass of normal air. So, when it is filled in the balloon, the balloon will be larger, lighter and less dense than the same amount of regular air. Hence it can displace more air than its weight, receive greater upthrust and rise. However, the balloon in which we blow air from the mouth contains carbon dioxide gas. It will be heavier, denser and occupy lesser volume than the same mass of regular air. So, it cannot displace air equal to its weight, receives lesser upthrust and falls to the ground. Lactometer

208 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 209Differences between Archimedes’ Principle and Law of Flotation.S.N. Archimedes’ Principle S.N. Law of Flotation1. When an object is partially or wholly immersed in a liquid, it experiences an upthrust which is equal to the weight of the displaced liquid.1. The law of floatation states that the weight of a floating body is equal to the weight of the liquid displaced. 2. Any object immersed in the fluid obeys Archimedes’ principle. 2. The law of floatation works only on floating objects. HOT SKILL HIGHER ORDER THINKING SKILL1. The objects which have more density than the liquid sinks and those which have less density than the liquid floats in liquid. Is this statement always true? Justify with an example. The objects having less density than water float and the objects having higher density sink in liquid but it’s not always true. If an object is shaped in such a way that, it can receive more upthrust and can displace water equal to its weight it can float in the water, even if it has higher density than the water. For example, Aluminium nail sinks in water because it has more density than water. But aluminium foil does not sink in water because it is flat, its shape helps it to receive more upthrust. It can easily displace water equal to its weight and float. 2. If equal upthrust is acting on three footballs put in the water, which physical quantity is equal for them?If all three footballs put in water receive the same upthrust, all of them push equal volumes of water. So, their equal volume is immersed in water. 3. While cooking rice, water boils at a temperature less than 100°C in mountainous regions. While cooking rice, water boils at a lesser temperature than 100°C in mountainous regions because atmospheric pressure is lesser than standard pressure. When atmospheric pressure is less, water molecules are freer to move. If the atmospheric pressure is less water molecules gain more kinetic energy for the same amount of heat received. So, they boil at a lower temperature than 100°C. 4. It is very difficult to submerge a football in water.It is very difficult to submerge a football in water because it is filled with air. Its density is too less than the density of the water. When we push it under water it can push very large weight of water than its own weight. It experiences a greater upthrust. So, its difficult for us to push it into the water. 5. Is it possible for objects denser than water to float? Describe the conditions required for that case. Yes, it is possible for the objects denser than water to float. For example, a steel plate, an iron pin, and a ship made of iron float in water. A pin carefully put in water floats because of surface tension. A steel plate put flat on water floats because it can displace water equal to its own weight. A ship made from iron can float in water because it is hollow inside, which decreases its apparent density below the density of water. It is streamlined so it can displace water equal to its weight. So, it can easily float in water.

210 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 2116. Compare between ascending hot air balloon and descending hot air balloon.The comparison between ascending hot air balloon and descending hot air balloon are:SN Ascending hot air balloon SN Descending hot air balloon 1 A hot air balloon goes up if it receives more upthrust than its weight.1 A hot air balloon goes down if it receives less upthrust than its weight.2 To go up the temperature of the hot air balloon should be increased, the density of air inside the balloon should be decreased and the volume should be increased.2 To go down the temperature of a hot air balloon should be decreased, density of the air inside the balloon should be increased and the volume should be decreased. 3 It pushes more weight of air than its weight.3 It pushes less weight of air than its weight. 7. What is hydrometer? What does it mean if it sinks more in a liquid? Hydrometer is an instrument which is used to check relative density of a liquid. It has a heavy bulb and narrow and long stem.If its stem has sunken more in a liquid, it means the density of liquid is less. 8. What happens to the weight of water displaced, if more load is added in the ship? 59 people died in Philippines in 2015 when ship capsize carrying 200 persons instead of its capacity of 141 people. Why does probability of a ship capsizing increase if it is overloaded? A ship can float by displacing water equal to its weight. So, if more load is added in the ship, it sinks more and displaces greater weight of liquid so that it can keep floating. If too much load is added, the ship keeps sinking and eventually drown. The probability of ship capsizing increases if it is overloaded because ship has to displace liquid equal to its total weight to float. If it is overloaded it has to sink more. So, there is a chance that it will drown and capsize. 4 STEPS EXERCISE EXERCISESTEP11. Multiple Choice Questions a. Why are there studs under the football shoes? i. to make shoes fashionable ii. to prevent slipping iii. to increase surface area iv. to decrease friction and pressure b. Which principle can describe submarines?i. Pascal’s law ii. Principle of simple machine iii. Law of inertia iv. Archimedes’ principle c. When the lever of the hydraulic jack is pressed down after closing the releasing valve, the load will rise up. What happens during the process? i. The top valve closes and the bottom valve opens for a one-way flow of liquid between the oil reservoir and pumping piston (left to right), when the lever is pressed down. ii. The top valve closes and the bottom valve opens for a one-way flow of liquid between the pumping piston and the lifting piston (right to left) when the lever is pressed down.

210 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 211iii. The top valve closes and the bottom valve opens for a one-way flow of liquid between the oil reservoir and pumping piston (left to right), when the lever is lifted up. iv. The top valve opens and the bottom valve closes for a one-way flow of liquid between the pumping piston and the lifting piston (right to left), when the lever is lifted up.d. What is the condition for flying a hydrogen balloon upwards in the air?weight of the balloon = weight of air displaced weight of balloon < weight of air displacedUpthrust experienced = weight of the balloon A BUpthrust experienced > weight of the balloonC D i. A ii. Biii. C iv. De. What is the working mechanism of the hydraulic machine? i. effort is applied in large piston → pressure is produced in large piston →same amount of pressure is transferred to small piston → force is increased at small pistonii. effort is applied in small piston → pressure is produced in small piston →same amount of pressure is transferred to larger piston → force is increased at large piston iii. effort is applied in small piston → force is produced in small piston → same amount of force is transferred to larger piston → pressure is increased at large pistoniv. pressure is applied in small piston → equal force is transferred in every direction → equal force is received by larger pistonf. If the cross-sectional area of the first piston of hydraulic lift is half of the second piston, calculate the amount of load lifted in the second piston when 1 N force is used in the first piston. i. 2 Kg ii. 0.2 N iii. 2 N iv. 20 N g. Special oil is used in hydraulic machines. Why? i. it has less intermolecular distance, is incompressible and transfers pressure equally in all directions ii. it has more intermolecular distance, is compressible and transfers pressure equally in all directions iii. it is incompressible, non-combustible, flammable iv. oil has less friction, is biodegradable, does not produce smokeh. Study the given statements and arguments. Statement : When a life belt used around the waist of a person to prevent drowning is put in water, it receives more upthrust and keeps us floating.

212 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 213Argument 1 : Upthrust keeps increasing, if we push it deeper into the water. Argument 2 : Upthrust keeps increasing, only until it is submerged completely. Which of the following is correct? i. Statement and argument 2 are correct but argument 1 is incorrect. ii. Statement and argument 1 are incorrect but argument 2 is correct.iii. Statement is incorrect but arguments 1 and 2 are correct. iv. Statement is correct but arguments 1 and 2 are incorrect. i. Which of the following are based on Pascal’s law?i. ship, boat, submarine ii. fish, life belt, hot air balloon iii. hydraulic jack, hydraulic brakes, aeroplane iv. hydraulic excavator, hydraulic jack, hydraulic press j. Which of the following does not satisfy Pascal’s law? i.F1F2 = A1A2 ii.F1A1 = F2A2iii. F1A2 = F2A1 iv.F1A2 = F2A1 a. ii b. iv c. ii d. iv e. ii f. iii g. i h. i i. iv j. iv Ans of MCQSTEP22. Very short questions a. Write the characteristics of Pascal’s machine. b. Write the name of A and B. objects immersed in fluid ABforce acting on immersed objects upwards opposite to centre of earthdown toward centre of earthdirection of forcesc. When an object is weighed in three different mediums, the respective measurements are listed in the table below. If we mix all three mediums in a single container, which one will float at the top and which one will sink to the bottom?Medium A B CWeight 14 N 15N 16Nd. What is the reading on the weight scale when the wooden block is added to the container filled with water? e. If a 20 N stone, displaces 5 N of water when it is placed in water, what is the upthrust?f. What is thrust?

212 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 213g. \"Liquid transmits the pressure equally in all directions.\" Which law does this statement represent?h. Name two instruments which can be constructed based on Pascal's law. i. If the pressure in piston A is 10 Pa, what will be the pressure in piston B?STEP33. Short questions a. Why can the ship carry more load in the ocean than in a river?b. If the weight of a body in the air is 20 N and in a liquid is 10 N, find the weight of the displaced liquid and mass of that liquid. [Ans: 10 N, 1.02 kg]c. Draw the diagram of hydraulic brakes. Label master cylinder, d. On which principle does the given diagram work? State the principle, too. e. An iron ball does not float in water but a bowl made from it can float in water. Why?f. If we put an egg in a glass of water ‘A’ and another in a glass of salt solution ‘B’, what will be observed? What happens to the egg if we keep adding salt in glass A? Why?g. When a rubber ball having small holes of the same diameter is filled with water and squeezed with our hand, it is observed that water comes out with equal force in all directions, why?h. Differentiate between flying of balloon when the air temperature inside the balloon is higher and flying of balloon when the air temperature inside the balloon is lower. STEP44. Long questions a. What is a hydraulic jack? Describe the working mechanism of the hydraulic jack with a diagram.b. The different weights of a piece of stone on weighing in three different media: air, water and salt solution are given in the table. Answer the following questions.i. Which is air, water and a salt solution out of the three media A, B and C?ii. If the weight of 1 kg of mass in the air is 10 N, find out the mass of the piece of stone. [Ans: 1.8 kg]iii. Find out the mass of water displaced by the piece of stone. [Ans: 0.2 kg]DBA C240N F =?A B 20 cm2 0.4 m2Medium WeightA 16 NB 18 NC 15 N

214 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 215c. Study the given diagram and answer the following questions.i. Which instrument is shown in the figure? ii. Write a use of this device. iii. Which principle does it work?iv. What happens if it is kept in pure water and then in a salt solution?d. According to Archimedes’ principle, upthrust is equal to the weight of the displaced liquid. Prove this statement with an activity along with the diagram.5. Numerical i. In a hydraulic machine, the cross-sectional area of a narrow cylinder is 80 cm2 and that of the wide cylinder is 8m2. What load is necessary on the wider piston to balance the 800 N force kept on the narrow piston? [Ans: 8 × 105N.]ii. If pistons A, B and C of the apparatus given in the diagram are supposed to be frictionless, what is the area of piston B? What force is exerted on piston C?[Ans: 30 cm2,125 N]iii. Calculate the upthrust on an object immersed in water, if the object displaces water of mass 2 kg. (g = 9.8 m/s2) [Ans: 19.6 N]iv. Observe the given figure and answer the following questions. (Use g = 9.8 m/s2)Spring balanceStoneWater5NBeakerUrekacan20NHow much is the weight of the body in the air? Find out the mass of the stone. How much is the upthrust exerted on the body immersed in water? Calculate the mass of the water displaced.♦♦♦C = 10 cm2B = 375N250NA=20 cm2

214 PRessuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 215Key terms and terminologies of the unit1. Heat: The flow of thermal energy from hotter object to colder object is called heat. 2. Thermal energy: The sum of kinetic energy of all the atoms or molecules in a substance is called thermal energy. 3. Temperature: The degree of hotness or coldness of a body is called temperature.4. Temperature based on kinetic energy: The average kinetic energy of all the atoms or molecules in a substance is called temperature.5. One calorie heat: The amount of heat required to raise the temperature of 1 gram of water by 10C is called one calorie heat.6. Flow of heat: Heat flows from a body at high temperature to a body at low temperature.7. Thermometer: The instrument which is used to measure the temperature is called thermometer.8. Principle of a liquid in glass thermometer: Principle of a liquid in glass thermometer states \"matter expands on heating and contracts on cooling.\"9. Calibration: The process in which the length between the upper fixed point and the lower fixed point is divided into 100 equal divisions and scales are written on it is called calibration.Sequence of Curriculum Issued by CDC Introduction to thermal energy, temperature and heat energy  Effect of heat on the volume of an object based on the kinetic energy of the molecules.  Introduction to anomalous expansion of water and its application in daily life. Introduction to specific heat capacity  Simple calculations using the heat equation Specific heat capacity in daily life Introduction to thermometer and its types (liquid thermometer, digital thermometer, radiation thermometer) Working principle and calibration of thermometerUNITHeat Energy 9Estimated teaching periods Theory Practical8 2Shiba Kumar Rai, born on January 26, 1953, in Bhojpur, Nepal, is a prominent Nepalese professor of medical microbiology known for his significant contributions to combating infectious diseases. As a member of the National Planning Commission, Rai influences health policies, while his leadership in organizations like the Shi-Gan Health Foundation focuses on improving healthcare access and quality nationwide. His expertise in infectious diseases has been pivotal in addressing health challenges and advancing public health initiatives in Nepal. Shiba Kumar RaiAbout the Nepalese Scientist

216 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 21710. Clinical thermometer: A thermometer which is used to measure our body temperature is called clinical thermometer.11. Digital Thermometer: Digital thermometer is a mercury free thermometer which contains thermistor inside the tip to measure temperature.12. Laboratory thermometer: A thermometer which is used to measure temperature of the different substances in the laboratory is called laboratory thermometer.13. Specific heat capacity: The specific heat capacity of a substance is defined as the amount of heat required to raise the temperature of 1kg mass of that substance by 1°C.14. Heat equation: According to heat equation, the amount of heat gained or lost by a body (Q) is equal to the product of its mass (m), specific heat capacity (s) and change in its temperature (dt), i.e. Q = m s dt15. Calorimeter: A calorimeter is a highly polished cylindrical pot of copper or aluminium which is used to measure the quantity of heat.16. Calorimetry: The branch of physics which deals with the measurement of heat energy is called calorimetry. 17. IR thermometer: An IR thermometer is an instrument that measures the temperature of a body through the detection of the intensity of thermal radiation.18. Principle of calorimetry: The principle of calorimetry states, \"When a hot body is mixed with a cold body, the heat lost by the hot body is equal to the heat gained by the cold body, provided that no heat escapes to the surroundings\".19. Liquid in a glass thermometer: The thermometer that absorbs heat and then the thermometric liquid in it expands to show the temperature is called liquid in glass thermometer.IntroductionWe use the words 'warm' and 'cold' frequently in our daily life. Summer days are hot and winter nights are cold. We often prefer a cup of hot tea in winter. Similarly, we like to wear woolen clothes and bask in the sun in winter days. As a result of these activities we can make our body warm. Thus, the form of energy which gives us a sensation of warmth is called heat.Heat affects the degree of hotness or coldness of a body. The quantity by which we compare the degree of hotness or coldness of a body is called temperature. Thermal energy flows from hotter objects to colder objects. Substances expand on heating and contract on cooling except for some substances such as water which contracts when heated from 0°C to 4°C. In this unit, we will learn the basic concept of thermal energy, heat, temperature, thermal expansion and anomalous expansion of water. Thermal energy, heat and temperature in Terms of Molecular MotionThermal energyLet’s take a cup of water which is in a vacuum chamber. Now it is not connected thermally to any other objects. As we know already, the water is matter. It has mass, occupies space and is made of molecules. A cup of water is also made of lots of water molecules. Each water molecule has its kinetic energy. If we take the summation of the kinetic energy of every molecule, then the final sum of kinetic energy is the total amount of energy contained in that cup of water. It is called thermal energy. The total amount of energy, kinetic energy contained in a closed system or object is called thermal energy.

216 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 217Factors affecting thermal energy Thermal energy depends upon the mass of the object, temperature of the object and nature of the substance. Relation between thermal energy and mass For example, if variable such as temperature and nature of the substance (specific heat capacity) is controlled, then the object which has more mass will have more thermal energy. It means thermal energy is directly proportional to the mass of the objects. Let’s take an example of a 35-year-old man and his 8-year-old daughter. Since both are humans their body temperature must be 37°C. However, the man has more thermal energy in the body than his daughter because he has more mass. On cold days, the daughter who has less mass needs to wear more clothes than the father with more mass, because she has less thermal energy due to less number of molecules in the body. Another example could be a cup of tea and a kettle of tea. Let’s suppose the tea is boiling. It means the tea is at 100°C. If we pour some of it into the cup and leave the remaining in the kettle, which one will cool down first? Of course, the cup of tea cools down first even though both are at the same temperature. The cup of tea cools down first because it has less thermal energy due to less mass, less number of molecules and less summation of kinetic energy. Relation between thermal energy and temperature (average speed of molecules)If the mass of both substances is equal, then the object at a higher temperature will have more thermal energy. For example, if we have a litre of lukewarm water and another litre of boiling water, then the litre of boiling water has more thermal energy because its molecules have more kinetic energy. Relation between thermal energy and the nature of the substanceSome substances naturally contain more thermal energy than other substances of the same mass and temperature because they have more specific heat capacity. Temperature Let’s take two cups and put water from the tap in both. Let’s put the first cup of water in the fridge and the second one in an electric kettle and switch it on. Now what happens in these two cups of tap water? Let’s observe the first cup of water which is put in the fridge. The cup of water in the fridge will start to lose thermal energy. When it loses thermal energy, the kinetic energy of the water molecules will start to decrease. They will vibrate less. The water molecules cannot vibrate further. The intermolecular distance will decrease. Its volume will start to decrease and the intermolecular force grows stronger. Some water molecules will have a bit more kinetic energy and some water molecules will have a bit less kinetic energy. However, the average kinetic energy of the molecules will be lesser than it was before. Hence, the water will be colder than before if we touch it.Now, let’s observe the second cup of water in the electric kettle. The cup of water in the electric kettle will start to gain thermal energy. When it gains thermal energy, the kinetic energy of the water molecules will start to increase. They will vibrate more. The water molecules will Degree Celsius is the most common unit of temperature. MEMORY TIPS

218 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 219vibrate further. The intermolecular distance increases. Its volume will start to increase and the intermolecular force will grow weaker. Some water molecules will have a bit more or a bit less kinetic energy. However, the average kinetic energy of the molecules will be greater than it was before. Hence, the water will be warmer than before if we touch it.Now, as we know the temperature is the degree of hotness and coldness of an object, from the above explanation we can tell that the temperature is the average kinetic energy of the molecules of the substance. The temperature is an effect of thermal energy. If an object receives thermal energy, its temperature will rise and if an object loses thermal energy, its temperature will fall. The SI unit of the temperature is Kelvin. Some other popular units of temperature are Fahrenheit and degrees Celsius. The temperature of the substances can be measured by the thermometer. Absolute zero temperature The lowest possible temperature is -273°C. It is also called absolute zero temperature. Absolute zero temperature is defined as the hypothetical situation in which every molecule of an object will come to rest and the thermal energy of that object will be zero. HeatNow, let's suppose we have two cups of water. One is cold kept in the fridge and another is warm boiled in an electric kettle. What is the difference between them? The difference is, that the cup of water in the fridge is at a lower temperature and it has less thermal energy. And the cup of water from the electric kettle is at a higher temperature and it has more thermal energy. What happens if they are in thermal contact? If we mix the cold cup of water and the hot cup of water, they will be in thermal contact. It means thermal energy can flow between them. The temperature determines the direction of the flow of thermal energy. The water at higher temperatures has a greater average kinetic energy of molecules. The water at lower temperatures has a lesser average kinetic energy of the molecules. So, when these two water molecules come in contact, the thermal energy from the hotter object will start to flow to the object at a lower temperature. When two objects of different temperatures come in thermal contact the heat flows from the hotter object to the colder object. Such a flow of thermal energy from hotter objects to colder ones is called heat energy. The heat can be measured by a calorimeter. The measurement of heat lost or gained by the object is called calorimetry. According to the principle of calorimetry, the heat lost by one object is equal to the heat gained by another object at thermal contact. When the thermal energy flows, the temperature of the hotter objects drops whereas the temperature of the colder object rises. The thermal energy will keep flowing until both objects are at the same temperature. The condition in which both objects at thermal contact gain the same temperature by losing or gaining thermal energy is called thermal equilibrium. The heat flows in many ways. Broadly the transfer of heat is classified as radiation, convection and conduction. The transfer of heat without medium with the help of electromagnetic waves is called

218 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 219radiation. The transfer of heat on liquids and gases where fluid molecules gain kinetic energy and travel up is called convection. The transfer of heat in solid molecules, where molecules vibrate and transfer thermal energy to another molecule is called conduction. FACTS WITH REASONS1. Steam burns deeper in the skin than boiling water.Steam burns deeper in the skin than boiling water because steam molecules have more kinetic energy than the molecules of boiling water. It can easily penetrate skin due to greater kinetic energy and burn deeper. 2. We should not touch hot pot with bare hands.Our hands are at a lower temperature than the hot pot. So, when we touch the hot pot with bare hands, the thermal energy will start to flow from the hot pot to our hands. Then the average kinetic energy of molecules in the skin will increase and we get painful burns. 3. Iron nails become warm when hammered. When we hammer the iron nail, its molecules will receive the kinetic energy. That kinetic energy will be distributed to all molecules within the nail. On further hammering, the iron molecules will collect more kinetic energy. Therefore, the total kinetic energy of molecules (thermal energy) as well as the average kinetic energy (temperature) of iron molecules increases. Hence, iron nails become warmer when hammered. A case in which more thermal energy but low temperatureThere are more molecules in a bucket of lukewarm water than that in a cup of boiling water. Each molecule has some thermal energy. So, there is more thermal energy in the bucket of lukewarm water. However, the average kinetic energy of the molecules in a bucket of lukewarm water is less than that in cup of boiling water. The cup of boiling water has fewer molecules but moving faster. So, the temperature of bucketful of lukewarm water is less than the cupful of boiling water.ACTIVITY 1Take three beakers. Fill cold water in a beaker and hot water in another. Measure temperature of water in these beakers separately. Now, mix both in the third beaker and measure final temperature. Do you find the final temperature of water less than that of the hot water and more than that of the cold water? This is due to the flow of thermal energy from the water at high temperature to the water at low temperature.FACTS WITH REASONSWe feel hot when we touch a hot body and cold when we touch a cold body, why?We feel hot when we touch a hot body because the thermal energy from the hot body transfers to our body. Similarly, when we touch a cold body we feel cold because thermal energy from our body transfers to the cold body. More thermal energy but less temperatureLess thermal energy but more temperature

220 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 221Differences between Heat and Temperature.Heat Temperature1. Heat is the flow of thermal energy from hotter object to the colder object.1. Temperature is the average kinetic energy of all the molecules in a substance.2. Heat is measured by a calorimeter. 2. Temperature is measured by a thermometer.Measurement of Heat: Units of HeatHeat is measured by a calorimeter in the same unit as energy. The SI unit of heat is joule (J). The more common unit of heat is the calorie. One calorie heatThe amount of heat required to raise the temperature of 1 gram of water by 1°C is called one calorie heat. That is, 1 calorie = 4.2 joulesThe branch of physics which deals with the measurement of heat energy is called calorimetry. A calorimeter is a highly polished cylindrical pot of copper or aluminium which is used to measure the quantity of heat. Its outer surface is kept shining which prevents the exchange of heat between the pot and the surrounding. Such arrangement is kept in a hollow wooden box. The gap between the pot and the box is filled with non-conductor of heat like, wool, cotton, etc. The stirrer kept in the pot is used to stir the mixture for uniform distribution of heat. MEMORY TIPSThermometerstirrercalorimeterEffects of HeatHeat energy is the flow of thermal energy. When an object loses or gains heat energy, some changes are observed in it. Some of them are changes in colour, volume, length, chemical reactivity, surface area, solubility, state of matter etc. Relation between heat and volume of the substanceThe volume of the substance is dependent upon the thermal energy present in it. If an object is heated, it receives the thermal energy. Its atoms and molecules will gain more kinetic energy. Their average kinetic energy (temperature) rises. So, the intermolecular force between the molecules decreases. They will vibrate faster and farther. Their intermolecular distance will grow. As a result, the entire volume of the object, the length of the object and its surface area too will increase. On, further heating the solid could change into liquid and the liquid can evaporate into gas. If we cool an object, it loses thermal energy. Its atoms and molecules will lose kinetic energy. Their average kinetic energy falls. So, the intermolecular force between the molecules increases. They will vibrate slower and nearer. Their intermolecular distance will decrease. As a result, the entire volume, length and surface area of the object will decrease. On further cooling, the If we add one calorie heat to one gram of water, its temperature will rise by 10C.MEMORY TIPS

220 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 221gas will condense into liquid and liquid will change into solid. The process in which substances change shape, size, volume, length and surface area when it is heated is called thermal expansion. Application of thermal expansioni. Gaps are left in the railway track.ii. The iron rim is heated before putting it in the wooden wheel.iii. The tight cap of the bottle can be opened by dipping in warm water.iv. Mercury and alcohol can be used as thermometric liquids in liquid glass thermometers. FACTS WITH REASONSGaps are left between two successive railway tracks.To allow the railway tracks to expand during their heating, a small gap is left between two successive railway tracks while laying on the surface. If it is not done, rails curve on heating which causes accident.In the middle of a day or a hot day in summer, the electric wires sag more, why?Electricity transmission line, and overhead telephone lines suspended from the poles expand on heating. So, in the middle of a day or a hot day in summer, the wires sag more. Disadvantage of thermal expansion i. Weathering of stone monuments. ii. Weathering of plaster of house and walls.iii. Thermos flask cracks when hot tea is put in its cold glass container. FACTS WITH REASONSThermos flask cracks when hot tea is put in its cold glass container.Thermos flask cracks when hot tea is put in its cold glass container because the inner side of the glass receives thermal energy from hot water. These molecules will gain kinetic energy, vibrate faster and expand. However, glass is an insulator. So, it cannot transfer thermal energy to the outer layer of glass. Here, the glass molecules of the inner layer have more average kinetic energy and the glass molecules at the outer layer have lesser average kinetic energy. This difference will create pressure and the glass will break. Anomalous expansion of water Let’s take one litre of water at 0°C, supply 4200 Joules heat energy and measure the temperature. It is found to be 1°C. Then again supply another 4200 joules of heat energy and measure the temperature again. The temperature is found to be 2°C. Let’s repeat the process up to 4°C. Now in this experiment, we supplied thermal energy to raise the temperature of a litre of water from 0°C to 4°C. Following changes were observed in the water. Fig. Pond freezes from top but liquid water remains at bottom

222 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 223Effect on water while heating from 0°C to 4°Ci. When heated from 0°C to 4°C, the volume of water decreased. ii. The water molecules that are being heated from 0°C to 4°C, tend to sink.iii. The density of water increased when heated from 0°C to 4°C. The water at 4°C is the densest. So, it remained at the bottom.According to the concept of thermal expansion, substances are supposed to expand when heated. Their volume should increase and their density should decrease when thermal energy is supplied. However, in this experiment, we noticed that the opposite is happening. When heated, water is contracting, volume is decreasing and density is increasing. It is an anomaly. Therefore, this property of the water, where it contracts on heating from 0°C to 4°C is called anomalous expansion of water.Does water show anomalous expansion of water at any temperature? No! it shows anomalous expansion from 0°C to 4°C only.Just to be sure, let's supply another 4200 Joules of heat energy to a litre of water at 4°C. Its temperature will rise and become 5°C. Let’s repeat the process until that litre of water is at 8°C. Now in this experiment, we supplied thermal energy to raise the temperature of a litre of water from 4°C to 8°C. Following changes were observed in the water. Effect on water while heating from 4°C to 8°Ci. When heated from 4°C to 8°C, the volume of water increased. ii. The water molecules that are being heated from 4°C to 8°C, tend to float.iii. The density of water decreased when heated from 4°C to 8°C.The water showed a regular, normal state of thermal expansion when heated beyond 4°C. Does water show anomalous expansion of water if it loses thermal energy? Yes! It shows anomalous expansion even if it is cooled. To be sure let's take a litre of water at 4°C, remove 4200 Joules of heat energy and measure the temperature. The temperature is found to be 3°C. Let’s repeat the process until the water is at 0°C. Now in this experiment, we removed thermal energy to decrease the temperature of a litre of water from 4°C to 0°C. Following changes were observed in the water. Effect on water while cooling from 4°C to 0°Ci. When cooled from 4°C to 0°C, the volume of water increased.ii. The water molecules that are being cooled from 4°C to 0°C, tend to float.iii. The density of water decreased when heated from 4°C to 0°C. The water at a lower temperature than 4°C is less dense and floats. So, water at 0°C is the least dense water and remains at the surface. As a result, water starts to freeze from the top. Advantages and disadvantages of anomalous expansion of waterThe advantage of the anomalous expansion of water is, that the bottom of the pond does not freeze where aquatic animals can survive. The disadvantage of anomalous expansion of water is, that the water pipes burst in winter. Application of anomalous expansion of wateri. Anomalous expansion of water is applied in the formation of ice, and freezing of ponds with

222 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 223liquid water underneath and its knowledge helps to design taps, water pipes etc. to prevent bursting. ii. Water pipes crack in cold places in the winter because first of all, the densest water at 4°C fills the pipe. Then on further cooling down the volume of water increases which puts pressure on the walls of the pipe. On further cooling down below 0°C, water changes into ice. The ice has the highest volume. So, it puts greater pressure on the pipes. So, the water pipes burst in winter. iii. Cold drink bottles, medicine bottles, milk bottles etc. contains a small space left because it helps to prevent the cracking of bottles. When the water in the cold drinks expands due to anomalous expansion of water, this little space will be filled. Therefore, an increase in volume does not put pressure on the glass and the glass will not break. iv. Due to anomalous expansion of the water, when the water cools down to 4°C during winter, it gains maximum density and sinks to the bottom. The process continues until the entire pond has water of 4°C. Then the surface further cools down to 3°C to 2°C to 1°C and then to 0°C. This water has less density than water at 4°C. So, it will stay at the top and freeze. Ice acts as an insulator and prevents further heat loss. As a result, fish can survive deep under the ponds and lakes even though the surface is frozen. Graphical plot of Anomalous expansion of water Graph of Volume-temperature curve Graph of Density-temperature curveMeasurement of TemperatureThe instrument which is used to measure temperature is called thermometer. ThermometerCalibration of the liquid in glass thermometer (Standard Points): Steam point (i.e. upper fixed point) and ice point (i.e. lower fixed point) are the two standard points fixed on the stem of a thermometer to make a thermometric scale. Thus, the process of

224 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 225making a number of equal divisions in between the upper fixed point and lower fixed point of a thermometer is called its calibration. The temperature at which water boils at standard atmospheric pressure is called the upper fixed point and the temperature at which pure ice melts at standard atmospheric pressure is called the lower fixed point.The different types of thermometers have different fixed points. For example, in Celsius scale the lower fixed point is 00C and upper fixed point is 1000C; in Fahrenheit scale the lower fixed point is 320F and upper fixed point is 2120F; similarly, in Kelvin scale the lower fixed point is 273K and upper fixed point is 373K.Types of thermometers based on thermometric liquidsOn the basis of the thermometric liquid used, there are two types of thermometers. They are mercury thermometer and alcohol thermometer.Types of thermometers on the basis of uses A thermometer is an instrument which is used to measure the temperature of a body. On the basis of uses, there are different types of thermometers. They are liquid in glass thermometer, digital thermometer and radiation thermometer. Liquid in glass thermometer : The thermometer which measures temperature due to the expansion of liquid kept in its bulb that rises in the capillary tube is called a liquid thermometer. The bulb, stem, capillary and thermometric liquid are its main components. The stem is usually long prismatic or cylindrical glass which has scales and calibration. There is a thin capillary in the stem. The thermometer consists of a very fine glass tube within the stem having a very small bore is called a capillary tube. At one end of the thermometer, a very thin-walled glass vessel filled with thermometric liquid and connected to the capillary tube is called a bulb. The substance used in a thermometer to measure the temperature is called thermometric liquid. They are Mercury and alcohol. Mercury is used as a thermometric liquid because it is a very good conductor, high density, low specific heat capacity and low specific heat capacity metal. It can respond to heat faster and expand uniformly without wetting glass. It does not need electricity to operate.Principle of liquid in the glass thermometer: The principle of the liquid in the glass thermometer is thermal expansion. The bulb absorbs heat and gives it to the high-density thermometric liquid (mercury). Then mercury expands on heating in a thin capillary tube of the stem. There is calibration in the stem. The position of Mercury will tell the temperature. When cooled mercury will contract and go back to bulb. FACTS WITH REASONSNowadays, the clinical thermometer is being replaced by the digital thermometer.Nowadays clinical thermometer is replaced by digital thermometer because digital thermometer shows temperature in exact numbers, are easy to use and does not contain poisonous mercury. Clinical thermometer

224 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 225ii) Digital Thermometer Digital thermometer is a mercury free thermometer which contains thermistor inside the tip to measure temperature. Nowadays, digital clinical thermometers are used widely to measure our body temperature. They provide highly accurate result of the body temperature. It reads temperature when the built-in sensor is kept in contact with the human body. They are equipped with beep alarm and memory function. This helps to record a wide range of temperature. It is a contact thermometer.Principle of digital thermometer: The principle of a digital thermometer is fluctuations in the resistance of the thermistor. When a thermistor collects heat, its resistance increases. Based on the amount of resistance temperature is displayed in the monitor. A digital thermometer has following advantages over the mercury-used clinical thermometer:1. It is more accurate than a bulb thermometer.2. It shows the temperature directly into digits. 3. It is toxic-free thermometer. If a mercury thermometer breaks, then the disposal of mercury creates a problem.iii) Infrared Radiation (IR) thermometerA radiation thermometer is an instrument that measures the temperature of a body through the detection of the intensity of infrared radiation. It works by collecting and concentrating infrared radiations. An infrared radiation thermometer is a non-contact thermometer. It contains a lens and heat sensor. It can collect infrared radiation from a person from far away, change it into an electric signal and display the temperature on the monitor. So, the Infrared radiation thermometer was widely used during the coronavirus pandemic because it is easy to use, avoids contact with an infected person and prevents infection. Principle of radiation thermometer:The principle of a radiation thermometer is the intensity of the infrared radiation emitted by an object. If more infrared is coming from an object, more heat is collected in the sensor. It generates electric signals and the corresponding temperature is shown on the monitor. FACTS WITH REASONSSensor of the infrared thermometer is turned toward the forehead of the person. The sensor of infrared thermometer must be turned toward the forehead of the person to collect as much infrared radiation as possible. It helps to collect more infrared radiation. Then more electric signal will be produced and correct temperature can be displayed in the monitor. ACTIVITY 2Make a 3D model of an infrared thermometer. Digital thermometerInfrared thermometers are quicker and easier to use than other thermometers. MEMORY TIPSRadiation thermometer

226 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 227Specific Heat CapacityMetals usually get hot very quickly. In kitchen, when we put a steel vessel on a gas flame then very soon it becomes too hot to touch. If there is water of the same mass and heated with the same amount of heat then it does not become as hot as the steel.FACTS WITH REASONSEqual masses of different substances require different amount of heat to be heated through the same rise in temperature. Why?Equal masses of different substances require different amount of heat to be heated through the same rise in temperature because atoms and molecules of the different substances have different nature. Atoms and molecules of these substances vibrate with different degrees if we supply same heat. ACTIVITY 31. Take two beakers of the same size and material.2. Take 100 g of water in a beaker and 100 g of alcohol in another beaker. 3. Insert thermometers in both the beakers and record the temperature of water and alcohol.4. Now, heat both the beakers for 5 minutes one after another by the same burner. Stir both during heating for uniform distribution of heat.5. Note, the final temperature of water and alcohol after 5 minutes. Calculate the change in temperature of water and alcohol. Do you find it same?6. Equal amount of heat is given to both water and alcohol. But, it is found that the rise in the temperature of alcohol is more than the rise in the temperature of water. 7. Again, note their cooling temperature in every 1 minute when they lose heat while cooling. Find the rate of decrease in temperature. Do you find it same?8. Alcohol cools faster than water. Thus, the substance which gets heated faster also cools faster.ThermometerWaterBunsen burnerStand StandAlcoholIn the above activity, alcohol requires less amount of heat to raise its temperature by 10C than the same amount of water. Thus, the specific heat capacity of a substance is defined as the amount of heat required to raise the temperature of 1kg mass of that substance by 1°C. The above discussion proves that, the heat capacity of alcohol is less than water. Units of Specific Heat CapacityIn SI units, the unit of specific heat capacity is J/kg oC or J kg-1 oC-1.In CGS system, its unit is Cal/g oC.Object with higher specific heat capacity Object with lower specific heat capacityIt takes longer time to be heated or cooled. It takes shorter time to be heated or cooled.It can absorb or release more heat. It can absorb or release less heat.It is a poor conductor of heat. It is a good conductor of heat.Specific heat capacity is the heat energy per kilogram per change in temperature.MEMORY TIPS

226 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 227Values of specific heat capacities of different substancesS.N. Substances Specific heat capacity S.N. Substances Specific heat capacity1. Lead 130 J/kg°C 10. Aluminium 897 J/kg°C2. Mercury 136 J/kg°C 11. Air 1010 J/kg°C3. Silver 236 J/kg°C 12. Gold 130 J/kg°C4. Brass 380 J/kg°C 13. Wood 1755 J/kg°C5. Copper 385 J/kg°C 14. Vegetable oil 2000 J/kg°C6. Steel 447 J/kg°C 15. Ice 2100 J/kg°C7. Iron 460 J/kg°C 16. Kerosene 2010 J/kg°C8. Glass 670 J/kg°C 17. Alcohol 2400 J/kg°C9. Sand 800 J/kg°C 18. Water 4200 J/kg°CMeaning of specific heat capacity of water is 4200 J/kg°CSpecific heat capacity of water is 4200 J/kgoC. It means that 4200 J heat is required to raise the temperature of 1 kg mass of water by 1°C.Advantage of high specific heat capacity of wateri. Water is used in cooling systemsWater has high specific heat capacity i.e. 4200 J/kg°C. It means that it can absorb more amount of heat. Therefore, water is used to cool the engines of automobiles and other machines.ii. Water is used in heating systemsWater has high specific heat capacity i.e. 4200 J/kg°C. It means that it can store more amount of heat and can radiate more amount of heat for longer period of time. So, it is used in hot water bottles to warm the bed, in a central heating system, etc.iii. High specific heat capacity of water causes sea breeze and land breeze:Air flows from a high air pressure zone to a low air pressure zone. During the day time, the land is hotter than the sea because the land has less specific heat capacity than the water. The air becomes hot and escapes from the land. So air pressure above the land is lower than above the sea. So, the air flows from the sea to the land during the day. It is called a sea breeze.But during the night, land cools faster than water. Water remains warm for a longer duration because it has a high specific heat capacity. There is high air pressure above If you supply 4200 J of heat to 1 kg of water, you will raise its temperature by 1°C.MEMORY TIPS

228 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 229the ground and low air pressure above the water. So, air moves from the land to the sea during the night. It is called a land breeze.Applications of high specific heat capacity of water are: i. It can be used in a hot water bag, lower fever, cool the engine of vehicles etc. ii. It makes pleasant weather near the seas. iii. Water present inside our body controls our body temperature. It is also used as coolant in nuclear powerplant, coolant in thermal power plant.Disadvantage of the high specific heat capacity of waterWater takes a very long time to boil, food does not cool down faster and boiled water remains hot for a longer time due to the high specific heat capacity of water. So it can be inconvenient sometimes. If boiling water burns our hand, it burns very badly because it supplies a large amount of heat to our flesh. FACTS WITH REASONSA wet handkerchief is used on the forehead of a person suffering from fever.A wet handkerchief is used on the forehead of a person suffering from fever because the specific heat capacity of water is very high, and it can absorb large amounts of heat energy with each degree of increase in temperature. It helps to lower our body temperature. Fluctuation in temperature during day and night is very less around the coastal areas, why?Due to high specific heat capacity of water i.e. 4200 J/kg0C, the sea water gets heated slowly during the day and cools during the night. Whereas the costal land having less specific heat capacity gets heated fast and cools down fast. So, the denser cold air from the sea blows towards the land during the day (sea breeze) and high density cold air from the land blows towards the sea during the night (land breeze). This makes less fluctuation in temperature during day and night around the coastal areas.In comparison to tap water, well water is warmer in the morning during the winter season.Well water is warmer in the morning in the winter season because water has a higher specific heat capacity compared to air and soil. Therefore, air and soil lose their heat energy faster than well water during the night. Well water loses heat slowly and it also absorbs heat from the earth. So, it remains warm. Factors Affecting Amount of Heat ContainedThe factors affecting heat lost or gained are mass of the object, change in temperature of the object and nature of the object. i. Heat lost or gained is directly proportional to the mass of the object. Q α mii. Heat lost or gained is directly proportional to the change in temperature. Q α dt iii. Heat lost of gained depends on nature of the substance called specific heat capacity.

228 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 229ACTIVITY 4To show that amount of heat contained in a body depends upon change in temperature and mass1. Fill 100 g of water in a beaker and immerse a thermometer into the water to measure its temperature.2. Supply heat for some time to raise its temperature upto 200C. Again heat the water for some more time. Does the temperature increase with the increase in amount of heat supplied? 3. Next time add 200 g of water in another beaker and heat both of them equally for the same time.4. Note the change in temperature in these two beakers. Do you observe the change in temperature more in case of 100 g water? 5. Take another beaker and partially fill it with 200 g of alcohol.6. Heat both 200 g of water and 200 g of alcohol equally for the same time and observe the change in temperature.7. Do you observe the rise in temperature of alcohol more? Heat equationWhen a cold body is heated, it absorbs heat and its temperature will rise. Similarly, when a hot body is cooled, it releases heat and its temperature will fall. But it has been proved that heat gained or lost (Q) by a body is directly proportional to the mass of the body and change in temperature.From the above statement, The heat gained or lost (Q) is directly proportional to its mass (m), i.e.Q ∝ m ………………. (i)The heat gained or lost (Q) is directly proportional to the change in temperature (dt), i.e.Q ∝ dt ………………. (ii)Combining relation (i) and relation (ii)Q ∝ m dtQ = m s dt (Where 's' is a proportionality constant called specific heat capacity.)This is heat equation. Thus, according to heat equation, the amount of heat gained or lost by a body (Q) is equal to the product of its mass (m), specific heat capacity (s) and change in its temperature (dt), i.e. Q = m s dt.From the heat equation, Q = m s dt, when 'Q' and 'm' are kept constant, dt ∝ 1sFrom the above relation, it is clear that on supplying equal amounts of heat, the rise in temperature will be more in the substances having low specific heat capacity, whereas the rise in temperature will be less in the substances having high specific heat capacity.MEMORY TIPS

230 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 231Calorimetry and its PrincipleIn winter season, people mix hot water with cold water for bathing. The temperature of the mixed water is less than the temperature of the hot water and more than the temperature of the cold water. In this process, the hot water loses heat and the cold water gains heat. The amount of heat lost or gained by an object is measured by using calorimetry. The principle of calorimetry states, \"When a hot body is mixed with a cold body, the heat lost by the hot body is equal to the heat gained by the cold body, provided that no heat escapes to the surroundings\".From the above statement, it is clear that Heat lost = Heat gained m1s1 (t1 – t) = m2s2(t–t2)Here, t = Final temperature of the mixturem1 = Mass of the hot body m2 = Mass of the cold bodys1 = Specific heat capacity of the hot body s2 = Specific heat capacity of the cold body t1 = Temperature of the hot body t2 = Temperature of the cold bodySolved Numerical 9.1Calculate the amount of heat required to change the temperature of 2 kg of iron from 30°C to 90°C. The specific heat capacity of iron is 460 J/kg °C.Solution: Given,Mass of iron (m) = 2.0 kg Specific heat capacity (s) = 460 J/kg°CChange in temperature (dt) = 90 ° C – 30 °C = 60 °C Amount of heat (Q) = ? We have, Q = m s dt = 2 × 460 × 60 = 55200 J∴ The required heat is 55,200 J.Solved Numerical 9.2Specific heat capacity of water is 4200 J kg-1 oC-1. Calculate the amount of heat energy released when 5 kg of water cools down from 80oC to 30oC.Solution: Given,Specific heat capacity of water (s) = 4200 J kg-1 oC-1Mass of water (m) = 5 kgInitial temperature (t1) = 80o CFinal temperature (t2) = 30o CChange in temperature (dt) = t1 - t2 = 80o C - 30o C =50o CWe have, Q = m s dt = 5 × 4200 × 50 = 1050000 = 1.05 × 106 J∴ The heat released is 1.05 × 106 J.Heat lost or gained is measured by calorimeter.MEMORY TIPS

230 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 231Solved Numerical 9.3100 g of water at 1000C is mixed with 200 g of water. If the mixture temperature becomes 500C, find the initial temperature of the cold water.Solution: Here, Mass of hot water (m1) = 100 g = 1001000 = 0.1kgInitial temperature of hot water (t1) = 100°CMass of the cold water (m2) = 200 g = 0.2 kgFinal temperature of the mixture (t) = 50°CInitial temperature of cold water (t2) =?From the principle of calorimetryHeat lost by hot water = Heat gained by cold water m1s1 (t1 – t) = m2s2(t–t2)or, 0.1 × (100 – 50) = 0.2 × (50 – t2)or,0.1 × 500.2 = 50 – t2or, t2 = 50 – 25 = 25°C∴ The initial temperature of cold water is 25°C.HOT SKILL HIGHER ORDER THINKING SKILL1. Substance expands on heating.When the substances are heated, the thermal energy enters them. The molecules of that substance receive the thermal energy. Their kinetic energy will increase. They start to vibrate faster and farther. Their intermolecular force will decrease and intermolecular distance will increase. As a result, their volume increases. It is called thermal expansion. Hence, substances expand on heating. 2. The temperature of mercury becomes more when an equal amount of heat is given to one kg of water and one kg of mercury each.Even if an equal amount of heat is given to the unit kilogram of water and the mercury, the mercury gains higher temperature because the specific heat capacity of mercury (126J/kg°C) is less than that of water (4200J/Kg°C). Mercury molecules gain more kinetic energy than water molecules and gain higher temperature than water, even if they absorb the same amount of heat. 3. Touching a cup of boiling water feels hot but touching a cup of water from the fridge feels cool. Explain it based on the motion of their molecules. A cup of boiling water feels hot because it is at a higher temperature than our body. The molecules of a cup of boiling water have more kinetic energy. They vibrate faster. The molecules of our skin are at low temperatures. They have lesser kinetic energy. They vibrate slowly. So, when the cup of boiling water and our skin comes in contact, the vibration of the cup of boiling water decreases and the vibration of molecules of our skin increases. Then the heat flows from a cup of boiling water to our skin. So, touching a cup of boiling water feels hot.

232 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 233But a cup of water from the fridge is cold. Its molecules have lesser kinetic energy. Compared to a cup of water from a fridge, molecules of our skin have more kinetic energy. So, when we touch a cup of water from a fridge, thermal energy flows from our hand to ice. The motion of molecules in our hand decreases and the motion of molecules in a cup of water from freedge increases. So, we feel cold. 4. Study the given graph where the relation between the volume of the water and the change in water temperature is shown graphically. i. What does the arc from 0°C to 4°C represent? The curve from 0°C to 4°C represents the anomalous expansion of water. ii. At what temperature water has the minimum volume? Why? Water has a minimum volume at 4°C.It happens because of anomalous expansion of water. The density of water increases and volume decreases when heated from 0°C to 4°C. iii. What happens if the water at O°C is filled in glass and warmed up to 6°C? If the water at 0°C is filled in glass and warmed up to 6°C, its density increases and volume will decrease from 0°C to 4°C. Then its density will decrease and volume will again start to increase if heated above 4°C.5. Why does sugar dissolve faster in hot water than the cold one?The sugar dissolves faster in the hot water than the cold water because the molecules of the water are moving faster, they have more kinetic energy, less intermolecular force and large intermolecular distances than the cold water. So, sugar molecules can enter in them. 6. Compare between thermal energy and heat energy. The differences between thermal energy and heat energy are:SN Thermal energy SN Heat energy 1 The total kinetic energy possessed by the molecules of the object is called thermal energy. 1 The flow of the thermal energy from hotter objects to colder objects on thermal contact is called heat energy. 2 It is the stored form of kinetic energy in an object.2 It is the form of energy which is transferring.3 Geothermal energy, nuclear energy, heating effect of electricity produces thermal energy.3 Burning fuels such as firewood, petroleum etc. produces heat energy. 7. When a hot iron nail is kept in cold water, the temperature of the nail is reduced, whereas the temperature of the water is increased. The nail is hot. It has more thermal energy. Its molecules have higher temperatures. Its molecules are vibrating at higher average kinetic energy. Whereas the water is cold. Its molecules are vibrating at lower average kinetic energy. So, when a hot iron nail comes in contact with cold water, the thermal energy will flow from the iron nail to the water. The iron nail starts to lose thermal energy. The kinetic energy of molecules will decrease. Therefore, the temperature (average kinetic energy of the molecules)

232 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 233decreases. However, cold water is receiving thermal energy. Its molecules will gain thermal energy. The kinetic energy of the molecules of cold water will increase. The water molecules will start to vibrate at greater average kinetic energy. Therefore, its temperature will rise. 8. Wooden or plastic spoon is used to eat ice-cream than a steel spoon. Why?Steel spoon is a very good conductor of the heat. It can transfer heat into the ice cream faster and melt the ice cream. However, wood is a bad conductor of heat. It won’t transfer heat into the ice cream and ice cream won’t melt. Therefore, wooden or plastic spoon is used to eat ice cream than a steel spoon. 9. During winter, in cold countries, water pipes burst in many places. What could be the reason? How do people deal with this problem?Water pipes usually burst in winters in cold countries. The reason is anomalous expansion of water. Due to anomalous expansion of water, water has highest volume at 0°C. When it turns to ice it has even greater volume. It exerts greater pressure on the walls of the water pipes and breaks them if left unchecked for long time. People usually leave the tap running, to prevent water from freezing in the pipes. It helps to prevent pipe from bursting. 10. Explain the relation between the specific heat capacity and the temperature change. The specific heat capacity of a substance is inversely proportional to the rate of change in its temperature. If the specific heat capacity of a substance is greater, the temperature will change slowly.If the specific heat capacity of a substance is less, the temperature will change faster.For example, Let’s put a cup of water on a plate and a cup of sand on a plate and put them both in the sunlight. The cup of water on the plate will heat slowly and gain less temperature because it has a more specific heat capacity.The cup of sand will heat faster and gain more temperature because it has less specific heat capacity. 4 STEPS EXERCISE EXERCISESTEP11. Multiple Choice Questions a. Why does water start to freeze from the top?i. anomalous expansion of water ii. fall in temperature iii. ice is lighter iv. specific heat capacity of water b. When does specific heat capacity equal the amount of heat? i. if the temperature of two unit masses increases by a unit degree Celsius ii. if the temperature of a unit mass changes by a unit degree Celsius iii. if an object is at absolute zero temperature iv. if the specific heat capacity of water is 4200J/kg°C c. Which of the following describes the anomalous expansion of water when it is heated from 0°C to 3°C?Increase in volume Decrease in volume Increase in density A BDecrease in density C D

234 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 235i. A ii. Ciii. B iv. D d. What happens if thermal energy is lost by the object?i. decrease in intermolecular distance, rise in temperature, change in state ii. increase in intermolecular distance, rise in temperature, change in state iii. increase in intermolecular distance, rise in temperature, increase in volume iv. decrease in the intermolecular distance, fall in temperature, decrease in volumee. Three substances of the same mass A, B and C were heated to equal temperature and put into a wax slab. After all of them lost all of the heat, A made a hole of 5 cm depth, B made a hole of 3 cm depth and C made a hole of 4 cm depth in the wax. They are arranged based on the magnitude of their specific heat capacity below. Which one is correct?i. C > B < A ii. A > B > C iii. C > A > B iv. B > C > A f. Water overflows in case A as well as case B. What describes this situation? A cooled B heated a cup full of water at 4°Ci. Water overflows in case A as well as in case B due to anomalous expansion of water.ii. Water overflows in case A as well as in case B due to an increase in thermal energy.iii. Water overflows in case A due to thermal expansion and water overflows in case B due to anomalous expansion.iv. Water overflows in case B due to thermal expansion and water overflows in case A due to anomalous expansion.g. The specific heat capacity of water is 4200 J/kg°C and the specific heat capacity of iron is 460J/kg°C. When the iron ball was dipped in the water, the iron ball lost 5432 Joules of heat. Calculate the amount of heat received by water. i. 4200 joules ii. 460 Joules iii. 5432 Joules iv. 49,596.521 Joules h. Why does heat flow from a hot object to a cold object?i. a hotter object has more thermal energy than a cold object ii. molecules of a hotter object have less kinetic energy than a cold object iii. hotter objects are bigger in volume than colder objectsiv. there is an invisible tube to transfer heat from the hotter to the colder objecti. Study the following statements and arguments and answer the following.Statement : When honey is cooled down below 4°C, its volume will change. Argument 1 : The volume decreases, density increases, freezes from the top. Argument 2 : The volume increases, density decreases, freezes from the top. Which of the following is correct? i. Statement is correct but arguments 1 and 2 are incorrect. ii. Statement and argument 1 are incorrect but argument 2 is correct.iii. Statement is incorrect but arguments 1 and 2 are correct. iv. Statement and argument 2 are correct but argument 1 is incorrect.

234 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 235j. What are the lower fixed points of the thermometer in Celsius, Fahrenheit, and Kelvin scales respectively?i. 0°C, 0°F, 0K ii. 0°C, 32°F, 273K iii. 0°C, 180°F, 373K iv. 0°C, 212°F, 373Ka. i b. ii c. iii d. iv e. i f. iv g. iii h. i i. iv j. ii Ans of MCQSTEP22. Very short questions a. What is thermal energy?b. Why is mercury used in liquid thermometers?c. What is the principle of ‘liquid in glass thermometer’?d. Is the object receiving thermal energy or losing thermal energy in this experiment? The kinetic energy of the molecules Intermolecular force Intermolecular distance Increases Decreases Increasese. Write the relationship between the amount of matter present in the object and the temperature change.f. The diameter of the copper ball and the inner diameter of the copper ring are the same. If Gravesande's cold copper ball does not pass through the cold copper ring, what should we do so that the ball can pass through the ring?g. The temperature of mercury becomes more when an equal amount of heat is given to 1 kg of water and 1 kg of mercury each, why?h. The specific heat capacity of mercury is 126 J/kg°C. What does it mean? i. Tilicho Lake freezes during winters but fish can be seen after the ice melts in warmer seasons. Why did not fish die?STEP33. Short questions a. If we have a litre of pure water at 4°C and a litre of pure water at 5°C, which one is heavier? Why? b. What is the relation between the vibration of molecules of a substance with its heat and temperature? c. How does the property of water shown in the graph differ from the property of other liquids?d. Out of a bucket full of water and a cup of water having the same initial temperature (suppose, 85° C), which one releases more amount of heat after 1 minute? Why?e. Differentiate between the digital thermometer and radiation thermometer.f. If a burning match-stick is thrown into a vessel full of boiling water, from where

236 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 237does the heat flow? Which of them contains more amount of heat?g. Ramesh was warming the water for bathing. He noticed that the water at the bottom of the bucket was warmer than the water at the top. What could be the cause? Explain how this happens.h. When we keep our right hand in lukewarm water and our left hand in icewater for the same duration then dip both hands into normal tap water. Which hand feels the tap water cold and which one feels it warm and why? STEP44. Short questions a. The specific heat capacity of three different substances are given below, answer the following questions. i. What do you mean by the specific heat capacity of A is 100 J/kg°C?ii. Which of the above substances will gain more temperature when they are heated with equal heat in their equal mass? Give reason.iii. Which one will penetrate the wax slab more while keeping the equal mass of these three substances heated for the same temperature of 100°C? Why?b. 200 g of water at 0°C is added to some hot water at 80°C to get the final temperature of 30°C. Calculate the mass of hot water only. (SHC of water is 4200 J /kg°C) c. Enlist the applications of anomalous expansion of water. The ghee starts to solidify from the bottom but the honey starts to solidify from the top. Explain this mechanism based on temperature, density and volume.d. Specific heat capacity of substance 'x' is greater than that of 'y'. Whose temperature will rise more if 4200 J heat is given to each? Why? If an equal amount of heat is given to two different substances, the change in their temperature is not the same. 5. Numericali. How much heat energy is required to change the temperature of 5 kg water by 20 °C ? [Ans: 4.2 × 105 J]ii. Water has a temperature of 5°C in winter. What quantity of heat energy is needed if 20 litres of water is to be heated to 35°C for bathing purposes? (specific heat capacity of water = 4200J/kg°C , mass of one litre = 1 kg) [Ans: 2.52 × 106 J]iii. A pressure cooker has a mass of 1.5 kg and a temperature of 30°C. The specific heat capacity of the alloy from which the pressure cooker is made is 1000 J/kg °C. What will be its temperature when 7.5 ×104 J of heat is supplied to it? [Ans: 80 °C]iv. The temperature of an iron ball of mass 4 kg changes by 10°C when 18.4 kJ heat energy is supplied to it. What is the specific heat capacity of the iron ball? [Ans: 460 J/kg°C]v. 200 ml of milk at 90°C is mixed with 10 ml of water at 15°C. Find the final temperature of the mixture. (Suppose the specific heat capacity of water = specific heat capacity of milk, 200 ml = 200 g) [Ans: 86.4°C]Substances Specific heat capacity (J/kg°C)A 100B 200C 300

236 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 237WaveUnit10IAirNRriOGlass Refracted rayRefraction of light Newton's color discRefraction during sun set Endoscopy

238 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 239Key terms and terminologies of the unit1. Light : Light is a form of energy that produces the sensation of vision.2. Optical medium : Optical medium is a material medium through which the electromagnetic waves propagate. 3. Rarer medium : The optical medium in which the velocity of the light is comparatively greater than another medium is called the rarer medium.4. Denser medium : The optical medium in which the velocity of the light is comparatively lesser than another medium is called a denser medium.5. Refraction of light : The phenomenon of bending of light as it passes obliquely from one optical medium to another is called the refraction of light. 6. Cause of refraction : The change in speed of the light wave upon crossing the boundary of two optical media is the cause of refraction.7. Refractive index : The refractive index of a material medium can be defined as the ratio of the speed of light in vacuum (c) to the speed of light in the medium (v). 8. Absolute refractive index :The refractive index of a medium, with respect to a vacuum, is called the absolute refractive index of the medium. 9. Real depth : The real position of the bottom or a point inside water is called a real depth.Sequence of Curriculum Issued by CDC Refraction of the light Laws of refraction of the light Total internal reflection of light: introduction, critical angle, condition for total internal reflection, application of total internal reflection of light(mirage and shining of the diamond) Introduction to light pipe, working mechanism of light pipe, application of light pipe on telecommunication, endoscopy, keyhole surgery etc. Dispersion of light: introduction and dispersion of light from a prismUNITLight10.1Estimated teaching periods Theory Practical6 1Kedar Lal Shrestha, born in 1938 and passing in 2020, was a revered physicist whose contributions left a lasting impact on academia and scientific institutions in Nepal. Serving as a professor at Tribhuvan University, he led the physics department from 1971 to 1986 and contributed significantly to the Research Centre for Applied Science and Technology (RECAST) during his tenure from 1979 to 1986. Additionally, Shrestha held the position of vice president at the Nepal Academy of Science and Technology from 1992 to 1997. Kedar Lal ShresthaAbout the Nepalese Scientist

238 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 23910. Apparent depth : The position of the bottom or a point inside water which is observed from outside is called an apparent depth.11. Critical angle : The angle of incidence in a denser medium for which the corresponding angle of refraction in the rarer medium is 900 is called the critical angle.12. Total internal reflection : The phenomenon of light returning into the same medium when it passes from an optically denser to an optically rarer medium with an angle of incidence greater than the critical angle (i > C) is called the total internal reflection.13. Mirage : Mirage is an optical illusion in which there is an appearance of water mainly on roads, deserts, etc. during the peak summer.14. Light pipe : A light pipe is a flexible fiber that confines a beam of light by the process of total internal reflection.15. Dispersion : The phenomenon due to which white light splits into seven colors (VIBGYOR), when passed through a prism, is called dispersion.16. Cause of the dispersion : The difference in speed of seven different wavelength colors inside a prism is the cause of the dispersion.IntroductionWe see a variety of objects in the world around us. This is because the light travels from the luminous sources to the object and then comes from them to our eyes. During the day, the sunlight that falls on objects gets reflected which enabling us to see them. Thus, light is a form of energy that produces the sensation of vision. In this unit, we will study the phenomena of bending and splitting of light. We will also discuss the total internal reflection and electromagnetic spectrum.Optically Denser Medium and Rarer MediumOptical medium is a material medium through which the electromagnetic waves propagate.Light does not travel with the same speed in all media. It travels the fastest in a vacuum, with its highest speed of 3×108m/s. In air, the reduction in speed of light is negligible, compared to that in a vacuum. It is reduced considerably in glass or water. The value of speed of light in water is 2.25 × 108m/s and that in glass is 2 × 108m/s. The medium in which light travels faster is called an optically rarer medium and the medium in which light travels relatively slower is called an optically denser medium. FACTS WITH REASONSWater is optically rarer medium as compared to glass, why?When light passes through water to glass, the value of speed of light in water is 2.25 × 108m/s and that in glass is 2 × 108m/s. Light travels faster in water than in glass. So water is an optically rarer medium than glass.Refraction of lightAccording to the ray model of light, it travels through space along straight lines. Light does not travel in the same direction in all media. When light enters obliquely from one transparent Light travels along a straight line. This property is called the rectilinear propagation. The straight line along which the light travels is called a ray.MEMORY TIPS

240 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 241medium to another, the direction of propagation of light in the second medium changes. This phenomenon is called refraction of light. Thus, the phenomenon of bending of light as it passes obliquely from one optical medium to another is called the refraction of light. Cause of RefractionLight travels more slowly when it enters into an optically denser medium from an optically rarer medium. On the other hand, when light travels from an optically denser to an optically rarer medium, it speeds up. The change in speed of the light wave upon crossing the boundary of two optical media is the cause of refraction.Refraction through Glass SlabThe rays of light will first incident in the surface of the glass. Since glass is transparent, the light will continue to pass through the glass. However, the velocity of the light will decrease inside the glass. So, the light will change the path and bends toward the normal. This bending of the light is called refraction of light. Then the refracted ray will fall on the opposite face of the glass slab and emerge from the glass. The emergent ray will bend away from the normal due to an increase in velocity. The emergent will be parallel to the incident ray. The perpendicular distance between the incident ray and emergent ray is called lateral shift. The figure alongside is a schematic diagram to show the refraction of light through the glass slab. Here the air is a rarer medium and the glass is a denser medium. The refractive index of air is 1 which is lesser than the refractive index of the glass which is 1.5. The ray of light AO is incident on the surface PQ. This ray of light AO is an incident ray. It has made an angle of measurement ‘i’ with the normal ray NN’. This angle is called the angle of incidence. The incident ray penetrates the transparent glass surface and travels along the path OB. The ray of light OB is called a refracted ray. It lies in the denser medium. Its velocity is lesser than the velocity of incident ray OA. So, it bends toward the normal ray to make an angle of refraction ∠NOB. The angle of refraction ∠NOB is smaller than the angle of incidence ∠AON. Then the light will reach the surface SR, emerge out from the glass slab and travel along BC. BC is an emergent ray. It is in a rarer medium so its velocity will increase and it will bend away from the normal. It is parallel to the incident ray. During the entire process, the ray of light has shifted away from the original path. It is called the refraction of light. The perpendicular distance between the incident ray and the normal is called lateral shift. It is the measurement of how far the light has bent due to the glass slab. IAirNRriOGlass Refracted rayRefraction of lightRefraction through glass slabiAPSQEmergent rayIncident rayRefracted rayRCDBqNN' N1N'1AIR OGlassRectangularglass slabDirection of original rayLateral displacementAIRNormalNormal

240 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 241Rules for the Bending of Rays at the Interface of Two Optical Mediai. When a ray of light travels from an optically rarer medium (air) to an optically denser medium (glass), it bends towards the normal at the interface of the two media.FACTS WITH REASONSFor a ray of light travelling from an optically rarer to an optically denser medium, the angle of incidence is always greater than the angle of refraction, why?A ray of light travelling from a rarer medium to a denser medium slows down and bends towards the normal at the point of incidence. So, for this ray of light, the angle of incidence is always greater than the angle of refraction.NormalriDenser (glass)Rarer (air)ii. When a ray of light travels from an optically denser medium (glass) to an optically rarer medium (air), it bends away from the normal at the interface of the two media.FACTS WITH REASONSFor a ray of light travelling from an optically denser to an optically rarer medium, the angle of incidence is always less than the angle of refraction, why?A ray of light travelling from a denser medium to a rarer medium goes faster and bends away from the normal at the point of incidence. So, for this ray of light, the angle of incidence is always less than the angle of refraction. iii. When a ray of light strikes the interface of two media at an angle of 90°, it does not bend from its path.Denser (glass)Rarer air If a ray of light travels from air to water along a certain path, it retraces the path, when light travels from water to air. The path of light is reversible.MEMORY TIPSLight passing through normalACTIVITY 1Objective: To show refraction through glass slab1. Fix a sheet of white paper on a drawing board using drawing pins.2. Place a rectangular glass slab over the sheet in the middle. Draw the outline of the slab with a pencil and name the outline as ABCD.3. Draw a line XO as an incident ray on the face AB. 4. Take four identical pins. Fix two pins, say P1 and P2, vertically on the line XO.5. Look for the images of the pins P1 and P2 through the opposite edge. Fix two other pins, say P3 and P4, such that these pins and the images of P1and P2 lie on a straight line.6. Remove the pins and the slab. Join the positions of the tip of the pins P3 and P4 and produce it up to the edge DC with the help of a ruler. Let P3P4 meets DC at O2.BMNM1N1eO2 P3P4riP1P2D CA OXYxy

242 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 2437. It is found that the light ray has changed its direction at points O on the surface AB and at a point O2 on the surface DC. When light enters into the glass slab from air, it bends towards normal and when light emerges out of the glass slab, it bends away from the normal.8. When light travels from denser medium to the rarer medium, it bends away from normal. The net distance shift by the emergent ray is called lateral shift. Here xy is lateral shift.Laws of Refractioni) The incident ray, the normal, and the refracted ray all lie in the same plane at the point of incident.ii) For a given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. This law is also known as Snell’s law of refraction. If 'i' is the angle of incidence and 'r' is the angle of refraction, then,sin isin r = constant (µ)This constant value is called the refractive index of the second medium with respect to the first. Refractive IndexThe degree of the change in direction that takes place in a given pair of media is expressed in terms of the refractive index. It refers to the degree to which a medium manages to slow down the wave that is transmitted through it. Thus the higher the refractive index of a material medium, the slower a wave passes through it. The refractive index indicates how much slower the electromagnetic wave passes through the medium with respect to its speed in vacuum (3 × 108 m/s).There is a relationship between the refractive index of a material and the speed of light in that material. The refractive index of a material medium can be defined as the ratio of the speed of light in vacuum (c) to the speed of light in the medium (v). µ = Speed of light in vacuumSpeed of light in the medium = cvThe refractive index of a medium, considered with respect to vacuum, is called the absolute refractive index of the medium. The absolute refractive index of a medium is simply called its refractive index. The refractive index of several media is given in the table.Snell's law is not applicable when the angle of incidence is equal to zero, i.e., when the ray is incident along the normal.MEMORY TIPS

242 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 243Medium Absolute Refractive IndexVacuum and air 1Ice 1.31Water 1.33Ethyl Alcohol 1.36Kerosene 1.44Glass 1.50Glycerine 1.47Diamond 2.42Solved Numerical 10.1.1The refractive index of diamond is 2.4. What is the speed of light in diamond if its speed in a vacuum is 3 × 108 m/s?Solution: Given,Speed of light in vacuum (c) = 3 × 108 m/sRefractive index = 2.4From the formula, µ = Speed of light in vacuum (c)Speed of light in the medium (v)or, 2.4 = 3 × 108Speed of light in the mediumor, Speed of light in diamond = 3 × 1082.4 = 1.25 × 108 m/sReal Depth and Apparent DepthLight ray bends away from the normal when it passes from water to air. This bending of light gives us a false impression of depth. When light rays pass from water to air, the light rays from the point at the bottom of a pond bend away from the air-water interface. The observer outside sees the point to be at a higher position above the actual position.The real position of the bottom or a point inside water is called a real depth. Whereas, the position of the bottom or a point inside water which is observed from outside is called an apparent depth. Apparent depth is due to the refraction of light that occurs at the water-air interface. The real depth and apparent depth are related to the refractive index of water by the given equation.µ = Real depthApparent depthThe refractive index of kerosene is 1.44 which is more than the refractive index of water (1.33). This means kerosene is a denser medium than water even though it has less density than water. An optical medium can be denser if it has less velocity of light than other medium. It does not depend upon the density of the matter.MEMORY TIPS1. Medium Velocity of light 2. Air 3 × 108 m/s3. Water 2.25 ×108 m/s4. Alcohol 2.19 × 108 m/s5. Kerosene 2.08 × 108 m/s6. Glass 2 × 108 m/s7. Diamond 1.24 × 108 m/s8. Glycerine 2.04 × 108 m/s MEMORY TIPS

244 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 245Solved Numerical 10.1.2An observer looks at a water tank. According to him, half of the tank is filled with water. If the height of the tank is 180 cm, find the real height of the water in the tank. (Refractive index of water = 1.33)Solution: Given,Height of the tank = 180 cmApparent depth = 1802 = 90 cmRefractive index of the water = 1.33From the formula, µ = Real depthApparent depthor, 1.33 = real depth90or, real depth = 90 × 1.33 = 119.7 cmThe real height of the water in the tank is 119.7 cm.FACTS WITH REASONSA spear hit by a fisherman does not hurt the fish inside water, why?When the rays of light refracted from the fish inside water pass from the water to the air, they bend away from the normal. Hence the fish appears at a position above its actual position. A person hitting at the apparent position of the fish inside water misses the target. So, a spear hit by the fisherman where fish is seen won't hurt the fish inside the water.Effects of Refractioni) A stick immersed obliquely in water appears to be bent.ii) A coin in a cup, just below the eye sight, becomes visible when the cup is filled with water.iii) A spear hit by a fisherman does not hurt the fish inside water.iv) When a thick glass slab is placed over a book page, the letters appear raised while viewing through the glass slab.v) Twinkling of stars in the sky.vi) The sun is visible to us about two minutes before the sunrise (i.e. advanced sunrise) and about two minutes after the sunset (delayed sunset).vii) The sun near the horizon appears flattened at the sunset and sunrise.FACTS WITH REASONSA stick immersed obliquely in water appears to be bent. Why?When rays of light from a part of the stick inside water pass from water to air, they bend away from the normal. The image of immersed part of stick is formed at an apparent depth. Hence, the stick appears to be bent at the air-water interface. WaterApparent bendApparent positionof stickWhen you look at the depth of a swimming pool, the bending of light can give you a false impression of depth.MEMORY TIPS

244 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 245ACTIVITY 21. Take a beaker and fill ¾ th of it with water.2. Place a pencil obliquely in the beaker and observe it from a side of the beaker.3. Does the pencil appear bent at the air-water interface?Real positionApparentpositionACTIVITY 31. Place a coin at the bottom of a glass tumbler filled with water.2. With your eye to a side above water, make an attempt to touch the coin with the help of a pencil. Did the pencil touch the surface of the coin exactly? WaterCoin (real position)Apparent position of the coinACTIVITY 41. Place a cup on a table in your classroom and put a coin in it. 2. Move away slowly from the cup and stop to move when the coin just disappears from your sight. 3. Ask a friend to pour water gently into the cup without disturbing the coin. 4. Keep looking for the coin from your position. Does the coin become visible again from your position? FACTS WITH REASONSA coin in a cup, just below the eye sight, becomes visible when the cup is filled with water, why?The direction of light reaching our eyes from the coin inside water through water (optically denser medium) to air (optically rarer medium) bends away from the air-water interface. Due to this, the coin seems just above of its actual position inside water in the cup. So a coin in a cup, just below the eye sight, becomes visible when the cup is filled with water.ACTIVITY 51. Place a glass slab over a page of a book on your table.2. Look at the letters under the slab from the sides. Do you see the letters raised?FACTS WITH REASONSWhen a thick glass slab is placed over a book page, the letters appear raised while viewing through the glass slab, why?The direction of light reaching our eyes from the letters under the glass slab through glass (optically denser medium) to air (optically rarer medium) bends away from the air-glass interface. Due to this, the letters seem just above of their actual position under the slab. So when a thick glass slab is placed over a book page, the letters appear raised while viewing through the glass slab.

246 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 247Twinkling of StarsTwinkling of stars is due to the atmospheric refraction. Light from distant stars undergoes a continuous refraction through layers of atmosphere. During the refraction in the atmosphere, the refracted rays bend towards the normal and the stars appear slightly higher than where their actual position is. The apparent position of stars fluctuates continuously due to the change in the earth's atmosphere. Stars appear brighter some times and fainter at the other times. So twinkling of stars is due to the fluctuation of the apparent positions of the stars during their refraction.FACTS WITH REASONSStars twinkle but planets do not, why?Compared to the planets, stars are very far. Light from the distant stars undergoes a continuous refraction through layers of atmosphere. It also fluctuates the apparent position of the stars and twinkle. But, such continuous refraction does not occur in planets. Hence planets do not twinkle. The sun is visible to us about two minutes before the sunrise (i.e. advanced sunrise) The actual sunrise means the actual crossing of the horizon by the sun. Light from the sun travels through layers of the atmosphere. When the sun is just below the horizon, its rays enter the earth's atmosphere from rarer to the denser medium and get refracted. But the observer on the earth thinks that the light is coming straight from the sun. Refracted rays that reach us make the sun appear on the horizon. This is the apparent sunrise. But the actual sunrise occurs when the sun reaches horizon a little later.Sun set occurs about two minutes after the sunset (delayed sunset)The apparent sunset occurs slightly later than the actual sun set. The light from the sun is already below the horizon. It gets refracted towards the earth. While travelling through rarer to denser medium of the atmosphere, the sunlight from apparent position of the sun reaches the observer on the earth. This enables us to see the apparent sunset for a while after the sun has already set. So, we can see the sun about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset because of atmospheric refraction. The sun near the horizon appears flattened at the sunset and sunriseWhen the light rays from the sun enter the earth's atmosphere, they get refracted towards the earth. Due to the atmospheric refraction, the rays from the top and bottom portions of the sun on horizon are refracted by different degrees. This causes the apparent flattening of the sun. But the rays from the sides of the sun on a horizontal plane are generally refracted by the same amount. So the sun still appears circular along the sides. EarthObserverHorizonApparent sunRising sun