Oasis School Science and Technology 10 New

Oasis School Science and Technology - 10 201 Pressure Pressure is defined as the thrust per unit area of a surface. When a thrust (F) is acting on a surface area (A) in contact, the pressure exerted is given by Fact File Pressure is a scalar quantity as it is taken perpendicular to the surface area. In case of liquid, the pressure is exerted equally in all directions, which means that pressure has no definite direction. ∴ P = F A Pr (P) ( ) ( ) essure Thrust F Area A = The SI unit of pressure is Nm-2 or pascal (Pa). Any substance having weight can exert pressure. Like solids, fluids, i.e., liquid and gas also have weight. The solid objects exert pressure on the surface on which they are placed. Fluids exert pressure on the walls and bottom of the container in which they are kept. Differences between Force and Pressure S. N. Thrust (force) S. N. Pressure 1. It is a push or pull acting on a body. 1. It is the thrust acting per unit area. 2. Its SI unit is newton (N). 2. Its SI unit is pascal (Pa). 3. It is a vector quantity. 3. It is a scalar quantity. 4. It is the cause of pressure. 4. It is the effect of force. Activity 1 • Take a brick and foam of thickness about 10cm to 20cm. Measure the dimension and weight of the brick. Put the brick on the foam with its broad surface as shown in fig.8.2(a). • Observe the depression made by the brick. Put the same brick on the foam with its narrow surface as shown in fig. 8.2 (b). • Observe the depression made by the brick and calculate the pressure in both cases. Also, write down the conclusion of this activity. (a) (b) Brick Foam Fig. Transmission of pressure Molecules of solids have strong intermolecular forces. They are arranged very close. They cannot be compressed. So they do not transfer pressure exerted at a point to other points. Molecules of liquid have relatively weaker intermolecular force than solid but stronger than gas. Their molecules can flow but cannot be compressed. Their density does not change when compressed. So liquids enclosed in a container can transfer pressure applied at a point without changing to every point within the container.

202 Oasis School Science and Technology - 10 Molecules of gas have very weak intermolecular forces. They are loosely arranged. They can be compressed easily. Their density changes significantly when compressed. Therefore they can transfer pressure applied at a point to any point in the container but partially. Activity 2 Observe the given diagram. Why is the water not falling down despite the glass being upside down? How is a thin paper able to hold the weight of the water? Think about the possible causes and learn the effect of atmospheric pressure. Liquid Pressure Any substance that has weight does exert pressure. Fluids (gas or liquid) have weight and exert pressure on the walls and bottoms of the container. If we consider a point within a liquid, it will be clear that there must be equal upward and downward pressure at that point. The thrust exerted by a liquid per unit area of the wall and base of the container is called liquid pressure. Properties of Liquid Pressure 1. Liquid pressure increases with depth. liquid pressure (p) ∝ height of liquid column(h) 2. Pressure applied on a liquid is transmitted equally in all directions. 3. The pressure at a point in a liquid does not depend on the volume of the liquid in a vessel. 4. The liquid pressure is independent of the shape of the vessel in which the liquid is kept. 5. The liquid pressure is directly proportional to its density. liquid pressure (p) ∝ density of liquid (d) 6. The liquid pressure is directly proportional to the acceleration due to gravity. liquid pressure (p) ∝ acceleration due to gravity (g) Pascal’s Law of Liquid Pressure When a liquid is completely enclosed in a vessel and a pressure is applied to it at any part of its surface, the pressure is transmitted equally throughout the liquid. This fact was first propounded by French scientist Blaise Pascal, which is popularly known as Pascal's law. Fact File Blaise Pascal (19 June 1623- 19 August 1662) was a French mathematician, physicist, inventor and writer, who propounded the famous law, which is commonly known as Pascal's law of liquid pressure.

Oasis School Science and Technology - 10 203 According to this law, "Pressure is transmitted equally in all directions when pressure is applied at a place on a liquid kept in a closed container.” Verification of Pascal's law a. Take a plastic bag. Fill the bag with water. Make a number of holes on the bag with a needle. Now, squeeze the bag as shown in the figure. Does water come out from all the holes with the same pressure? It can be seen that water comes out through all the holes simultaneously with the same pressure. b. Let us consider a spherical glass vessel with four water-tight frictionless pistons P, Q, R and S having the same cross-sectional area (A). If the piston (P) is pushed by applying force (F), it exerts a pressure. The pressure exerted on the pistons Q, R and S will be the same as the pressure exerted on piston P on which force is applied. These experiments prove Pascal's law of liquid pressure. Application of Pascal's law Hydraulic machines like the hydraulic jack, hydraulic press and hydraulic brakes are constructed on the basis of Pascal's law of liquid pressure. Fact File Pascal's law of liquid pressure is applicable for incompressible gases also. Fig. Spherical glass vessel Q S P R 1. Hydraulic jack It is used in raising automobiles, like trucks, buses, cars, motorcycles, etc. for repair works in service stations. The construction of the hydraulic jack is shown in the figure 8.5. This machine works on the basis of Pascal's law. When a small effort is applied on a small piston, a large force is developed on the large piston. As a result, the vehicle placed on the large piston is raised upwards. The hydraulic jack is also called a hydraulic lift. 2. Hydraulic press Hydraulic press is used for compressing cotton bales, extracting oil from oil seeds, punching holes in metals, giving specific shapes to metal sheets, etc. It also works on the basis of Pascal's law. Fig. Plastic bag with water Polythene bag Water Fig. Hydraulic jack Liquid F2 Valve 1 Valve 2 Applied force F1

204 Oasis School Science and Technology - 10 The construction of the hydraulic press is given in the figure. It has two cylinders X and Y. The cylinder X is smaller than Y. Those cylinders are fitted with water-tight frictionless pistons P1 and P2 respectively. The cylinders are filled with a liquid. A rigid ceiling is placed over the large piston. When a small effort is applied on the small piston, a large force is developed on the large piston and it is raised upwards. As a result, the substance placed on the large piston is lifted and gets compressed between the platform of the large piston and the ceiling. 3. Hydraulic brakes Hydraulic brakes are used in heavy automobiles, like trucks, buses, aeroplanes, cars, motorbikes, etc. to stop them by applying a small effort. The construction of hydraulic brakes is also based on Pascal's law. The construction of a hydraulic brake is shown in the following figure 8.7. When the driver presses on the foot pedal, a piston moves inwards, pressing the column of liquid contained in the master cylinder. This helps to expand the brake holes with the help of the wheel cylinder. When the pressure on the foot pedal is released, the brake shoe pulls up due to the force of the spring. The wheel pistons move back into the cylinder and the liquid returns to the master cylinder. (fig. 8.7). The advantage of this system is that the pressure set up in the master cylinder is transmitted equally to all wheels so that the braking effort is equal on all wheels of the vehicle. 4. Hydraulic excavator It is a construction machine. It contains a hydraulic actuator. The actuator helps to move the arm of the excavator like a hand. The hydraulic actuator contains a cylinder and piston filled with hydraulic fluid. The force applied at one end of the actuator is multiplied Fig. Hydraulic press F2 A2 A1 F Y 1 X Liquid Piston P1 Platform Rigid ceiling Material for pressing Pivot Piston P2 Fig. Hydraulic brake Foot pedal Liquid Piston Pipeline To other wheel Brake shoe Hinge Return spring Brake-shoe Wheel cylinder Master cylinder Fig: excavator

Oasis School Science and Technology - 10 205 by many folds and it exerts a considerable force for working. 5. Hydraulic lift The hydraulic lift is a hydraulic machine. It has two pistons. The first piston has a small crosssectional and the second one has a larger cross-sectional area. It is filled with incompressible liquids. When a small force is used in a small piston, the force is multiplied many folds and sent to the larger piston. The piston rises and lifts the heavy loads. Later a valve is opened to release the fluid. Then the larger piston will slowly fall due to gravity. Hydraulic Machine: A Force Multiplier Vessel Let us consider a hydraulic machine having two vessels of cross-sectional area 'A1 ' and 'A2 ' respectively. These vessels are fitted with water-tight frictionless pistons. The cross-sectional area of the small vessel is A1 and that of the large vessel is A2 . So A2 > A1 . When a force F1 is applied on the small piston of area A1 , a pressure P1 is exerted on the small piston. Principle of a hydraulic machine The 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." According to Pascal's law, Pressure on the small piston = Pressure on the big piston i.e. P1 = P2 F A 1 1 = F A 2 2 or, F1 ×A2 = F2 × A1 Since, A2 > A1 Thus, F1 < F2 Hence, a hydraulic machine is a force multiplier. Fig: Hydraulic lift for motor cycle Fig: hydraulic lift Fig. Hydraulic machine F1 F2 A1 A2

206 Oasis School Science and Technology - 10 Activity 3 Take an empty plastic bottle. Close the mouth of the bottle with an airtight stopper. Put it in a bucket filled with water. You see that the bottle floats. Push the bottle into the water. You feel an upward push. Try to push it further down. You will find that it is difficult to push it deeper. This indicates that water exerts a force on the bottle in the upward direction. The force exerted by the water goes on increasing as the bottle is pushed deeper till it is completely immersed. The upward force exerted by a liquid is called upthrust. Worked out Numerical 1 Study the given figure and answer the following questions: i. Name the equipment shown in the figure. On the basis of which law does it work? ii. Calculate the pressure exerted at A. iii. Calculate the effort (F2 ) on piston B. 0.4 m2 240N F2 = ? A B 20 cm2 Ans: i. The equipment shown in the given figure is a hydraulic machine. It works on the basis of Pascal's law. ii. Solution: Given, Force on piston A (F1 ) = 240 N Area of piston A (A1 ) = 20 cm2 = 20 × 10-4 m2 [ ∵ 100 cm = 1m] We know, P1 = F1 A1 = 240 20 10 12 10 1 2 10 4 4 5 × = × = × − . Pa ∴ Pressure at A ( P1 ) = 1.2 × 105 Pa

Oasis School Science and Technology - 10 207 iii. Effort on piston B (F2 ) = ? Area of piston B (A2 ) = 0.4 m² We know, F1 A2 = F2 A1 or, F2 = F A A 1 2 N 1 4 240 0 4 4 20 10 = 4 8 10 × × = × − . . ∴ Effort on piston B (F2 ) = 4.8 ×104 N Upthrust When a body is placed in a liquid, the liquid exerts an upward force on it. For example, when a piece of cork is held below the surface of water and then released, the cork immediately rises to the surface. It appears as if some upward force is exerted by water on the cork which pushes it to the surface. Similarly, if we lift a bucket lying at the bottom of a pond, it appears to be light as long as it is being lifted inside the water. But as soon as the bucket is lifted out of the water, the same bucket seems to get heavier. This is because of the upward force of the liquid on the bucket. In general, whenever an object is partially or wholly immersed in any liquid, it loses some weight and becomes lighter. When an object is immersed in a fluid (liquid or gas) the object experiences a resultant force due to the liquid pressure. The resultant upward thrust exerted by a fluid is called upthrust. Since upthrust is a force acting upward, it is measured in newton (N). Causes of Upthrust In the above figure, a cubical object is kept in water. It experiences force from all directions. The force applied from right and left of the cube is equal and opposite. The pressure and force applied from down to up is greater than the pressure and force applied from up to down. This resultant force is called upthrust. It is always in upward direction. Thus, the difference in depth between upper surface and lower surface of the object has different pressure and thrust. This is the main cause of upthrust. The Formula of the Upthrust Let us consider a cylinder of height 'h' and uniform cross-sectional area ‘A’ is immersed in a liquid of density ‘d’. The height of the liquid column above the upper surface of the cylinder is 'h1 ' and that of the liquid above the lower surface of the cylinder is 'h2 '. Now, Force on upper face (F1 ) = P1 × A = h1 dg A [∵ P1 = h1 dg] Force on lower face (F2 ) = P2 × A = h2 dg A [∵ P2 = h2 dg] [g = acceleration due to gravity] Upthrust (U) = F2 – F1 = h2 dgA – h1 dgA Fact File Upthrust is applicable both in liquid and gas. The combined word of liquid and gas is called fluid. Upthrust Small forces on the top Water Large forces on the bottom Weight of box h1 h2 h Fig. A cylindrical body in a liquid ∴ U = dgA (h2 – h1 )

208 Oasis School Science and Technology - 10 h2 – h1 = height of the body (h) ∴ [ ∵ V = l × b × h = A × h] Again, U = mg [∵ d = m V ] ∴ U = mg where, 'mg' is the weight of the liquid displaced. Factors affecting upthrust The upthrust due to a fluid on a body immersed in the fluid depends on the following factors: 1. The volume of the submerged part of a body: Upthrust (U) is directly proportional to the volume (V) of the submerged part of the body in a fluid, i.e., U ∝ V . 2. Density of the fluid: Upthrust (U) is directly proportional to the density (d) of the fluid in which the body is submerged, i.e., U ∝ d . 3. Acceleration due to gravity: Upthrust (U) is directly proportional to the value of acceleration due to gravity (g), i.e., U ∝ g . How to calculate upthrust of a liquid Upthrust of a liquid on an immersed body is measured by finding the difference in the weight of the body in air and in the liquid. Upthrust (U) = Weight loss in the liquid or, U = Weight in air (Wa ) – Weight in liquid (Wl ) ∴ U = Wa – Wl Activity 4 • Take a piece of stone and tie it to one end of a rubber string on a spring balance. Suspend the stone by holding the balance on the string as shown in the figure. Note the reading on the spring balance due to the weight of the stone. Now, slowly dip the stone in a container as shown in the figure. Observe what happens to the reading on the balance. Fig. Stone Water Spring balance Wa Wi U = dgAh U = V dg Fact File 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.

Oasis School Science and Technology - 10 209 Reasonable Fact An egg sinks in fresh water but floats in a strong solution of salt. Fresh water has less density than that of the egg so the egg cannot displace water equal to its weight due to less upthrust and it sinks. But salt on mixing with water increases the density of water and hence the upthrust. As a result, the egg can displace water equal to its weight and floats. Reasonable Fact A balloon filled with hydrogen gas rises up, but a balloon filled with air falls down. The weight of the air displaced by the balloon filled with hydrogen gas is more than the weight of the balloon. So the upthrust of the air is also more than the weight of the balloon. As a result, the balloon filled with hydrogen gas rises up. But the weight of the balloon filled with air is more than the upthrust of the air. As a result, the balloon filled with air falls. Activity 5 • Take two beakers, one with salt water solution and another with pure water. Put an egg each in both the beakers. In which liquid does the egg float and why? The density of the salt water solution is higher than that of the pure water. So, the upthrust of the salt water solution is more than that of the pure water. Please note that upthrust is directly proportional to the density of a liquid. Fig. Egg Egg Salt Water solution Pure Water Archimedes’ Principle When a solid object is immersed in a fluid, an upward force acts on it. The magnitude of the force is given by Archimedes' principle. This principle was propounded by Archimedes, a famous Greek scientist. Archimedes' principle states, "When a body is partially or wholly immersed in a fluid, it experiences an upthrust which is equal to the weight of the fluid displaced by it.” Theoretical Proof of Archimedes’ Principle Let us consider a cylinder of height 'h' and uniform cross-sectional area 'A' is immersed in a liquid of density 'd'. If h1 and h2 are the depths of the upper and lower surfaces of the immersed cylinder, then Force on upper face (F1 ) = P1 × A = h1 dg A Force on lower face (F2 ) = P2 × A = h2 dg A h1 h2 h Fig.

210 Oasis School Science and Technology - 10 [g = acceleration due to gravity] Now, Upthrust (U) = F2 – F1 = h2 dgA – h1 dgA = (h2 – h1 ) Adg Let h2 – h1 = h, then, U = hAdg U = Vdg. [∵ V = h × A] U = mg [∵ V.d = m] ∴ Hence, Archimedes’ principle is proved. Experimental Verification of Archimedes' Principle i. Place a beaker under the spout of the ureka can. Pour water into the can until it starts overflowing through the spout. When the water stops dripping, keep a beaker under the spout. ii. Now suspend a stone by the hook of a spring balance. Let the reading be W1 on the scale of the balance (wt. of the stone in the air). iii. Gently lower the stone into the water in the ureka can. It displaces water which overflows through the spout and collects in the empty beaker. When the stone is completely immersed in water and the water has stopped dripping, let the reading on the spring balance be W2 . You will find that W2 is less than W1 . iv, The water collected in the beaker is the liquid displaced by the stone. Find the weight of the liquid displaced. Let Weight of the stone in air = W1 Weight of the stone in water = W2 ∴ Upthrust = W1 – W2 ......................... (i) Weight of the beaker with displaced water = W3 Weight of the beaker = W4 ∴ Weight of the displaced water = W3 – W4 ............. (ii) The value of (i) and (ii) will be equal. This verifies Archimedes' principle. Upthrust (U) = Weight of the liquid displaced Fig. Spring balance Stone Water Top pan balance Beaker Ureka can W2 W1

Oasis School Science and Technology - 10 211 Worked out Numerical 2 Study the given figure and answer the following questions: i. What is the upthrust given by the liquid? ii. Calculate the weight of the stone in air. iii. Which principle is this experiment based on? Solution: i. The upthrust given by the liquid (U) = 5 N [ ∵ Archimedes' principle] ii. The weight of stone in air = Weight of stone in water + Weight of water displaced = (20+5) N = 25N iii. This experiment is based on Archimedes' principle. Law of Flotation The law of flotation states, “A body floats on a liquid if it can displace the liquid equal to its own weight.” For a floating body, Wt. of the floating body = Wt. of the liquid displaced The weight of the solid floating in a liquid is equal to the weight of the liquid displaced by the immersed part of the solid. In the floating condition, the apparent weight of the body will be zero, and the body will be weightless. Verification of the Law of Flotation According to the law of flotation, "The weight of the liquid displaced is equal to the weight of a floating body". This can be verified with the help of a simple experiment. Pour water into an overflow can to the level of the spout and place a beaker under the spout. Take a solid block (wood) which floats on the liquid and weigh it. Let it be W1 . Now, immerse the block in the overflow can. It displaces the water which overflows through the spout and collects in the empty beaker. Now, weigh the displaced water. Let it be W2 . Finally, you will get that Wt. of the floating body = Wt. of the liquid displaced Spring balance Stone Water 5N Beaker Ureka can 20N Fig. Wood Water Wt. of displaced water = W2 Wood W1

212 Oasis School Science and Technology - 10 ∴ Wt. of the floating body = Wt. of the liquid displaced Forces Acting on an Immersed Body When a body is immersed in a liquid, it is acted upon by two forces. They are: i. The weight of the body 'W' acting vertically downwards. ii. The upthrust due to the liquid 'U' acting vertically upwards. These two forces are acting just opposite of each other. The movement of the body is in the direction of the resultant force. Conditions that Determine the Sinking or Floating of a Body 1. When the weight of a body (solid) is more than the upthrust, the body sinks in the liquid. It happens so when the density of the body is greater than that of the liquid (fig. 8.15). In short, density of solid > density of liquid 2. When the weight of a body (solid) is equal to the upthrust, the body floats below the surface of the fluid. It happens so when the density of the body is equal to the density of the liquid (fig. 8.16). In short, density of solid = density of liquid 3. When the weight of a body (solid) is less than the upthrust, the body floats partially above the surface of the fluid. It happens so when the density of the body is less than that of the liquid (fig. 8.17). In short, density of solid < density of liquid Reasonable Fact a) When W> U: The body sinks down b) When W = U: The body floats freely c) When W < U: The body floats in the condition of being partially immersed. Fig. Fig. Fig.

Oasis School Science and Technology - 10 213 Applications of the Law of Floatation Floatation in liquid 1. Flotation of ships made of iron An iron nail sinks in water but a ship made of iron floats. The density of an iron nail is more than that of water. Therefore, the weight of the iron nail is more than the weight of the water displaced by it, so it sinks. But a ship made of iron does not sink because it is made hollow from inside so that its average density becomes less than that of water. So the weight of water displaced becomes equal to the weight of the ship, and the ship floats. Reasonable Fact An iron nail sinks in water but floats in mercury, why? The density of iron is more than that of water. The iron nail cannot displace the water equal to its weight and it sinks. But the density of mercury is much more than that of the water. It gives more upthrust, which helps the iron nail to float on mercury. 2. Flotation of submarines A submarine displaces water equal to its own weight when floating on the surface. To make it dive water is allowed to run into the ballast tanks inside it so that the weight of the submarine is more than the water displaced by it. 3. Flotation of a man The density of the human body with empty lungs is 1.07 g/cm3 while with lungs filled with air it is 1 g/cm3 . A good swimmer with filled lungs can displace water nearly equal to his own weight. Because of this s/he can swim easily in water. It is easier to swim in sea water than in river water due to higher density of sea water as it contains dissolved salts, minerals, etc. Greater upthrust is applied on the swimmer by the sea water as compared to river water, making it easier to swim in sea water. In the Dead Sea, the density of water is 1.16 g/cm3 , which offers greater upthrust. Therefore, chances of drowning in the Dead Sea are negligible. Fig. A ship made of iron floats on water Fig. A submarine dives in water Fig. A man swimming

214 Oasis School Science and Technology - 10 4. Flotation of icebergs The density of ice (0.9 g/cm3 ) is less than that of water (1 g/cm3 ). So, the weight of water displaced by ice is more than the weight of the ice. As a result, iceberg floats in water. 5. Flotation of fish Fish put good use of Archimedes' principle. It has air sacs in its body. It collects more air in air sacs, which decreases its density and increases volume. So it can displace more volume of liquid. It experiences more upthrust and floats higher. When it removes air from the sac, its density increases and its volume decreases. It displaces a lesser volume and weight of liquid than earlier. So gravity pulls it down. 6. Life belt Life belt is a big hollow tube of rubber. When air is filled in it, its volume increases. If a man wearing life belt jumps in water, then the weight of the water displaced becomes more than the weight of the man and belt. Thus, the man remains safe from being drowned. 7. Hydrometer A hydrometer is a device used to measure the density of a liquid or relative density of a liquid. It works based on the law of flotation. It has a very heavy bulb and a long stem. The bulb is wide and filled with heavy metal. The stem is long and thin. It is calibrated. When it is put in water, the heavy and broad bulb displaces a large weight of water. So it experiences greater upthrust and floats in liquid. If the density of the liquid is less, it sinks more and vice versa. Reasonable Fact An iceberg made up of water floats in water. The density of ice (0.917 g/cm3 ) is less than that of water (1 g/cm3 ). Iceberg is able to displace water equal to its weight. Therefore, iceberg floats in water. A ship can carry more loads on sea water than on the river water. The density of salty water (1.026 g/cm3 ) is more than that of fresh water (1.00 g/cm3 ). So, sea water offers more upthrust to the ship than that of river water. As a result, a ship can carry more loads on sea water than on river water. A special hydrometer that can measure purity (relative density) of the milk is called lactometer. Fig: hydrometer Floatation in Atmosphere The average density of the atmosphere is 1.204 kg/m³, which is far too less than the density Life belt

Oasis School Science and Technology - 10 215 of water (1000kg/m³). Upthrust exerted by the fluid on the immersed object is directly proportional to the density of the fluid. So, an object in the air experience less upthrust than in the water. As a result, denser objects such as wood and ice cannot float in the air even though they can float in water. However, objects that have lesser density than the air can float in the air. For example, a hot air balloon and hydrogen filled balloon. 1. Hot air balloon Hot air balloon uses propane gas to float. About 15 gallons of liquid propane is used for an hour of flying. When the burner is lit, the liquid propane will change into very hot gas and fills the balloon. The density of the air inside the balloon will be lesser than the density of the air outside the balloon. The balloon experiences greater upthrust than its weight. So, the hot air balloon starts to float. The hot air balloon pilot increases the size of the flame to ascend the balloon. When the flames are larger, the gas in the balloon becomes hotter and thinner. Due to a decrease in density of air inside the balloon on heating, hot air balloon experiences more upthrust and rises in the atmosphere. When flames are lowered, the gas in the balloon becomes colder and thicker. Due to an increase in density of air inside the balloon on cooling, a hot air balloon experiences less upthrust and descends. Ascending and descending of a hot air balloon can be controlled by controlling the size of the flame. 2. Rising of hydrogen filled balloons When balloons are filled with lighter gases, like hydrogen or helium, the weight of the air displaced by the balloon becomes more than the weight of the gas filling the balloon. As a result, the balloon rises. As the balloon rises up, the density of air decreases and, hence, the weight of the air displaced also becomes less. Ultimately, when the weight of the displaced air becomes equal to the weight of the balloon, it does not rise any further, or it becomes stationary at a certain height. Differences between Archimedes’ Principle and Law of flotation S.N. Archimedes’ Principle S.N. Law of flotation 1. When a body is partially or wholly immersed in a liquid, the upthrust on the body is equal to the weight of the liquid displaced by it. 1. A body floats on a liquid if it can displace the liquid equal to its weight. 2. It applies equally to floating as well as sinking bodies. 2. It applies only to floating bodies. Fig. Fig: hydrogen filled flying balloon Weather Balloon Recovery Parachute Payload Module

216 Oasis School Science and Technology - 10 Activity 6 • Take a beaker and fill it with water. Take a piece of cork and an iron nail of equal mass. Place them on the surface of the water. Observe what happens. The cork floats, but the iron nail sinks since the cork displaces the liquid equal to its weight but the iron nail cannot displace the liquid equal to its weight. Cork Iron Nail Water Fig. Reasonable Thinking Skill Reasonable Thinking Skill R T S 1. It is easier to swim in ocean water than in river water. It is easier to swim in ocean water than in river water because ocean water has more upthrust due to its higher density because of the presence of salt which is absent in river water. 2. It is observed that the ship floats higher when the goods are unloaded. According to the law of floatation, an object can float in water if it can displace water equal to its total weight. Here, when the loads are removed from the ship, its overall weight decreases. So, it can manage to float by pushing a lesser amount of water than before. Hence the ship rises a bit higher than before. 3. Although a ship is made up of iron, it can float on water. Although a ship is made up of iron, it can float on water because it is shaped in such a way that it holds lots of air within the ship. Due to this, overall density of the ship is lesser than that of the water. But most importantly, it can displace water equal to its own weight due to its shape. So, it can float in water. 4. Ramesh noticed that his child is lighter to carry in the waters of the swimming pool than in the playground. Ramesh noticed that his child is lighter to carry in the water of the swimming pool than in the playground. The reason is upthrust. The upward resultant force that pushes objects upward is less in the air. However, water can exert more upthrust than air. The upthrust can decrease the apparent weight of an object. So, the weight of a person drops significantly when it is in water than in air. 5. Study the given diagram and answer the following questions.

Oasis School Science and Technology - 10 217 i. Which liquid among A, B and C has the highest upthrust? Liquid A has the highest upthrust among them. It is because the hydrometer is floating more in it. It means that liquid has more density. Upthrust is directly proportional to the density of liquid. So, liquid A must have more upthrust. ii. What happens to the hydrometer, if salt is added continuously to liquid C? If salt is added continuously to liquid C. Then the hydrometer will rise due to an increase in density and upthrust of the liquid. iii. On which principle is this device based on? This device, hydrometer, is based on law of floatation. 6. Study the given diagram and answer the following questions. i. 2 kg of water is displaced when 5 kg mass was introduced in the water. How much is the upthrust of the water acting on the 5 kg mass? Mass of the liquid displaced = 2 kg Acceleration due to gravity (g) = 9.8 m/s2 Weight of the liquid displaced = mass of the liquid displaced × acceleration due to gravity = 2 × 9.8 = 19.6 N Upthrust acting on 5 kg mass =? Now, Upthrust = weight of the liquid displaced = 19.6 N ii. What will be the weight of the solid inside the water and in the air? The mass of the object is 5 kg. Acceleration due to gravity (g) = 9.8m/s2 Upthrust of water = 19.6 N Weight of the object in air =? Weight if the object in water =? Now Weight of the object in air = mass of the object x acceleration due to gravity = 5 kg × 9.8 m/s2 = 49 N Again Weight of an object in water = weight in the air – upthrust = 49 N – 19.6 N = 29.4 N iii. Which law is the above experiment based upon? This experiment is based on Archimedes’ principle. 7. Air offers upthrust similar to the water, but a piece of wood cannot float in air but floats in water. A piece of wood does not float in air because it has more density than air. Due to lesser density, air cannot produce sufficient upthrust to carry wood. A piece of wood floats in water because it has less density than the water. Due to greater density, water can produce sufficient upthrust to carry wood.

218 Oasis School Science and Technology - 10 8. It is difficult to submerge a football into water than a table tennis ball? The upthrust acting on a body is directly proportional to the volume of object. Since football is bigger than the table tennis ball, it experiences greater upthrust (resultant force that pushes upward) than table tennis ball. So, it will be difficult to submerge a football into water than a table tennis ball. Exercises 1. Choose the best answer from the given alternatives. a. On which factor liquid pressure does not depend? i. height of the liquid column. ii. density of the liquid iii. acceleration due to gravity iv. surface area of the liquid b. On which principle does hydraulic jack work? i. pascal’s law ii. Archimedes’ law iii. principle of simple machine iv. electromagnetic induction c. How much pressure is produced by the force of 240 N on the piston of a surface area of 20 cm2 ? i. 1.2 Pa ii. 1.2 × 105 Pa iii. 12 Pa iv. 4800 Pa d. Pressure exerted by a litre of water at the bottom of a bottle is more than that by the litre of mustard oil in an identical bottle. Why? i. water has more height than the liquid ii. water experience more acceleration due to gravity than oil iii. water has more density than the oil iv. water has more volume than the oil e. Why does ice float in the water? i. ice floats because it is big and white. ii. ice floats because water can carry anything iii. ice has less density than water iv. ice has less volume than water f. If piston A is double the area of piston B, how much load on piston A can be balanced by 5 N force on piston B? i. 5 N ii. 2.5 N iii. 10 N iv. 25 N 2. Define the following terms with required examples. a. Pressure b. Liquid pressure c. Thrust d. Pascal’s law e. Hydraulic machine f. Upthrust g. Archimedes’ principle h. Law of floatation i. Hydrometer 3. Answer the following questions in very short. a. Write down the formula and SI unit of pressure. b. Name the two factors on which the pressure exerted by a body depends? c. On which factors does hydraulic pressure depend?

Oasis School Science and Technology - 10 219 d. Write down the use of the hydraulic lift. e. Name any two instruments based on Archimedes’ principle. f. What is the resultant force acting on floating object called? In which direction does it act? g. What does it mean if hydrometer sinks more in river water than sea water? 4. Give reasons. a. Heavy vehicles have multiple flat wheels. b. The pointed end of a nail can pierce wood more easily than a blunt end. c. Base of the house is made wider. d. Pascal’s machine is called a force multiplier. e. Weight of an object decreases when it enters the water from the air. f. Liquid pressure at the bottom of the oil tanker decreases as it drives from India to the hilly region of Nepal. g. An egg sinks in freshwater but floats in salt water. h. A ship sinks more when it enters the river from the ocean. i. An iron sinks in water but a steel plate floats in it. j. If a hot air balloon pilot make a larger fire, the balloon rise up. k. Hydrometer can check the purity of petroleum. l. An iron nail sinks in water but floats on mercury. m. It is difficult to submerge a swimming tube under water. n. It is easy to lift a bucket of water only when it is inside the water but gets harder when it is out of the water. o. Hydrogen gas is collected by downward displacement of air in an inverted gas jar. p. If we swim in a river, sea water and dead sea, where will it be easiest for us to swim? q. Hydraulic fluid is used as brake oil. 5. Differentiate between the following. a. Force and pressure b. pressure and upthrust c. Pascal’s law and Archimedes’ principle d. Sinking of iron nail and floatation of iron ship e. Rise of hydrogen in air and sinking of carbon dioxide in air f. Empty ship floating in sea and loaded ship floating in sea g. Air filled balloon and hydrogen filled balloon released in air 6. Answer the following questions in short. a. Write down the principle of the hydraulic press. b. Name the equipment shown in the given diagram. Also, write down the principle on which this equipment works. Calculate the force produced in piston B. 320cm2 20N F2 A B 8 cm2

220 Oasis School Science and Technology - 10 c. What is the difference between rise of hot air balloon and sinking of cold air balloon? d. It is easier to lift a heavy stone underwater than in the air. How can we explain this? e. In the given figure, which is seawater and which is river water, why? f. What will be the effect on the weight of displaced water if the load is added to the ship floating in the ocean? The probability of sinking of the ship increases if it is overloaded. Write with reason. g. Why does a ball try to come up when it is pressed in water? Describe. h. What are the properties of the liquid pressure? i. Derive the relation U = vdg where letters have the usual meaning. j. Usually, the objects denser than water sinks in water. Can objects denser than water float in water? What are the conditions required for that? k. Write properties of hydraulic fluid. l. Write down the applications of pascal’s law and Archimedes’ principle. m. Why do water tanks at the top of the building fill buckets faster in the ground floor? 7. Answer the following questions on the basis of the given figure. Which instrument is shown in the figure? If the crosssectional area of piston A is 40cm2 and that of piston B is 4m2 , what load is necessary on piston ‘B’ to balance the 600N force kept on piston A? (Ans: 6 × 105 N) 8. Answer the following long questions. a. On which principle is hydraulic brake based? Prove that the hydraulic press is a force multiplier. b. Describe in brief the working mechanism of a hydraulic brake. c. A ship coming from the sea enters the river, will its hull sink more or less in river water? Give reason. The weight of an object is 20N in air and the weight of that object in the water is 12N only then: i) What is the value of upthrust on the object by water? (Ans: 8N) ii) What is the weight of the displaced water by it? (Ans: 8N) d. Study the given table and answer the following questions: Substances Density (in g/cm3 ) W 0.8 X 13.6 Y 1 Z 0.9 i. If the volume of all substances is equal, which substance has the greatest mass? Egg Water Fig-1 Fig-2 B A Liquid

Oasis School Science and Technology - 10 221 ii. Among the substances in the table, which substance sinks in water?Why? iii. Among the given substances, name the substance that gives the least upthrust. Why does it have the least upthrust? e. What are the factors that affect upthrust? Explain. f. Let’s suppose a new boat entered the lake. What are the conditions in which this boat can either float or sink? Write conditions for both cases. g. On which law is the given diagram based? State the law. Calculate the area of Piston B and the force exerted in piston C. h. Answer the following questions based on the given figure. i. On which principle is the experiment based? State the principle. ii. What is the weight of the stone? iii. What is the reason that the weight of the object is less in water than in air? i. Three objects with different densities A, B and C are in the water as shown in the figure. Now, answer the following questions: i) Which object has a higher density than the water? Why? ii) If the mass of object A is 1 kg. How much mass of water is displaced by the object? iii) Which law is applicable for object B? State the law. j. When Bishal entered a hot water bath in tatopani, he noticed that his body feels lighter. Is it carried by water? What is that force that pushes the body upward called? Why does it act on immersed object? Explain the physics behind it with the help of a diagram? k. An iron nail sinks in water. Does it not experience upthrust? Discuss. l. The cross-sectional area of a small piston and large piston are 0.5m2 and 5m2 respectively. How much effort is necessary to lift 200 kg mass at large piston? C = 10 cm2 B = 375N 250N A=20 cm2 Spring balance Stone Water 5N Beaker Ureka can 20N A B C

222 Oasis School Science and Technology - 10 Key terms and terminologies 1. Thermal energy : The sum of kinetic energy of all the atoms or molecules in a substance is called thermal energy. 2. Temperature : The average kinetic energy of all the atoms or molecules in a substance is called temperature. 3. Heat : The resultant thermal energy which is transferred from the body of high temperature to the body of low temperature is called heat. 4. 1 calorie heat : The amount of heat required to increase the temperature of 1 gram of pure water by 1°C or 1 K is called 1 calorie heat. 5. Anomalous expansion of water: The unusual behaviour shown by water between 0°C and 4°C is called anomalous expansion of water. 6. Heat equation : The equation that shows the relation among the amount of UNIT 9 HEAT ENERGY Estimated teaching periods Theory 8 Practical 2 • 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 thermometer The Sequence of Curriculum Issued by CDC James Prescott is popular for the development of the first law of thermodynamics. He was born in England on 24th December 1818 and died on 11th October 1889. He was a physicist and mathematician. He studied the nature of heat and its effect on mechanical work. He discovered the law of conservation of energy and the first law of thermodynamics. He was honoured with Royal Medal, Copley Medal and Albert Medal. About the Scientist James Prescott Joule

Oasis School Science and Technology - 10 223 heat lost or gained by a body, the mass of the body, specific heat capacity and change in the temperature is called the heat equation. 7. 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. 8. Land breeze : During night, the colder air from the land travels to the sea. It is called land breeze. 9. Sea breeze : During day, the colder air from the sea travels to the land. It is called sea breeze. 10. 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". 11. Digital thermometer : A digital thermometer is a modern thermometer which shows the temperature in digits. 12. Radiation thermometer : An IR thermometer is an instrument that measures the temperature of a body through the detection of the intensity of thermal radiation. 13. Calibration : The process of making a number of equal divisions in between the upper fixed point and lower fixed point of a thermometer is called its calibration. 14. Upper fixed point : The temperature at which water boils at standard atmospheric pressure is called the upper fixed point. 15. Lower fixed point : The temperature at which water freezes at standard atmospheric pressure is called the lower fixed point. Introduction Every living organism on this earth requires heat. We cook our food by using heat. We dry our clothes by using the heat of the sun. Heat is a form of energy which produces the sensation of warmth. It makes substances hot. Heat is the cause of change in temperature. The temperature of a body increases due to the application of heat. When heat is given to a substance, the kinetic energy of the molecules increases, and they move with greater speed at higher temperature and vice versa. The heat of a body is considered as molecular motion. Heat flows from a hot body to a cold body until both of them have equal temperature. In the SI system, heat is measured in joule (J) and in CGS system, it is measured in calorie (cal.). Heat energy is measured by using calorimeter. Thermal energy, heat and temperature Thermal energy Atoms and molecules are the building blocks of all kinds of matter. These atoms or molecules are moving in an unexpected way. Kinetic energy comes from the random movement of the atoms or molecules in the body. Whenever the kinetic energy of the atoms or molecules in a substance increases, it becomes warmer. So, thermal energy is a

224 Oasis School Science and Technology - 10 form of energy which is due to the motion of the atoms and molecules of the substance. The sum of kinetic energy of all the atoms or molecules in a substance is called thermal energy. For example, a bucket of hot water has more thermal energy than that in a cup of the same water. This is because more molecules means more motion, more kinetic energy and more thermal energy. Reasonable Fact The air inside the air pump becomes warmer on rapidly compressing. The sum of kinetic energy of all molecules in a body is called thermal energy. When air is compressed rapidly, its molecules move faster. As motion of the air molecules increases, the kinetic energy also increases. Increase in kinetic energy results in the increase in temperature. So, the air inside the air pump becomes warmer on rapidly compressing it. Temperature The average kinetic energy of the molecules in a substance is related to its temperature. Changes in the average kinetic energy of the molecules make a body hotter or colder. Whenever the average kinetic energy of the molecules in a substance increases, its degree of hotness also increases. So, the average kinetic energy of all the atoms or molecules in a substance is called temperature. The average speed of molecules in a hot body is faster and the average speed in a cold body is slower. That is why, the higher the temperature of a substance, the faster the motion of its molecules. Reasonable Fact An iron nail becomes hot on frequently hammering it. Striking an iron nail with a hammer causes the molecules in the iron nail vibrate faster. It increases the kinetic energy of the atoms of iron nail. So, an iron nail becomes hot when hammered frequently. Heat: Flow of thermal energy Take 5 litre of water at 700 C and mix it with 10 litre of water at 250 C. The thermal energy transfers from hot water to the cold water. This transferred thermal energy is called heat. Thus, the resultant thermal energy which is transferred from the body of high temperature to the body of low temperature is called heat. Its SI unit is joule(J) and measured by calorimeter. Heat does not store in a body, but it can move from one body to another. More thermal energy will be in the area of the body where heat moves. In the same way, the area of the body that loses heat will have less thermal energy. Reasonable Fact We 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 thermal energy from our body transfers to the cold body.

Oasis School Science and Technology - 10 225 Factors Affecting Thermal Energy Contained in a Body Molecules in a substance are in constant motion. The liquid molecules have more freedom of movement than those in a solid. Molecules in a gas have the greatest degree of motion. Hence, thermal energy of a body depends upon: i. average kinetic energy of the atoms or molecules in a substance. ii. the number of atoms or molecules of the substance. (i.e., the size or the mass of the body) The total quantity of thermal energy contained by a body is directly proportional to the average kinetic energy (K.E.) of the molecules and mass of the body. i.e. Quantity of thermal energy (Q) ∝ average kinetic energy × mass of the body Stirring stick Outer vessel Insulated ring Inner vessel Air space Insulated lid Fig. Calorimeter Different bodies may have different temperature The properties of the molecules of different matters are different. So, when an equal amount of heat is given to an equal mass of different substances, the average kinetic energy of the molecules is also different. Thus, they do not have the same temperature. Heat is required to keep our body warm. Living organisms require heat energy to survive. Heat is used to cook our food, run various means of transportation, such as vehicles, aeroplanes, rockets, etc. Reasonable Fact If equal amount of heat is given to two different substances, the change in their temperature is not same. Quantity of thermal energy (Q) ∝ average kinetic energy × mass of the body The nature of particles is different in different substances. So, the average kinetic energy is not same when equal heat is given to them. Therefore, even if equal amount of heat is given to two different substances, the change in their temperature is the not same.

226 Oasis School Science and Technology - 10 One calorie heat The amount of heat required to increase the temperature of 1 gram of pure water by 1°C or 1 K is called 1 calorie heat. Similarly, 4200 J heat is required to change the temperature of 1 kg mass of pure water by 1°C or 1 K. From this relation, it becomes clear that 4.2 joules of heat energy is required to raise the temperature of one gram of pure water by 10 C or 1K. Thus, 1 calorie = 4.2 joules . Effects of Heat Heat can show the following effects on different substances. i. Heat changes the temperature of a body. ii. Heat changes the physical state of a body. iii. Heat changes the solubility of a substance. iv. Heat brings about a chemical change. v. Heat changes the volume of a body. Relation between heat and volume of the substance When objects absorb heat energy, their thermal energy rises. Their molecules get more kinetic energy. The intermolecular force between the molecules will decrease. As a result, the intermolecular distance among the molecules increases. Hence an object expands on heating. On further heating, it can change states due to a further increase in intermolecular distance. On cooling, objects lose thermal energy. Their molecules lose kinetic energy. Intermolecular distance will decrease and the objects shrink. Applications of Expansion i. Thermometer is based on the principle that matter expands on heating and contracts on cooling. ii. A jammed glass bottle can be loosened by heating its neck. iii. Gaps are left between two successive rails. To prove that volume of the object changes upon heating. Materials required: A ring, a ball, Bunsen burner and a glass of cold water. Procedure i. Take a copper ball and pass it from copper ring. It will pass. ii. Warm the ball and try to pass it from the copper ring again. It won’t pass. iii. Drop the hot copper ball in cold water. Later pass it again from the copper ring. It will pass. Observation: The cold ball passed from the copper ring. The warm ball did not pass from copper ring. Analysis: Volume of the copper ball increased on heating. Volume of the hot copper ball decreased on cooling. Result: The volume of the substances is directly proportional to the temperature.

Oasis School Science and Technology - 10 227 Activity 1 Boil water in kettle. Pour it in a cold glass cup in winter. Did it break? When hot water is poured in cold cup in winter, the glass cup occasionally breaks. Why? Discuss. Hot water and broken glass can hurt us. Be careful! Some cases where expansion can trouble us i. A thick glass vessel cracks when boiling water is poured into it. ii. Soda glass bottle bursts in very hot weather. iii. Time measured by an ordinary pendulum clock fluctuates in summer and winter. iv. In cold countries, sometimes water pipes burst during winter. Reasonable Fact A thick glass vessel cracks when boiling water is poured into it. When boiling water is poured in a thick glass vessel, its inner walls suddenly expand while the outer walls do not. This uneven expansion breaks the glass vessel. Time measured by an ordinary pendulum clock fluctuates in summer and winter. In summer and winter the length of pendulum changes due to expansion and contraction. So, time measured by an ordinary pendulum clock fluctuates in summer and winter. Anomalous Expansion of Water Generally, liquids expand on heating and contract on cooling. But a different behaviour is observed in the case of water. If the water at 0°C is heated, it contracts until the temperature becomes 4°C. But the expansion is observed after 4°C only. Thus, the unusual behaviour shown by water between 0°C and 4°C is called anomalous expansion of water. Effects of anomalous expansion of water 1. Fishes can survive under a frozen pond At 4°C, water has maximum density and minimum volume. This property of water is beneficial for aquatic animals in the cold countries. During winter season, the temperature of water in the cold countries falls to 4°C. On further cooling, the water in the upper layer becomes light and remains on the surface. The surface water gradually cools to 0°C and finally freezes but the water of temperature 0°C to 4°C is in the pond from top to bottom. Water has maximum density at 4°C. So, the layer of water having 4°C remains at the bottom and above this, the layers of water have 3°C, 2°C, 1°C and 0°C are formed. Once the ice is formed at the top, it helps Fig. Pond in very cold countries

228 Oasis School Science and Technology - 10 to trap the heat since it is a bad conductor of heat. Thus, anomalous expansion of water is beneficial for aquatic animals to survive in the pond of very cold countries even through the surface of the pond has frozen into ice. Reasonable Fact A little room is left in the bottle of coke. Coke is a cold drink. It is usually kept in refrigerator. The volume of water in it increases when cooled below 4°C due to anomalous expansion of water. It will need more volume if it freezes into ice. So to prevent bottle from cracking, a room for expansion of liquid is left in the bottle. Fig: Graphical representation of Anomalous expansion of water 2. Water pipe burst in winters Water at 4°C has lowest volume and highest density due to anomalous expansion of water. So the most dense water is collected in pipes, which increases liquid pressure at the walls of pipe. In addition to that, when temperature falls lower than the 4°C, volume starts to increase. At 0°C water turns into ice and has maximum volume. So it exerts greater pressure on the walls of pipes. As a result pipe burst in winters. Factors Affecting Amount of Heat Contained The amount of heat contained in a body depends upon the following factors. i. Change in temperature : The heat energy contained in a body is directly proportional to the change in temperature, i.e Q α dt. ii. Amount of mass : The heat energy contained in a body is directly proportional to the mass of the substance, i.e. Q α m. iii. Nature of the body : Specific heat capacity of the different substances is different. So, the heat contained in a body depends upon the specific heat capacity of the body.

Oasis School Science and Technology - 10 229 Heat Equation When a hot body is cooled, it gives out heat. As a result, the temperature of the body falls. But when a cold body is heated, it absorbs heat and its temperature rises. It has been proved that heat gained or lost (Q) by a body is directly proportional to: i. Mass of the body (m), i.e., Q ∝ m ________ (i) ii. Change in temperature (dt), i. e., Q ∝ dt ________ (ii) Combining (i) and (ii), we get Q ∝ mdt or, Q = msdt (where 's' is a constant called specific heat capacity of the body) ∴ Q = msdt _______ (iii) The amount (quantity) of heat lost or gained by a body (Q) is equal to the product of mass (m), the specific heat capacity (s) and change in the temperature (dt) of that body. The above equation (iii) is called the heat equation. The equation that shows the relation among the amount of heat lost or gained by a body, the mass of the body, specific heat capacity and change in the temperature is called the heat equation. It is Q = msdt. The amount (quantity) of heat lost or gained by a body depends on: i. Mass of the body (m). ii. Specific heat capacity of the body (s). iii. Change in temperature (dt). 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. It is also called specific heat. The SI unit of specific heat capacity is J/kg°C or J/kg K. Values of specific heat capacities of different substances S.N. Substances Specific heat capacity S.N. Substances Specific heat capacity 1. Lead 130 J/kg°C 10. Aluminium 900 J/kg°C 2. Mercury 140 J/kg°C 11. Petrol 1670 J/kg°C 3. Silver 234 J/kg°C 12. Wood 1755 J/kg°C 4. Brass 380 J/kg°C 13. Vegetable oil 2000 J/kg°C 5. Copper 400 J/kg°C 14. Kerosene 2200 J/kg°C 6. Steel 447 J/kg°C 15. Ice 2100 J/kg°C 7. Iron 460 J/kg°C 16. Alcohol 2400 J/kg°C 8. Glass 670 J/kg°C 17. Water 4200 J/kg°C 9. Sand 800 J/kg°C

230 Oasis School Science and Technology - 10 From the above table, we see that the specific heat capacity of water is 4200 J/kg0 C. It means that 4200 J heat is required to raise the temperature of 1 kg mass of water by 1°C. It should be noted that the specific heat capacity of water is higher than that of all other common substances. Mercury has low specific heat capacity, i.e., 140 J/kg0 C, and it is a good conductor of heat. So, it is used as a thermometric liquid. It was found that when different substances having equal masses are given equal amounts of heat, the rise in temperature will be different. This is due to the different specific heat capacity. Thus, 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. This fact becomes clear from the following mathematical relation: Q = msdt or, dt = Q ms If 'Q' and 'm' are constants, then ∴ ∴ dt dt ∝ s α 1 The above relation shows that the change in temperature is inversely proportional to the specific heat capacity of a substance. Reasonable Fact Water is used as a coolant in the radiator (engine) or vehicles. Water absorbs a large amount of heat from the per degree celsius rise in temperature because as it has very high specific heat capacity (i.e., 4200 J/kg°C). Due to this, the radiator (engine) does not get heated up when filled with water. Therefore, water is used in the radiator of vehicles to prevent the engine from excess heating. Reasonable Fact A wet handkerchief is kept on the forehead of a patient suffering from fever. When a person is suffering from fever, a wet handkerchief is kept on his forehead to lower his body temperature. As the wet handkerchief contains water, it can absorb a large amount of heat per degree celsius rise in temperature due to its high specific heat capacity. As a result, the temperature (i.e., high fever) of the patient decreases. Therefore, a wet handkerchief is kept on the forehead of a patient suffering from fever. Reasonable Fact Well water feels warmer in the morning during the winter season. The specific heat capacity of soil (land surface) is less compared to that of water. So, during the winter night, the temperature of the soil decreases faster than that of water. As a result, heat flows from well water to our body when we touch well water. Therefore, well water feels warmer in the morning during the winter season. Fact File Due to the high specific heat capacity (4200J/kg°C), water is used for heating purposes (as in hot water bags) and for cooling purposes (as in the radiator of vehicles).

Oasis School Science and Technology - 10 231 Reasonable Fact In a desert, it is very hot during the day and very cold during the night. The surface of the desert is made up of sand and the specific heat capacity of sand is low (i.e. 800 J/ kg°C). Due to this, the sand becomes very hot during the day as its temperature rise faster due to heat of the sun whereas the hot sand loses its heat and temperature very soon during the night. As a result, it is very hot during the day and very cold during the night in the desert. Impact of Specific Heat Capacity of Water in Climate of Coastal Area Specific heat capacity is the nature of a substance to retain or lose heat per unit mass per unit change in temperature. The specific heat capacity of water in the sea is very high compared to the land nearby in coast. So the water heats up slowly and cools down slowly but absorbs and release large amount of heat. The land heats up faster and cools down faster but absorbs and release less amount of heat. The water absorbs larger quantity of heat from surrounding during day but is still at lower temperature than the land. It creates sea breeze. The sea breeze travels from sea to the land. So, the coast remains cooler. During nights lands are very cold due to low specific heat capacity but the ocean is warm because it cools down slowly. It releases huge quantity of heat into the surrounding. So, the coast gets warmer. The colder air from the land travels to the sea. It is called land breeze. Land breeze is weaker than sea breeze so it does not have much effect on climate. Hence the average temperature during the day and average temperature during the night is almost same in coastal areas. Fig: Sea breeze Fig: Land breeze Reasonable Fact Water is used in hot water bag. Water has high specific heat capacity (4200 J/kg°C). It can radiate large amount of heat slowly per degree celsius fall in temperature for a very long time. So, water is used in hot water bag. Calorimetry and Its Principle Calorimetry is the measurement of heat lost or gained by an object. 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", i.e.,

232 Oasis School Science and Technology - 10 Heat lost = Heat gained or, m1 s1 (t1 – t) = m2 s2 (t–t2 ) where, m1 = Mass of the hot body s1 = Specific heat capacity of the hot body t1 = Temperature of the hot body m2 = Mass of the cold body s2 = Specific heat capacity of the cold body t2 = Temperature of the cold body t = Final temperature of the mixture The condition in which two objects in thermal contact gain equal temperature and no heat flows between them is called thermal equilibrium. Worked out Numerical 1 Calculate the amount of heat required to change the temperature of 1.5 kg of iron from 20°C to 100°C. The specific heat capacity of iron is 460 J/kg °C. Solution: Given, Mass of iron (m) = 1.5 kg Specific heat capacity (s) = 460 J/kg°C Change in temperature (dt) = 100 ° C – 20 °C = 80 °C Amount of heat (Q) = ? We have, Q = msdt = 1.5 × 460 × 80 = 55200 J. ∴ The required heat is 52200 J. Worked out Numerical 2 The initial temperature of a pressure cooker of mass 2 kg is 30°C. The specific heat capacity of the alloy of the pressure cooker is 1000 J/kg°C. Calculate the final temperature of the pressure cooker. The amount of heat supplied is 7.5 ×104 joule. Solution: Given, Mass of pressure cooker (m) = 2 kg Specific heat capacity (s) = 1000 J/kg 0 C Amount of heat (Q) = 7.5 × 104 J Initial temperature (t1 ) = 30 0 C Change in temperature (dt) = ? We have,

Oasis School Science and Technology - 10 233 Q = msdt Or, dt = Q m s C × = × × = 7 5 10 2 1000 37 5 4 . . º Now, Final temperature (t2 )= t1 + dt = 300 C + 37.50 C = 67.50 C ∴ The final temperature of the pressure cooker is 67.50 C. Worked out Numerical 3 A bucket contains 10 kg of hot water at 700 C. The water is cooled for bathing by mixing 20 kg of water at 100 C. Calculate the final temperature of the mixture. [Neglect the heat absorbed by the bucket]. Solution: Given, Mass of hot water (m1 ) = 10 kg Temperature of hot water (t1 ) = 700 C Specific heat capacity of water = s (both hot and cold) Mass of cold water (m2 ) = 20 kg Temperature of cold water = (t2 ) = 100 C Let, the final temperature of the mixture be t 0 C. Now, According to the principle of calorimetry, Heat lost = Heat gained or, m1 × s × (70–t) = m2 × s (t–10) or, 10 × s × (70–t) = 20 × s × (t–10) or, 10 × (70–t) = 20 (t-10) or, 700 – 10t = 20t – 200 or, 700 + 200 = 20t + 10t or, 900 = 30t or, t = 900 30 = 30 or, t = 300 C ∴ The final temperature of the mixture is 300 C. Temperature The degree of hotness or coldness of a body is called temperature. The normal temperature of the human body is 98.6°F or 37°C. Temperature is measured in kelvin (K), degree Celsius (°C) and degree Fahrenheit (°F). The temperature of a body is measured by using a thermometer. Fact File Steam molecules have higher temperature than our skin. They have greater average kinetic energy, so they can easily penetrate our skin and burn us. Fact File If an equal amount of heat is given to unequal masses of the same body, the temperature of the body with less mass will be higher and vice-versa.

234 Oasis School Science and Technology - 10 When heat is supplied to a body, it becomes hot and as the heat is removed, it becomes cold. Obviously, a hot body has more temperature than a cold one. Definition of Temperature in Terms of Kinetic Energy The average kinetic energy of the molecules of a body is called the temperature of that body. The temperature of a body is directly proportional to the molecular vibration of the body. Activity 2 Take two beakers of the same size. Put 150 ml of mustard oil in one beaker and 150 ml of water in another. Measure the initial temperature. Heat those liquids using an equal-sized lamp for five minutes. Measure the temperature of each liquid every one minute. Which liquid shows faster change in the temperature? The rate of rise in temperature depends on the nature of the body. The temperature of oil will increase faster than that of water. This is due to the nature of the molecules of oil and water. The heat retention capacity of the molecules of these liquids is different. So, they have different capacity to absorb and lose heat. It is found from the experiment that 1 kg of water requires 4200 J of heat energy to raise its temperature by 1° C. But 1 kg of oil requires less amount of heat (about 2000J) than that of water to increase its temperature by 1° C. Activity 3 • Take two beakers of equal size and put different amounts of water in those beakers. • Heat the beakers with the same sized lamp. • Observe the reading of the thermometer dipped into these liquids after a certain time interval. What do you observe ? • The beaker having less amount of water has a higher temperature if an equal amount of heat is supplied for an equal interval of time. Thermometer Water Bunsen burner Stand Stand Fig. 4.3

Oasis School Science and Technology - 10 235 Differences between Heat and Temperature S.N. Heat S.N. Temperature 1. The flow of thermal energy from object at higher temperature to the object at lower temperature is called heat energy. 1. The average kinetic energy of the molecules of a body is called the temperature of that body 2. Heat is measured by a calorimeter. 2. Temperature is measured by a thermometer. 3. The SI unit of heat is joule. 3. The SI unit of temperature is kelvin. 4. Heat flows from a hot body to a cold body. 4. Temperature gives the direction of flow of heat. 5. Heat is the cause of change in temperature. 5. Temperature is the effect of heat. Measurement of Temperature The device used for measuring the temperature of a body is called the thermometer. Various types of thermometer are available based on their uses. Liquid thermometer, digital thermometer and radiation thermometer are commonly used to measure temperature. Temperature is expressed in three different scales after measurement. They are: i. Celsius scale ii. Fahrenheit scale iii. Kelvin scale Relation among °C, °F and K Scale – Lower fixed point Upper fixed point – Lower fixed point C – 0 100 – 0 = F – 32 212 – 32 = K – 273 373 – 273 C – 0 100 = F – 32 180 = K – 273 100 upper fixed point 373.15 273.15 0 kelvin -273.15 Celsius -459.7 Fahrenheit 100 0 212 32 100 K 100 0 C 180 0 F Lower fixed point Absolute Zero Types of Thermometers Based on Uses 1. Liquid thermometer The glass thermometers that work on the principle of thermal expansion of thermometric liquid are called liquid thermometers. The working principle of the thermometer is, "Liquid expands on heating and contracts on cooling." They usually contain coloured alcohol or mercury as a thermometric liquid. A liquid thermometer could be a laboratory thermometer, clinical thermometer or maximum-minimum thermometer.

236 Oasis School Science and Technology - 10 Activity 4 Now, let us observe the boiling and freezing points of alcohol and mercury. Which of the following thermometric liquid is the right liquid to measure 100°C? Why? Substances Freezing point Boiling point 1. Mercury – 39°C 357°C 2. Alcohol – 115°C 78° C Structure of Liquid Thermometer A liquid thermometer consists of a glass tube in which a bulb is attached with a capillary tube. The bulb of the thermometer is filled with a thermometric liquid. When heat is supplied to the bulb, the liquid in the bulb expands and rises up in the capillary tube. After sometime, it shows a constant reading, and the reading shows the temperature of the body. The clinical thermometer has a bent near bulb in its capillary to prevent back flow of mercury. Clinical thermometer is short and ranges from 35°C to 42°C or 95°F to 107.6°F. It is used to check temperature of human body. The laboratory thermometer is long and ranges from 10°C to 110°C. It is used to measure temperature in science laboratory. The maximum-minimum thermometer is U-shaped. It is used to measure the highest and lowest temperature of the day. The structure of a liquid thermometer is shown below: Lower fixed point Bulb containing thermometric liquid Glass stem Scale Capillary tube Upper fixed point Fig. 4.4 Liquid thermometer Laboratory thermometer Reasonable Fact The boiling temperature of water cannot be measured with the help of an alcohol thermometer. The boiling point of alcohol is 78°C whereas the boiling point of water is 100°C. Since the boiling point of water is more than that of alcohol, the boiling temperature of water cannot be measured with the help of an alcohol thermometer. Reasonable Fact Two reasons for using mercury as a thermometric liquid. Mercury is used as a thermometric liquid because: i. It is a good conductor of heat. ii. It has low specific heat capacity, i.e., 140 J/ kg0 C.

Oasis School Science and Technology - 10 237 2. Digital thermometer A digital thermometer is a modern thermometer which shows the temperature in digits. It does not have any calibration on it. It has a built-in heat sensitive thermistor. When the bulb of the thermometer comes in contact with a body while it is on, it shows the temperature on the monitor. Fig. Digital thermometer The principle of operation of the thermometer is recording the change of electrical resistance of a conductor when thermistor is heated and converting it into a numeric value. The digital thermometer operates through the contact method, as the mercury thermometer. To obtain accurate results, it is necessary to ensure close contact of the thermometer with the human body. A digital thermometer monitors temperature fluctuations. When the figures stabilizes, the thermometer beeps and a digital display shows the results. 3. Radiation thermometer Every hot object radiates thermal energy in the form of infrared radiation. The intensity of infrared radiation can be measured by an instrument such as an infrared radiation thermometer. IR gun is not a thermometer like that we used traditionally. It is actually a detector based on the principle to measure the intensity of thermal radiation from hot objects. Thus, an IR thermometer is an instrument that measures the temperature of a body through the detection of the intensity of thermal radiation. Working mechanism of radiation thermometer We simply hold an IR gun, press the trigger, and point the laser towards the target to measure the surface temperature of the body. The lenses will converge radiations into the thermopile. Thermopile generates heat which is converted into electricity and sent to the detector. The higher the amount of current detected higher will be the temperature of the object. The reading is displayed in the monitor. Reasonable Fact Infrared radiation thermometer was widely used during COVID-19 pandemic. The infrared radiation thermometer was widely used during COVID-19 pandemic because it is a non-contact thermometer that can measure temperature from far away. Figure of radiation thermometer Fact File Radiation thermometer was mostly used during corona pandemic to check temperature of people, students and office staffs.

238 Oasis School Science and Technology - 10 Activity 5 Make a model of radiation thermometer and explain how it work. Calibration of Thermometer The thermometers have bulb, stem and capillary tube. The stem have calibration in it. It helps to take reading. To calibrate thermometer, first of all lower fixed point and upper fixed point is marked. Then the gap between upper fixed point and lower fixed point is divided into required number of equal divisions based on scale. The process of marking upper and lower fixed point then dividing the length between them into required equal divisions is called calibration of thermometer. Upper fixed point: The temperature at which water boils at standard atmospheric pressure is called the upper fixed point. Lower fixed point The temperature at which water freezes 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 00 C and upper fixed point is 1000 C; in Fahrenheit scale the lower fixed point is 320 F and upper fixed point is 2120 F; similarly, in Kalvin scale the lower fixed point is 273K and upper fixed point is 373K. Reasonable Fact The bulb of a thermometer is made thin-walled. The bulb of a thermometer consists of thermometric liquid, either mercury or alcohol. A thin walled bulb helps to conduct heat from outside to the thermometric liquid. So, the bulb of a thermometer is made thin. Activity 6 OBJECTIVE : To find the lower fixed point of a thermometer. We should put the thermometer in a funnel that contains pure melting ice at 1 atm pressure. The mercury will shrink by losing heat and becomes stable. We should mark the level of mercury. It is now lower fixed point (ice point) for thermometer. Lower fixed point is also defined as the temperature of the pure melting ice at sea level.

Oasis School Science and Technology - 10 239 Activity 7 OBJECTIVE : To find the upper fixed point of a thermometer. We should put the thermometer in a round bottom flask that contains pure boiling water at 1 atm pressure. The mercury will expand by absorbing heat and becomes stable. We should mark the level of mercury. It is now upper fixed point for thermometer. Upper fixed point is also defined as the temperature of pure boiling water at sea level. Reasonable Thinking Skill Reasonable Thinking Skill R T S 1. The pond was frozen. Mr. Harka cut open a hole in the ice and started fishing. When Ekta went near him and observed she saw that there is water below. The pond was frozen but Harka was still able to collect fish by cutting a hole in the ice. There was water under the ice along with aquatic animals and plants. It is possible only because of the anomalous expansion of water. Materials expand on heating but water contracts when heated from 0°C to 4°C. As a result, water at 4 °C has the highest density and sinks to the bottom. Water starts freezing from the top. The layer of ice at the top acts as an insulator and prevents further cooling of water below. As a result, liquid water at 4°C remains at the bottom of the pond where aquatic animals and plants can survive. 2. Ice melts on heating. What happens to kinetic energy of molecules, intermolecular force and intermolecular distance upon heating? The ice is a solid. Its molecules are bound together by hydrogen bonds. When heat is supplied, the average kinetic energy of the molecules increases. Molecules vibrate faster and bonds are broken. Intermolecular force decreases. The intermolecular distance increases. The molecules travel faster and farther. Then solid ice melts into liquid water. 3. The clinical thermometer is not cylindrical but prismatic. Clinical thermometer is made prismatic instead of cylindrical because it helps to refract more light to the mercury. So, that mercury can shine and it will be easy to take a reading. 4. There is a kink in the clinical thermometer but not in a laboratory thermometer. The clinical thermometer has a kink because it helps to prevent the backflow of mercury. So, doctors can remove the thermometer from patients and observe a few durations later. However, readings can be taken in a laboratory thermometer without removing them from the experiment. So, a laboratory thermometer does not need a kink.

240 Oasis School Science and Technology - 10 5. In winter we can warm water and keep it in a hot water bag. Then it is kept with us in bed. It keeps us warm all night long. Usually, people boil water and fill it in a hot water rubber bag to keep them warm on cold nights. It is possible only because the water has a high specific heat capacity. When boiled water can absorb a large quantity of heat per each degree Celsius rise in temperature. And later, it will lose the same quantity of heat per each degree Celsius fall in temperature. So, it can keep us warm for a very long time. 6. Pundarika woke up in the morning. She felt cold after leaving the bed. But when she returned to her room, after her morning walk she felt warm in the same room. She is wondering why the air in same room felt cold earlier but feels warmer after a morning walk. Explain it to her. She found the air of same room colder at first but warmer after returning from her morning walk. The main reason is the principle of calorimetry. According to the principle of calorimetry, heat flows from a hotter object to a colder one. When we wake from bed our body is warmer than the air in the room. So, we lose heat and feel cold. During morning walk, our skin becomes colder due to the air. So, when we return to the room, heat from the air enters our bodies. Hence, we feel warm. 7. Study the given graph where the relation between the density of the water and the change in water temperature is shown graphically. i. What does the curve 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? Water has a minimum volume at 4°C. The volume of the substance is inversely proportional to the density of the liquid. The density of water is maximum at 4°C. So opposite to that, the volume of water must be minimum at 4°C. iii. What happens if the water at 0°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. 8. Average temperature during the day and average temperature during the night is almost same in coastal areas. So weather is pleasant there. What is the role of specific heat capacity for such climate? Describe. Specific heat capacity is the nature of a substance to retain or lose heat per unit mass per unit change in temperature. The specific heat capacity of water in the sea is very high compared to the land nearby in coast. The water absorbs larger quantity of heat from surrounding during day but is still at lower temperature than the land. It creates sea breeze. So, the coast remains cooler. During nights lands are very cold due to low specific heat capacity but the ocean is warm Max 0°C 4°C Density Temperature

Oasis School Science and Technology - 10 241 because it cools down slowly. It releases huge quantity of heat into the surrounding. So, the coast gets warmer. Hence the average temperature during the day and average temperature during the night is almost same in coastal areas. 9. Icebergs in ocean are at 0°C yet have more thermal energy than the cup of boiling water. How is it possible? Even though icebergs in ocean are at 0°C, they have more thermal energy than the cup of boiling water. It is because icebergs have greater mass than the cup of boiling water. Since thermal energy is directly proportional to the total amount of mass. Exercises 1. Choose the best answer from the given alternatives. a. What is measured by the calorimeter? i. temperature ii. calorie iii. heat iv. radiation b. On which factor amount of thermal energy present on the body does not depend? i. average kinetic energy of the molecules ii. mass of the molecule iii. nature of the substance iv. acceleration due to gravity c. What is the principle of a thermometer? i. substances expand on heating and cooling. ii. substances expand when heated from 0 °C to 4 °C. iii. substances expand on heating and contract on cooling. iv. substances sometime expand and sometime contract on heating. d. What is thermal energy? i. total kinetic energy ii. average kinetic energy iii. flow of kinetic energy iv. heat energy e. Why does water remain hot for a longer duration than the same mass of oil at an equal temperature? i. it has more temperature than the oil ii. it has more mass than the oil iii. it has more density than the oil iv. it has a more specific heat capacity than the oil 2. Define the following terms with required examples. a. Heat b. One calorie heat c. Temperature d. Thermal expansion

242 Oasis School Science and Technology - 10 e. Thermometer f. Clinical thermometer g. Digital thermometer h. Radiation thermometer i. Heat equation j. Specific heat capacity k. Thermometric liquid l. Principle of calorimetry m. Anomalous expansion of water n. Calibration of thermometer 3. Answer the following questions in very short. a. State the factors on which the amount of heat present in a body depends. b. Which physical quantity of heat equation depends upon nature of the substance? c. What is the sum of kinetic energy of each molecules called? d. Write down formula and SI unit of specific heat capacity. e. Name any two liquids which are used in the thermometer. f. Name the instrument which is made based on the expansion of a liquid on heating. g. What is the body temperature of a healthy human? h. What is the value of the specific heat capacity of water? i. What is the relation between the specific heat capacity and the rate of increase or decrease in its temperature? j. What is the relation between the amount of heat energy contained by a body, its mass and kinetic energy of the molecules? k. Give a positive effect of anomalous expansion of water. l. Give two examples of positive effects of high specific heat capacity of water. m. What is the relation between melting point of water and atmospheric pressure? n. What is the lower fixed point and upper fixed point of thermometers in different scales? 4. Give reasons. a. Water is used on hot bags to keep us warm on cold nights. b. A new quilt is warmer than the old one. c. A wet handkerchief is kept on the forehead of the patient suffering from fever. d. We wear woollen clothes in the winter. e. When a hot iron nail is kept in cold water, the temperature of iron decreases, whereas the temperature of water increases. f. The temperature of mercury is more when an equal amount of heat is given to one kilogram of water and mercury each. g. Specific heat capacity of objects differs. h. The temperature difference is low near the sea. i. The water at the bottom of the frozen pond is warm enough to sustain life. j. Water pipes burst open in cold winters in cold countries. k. A glass full of water at 4°C will overflow if it is either heated or cooled. l. A little vacuum is left in glass bottles of cold drinks. n. Night in deserts is very cold and day in that place is very hot.

Oasis School Science and Technology - 10 243 o. Water boils at lower temperatures in higher altitudes. q. Metal sheet gets warmer when hammered. r. Coastal area have fine weather and temperature day and night. 5. Differentiate between the following. a. Heat and temperature b. Thermal energy and heat energy c. Radiation thermometer and digital thermometer d. Lower fixed point and upper fixed point 6. Answer the following questions in short. a. What is the meaning of the statement that the specific capacity of water is 4200 J/kg°C? b. What is used to cool the engine of the vehicle and why? c. If an equal amount of heat is given to the equal mass of water and alcohol, which warms up faster and why? d. Objects expands on heating. What happens to the kinetic energy of molecules, intermolecular force and intermolecular distance on heating? e. If we boil water and pour some in a cup. Will both have same temperature? Why? Will both have equal amount of heat? Why? f. Metallic door knobs are colder to touch than the wooden parts of the door in winter mornings. Explain the reason based on kinetic molecular theory. g. If you cannot open lunch box at your home because its too tight. How can you open it without breaking? Can we use heat to open it? How should we do it? Why does it work? Explain. h. When a cup of warm water is allowed to cool, the bottom of the cup has colder water than the top. Explain the reason behind it. i. Even though the ponds freeze in winters in cold countries, aquatic life yet manages to survive under the frozen pond. How is it possible? j. How is solidification of ghee different from solidification of water? Explain. k. Do water and other liquids expand similarly? What is the process in which water expands differently from other liquid called? l. Describe the ball and ring experiment to demonstrate thermal expansion. m. Draw the diagram, describe the structure and explain working mechanism of clinical thermometer, digital thermometer and radiation thermometer. 7. Study the given figure and answer the following questions. i. What is the direction of heat and why? ii. What will be the equilibrium temperature if stopper is opened, assuming there is no heat gain or loss in the system? (Ans: 66.66°C) 8. Answer the following long questions. a. Write any three advantages of high specific heat capacity of water. 10 kg of water at 600 C 5 kg of water at 800 C A B Stopper

244 Oasis School Science and Technology - 10 b. Describe an activity to show that heat always flows from the body at high temperature to the body at low temperature. c. How is a thermometer calibrated? d. Even though child and parents have same body temperature of 37°C, why do children have less heat in the body? Why should they wear more clothes in winter? e. Study the given table and answer the following questions. Metal Specific heat capacity A 140 J/kg 0 C B 460 J/kg 0 C C 380 J/kg 0 C i. The specific heat capacity of metal ‘A’ is 140 J/kg°C. What does it mean? ii. If the same amount of heat energy is given to the same mass of all three metals at the same temperature, which one will gain the lowest temperature, why? iii. Which metal will penetrate the greatest depth if each with equal mass is put on a wax slab after heating to 75°C each? Why? f. Three liquids ‘A’, ‘B’ and ‘C’ of equal mass are kept in the same type of container and placed in the sun for 30 minutes. The increase in temperature is given in the table. Liquid Increase in temperature A 170 C B 300 C C 230 C i. Which liquid has the highest specific heat capacity? Why? ii. If an equal mass of all three liquids at the same temperature is cooled, which one will cool down faster? Why? g. Specific heat capacities of three different substances are given below: Substances Specific heat capacity A 910 J/kg 0 C B 380 J/kg 0 C C 470 J/Kg 0 C i. What do you mean by the specific heat capacity of A is 910 J/kg°C? ii. Which of the above-given substance will gain the least temperature while heating the equal mass of all the three substances supplied with an equal amount of heat? Give reason. iii. Which one will go down least in depth while keeping equal mass at the three substances heated for the same temperature of 100°C and kept on a wax slab, why?

Oasis School Science and Technology - 10 245 9. Numerical a. Write the relationship between heat gained or lost by an object with its mass. An iron ball of mass 5 kg at 100°C temperature is dipped in water of mass 10 kg at 20°C. Find the final temperature of water. (The specific heat capacity of water and iron are 4200 J/kg°C and 470 J/kg°C respectively.) (Ans: 24.33°C) b. What will happen if oil is used in hot water bag instead of water? Explain with reason. Hot water of mass 10 kg at 90°C is cooled for taking bath by mixing 20 kg of water at 20°C, what is the final temperature of the water? (Specific heat capacity of water is 4200 J/kg°C) (Ans: 43.33°C) c. Why does frostbite occur in mountain climbers? What will be the final temperature of a mixture of 10kg of water at 70°C and 20 kg of water at 10°C? Neglect the heat absorbed by the container. d. Write any two advantages of high specific heat capacity of water. Calculate specific heat capacity of the alloy of a pressure cooker of mass 1.5 kg, if the quantity of heat necessary to raise its temperature by 60°C is 81 kJ. (Ans: 900 J/kg°C) e. A 1000 kW heater gives a heat of 4.2 × 105 kJ in an hour. If this amount of heat is supplied to 20 kg of water at 30°C, find the final temperature of the water. [Ans: 80°C] f. 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] g. 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 up is 1000 J/ kg°C. What will be its temperature when 7.5 ×104 J of heat is supplied to it? [Ans: 80°C] h. How much heat energy is required to raise the temperature of 5 litres tap water at 20°C to 35°C for bathing?

246 Oasis School Science and Technology - 10 10 WAVE UNIT Real depth Apparent depth Water Coin I O A A N N' i r

Oasis School Science and Technology - 10 247 Key terms and terminologies 1. Light : Light is a form of energy that produces the sensation of vision. 2. Rarer medium : The medium in which light travels faster is called optically rarer medium. 3. Denser medium : The medium in which light travels comparatively slowly is called optically denser medium. 4. Refraction of light : The bending of light when it passes obliquely from one transparent medium to another is called refraction of light. 5. Causes of refraction : Refraction of light takes place while going from one medium to another because the speed of light in different media is different. 6. Lateral shift : The perpendicular distance between the original path of the incident ray and the emergent ray is called lateral displacement or lateral shift. 7. Refractive index : The ratio of the sine of angle of incidence to the sine of angle of refraction for a given pair of media is constant which is called refractive index. UNIT 10.1 LIGHT Estimated teaching periods Theory 6 Practical 1 • 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 prism The Sequence of Curriculum Issued by CDC Willebrord Snellius was well known for discovering Snell’s law of refraction of light. He was born in the Dutch Republic on 13th June 1580 and died on 30th October 1626. He was a Dutch astronomer and mathematician. He along with his friends used the method of triangulation to measure the circumference of the earth. For his honour, a glacier in Antarctica is named Snellius Glacier after his name. About the Scientist Willebrord Snellius

248 Oasis School Science and Technology - 10 8. Real depth : The actual depth of the pond is called real depth. 9. Apparent depth : The virtual depth at which a body appears due to refraction of light is called apparent depth. 10. Critical angle : The angle of incidence in the denser medium for which the corresponding angle of refraction in the rarer medium is 90° is called critical angle. It is denoted by C or ic . 11. Total internal reflection : When the angle of incidence is further increased beyond the critical angle, the ray of light reflects back to the same denser medium instead of refraction. This phenomenon is called total internal reflection of light. 12. Light pipe : A transparent hollow tube made of glass or plastic is called light pipe. 13. Mirage : Mirage is an optical illusion of water in which a pond-like structure is seen on the surface of coal-tarred roads or in the hot desert. 14. Optical fibre : Optical fibre is a very thin, transparent tube slightly thicker than human hair made from silicon or plastic. 15. Endoscopy : Endoscopy is the process in which an endoscope is used to examine the interior of the intestine, blood vessels, lungs etc. 16. Colonoscopy : Colonoscopy is the process in which an endoscope is passed through the anus into the large intestine to check the health condition. 17. Keyhole surgery : Keyhole surgery is a surgical procedure in which the surgeon can access the abdomen and pelvis with the help of a laparoscope. 18. Dispersion of light : The phenomenon of splitting white light into its constituent colours on passing through a glass prism is called dispersion of light. 19. Newton's colour disc : The circular disc which has seven segments with seven colours of rainbow in each segment is called Newton's colour disc. 20. Rainbow : A rainbow is a semi-circular or circular colourful meteorological phenomenon seen in the sky. Introduction Light makes it possible for us to see things around us. Thus, light is a form of energy that produces the sensation of vision. It always moves in a straight line as long as the light rays are in a medium with the same density. Light travels from one place to another with a very high speed. It was found that the speed of light in a vacuum is about 3×108 m/s. This is the maximum speed of light. The speed of light in air medium is slightly less than that in the vacuum. For the sake of simplicity, the speed of light in air is taken to be equal to that of the speed in the vacuum. The speed of light in glass medium is 2 × 108 m/s whereas the speed of light in water medium is 2.25 × 108 m/s. Optically Denser and Rarer Medium The speed of light in different media is different. A medium is a transparent substance like air, glass, water, oil etc. in which light travels. The medium in which light travels faster is called optically rarer medium. The medium in which light travels comparatively slowly

Oasis School Science and Technology - 10 249 is called optically denser medium. For example, air is an optically rarer medium as compared to glass because velocity of light in air is 3×108 m/s, which is greater than the velocity of light in glass (denser) medium which is 2×108 m/s. The density of the medium has no role in determining an object as rarer or denser medium. For example, water has density of 1000kg/m³ which is greater than the density of oil which is 950 kg/m³. But water is optically rarer medium compared to oil despite of having greater density. Refraction of Light When light rays are made to go obliquely from one transparent medium to another, they change their direction at the boundary separating the two media. For example, when light rays travelling in air enter into water medium, they bend or change their direction on entering the water medium. This process is called refraction of light. The bending of light when it passes obliquely from one transparent medium to another is called refraction of light. Refraction of light is responsible for a number of optical illusions. For example, the stick or pencil immersed partially in water appears to be bent. The depth of a pond appears less than the real depth, mirage can be seen on coal-tarred roads or deserts in hot sunny days and so on. Similarly, various optical instruments like camera, microscope, telescope, etc. work on the basis of refraction of light through glass lenses. Cause of Refraction of Light We know that the speed of light is different in different media. It is less in water and more in air. When light travels from one medium to another, the speed of light changes and this change is the cause of refraction of light. Hence, refraction of light takes place while going from one medium to another because the speed of light in different media is different. Reasonable Fact Water is optically rarer medium as compared to glass. When light passes through water to glass, the value of speed of light in water is 2.25 × 108 m/s and that in glass is 2 × 108 m/s. Light travels faster in water than in glass. So water is an optically rarer medium than glass. Refraction through Glass Slab General Terms Used in Refraction of Light i) Incident Ray : A ray which strikes the surface of separation of two optical media is known as the incident ray. In the given figure, AO is the incident ray. ii) Refracted Ray : The ray which travels in the second optical medium, with a change in direction, is called the refracted ray. In the given figure, OB is the refracted ray. iii) Normal : A perpendicular line drawn at the point of incidence is called the normal. In the given figure, NN' is a normal line. I Air N R r i O Glass Refracted ray Fig 5.1 Refraction of light

250 Oasis School Science and Technology - 10 iv) Angle of Incidence : The angle made by an incident ray with the normal, at the point of incidence, is known as the angle of incidence. In the given figure 'i' is the angle of incidence. v) Angle of Refraction : The angle made by refracted ray with the normal, at the point of incidence, is known as the angle of refraction. In the given figure, 'r' is the angle of refraction. vi) Emergent Ray : The ray of light which comes out from the denser medium to the same rarer medium after its refraction is called emergent ray. vii) Emergent Angle : The angle made by emergent ray with normal is called emergent angle. vii) Lateral Shift: When light travels from denser medium to the rarer medium, it bends away from normal towards the direction of incident ray. The net distance shifted by the emergent ray relative to incident ray is called lateral shift. Laws of Refraction of Light When light travels from one medium to another, it follows certain rules. These rules are popular as laws of refraction of light. The laws of refraction of light are as follows: 1. The incident ray, normal and the refracted ray all lie on the same plane at the point of incidence. 2. The ratio of the sine of angle of incidence to the sine of angle of refraction for a given pair of media is constant, i.e. sin i sin r = µ (constant) Fig. Refraction through glass slab i A P S Q Emergent ray Incident ray Refracted ray R C D B q N N' N1 N'1 AIR O Glass Rectangular glass slab Direction of original ray Lateral displacement AIR Normal Angle of incidence(i) Incident ray Angle of refraction (r) Refracted ray Normal Air medium (rarer) Glass medium (denser) Fig. Refraction of light B O r 900 i A