Oasis School Science and Technology 8 New

Oasis School Science and Technology - 8 147 Ideal Machine The machine without friction during its operation is called an ideal (or perfect) machine. In such a machine, the output work is always equal to the input work. For an ideal machine, Output work = Input work Also, ŋ = Output work Input work × 100 % = 100% [∵ Output work = Input work] The efficiency of an ideal machine is 100%. It is to be noted that in practice no machine is 100% efficient. The output work of a machine is always less than the input work. A machine cannot be 100% efficient because of the following reasons: (i) A part of the work done or energy supplied to the machine is wasted in overcoming the friction between the movable parts of the machine. (ii) A part of work done or energy supplied to the machine is wasted in moving the parts of the machine. Please note that no machine is weightless. Therefore, the efficiency of a practical machine is always less than 100%. Meaning of 80% efficiency of a machine A machine has 80% efficiency means that 80% of the input work is converted to useful output work and the remaining 20% of the input work is wasted to overcome the friction and to move the parts of the machine. Differences between Practical machine and Ideal machine S.N. Practical machine S.N. Ideal machine 1. The efficiency of a practical machine is less than 100%. 1. The efficiency of an ideal machine is 100%. 2. In a practical machine, output work is less than input work. 2. In an ideal machine, output work is equal to input work. 3. In a practical machine, MA is less than VR. 3. In an ideal machine, MA is equal to VR. Reasonable Facts It is impossible to get a perfect machine in practical life. Due to friction and the weight of a machine, the output work of a machine is always less than the input work. Therefore, it is impossible to get a perfect machine in practical life.

148 Oasis School Science and Technology - 8 Worked out Numerical 1 A load of 900 N is lifted by a machine applying 300 N effort. If the effort distance and load distance are 80 cm and 20 cm respectively, calculate the mechanical advantage, velocity ratio and efficiency of the machine. Solution: Given, Load (L) = 900 N Effort (E) = 300 N ∴ Mechanical advantage (MA) = L E = 900 N 300 N = 3 Effort distance (E.d.) = 80 cm Load distance (L.d.) = 20 cm ∴ Velocity ratio (VR) = E.d. L.d. = 80 cm 20 cm = 4 Now, Efficiency (ŋ) = MA VR × 100% = 3 4 × 100% = 75% ∴ The MA, VR and ŋ of the machine are 3, 4 and 75% respectively. Types of Simple Machines On the basis of structure and use, there are six types of simple machines. They are: 1. Lever 2. Pulley 3. Wheel and axle 4. Inclined plane 5. Screw 6. Wedge 1. Lever A lever is a rigid bar which moves freely about a fixed point called the fulcrum. In a lever, effort is applied at one point to lift a load on another end. A lever has a fulcrum, effort distance and load distance. The fixed point about which a lever can rotate freely is called fulcrum. The distance between the fulcrum and the point at which effort is applied is called effort arm. Similarly, the distance between the fulcrum and the point at which the load acts is called the load arm. Lever was invented around 240 BC. Fig. 3.2 Parts of a typical lever Load arm Load Effort arm Effort Fulcrum

Oasis School Science and Technology - 8 149 Working Principle of Lever When a lever is in balanced state, input work is always equal to output work. For an ideal lever, the input work is always equal to the output work. It is called the principle of lever. When a lever is in a balanced condition, Input work = Output work or Effort × effort arm = Load × load arm Types of Lever Depending on the position of the fulcrum, effort and load, levers are of three types. They are as follows: (i) First class lever (ii) Second class lever (iii) Third class lever a. First class lever: The lever in which fulcrum lies in between load and effort is called first class lever. For example, crowbar, see-saw, scissors, pliers, nail cutter, etc. Fig. Some levers of the first class Crowbar See-saw Scissors Pliers Nailcutter First class lever can increase the rate of doing work, change the direction of force and multiply the force. In this type of lever, the effort arm may be less, equal to or greater than the load arm. The MA of first class lever may be more than one, one or less than one. Reasonable Facts The cutting edges of metal-cutting scissors are made shorter but those of cloth-cutting scissors are made longer. Metal is harder than the cloth. So, we have to apply greater effort to cut metal than the cloth. If the cutting edges of metal-cutting scissors are made shorter then the load distance is shorter. As a result of which greater effort is created on that edge and it is easy to cut the metal piece. But it is easy to cut cloth even the cutting edges of cloth cutting scissors are made longer. Therefore, cutting edges of metal-cutting scissors are made shorter but those of cloth cutting scissors are made longer. b. Second class lever: The lever in which load lies in between fulcrum and effort is called second class lever. For example, wheel barrow, nutcracker, Fig. Some levers of the second class Wheel barrow Nutcracker Bottle opener

150 Oasis School Science and Technology - 8 bottle-opener, etc. In second class lever, effort arm is always greater than the load arm. So a small effort applied on the second class lever can lift a heavy load. Therefore, MA of second class lever is always more than one. Reasonable Facts It is easier to lift a load when shifted towards wheel in a wheel barrow. A wheel barrow is a second class lever. The mechanical advantage is increased if the load is shifted towards the fulcrum, which increases the effort arm. Therefore, it is easier to lift a load when shifted towards wheel in a wheel barrow. c. Third class lever: The lever in which effort lies in between load and fulcrum is called third class lever. For example, shovel, fishing rod, fire tongs, broom, etc. In third class lever, effort distance is always less than load distance. So, more effort should be applied to overcome a smaller load. Therefore, MA of a third class lever is always less than one. Worked out Numerical 2 Study the given figure and calculate MA, VR and ŋ of the lever. Solution: Effort = 200 N Load = 600 N Effort arm = 20 cm + 60 cm = 80 cm Load arm = 20 cm MA = ? VR = ? Fig. Some levers of the third class Shovel Fishing rod Fire tongs Broom Fact File Velocity ratio and mechanical advantage of first class lever can be either less than one or one or more than one, based on position of fulcrum. 600 N 200 N 20 cm 60 cm

Oasis School Science and Technology - 8 151 ŋ = ? We have, MA = Load Effort = 600 200 = 3 VR = Effort distance Load distance = 80 20 = 4 Now, ŋ = MA VR × 100% = 3 4 × 100% = 75% ∴ The MA, VR and ŋ of the lever is 3, 4 and 75% respectively. Activity 2 • Take a scale of length 30 cm as shown in the figure. Make a hole at the mid-point of the scale such that the tip of a ball pen can be easily inserted in it. Keep the scale in the balanced condition by inserting the tip of a ball pen in the hole and fixing it on the stand. Also, keep the scale in the balanced condition by different masses on its both left as well as right sides. Convert the mass into effort. It is to be noted that mass of 100g equals to 1 N effort. Let us consider that mass of the right side is the load and that of left side is the effort. Keep the load in different distances from the fulcrum and balance it by the effort. Complete the following table on the basis of your experiment. Left Side Right Side Effort (N) Effort arm Effort × Effort arm Load (N) Load arm Load×Load arm • If the method of experiment is accurate, effort x effort arm = load x load arm. But due to the friction during the experiment, negligible differences may occur. Fulcrum Effort Stand Load Fig. Reasonable Thinking Skill R T S 1. Mechanical Advantage of the third class lever is always less than 1. The mechanical advantage of a third class lever is always less than one because it has a longer load arm than the effort arm. 2. Efficiency of a machine is always less than 100%. The efficiency of a machine is always less than 100% because friction decreases the output work.

152 Oasis School Science and Technology - 8 3. What is the percentage ratio of output work to input work called? The percentage ratio of output work to input work is called efficiency. 4. Uncle used a long metal bar to turn over a heavy stone by putting it between the rock and the fulcrum. How does it work? Uncle used a long metal bar to turn over a heavy stone. He used it as a crowbar. He put it between the rock and fulcrum so that it works as first class lever with a longer effort arm. Its mechanical advantage will be more than one. It multiplies force and helps to turn over the rock. 5. Why are second class levers and third class levers used? The second class lever is used because it makes work easier by multiplying force. The third class lever is used because it makes the load move faster. 6. VR of a machine is 2. What does it mean? VR of a machine is 2. It means the effort distance is 2 times longer than the load distance. 7. Calculate the efficiency of a wheelbarrow, when 150 N effort is used to carry a 450 N load. The load and effort are 0.4 m and 1.5 m far away from the wheel. Solution: Effort (E) = 150 N Load (L) = 450 N Effort distance (ED) = 1.5 m Load distance (LD) = 0.4 m Efficiency = ? Using formula, Efficiency = (load × load distance) (effort × effort distance) × 100% = (450 × 0.4) (150 × 1.5) × 100% = 80% 8. Fire tong belongs to the third class lever. Why? Can it multiply force? Why? Why not? Fire tong belongs to the third class lever because we have to apply effort at the middle part of the tool which is between the load and the fulcrum. It cannot multiply force because it has a shorter effort distance than the load distance. Exercises 1. Choose the best answer from the given alternatives. a. Which of the following is a crowbar? i. first class lever ii. second class lever iii. wheelbarrow iv. third class lever

Oasis School Science and Technology - 8 153 b. What is the distance between the fulcrum and the load called? i. load arm ii. effort arm iii. fulcrum iv. effort distance c. What is the force applied to a machine to do work called? i. output ii. input iii. load iv. effort d. Which kind of lever cannot multiply force? i. second class lever ii. third class lever iii. first class lever iv. fourth class lever e. What class of lever does a spoon belong to? i. second class lever ii. third class lever iii. first class lever iv. fourth class lever 2. Define the following with required examples. a. Machine b. Simple machine c. Effort d. Load e. Mechanical advantage f. Velocity ratio g. Output work h. Input work i. Efficiency j. Ideal machine k. Lever l. Principal of lever m. First class lever n. Second class lever o. Third class lever 3. Very short questions a. Which class of lever does a beam balance belong to? b. Which class of lever does a fishing rod belong to? c. Write the mathematical equation to calculate mechanical advantage. d. What is the ratio of effort distance to the load distance called? e. What kind of machine have 100% efficiency? 4. Give reasons. a. A bottle-opener is called the second class lever. b. Second class lever can multiply force. c. Spoons cannot multiply force. d. MA of the broom is less than 1. e. Arm of the crowbar at the side of the stone is made shorter than the arm where we use force. f. Ideal machine does not exist. g. It will be easier to carry the load if the load is pushed toward the wheel of the wheelbarrow. 5. Differentiate between: a. First class lever and Third class lever b. Input work and Output work

154 Oasis School Science and Technology - 8 6. Short question answers a. Classify the given levers. Draw their diagrams showing fulcrum, points of application of effort and load. i) Scissors ii) Fire tongs (iii) Nail cutter iv) Spoon v) Nut-cracker vi) Forceps b. Write any two uses of simple machines. c. In which condition does a lever multiply force? Give two examples of such levers. d. In which condition a lever increases velocity? Give two examples of such levers. e. Draw the diagram of first class lever. 7. Long question answers a. Write down any three advantages of using a simple machine. b. Show the relation between MA, VR and efficiency. c. Third class lever cannot multiply force because it has a shorter effort arm. It has a mechanical disadvantage. But also, it is widely used in daily life. Why? d. Describe the importance of simple machines in our daily life. 8. Numerical Problems a. A load of 1000 N can be lifted by applying an effort of 250 N. If the load arm is 25 cm, calculate the effort arm. [Ans: 100 cm] b. An effort of 20 N is applied to lift a load. If the load arm and effort arm are 15 cm and 60 cm respectively, calculate the load. [Ans: 80 N] c. Neha and Reha are playing see-saw. Neha is sitting 60 cm away from the fulcrum and Reha is sitting 40 cm away from the fulcrum. Calculate the effort that Reha should apply to lift Neha. The weight of Neha is 360 N. [Ans: 540 N] d. Study the given figure and calculate the load distance. [Ans: 20 cm] 325 N 40 cm ? 650N e. An effort of 4 N is applied to lift a load of 20 N. If the effort arm is 5m, calculate the following: (i) load arm (ii) input work (iii) output work [Ans: (i) 1m (ii) 20 J (iii) 20 J

Oasis School Science and Technology - 8 155 Key terms and terminologies 1. Pressure : The force acting perpendicularly on a unit area of a surface is called pressure. 2. Thrust : The total force acting perpendicularly on a given surface is called thrust. 3. One pascal pressure : One pascal pressure is defined as the pressure exerted by 1 N force if it acts perpendicularly on a unit area (1m²). 4. Atmosphere : The earth is surrounded by a thick layer of air which is called the atmosphere. 5. Atmospheric pressure : The pressure exerted by the atmosphere due to its weight is called atmospheric pressure. 6. Standard atmospheric pressure : The atmospheric pressure at the sea level is called standard atmospheric pressure. 7. Liquid pressure : The thrust exerted by a liquid per unit area of the surface of the wall or bottom of the container is called liquid pressure. 8. Compressed air : Compressed air is a gas or mixture of gases stored under greater UNIT 6.3 PRESSURE Estimated teaching periods Theory 4 Practical 1  Introduction to pressure, unit of pressure and mathematical equation of pressure  Simple calculations related to pressure  Application of pressure in daily life  Liquid pressure and its application in daily life  Introduction of compressed air, its application and measurement  Introduction of atmospheric pressure, its importance and uses The Sequence of Curriculum Issued by CDC Blaise Pascal is noted for the study of liquid pressure and for proposing Pascal’s law. He was born in France on 19th June 1623 and died on 19th August 1662. He was a French mathematician, physicist, inventor, philosopher and writer. His notable ideas are probability theory, Pascal’s wager, Pascal’s triangle, Pascal’s law, Pascal’s theorem and Pascal’s calculator. He studied at Cartesianism, Jansenism and Fideism schools. About the Scientist Blaise Pascal

156 Oasis School Science and Technology - 8 pressure than normal atmospheric pressure. 9. Pressure gauge : The pressure gauge is a device that measures the pressure exerted by a fluid on the walls of its container. 10. Manometer : A Manometer is a device to measure fluid pressures. Introduction We prefer a sharp knife for cutting vegetables to a blunt one. Asharp knife cuts vegetables better due to its thin edge. The force of our hand falls over a small area of the object producing a bigger pressure which cuts the object easily. But a blunt knife does not cut an object easily due to its thick edge. The force of our hand falls over a large area of the object and produces less pressure which cuts the object with difficulty. Thus, the effect of the same force on different areas is different. The force acting perpendicularly on a unit area of a surface is called pressure. In SI system, pressure is measured in newton per square metre (N/m²). It is also called pascal (Pa). Pressure is a scalar quantity. Measurement of Pressure The value of pressure is obtained by dividing the force acting perpendicularly on an object by the area of the object on which the force acts. The total force acting perpendicularly on a given surface is called thrust. So pressure can also be represented as follows: Pressure (P)= Thrust (F) Area (A) From the above relation, it can be concluded that the pressure depends on the following two factors: (i) Force applied or the thrust (ii) Area over which the force acts The same force can produce different pressures depending on the area over which it acts. When a force acts over a large area of a surface, it produces a small pressure. But if the same force acts over a small area, it produces a large pressure. When the force acting on a surface increases, the pressure also increases and vice-versa. One Pascal Pressure One pascal pressure is defined as the pressure exerted by 1 N force ifit acts perpendicularly on a unit area (1m²). Differences between force and pressure Force Pressure 1. Force is the push or pull which changes or tries to change the position of an object. 1. The force acting perpendicularly on a unit area of a surface is called pressure. 2. Its SI unit is N. 2. Its SI unit is Pa or N/m2 . Fig. Sharp knife produces greater pressure to cut vegetables

Oasis School Science and Technology - 8 157 Worked out Numerical 1 A wooden block of 800N occupies 2m² surface area. Calculate the pressure exerted. Solution: Given, Force (F) = 800N Area (A) = 2m² Pressure (P) = ? According to the formula, ∴ The pressure exerted (P) = 400 Pa. Application of pressure in daily life i. Knives have sharp ends. Sharp ends has lesser area, so it can exert greater pressure and cut vegetables easily. ii. Tyres of vehicles have strips. The strips decreases the area of contact between tyre and ground. It helps to increase pressure and friction. As a result vehicle can move without slipping. iii. Trucks have many pairs of wheels than micro vans. Multiple tyres help to increase the area in contact with ground. It helps to distribute weight of truck so that each tyre has to carry less weight. It helps to decrease pressure and prevent sinking in dirt and soil and slippery roads. iv. Base of retaining wall is made wider. Wider base has more area so it can withstand the weight of the wall. It decreases pressure and prevent sinking. Atmospheric Pressure The earth is surrounded by a thick layer of air which is called atmosphere. Air has weight and it exerts pressure. The pressure exerted by atmosphere due to its weight is called atmospheric pressure. The atmospheric pressure at the sea level is called standard atmospheric pressure which is about 101300 N/m2 or 760 mmHg. The atmospheric pressure decreases with an increase in altitude. So, the atmospheric pressure is less at the top of Mt. Everest than that of the Terai. Atmospheric pressure acts on each and every living organism including different objects on the earth surface. But we do not feel the atmospheric pressure because it is neutralized by the air pressure inside lungs, pressure in the blood vessels and internal pressure from several other organs. Thus, the pressure exerted by blood in our body is almost equal to the atmospheric pressure. Fact File Solid put pressure on the point of contact. Liquid and air exert pressure in all direction. P = P = F 400 N/m² A P = 800 2

158 Oasis School Science and Technology - 8 Variation of atmospheric pressure with altitude Effect of gravity is less at high altitude and there is thin atmosphere compared to the sea level. This reduces density of the air as well as the atmospheric pressure. So, atmospheric pressure decreases with increase in altitude Nose bleeding occurs as we go to a higher altitude since the pressure in our body remains constant but the external atmospheric pressure decreases. Due to this reason, body pressure is more than that of the atmospheric pressure and hence bleeding of nose starts. Different altitudes have different atmospheric pressure. This helps blowing of air from one place to another. The pressure in the aeroplanes and jet planes is adjusted so that the passengers feel comfortable and they can breathe easily though they fly in a higher altitude. Activity 1 • Take a water trough and dip an empty glass as shown in the figure. Tilt the glass gradually. Observe the activity. Air bubbles come out of the glass. The glass which is dipped into the water trough contains air though it seems empty. Fig. 4.2 Reasonable Facts Air blows from one part to another on the surface of the earth. Air blows from one part to another on the surface of the earth due to change in atmospheric pressure of earth's surface and due to effect of gravity. Reasonable Facts A glass does not immerse in water when dipped straight downwards. When a glass dipped straight downwards in water, the air present inside the glass cannot flow outside the glass. Therefore, a glass does not immerse in water when dipped straight downwards. Activity 2 • Take a tin can and fill it with water leaving some space. Boil the water for few minutes and close the lid. Allow it to cool by pouring cold water over it. What do you observe? When the water in the tin can boils, it drives the air out of the can. On cooling the can by closing the lid, a partial vacuum is formed inside the can. As a result, the tin can shrinks due to the atmospheric pressure. Water Partial vaccum Tin Water Atmospheric vapour pressure Lid Fig. atmospheric pressure can crush empty can because empty can has no air pressure

Oasis School Science and Technology - 8 159 Activity 3 • Take a glass. Fill it with water up to the brim so that there is no space for air. Cover the glass with a postcard. Invert the glass as shown in the figure. Now, remove the hand from the postcard gently. What do you observe? It is found that water does not fall from the glass immediately. The atmospheric pressure helps to hold water in the glass by pressing the postcard upward. Postcard Glass Water Fig. atmospheric pressure holds the weight of water Activity 4 • Take water in a glass. Suck the water slowly with a straw pipe. How is it possible to draw water from the glass? Think! When air in the straw is sucked, it creates partial vacuum inside the pipe. So, it helps to reduce the pressure in the straw and atmospheric pressure helps to raise the water level upward. Asimilar process takes place while filling the ink in a fountain pen. Fig. atmospheric pressure helps to push liquid into straw Importance of Atmospheric Pressure Atmospheric pressure is very important for our day to day activities. We are able to use various equipment due to the presence of atmospheric pressure. i. It is important to balance our body pressure. ii. It is important to fill medicine in a syringe. iii. It is important to fill air in a bicycle tube or tube of vehicles. iv. It is important to lift water by using a water pump. v. It is important to draw soft drinks through a straw. vi. It is important to use rubber suction pads on the walls to hang clothes, calenders, etc. Application of atmospheric pressure i. Fill ink in a fountain pen. When we press the ink chamber of the fountain pen, the air will be removed from it. There will be zero pressure inside the rubber chamber. But the ink in the bottle is being pushed by atmospheric pressure. So it rises from high pressure region to ink chamber which is at low pressure. ii. Fill medicine in a syringe.

160 Oasis School Science and Technology - 8 When we pull up the piston of the syringe, vacuum is created in the cylinder. There is no air pressure in the cylinder, so medicine which are pushed by atmospheric pressure rise in the needle to fill vacuum. iii. Filling air in a tube of football, tubes of tyre, etc. When we pull up the piston of the air pump, vacuum is created in the cylinder of the pump. The air from the atmosphere enters into the air pump. As we push the piston of the pump, air enters into the tube of tyre and football. Liquid Pressure Liquids stored in vessels have their own weight. The weight acts on the bottom of the vessel. Thus, liquids exert pressure at the bottom of the vessel due to their weight. The thrust exerted by a liquid per unit area of the surface is called liquid pressure. The total force exerted by a liquid perpendicularly on a surface is called thrust of the liquid. If a thrust (F) is acting on a surface area (A) in contact with a liquid, the pressure exerted by the liquid on the surface is given by Pressure (P) = Thrust (F) Area (A) P = F ∴ A The SI unit of liquid pressure is N/m2 or Pa (pascal). Pressure exerted by liquid column Let us consider a cylindrical vessel of base area 'A' is filled with a liquid of density 'd'. The pressure is exerted at the bottom of the vessel due to the weight of the liquid which is given by Pressure (P) = Force (F) Area (A) = Weight of the liquid (F) Base area of the container (A) [∵W = F] = m.g A m - mass of liquid g - acceleration due to gravity [∵F = mg] = d.V.g A mass (m) volume (V) ∵ density (d) = = A.h.d.g A (∵ V = l × b × h = A × h) h A d Fig 4.6

Oasis School Science and Technology - 8 161 ∴ P = hdg Proved From the formula P = hdg, it can be noted that liquid pressure depends on the following factors: i. depth of the liquid from its free surface (h) ii. density of the liquid (d) iii. acceleration due to gravity (g) Worked out Numerical 2 Calculate the pressure exerted by water at a depth of 2 m. The density of water is 1000 kg/m3 . Solution: Density (d) = 1000 kg/m3 Depth (h) = 2 m Acceleration due to gravity (g) = 9.8 m/s2 Pressure (P) = ? We have, P = hdg = 2×1000×9.8 = 19600 Pa ∴ The pressure exerted by the water is 19600 Pa. Worked out Numerical 3 A tank of dimensions 3m × 2m × 1m is filled with water. Calculate the pressure exerted by water at the bottom of the tank. Solution: Depth (h) = 1m [ ∵ The height of the tank is 1m.] Density (d) = 1000 kg/m3 Acceleration due to gravity (g) = 9.8 m/s2 Pressure (P) = ? We have, P = hdg = 1 × 1000 × 9.8 = 9800 Pa. The pressure exerted by water is 9800 Pa. Fact File Liquid pressure increases about 1 atmosphere for every 10 meter depth in ocean.

162 Oasis School Science and Technology - 8 Worked out Numerical 4 A drum is filled with a liquid of depth 2m. Calculate the density of the liquid if the pressure exerted by it is 5000 N/m2 . Solution: Depth (h) = 2m Pressure (P) = 5000 N/m2 Acceleration due to gravity (g) = 9.8 m/s2 Density (d) = ? We have, P = hdg or, d = P hg = 5000 2×9.8 = 255.1 kg/m3 ∴ The density of the liquid is 255.1 kg/m3 . Properties of Liquid Pressure 1. Liquid pressure is transmitted equally in all directions, if pressure is applied to the liquid kept in a closed vessel. Activity 5 • Take a plastic bag and make holes on it with a pin. Fill the bag with water and close the lid. Press the bag as shown in the figure and observe the flow of water through the holes. What do you know from this activity ? We can see that the force of water coming out of each hole is the same. This is due to the property of liquid that liquid pressure is transmitted equally in all directions. This law is called Pascal's law. This law was propounded by Blaise Pascal. Fig. 2. Liquid pressure increases with increase in depth. Compare this property with the above property. If the liquid is acted by external pressure, liquid pressure is transmitted equally in all directions. If not so, liquid pressure increases with increase in depth. Liquid pressure (P)is directly proportional to the height (h) of the liquid column, i.e. P ∝ h. Therefore, water dams are made thicker at the base so that the bottom can withstand more pressure due to water.

Oasis School Science and Technology - 8 163 Activity 6 • Take a bottle and make 4 different holes A, B, C and D at different heights as shown in the figure. Close the holes by using cork and fill the bottle with water. Open all these holes simultaneously and observe the flow of water. • As we go from top to bottom of the bottle, i.e.Ato D, the pressure of the flow of water increases. • Due to this property, the bucket in downstairs tap fills faster than upstairs tap. Also, the base of a dam is made wider to hold more pressure given by the water. But it is to be noted that if the bottle is pressed, the rate of flow of water through all holes remains the same. Fig. A B C D 3. Liquid determines its own level. Activity 7 • Take a Pascal tube and fill it with water. Observe the level in all the tubes. What do you find? You will find the same height of liquid in all the tubes. Fig. Liquid maintains its own level 4. Liquid pressure is directly proportional to the density of the liquid. Liquid having less density exerts less pressure and liquid having more density exerts more pressure. So, if mercury and water are kept in two similar vessels up to the same height, the pressure exerted by mercury is more than that by the water. Activity 8 • Take a beaker and fill it with water. Keep an egg in the beaker. Does the egg sink? Now, dissolve some salt in the beaker and form a concentrated salt solution. Does the egg float? What can you conclude from this activity? Fig. effect of liquid density on sinking and floating (a) egg egg (b)

164 Oasis School Science and Technology - 8 Reasonable Facts A balloon bursts when filled with more air. When a balloon is filled with more air, the pressure of air inside the balloon is greater than that of the atmospheric air pressure and that air tries to escape out from the balloon. As a result, balloon bursts when filled with more air. Application of liquid pressure in daily life i. Water tank is kept at the top of roof so that there will be more liquid pressure and water can flow fast through the tap in lower floors. ii. Liquid pressure is used to operate hydraulic machines such as hydraulic press and hydraulic lift. iii. The wall of dam is made thickerin the bottom than in the top to withstand increasing liquid pressure. iv. The sea divers wear special diving suit while going deep under the sea because there is very high liquid pressure. v. Liquid pressure in hydraulic brakes can stop heavy trucks. Compressed Air Compressed air is an alternative to fossil fuel. The normal air is collected and compressed into a smaller volume. A rotary screw air compressor or reciprocating air compressor is used to compress air. Compressed air is hotter because molecules are forced to stay in small volumes which increases the rate of collision. It is used as a source of renewable energy. Compressed air is a gas or mixture of gases stored under greater pressure than normal atmospheric pressure. It is more expensive than hydroelectricity and petroleum. It is a clean, safe and simple source of energy. It is widely used in industries, scuba diving, medical fields, transportation, power tools, painting and cleaning. Importance of compressed air i. Instruments such as tire pumps, air rifles and aerosol are based on application of compressed air. ii. It is also used for power tools such as air hammers and drills. iii. Compressed air car runs by an engine that contains a pressure vessel filled with compressed air. iv. Compressed air brakes are used in trains, heavy trucks and buses. v. Underwater diving uses compressed air for breathing, maintaining buoyancy and lifting bags. air brakes air hammer air car engines Fig: uses of compressed air

Oasis School Science and Technology - 8 165 Measurement of pressure of compressed air Mishandling of compressed air can damage eyes, ears and noses. It can sometimes kill us. A suitable magnitude of pressure is safe to us. We should know the pressure created by compressed air on the tools to avoid overpressure. An air compressor gauge (pressure gauge) can be used to measure the pressure due to compressed air in the system. It can tell if there is overpressure or pressure drop in the system. Pressure due to compressed air is usually expressed in pounds per square inch (PSI), milibar or mmHg. Activity 9 Make a list of appliances used in our community that works with compressed air. Pressure gauge The pressure gauge is a device that measures the pressure exerted by a fluid on the walls of its container. The term pressure gauge usually refers to a self-contained indicator that converts the detected pressure into the mechanical motion of a pointer. Pressure gauges are used by industry professionals to troubleshoot fluid power machines which are designed to work within a set pressure range. The change in the reading of the pressure gauge suggests leaks or overflow of fluid. Pressure gauges can either be mechanical or digital. A pressure gauge is also known as a pressure meter and vacuum gauge. Most pressure gauges calculate the pressure relative to atmospheric pressure as the zero point. Most pressure gauges are filled with liquid glycerine and silicon. Reasonable Facts Some pressure gauge is filled with liquids. Some pressure gauge is filled with liquids to reduce the vibration so that readings can be more reliable. Activity 10 Make a model of a pressure gauge. Fact File Compressed air and atmospheric gas have same composition. Fig: pressure gauge Fact File A pressure gauge is a fluid intensity measurement device.

166 Oasis School Science and Technology - 8 Manometer A Manometer is a device used to measure fluid pressures. A common simple manometer consists of a U-shaped tube of glass filled with some liquid. Typically, the liquid is mercury because of its high density. A manometer is used for measuring the fluid pressure with respect to an outside source which is usually considered to be the earth’s atmosphere. A device that uses the surface area and weight of a liquid column to measure and indicate pressure is known as a manometer. U-Tube Manometer, EnlargedLeg Manometer, Well-Type Manometer and Inclined-Tube Manometer are a few types of manometers. Reasonable Thinking Skill R T S 1. Base of the building is made wider. Base of the building is made wider because the wider base has more area, which helps to reduce the pressure of the house on the ground. As a result, the building won’t sink. 2. How does liquid pressure vary when a water tank is carried from sea level to mountain? Liquid pressure is directly proportional to acceleration due to gravity. When a water tank is brought from sea level to the mountains, liquid pressure at the bottom of the container must decrease because the mountain region has a lesser magnitude of the acceleration due to gravity. 3. Why does the bucket fill faster on the lower floor than on the top floor? The height of the liquid column from a water tank on the roof to the tap on the ground floor is more, so there is more liquid pressure and water flows faster than on the top floor. So buckets fill faster on the lower floor than on the top floor. 4. A thin paper can prevent the water from falling from upside-down glass. Why? A thin paper can prevent the water from falling from upside down glass because the atmospheric pressure pushes the paper upward against the glass. 5. Compressed air is used as fuel. Why? Compressed air is used as fuel because it is renewable, does not pollute the environment and is efficient. 6. Most compressed air technology uses a pressure gauge. What is its purpose? Most compressed air technology uses a pressure gauge because it helps to know the Fig: manometer

Oasis School Science and Technology - 8 167 exact pressure of the fluid in the machine. Variation in pressure suggests the presence of defects. 7. Astronauts must wear space suits in space. What could be the reason? Astronauts must wear space suits in space because the suit helps to produce suitable pressure around the body. Otherwise, due to the absence of atmospheric pressure, the lungs and intestines would hurt or burst. 8. A water tank is 2 m tall. If it is half filled with water of density 1000 kg/m³, calculate the pressure it exerts at the bottom of the container. Solution: Height of liquid (h) = half of the water tank = 1 meter Density of water (d) = 1000 kg/m³ Acceleration due to gravity (g) = 9.8 m/s² Liquid pressure (P) = ? Using formula, P = hdg = 1 × 1000 × 9.8 = 9800 Pa Exercises 1. Choose the best answer from the given alternatives. a. What is the value of atmospheric pressure at sea level? i. 11300 N/m² ii. 101300 N/m2 iii. 30110 N/m iv. 110300 N/m2 b. What is the mathematical equation for liquid pressure? i. P = hdg ii. P = Adg iii. P = hdv iv. P = dAhg c. What is the direction of liquid pressure? i. upward direction ii. downward direction iii. all directions iv. one direction d. What is used as the fluid in manometer? i. alcohol ii. water iii. mercury iv. hydrogen peroxide e. Which one of the following technologies uses compressed air? i. air brakes ii. hydraulic brakes iii. disc brakes iv. hydraulic lift

168 Oasis School Science and Technology - 8 2. Define the following terms with required examples. a. Pressure b. Thrust c. One pascal pressure d. Atmospheric pressure e. Standard atmospheric pressure f. Liquid pressure g. Compressed air h. Pressure gauge i. Manometer 3. Very short questions. a. What is the pressure exerted by the weight of the air column of the atmosphere called? b. Write an use of manometer. c. Write down the SI unit of pressure. d. What is the relation between pressure and area? e. If oil and water are kept in a bottle of the same height, which one exerts more pressure at the bottom of the container? 4. Give reasons. a. Atmospheric pressure decreases with an increase in altitude. b. The base of a dam is made wider. c. A balloon bursts when more air is filled in it. d. Water tanks are kept at the top of the building. e. Scuba divers wear a special suit. f. Industries measure the air pressure of compressed air continuously in factories. g. Camel can walk easily in the desert. h. A tin can is crushed when the air inside it is removed. i. Atmospheric pressure is less in the Himalayan region than in the terai. j. Nose bleeding occurs as we go to a higher altitude. 5. Differentiate between: a. Atmospheric pressure and Liquid pressure b. Force and Pressure 6. Short question answer a. Write down the applications of pressure in daily life. b. Enlist any two applications of atmospheric pressure. c. How does atmospheric pressure help in filling medicine in a syringe? d. What are the factors on which pressure depends upon? e. Which factors affect the liquid pressure? f. How does liquid pressure vary from the ground floor to the top floor of the building and why, if the water tank is on the roof? g. Enlist the precaution while using compressed air technology. h. Draw the diagram of the manometer and pressure gauge.

Oasis School Science and Technology - 8 169 7. Long question answer a. Write any four applications of liquid pressure. b. Write any four uses of compressed air. c. Discuss the importance of atmospheric pressure. d. Derive the equation p = hdg where letters have the usual meaning. e. Describe an experiment to show the presence of atmospheric pressure. f. Describe an experiment to show that liquid pressure increases with an increase in depth. 8. Numerical problems: a. The mass of a box is 300 kg and its base area is 15 m2 . Calculate the pressure exerted by the box on the ground (g = 9.8 m/s2 ). [Ans: 196 N/m2 ] b. Calculate the force applied if 100 N/m2 pressure is exerted over the area of 0.2 m2 . [Ans: 20 N] c. Calculate the pressure of water in a well if the depth of the water is 10 m. [Ans: 98000 Pa] d. Calculate the pressure exerted at the bottom of the given container. [Ans: 8820 Pa] 3 m d = 300 kg/m3 A e. If a force of 2000N acting on a surface exerts 200 N/m2 pressure, calculate the area of the surface. [Ans: 10 m2 ] f. Calculate the pressure exerted on the ground by a boy of mass 60 kg if he stands on one foot. The area of the sole of his shoe is 150cm2 .[Ans: 39200 Pa] g. The dimension of a water tank is 5m×4m×2m. Calculate the pressure at the bottom of the tank when it is half-filled. The density of water is 1000 kg/m3 and acceleration due to gravity at that place is 9.8 m/s2 . [Ans: 9800 Pa]

170 Oasis School Science and Technology - 8 7 ENERGY IN THE DAILY LIFE UNIT

Oasis School Science and Technology - 8 171 UNIT 7.1 HEAT Estimated teaching periods Theory 5 Practical 1  Introduction to transmission of heat by conduction and its application in daily life, introduction to conductors and insulators  Introduction to transmission of heat by convection and its uses, Convection current and the weather  Introduction to transmission of heat by radiation and its uses  Introduction of waves [mechanical waves and electromagnetic waves]  Transmission of electromagnetic waves and heat by radiation, effect of colour of object on transmission of heat by radiation and its application  Working mechanism of green house and greenhouse effect  Structure and utility of thermos flask 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 Key terms and terminologies 1. Heat : Heat is a form of energy which causes the sensation of warmth or coldness. 2. Transmission of heat : The flow of heat energy from a body at a higher temperature to another body at a lower temperature is called transmission of heat. 3. Conduction : The process of transfer of heat from one particle to another without actual movement of the particles is called conduction. 4. Good conductors : The substances which conduct heat easily are called good conductors of heat.

172 Oasis School Science and Technology - 8 5. Bad conductors : The substances which do not conduct heat easily are called bad conductors of heat. 6. Convections : The process of transmission of heat by the actual movement of molecules of a medium is called convection. 7. Convection of air : The movement of hot and light air upwards and heavy air downwards is called convection of air. 8. Radiation : Radiation can be defined as the transmission of heat from a hot body to a cold body without affecting the medium. 9. Radiant heat : The heat energy emitted by a hot body is called radiant heat. 10. Thermos flask : The device which keeps hot liquids hot and cold liquids cold for several hours is called a thermos flask. 11. Wave motion : Wave motion is a vibratory disturbance travelling through a medium in which energy is carried from one place to another. 12. Mechanical wave : The wave which requires a material medium for its propagation is called a mechanical wave. 13. Electromagnetic waves : The waves which travel by making an electric field and magnetic field are called electromagnetic waves. 14. Greenhouse effect : The phenomenon of increasing the temperature of the earth when greenhouse gases trap solar radiation is called the greenhouse effect. 15. Global warming : The process in which the temperature of the earth is increasing gradually due to the greenhouse effect is called global warming. Introduction When we touch a burning candle, we feel hot and when we touch an ice-cube, we feel cold. Similarly, when we rub our hands against each other, they become warm. It is because the kinetic energy changes into heat energy and we feel hot. So, heat is a form of energy which causes the sensation of warmth or coldness. When heat flows into our body, we get the sensation of warmth and when it flows out from our body, we get the sensation of coldness. Heat is produced due to the movement of molecules. Heat is measured in joule (J), calorie (cal.), etc. In our daily life, we use heat for various purposes, such as cooking, drying clothes, running engines, heating water, etc. The sun is the main source of heat energy. We also obtain heat energy from fire, electric heater, diesel, coal, etc. When a body is heated, the following effects can be observed in the body: i. increase in volume (expansion) ii. increase in temperature iii. change of state Transmission of Heat It is a well-known fact that water flows from a higher level to a lower level. Similarly, heat flows from one place to another due to the difference in temperature between them. When two bodies at different temperature are brought into contact, heat flows from the Fact File Heat is also defined as the flow of thermal energy from hotter objects to colder objects.

Oasis School Science and Technology - 8 173 body at higher temperature to the body at lower temperature. In our daily life, we observe the transmission of heat. For example, the sun is extremely hot. The heat of the sun flows to the earth as the sun is at higher temperature than the earth. When a body gets heat, it becomes hot and if the body loses heat, it becomes cold. The flow of heat energy from a body at a higher temperature to another body at lower temperature is called transmission of heat. This process also takes place from one part of a body to another part of the same body. There are three distinct modes of transmission of heat. They are conduction, convection and radiation. 1. Conduction When one end of an iron rod is heated, its another end also becomes hot. This method of transmission of heat from one end of the rod to another is called conduction. All solids get heated through conduction. The process of transfer of heat from one particle to another without actual movement of the particles is called conduction. In this method, no physical movement of the particles within the material medium takes place. In solids, molecules are very closely packed together. If we heat one end of a metallic rod, the molecules at that end absorb heat and start vibrating. These vibrating molecules collide with their neighbouring molecules and transfer a part of heat energy to them. These particles start vibrating more rapidly and transfer heat energy to their neighbouring particles and so on. In this way, transmission of heat energy takes place in solids. Such method of transmission of heat is called conduction. This process continues till both ends of the rod are at the same temperature. Experiment 1 Objective: To demonstrate the transmission of heat in solids by conduction Materials required: Stand, iron rod, wire, iron nails, Bunsen burner and wax Procedure • Bring a small piece of iron rod and clamp it to a stand. Fix 4-5 iron nails on the iron rod with the help of molten wax as shown in the fig. 6.1 (a). Heat the iron rod at one end with the help of a Bunsen burner. What do you observe after 2-3 minutes? Fig : (a) Before heating and (b) After heating (a) (b) Iron rod Direction in which heat travels Bunsen burner Stand Iron nails falling down Iron nails attached to the rod Stand

174 Oasis School Science and Technology - 8 Observation After a few minutes, it can be observed that the iron nails fall one by one starting from the end which is near the flame of the burner [fig. 6.1 (b)]. Conclusion This experiment proves that the heat is transferred from the hot end to the cold end of a body by conduction. Reasonable Facts Metal can conduct heat without actual movement of molecules. Metal can conduct heat without actual movement of molecules because it has free electrons that can move freely. Activity 1 · Take a metal rod and a wooden stick. Hold one end of the metal rod in one hand and the wooden stick in another. Now, heat the other end of the rod and wooden stick by a Bunsen burner. Wait for a while. What do you feel? After some time, it can be felt that the wooden stick can be held for a longer time than the metal rod. It is because the metal rod becomes hot very soon. So, we cannot hold it for a long time. In other words, the metal rod conducts heat easily. So, it becomes hot very soon. But the wooden stick does not conduct heat easily. So, it remains cold and we can hold it for a long time easily. Conduction capacity of various objects The property of an object to transfer heat is called conductivity. The conduction capacity of an object depends upon nature of the substance and temperature of the substance. Each object has different conduction capacity. Usually, metals have higher conduction capacity than non-metals and insulators. Good conductors and Bad conductors Each type of material has different conduction capacity. Conduction of heat depends upon nature of the substances. Metals have higher thermal conductivity than non-metals. The substances which conduct heat easily are called good conductors of heat. All metals and their alloys conduct heat easily. So, they are good conductors. Copper, aluminium, iron, silver, gold, etc. are some examples of good conductors. Silver is the best conductor of heat. The substances which do not conduct heat easily are called bad conductors or poor conductors of heat. All liquids except mercury and gases are poor conductors of heat. Cloth, glass rod, wood, paper, plastic, etc. are some examples of bad conductors. Bad conductors are also called insulators.

Oasis School Science and Technology - 8 175 Applications of Thermal Conductivity i. Quilts are filled with fluffy cotton : Air enters the pores of cotton. Both cotton and air are bad conductors of heat. So the cotton does not allow the heat to flow from our body to the surrounding. It provides good heat insulation. As a result, our body remains warm. ii. Cooking utensils are made of metals : Metals are good conductors of heat. Heat can get conducted easily through the base of the utensils. iii. Handles of cooking utensils are made from insulators: Ebonite insulators on the handles of cooking utensils do not allow heat to get conducted from the utensils to our hands. iv. Very hot water poured into a thick glass tumbler cracks it : Glass is a bad conductor of heat. When very hot water is poured into a glass tumbler, the inner surface expands due to the conduction of heat. This heat does not reach the outer surface quickly. Hence, the inner surface expands but the outer surface does not and the glass cracks. v. The walls of cold storage rooms are made thicker : Bricks are bad conductors of heat. Thick walls of the brick reduce the flow of heat from the surroundings to the rooms. vi. To prevent ice from melting, it is covered with jute and sawdust : Sawdust and jute are bad conductors of heat. A large amount of air is trapped in them. This trapped air acts as a layer of insulation. It does not let the ice gain heat from the surroundings. vii. Woollen clothes are warm : Wool and air are bad conductors of heat. The fine pores in woollen clothes are filled with air. Therefore, woollen clothes check the flow of heat from our body to the surroundings and keep our body warm. viii. In cold countries, windows have two panes or three panes with a thin layer of air in between : Air in between two panes of windows is a bad conductor. It checks the conduction of heat from the room to the surroundings 2. Convection Convection is the process by which heat is transmitted through liquids and gases from one point to another due to the actual movement of molecules. In liquids and gases, heat is transmitted by convection. It is because the molecules of liquids and gases are free to move about. Convection is not possible in case of solids because the molecules in solids are very closely packed. The process of transmission of heat by the actual movement of molecules of a medium is called convection. There are two types of convection. They are:

176 Oasis School Science and Technology - 8 a. Natural convection: The convection due to change in temperature of the liquid and gas molecules is called natural convection. It results see breeze, cloud, thunder, etc. b. Convection due to external force: The conduction due to heat, electricity, chemicals, etc. is called convection due to external force. Running fan, geyser, water heater, etc. are convection due to external force. When water is heated, the water molecules at the bottom of the container get heated first. These molecules expand and become lighter. Those hot molecules have more kinetic energy and rise up whereas cold water molecules, being heavier, come down. In this way, each water molecule at the bottom gets heated and rises up and cold molecules fall down at the bottom. This action sets the flow of water molecules which is known as convectional current. This current transfers heat to the entire mass of water. Therefore, the transmission of heat takes place by the actual movement of hot water molecules. In coastal regions, breeze generally blows from the sea to the land during the day and from the land to the sea during the night. This is an example of convection current of air or gases. Reasonable Facts Gravity plays role in convection. The gravity pulls colder and denser water molecules down whereas the warmer and less denser water moves up. Experiment 2 Objective: To demonstrate the transmission of heat (convection) in liquids Materials required: Beakers, water, a few crystals of potassium permanganate, tripod stand, Bunsen burner, wire gauze Procedure • Take a beaker and fill it partially with water. Put a crystal of potassium permanganate into it without disturbing the water. Keep the beaker on a table and wait for a while [fig. (a)]. What do you observe? • Take another beaker and fill it partially with water. Put a crystal of potassium permanganate. Now, heat the beaker by using a Bunsen burner [fig. (b)]. What do you observe? Fact File Convection occurs because fluids tend to expand on heating to be less denser. Figure: convection (a) (b) Water Water Beaker Beaker Convection current Wire gauze Tripod stand Potassium permanganate Potassium permanganate Burner

Oasis School Science and Technology - 8 177 Observation In the first beaker [Fig. (a)], the crystal of potassium permanganate dissolves in water and the colour of the crystal moves uniformly but convectional current cannot be seen. In the second beaker [fig. (b)], it can be observed that the coloured water rises from the place where the heat is being applied. After moving some distance, the coloured water spreads and comes down along the sides of beaker. In this beaker, convectional current can be observed. This current transfers heat to the entire mass of water by actual movement of heated water molecules. Conclusion This experiment proves that the transmission of heat takes place by the actual movement of heated molecules or convection in liquids. Blowing of Air or Wind During the day, the air on the surface of the earth gets heated due to the heat of the sun. So, the hot air becomes lighter and moves upwards while its place is occupied by cold air. This movement of air is called wind or blowing of air. The movement of hot and light air upwards and heavy air downwards is called convection of air. This phenomenon causes the wind and weather. Thus, the movement of wind from more density, more pressure and less temperature region to less density, less pressure and more temperature region is responsible for wind, weather, thunder and rain. Applications of Convection a. Ventilators in a Room Ventilators are made on a ceiling or near the ceiling on the wall of a room. A ventilator in a room maintains a fresh flow of air in the room. Fresh air is important for us to breathe. The air that we breathe out is warmer and hence lighter. The fresh air enters our room through the window and ventilation and the warm air goes out through the ventilator. This sets up a convection current and maintains a continuous flow of fresh air in the room. b. Chimneys In factories, chimneys are fitted to remove undesirable smoke and fumes. The light smoke, fumes and hot gases rise through the chimneys. c. Installation of Air-conditioners For effective cooling, air-conditioners are at a height above the level of the windows. The cool air from the air-conditioner sinks, whereas the warm air of the room rises. d. Installation of Room Heater For effective heating, room heaters are at the ground level. Air molecules in a room, when they come in contact with the heater, get heated and gain kinetic energy. Such hot and light molecules rise upward. The cool air molecules occupy the space of hot molecules. This sets up convection currents in the room and heat transfers to different parts of the room.

178 Oasis School Science and Technology - 8 e. Installation of the Freezer In a refrigerator, the freezer is always made at the top. The cold air which sinks down from the freezer cools the things kept in the lower part of the refrigerator. At the same time, the heat released in the lower part of the refrigerator makes the air warm and it rises up to the freezer. Thus, the freezer at the top sets up a convection current and keeps all parts of the refrigerator cool. f. Land Breeze and Sea Breeze The land gets heated faster as well as loses the heat faster. Water, however, has just the opposite characteristic. During the day, the land becomes hot and the air around it also becomes hot. Such hot and light air rises up. This place is occupied by the cold air that comes from the sea. Thus, during the day, cold air above the sea blows towards the land which is called a sea breeze. During the night, the land becomes cold and the air around it also becomes cold. But the air above the sea remains hot. The hot and light air rises up. This place is occupied by the cold air that comes from the land. Thus, during the night, cold air above the land blows towards the sea which is called a land breeze. 3. Radiation We feel very hot when we stand in the sun during summer season. We know that there is no material medium between the earth and the sun except the earth’s atmosphere. Then how does the heat travel from the sun to the earth? It is because of the third mode of heat transmission which does not require a material medium. This mode of heat transfer is called radiation. So, radiation can be defined as the transmission of heat from a hot body to a cold body without affecting the medium. A material medium is not essential for radiation. So, radiation also takes place in vacuum. Transmission of heat energy from the sun to the earth, from an electric heater to its surroundings, etc. are some examples of radiation. The heat energy emitted by a hot body is called radiant heat. This heat energy propagates in a straight line with the speed of light, i.e. three lakh kilo meters in one second. Fig: Radiation Fact File Heat travels fastest by radiation.

Oasis School Science and Technology - 8 179 Applications of Radiation a) Electric room heater The curved polished surface behind the rod-like heating element reflects the radiation falling on it. We feel the warmth even if we stand a few feet away from the heater. b) Choice of clothes White clothes absorb less heat radiation than clothes of other colours. So, we wear white or light-colour clothes in summer. On the other hand, we wear black and dark-colour clothes in winter. Such clothes absorb more heat and keep us warm. c) Solar heaters and solar cookers The heat from the sun is used to heat water in a solar water heater. The inner surface of the solar cooker is polished black. The black surface absorbs the heat. Reasonable Facts Cooking pots and pans have black base. The cooking pots and pans have black base because it can absorb huge quantity of heat and helps in cooking faster. It also saves fuel. d) Radiators of cars and air conditioners: The black coating on radiators of cars and air-conditioners absorbs maximum heat radiation and produces a cooling effect. Wave If a piece of stone is thrown into a pond of still water, circular ripples are produced on the surface of the water. These ripples carry energy from the stone to all parts without actual movement of the water from one point to another. This type of motion is simply called wave motion. The material in which a wave is traveling is called medium of the wave. Water of the pond is the medium. The wave travels due to the repeated vibrations of the particles of the medium. Wave motion Activity 2 Let's put some pieces of paper on the surface of still water. Drop a piece of stone into the water. Observe the motion of the pieces of paper. When the water waves reach the paper, the paper starts vibrating up and down with the water surface. The paper remains where it was placed and is not carried away to one side of the pond. It proves that water waves do not carry water from one place to another but only energy. Fact File Black bodies are good absorbers and good radiators of heat. Shining smooth bodies are poor absorbers and poor radiators of heat.

180 Oasis School Science and Technology - 8 When a piece of stone is thrown into the water, the energy of the stone disturbs the water molecules close to the stone. The stone transfers energy to the nearby molecules of the water and the water molecules start vibrating. The vibrating molecules transfer some of the energy to the neighbouring molecules which also start vibrating and so on. In this way, all the water molecules start vibrating and give rise to a water wave. So, a wave motion is a vibratory disturbance travelling through a medium in which energy is carried from one place to another. Wave motion transmits energy from one place to another by means of a periodic disturbance between the two points. Water wave, sound wave, light wave are some examples of waves. In a sound wave, the sound energy carried out by speech (vocal chord) disturbs the air molecules. The air molecules start vibrating and carry energy from one place to another. When the vibrating air molecules strike the ear drum, it also vibrates and stimulates the sensory cells located in the inner ear. These sensory cells transmit sound impulses to the brain through auditory nerves. As a result, we hear the sound. Characteristics of Wave Motion i. Wave motion is a periodic disturbance produced by a vibrating body. ii. In a wave motion, the particles of a medium do not travel from one place to another. They only vibrate about a fixed position called mean position. iii. A wave motion travels with the same speed in all directions. iv. A wave transfers energy from one point to another but not the matter or particles. v. The velocity of a wave depends only on the nature of the medium. Types of Wave Some waves can travel only through a material medium while some can travel even through vacuum. On the basis of requirement of medium, waves are classified into two types. They are: 1. Mechanical wave 2. Electromagnetic wave Mechanical wave The wave which requires material medium for its propagation is called mechanical wave. This wave is also called elastic wave because it depends on the elastic nature of the medium. Sound wave, water wave, etc. are mechanical waves as they cannot travel through vacuum. Sound wave is a mechanical wave. The velocity of sound is 332 m/s in air, 1451 m/s in water and 5120 m/s in iron. Fig: Ripples produced in a pond

Oasis School Science and Technology - 8 181 Electromagnetic Waves The waves which travel by making electric field and magnetic field are called electromagnetic waves. Electromagnetic waves can travel through vacuum. The solar radiation can travel through vacuum. So, it is a type of electromagnetic wave. Gamma rays, X-rays, UV-rays, visible light, infrared ray, radio wave, etc. are the electromagnetic waves. Electromagnetic waves of a short wave length can carry more energy. So, they can easily penetrate a body of a certain thickness. These rays are harmful to our body. They have higher frequency since frequency is inversely proportional to the wavelength of the wave. It is due to the reason that electromagnetic waves have the same speed in the vacuum. The speed of the electromagnetic waves can be calculated by the given wave equation. Speed (v) = frequency (f) × wavelength (λ) Properties of Electromagnetic Waves i. Electromagnetic waves travel at the same speed in vacuum ( i.e. 3 × 108 m/s). ii. They are transverse waves. iii. They do not need any material medium for propagation. iv. They obey laws of reflection and refraction of light. v. They are not affected by any electric and magnetic field. Uses of electromagnetic waves i. Visible light is used in photography, photosynthesis, etc. ii. Infra-red radiations are used for heating, in night vision devices, treating muscular strain, etc. iii. Microwaves are used in microwave ovens for cooking/warming food, communication in RADAR, etc. Electromagnetic waves and radiation of heat Electromagnetic waves such as infra-red rays, ultraviolet rays, gamma rays etc. can transfer heat energy from source to other places by radiation. They travel at the speed of 3 lakh kilometers per second and reach earth from sun at 8 minutes and 20 seconds. Colour of the object and radiation of heat and light It is observed that the colour of the objects affect the radiation and absorption of heat and light. i. The black colour usually absorbs all rays of light and hardly reflects any. So black colour can absorb and release large amount of heat in short duration. So, people wear black or dark clothes in winter. Black kitchen utensils are used as frying pans so that it can absorb more radiation of heat and supply to the food. ii. The white colour cannot absorb most of the radiation. It reflects almost all of the radiation. So, people wear white or bright shirts in summer. ∴ v = f × λ

182 Oasis School Science and Technology - 8 Thermos flask The device which keeps hot liquids hot and cold liquids cold for several hours is called a thermos flask. It was invented by James Dewar, a British scientist in 1890 AD. Structure Athermos flask consists of a double-walled glass vessel, outer case, cork support, stopper and a lid. There is a vacuum in between the two walls of the thermos flask. This vacuum reduces heat loss due to conduction and convection. The outer surface of the inner wall and the inner surface of the outer wall are silvered. This silvered surface reduces heat loss by radiation. The mouth of the vessel is closed by a cork, which is a bad conductor of heat. The glass vessel is kept on cork pad and surrounded with an outer case. It is thus thermally insulated and keeps hot liquids hot and cold liquids cold for a long period of time. Use A thermos flask is used to keep hot liquids such as warm water, cofee and tea. Activity 3 • Bring a mercury thermometer and an alcohol thermometer. Study their structure carefully and draw a labelled diagram of each. • Measure your body temperature by using both of them one by one. What do you find? Reasonable Facts Water remains warm for longer duration in thermos flask. Water remains warm for longer duration in thermos flask because it is properly insulated. For example it has vacuum, silvered layers, insulating cork etc. Greenhouse Effect Different types of gases like carbon dioxide, chlorofluorocarbon, methane, oxides of nitrogen, water vapour, etc. are present in the atmosphere. These gases cover the earth's surface making dense canopy similar to the glass frame of an artificial greenhouse. This dense canopy of different gases allows the solar radiations to enter into the earth surface but does not allow them to escape out after reflection. Ozone layer also prevents solar energy from escaping. As a result, these solar radiations absorbed in the atmosphere and increase temperature of the earth. The phenomenon of increasing the temperature of the earth due to greenhouse gases is called greenhouse effect. The gases which are responsible Fig: Thermos flask Fact File The vacuum works as a insulator in a thermos flask.

Oasis School Science and Technology - 8 183 for the greenhouse effect are called greenhouse gases. The earth is a natural greenhouse. The temperature of the earth is increasing gradually due to greenhouse effect. This process is called global warming. According to the researches, the temperature of the earth has increased less as compared to that of the polar region. As a result, a large mass of ice in the polar caps melts which increases the level of the sea. It affects the lives of people in coastal areas. Similarly, the temperature of higher regions is increasing more as compared to the lower regions due to the drying of the water resources and reduction in the amount of ice in the Himalayan region. Artificial Greenhouse The name greenhouse effect comes from the process that occurs in an artificial greenhouse. An artificial greenhouse is a house made up of glass or plastic that can trap solar heat the same way as the earth does. It is used to grow summer crops in winter or other off season. Reasonable Facts Roof of artificial green house is slanted. The roof of artificial green house is slanted so that sun-rays falls perpendicularly on roof and maximum solar energy enters inside the green house. Activity 4 Make a 3D model of artificial green house. Sources of greenhouse gases The greenhouse gases are released from different human activities as well as by some natural processes. Some of them are listed below: i. Carbon dioxide gas (CO2 ) is produced by burning of fossil fuels (like coal, oil, natural gases), industrial activities, automobiles, aircraft, etc. ii. Methane (CH4 ) is produced from the decomposition of domestic wastes and sewage. iii. Chlorofluorocarbons (CFCs) is released from refrigerators, air conditioners, aerosol spray cans, etc. iv. Water vapour (H2 O) is released from the oxidation of organic compounds as well as vaporization of water from water sources. Importance of greenhouse effect If there were no greenhouse gases in the atmosphere, all the solar radiations entering the earth surface would escape into the space. As a result, heat would not be stored in the earth. Consequently, the temperature of the earth would be less by –180 C than today's Fig: Artificial greenhouse Fact File It is warmer inside the green house even in the winters.

184 Oasis School Science and Technology - 8 temperature. Ice would cover the earth and there would not be any organism on the earth. So, greenhouse effect is very important for existence of lives on the earth. Adverse impact of greenhouse effect Due to various human activities, the concentration of greenhouse gases is increasing day by day. It increases the temperature of the earth. The adverse impact of greenhouse effect are given below: a. Effects on animals i. It helps to spread different types of diseases like malaria, filariasis, cholera, diarrhoea, etc. ii. It helps to increase the number of pathogenic insects. b. Effects on plants i. It affects water cycle, soil moisture and soil composition. As a result, there is a change in the cultivation and harvesting periods. ii. Due to the effect of global warming, tropical plants are seen in the temperate region. iii. It helps in breeding, growth and development of disease causing insects. c. Effects on climate i. It increases the temperature of the earth. ii. It causes melting of ice in the polar region, which increases the sea level and may submerge the low land areas of the earth. iii. It changes the pattern of rain fall and weather condition. Ways of controlling greenhouse effect Greenhouse effect can be controlled by: i. Decreasing deforestation and increasing afforestation. ii. Reducing the use of fossil fuels like petrol, diesel, kerosene, coal, etc. iii. Banning the production and use of CFCs. iv. Using solar energy, wind energy, biogas, etc. Difference between Natural greenhouse and Artificial greenhouse Natural greenhouse Artificial greenhouse 1. Some of the gases (e.g. carbon dioxide, methane, etc) in the lower atmosphere cover the earth in the form of dense canopy like an artificial greenhouse, which is called natural greenhouse. 1. The artificial house made in gardens using glass or plastics is called artificial greenhouse. 2. The possibility of all living organism on the earth is the result of natural greenhouse. 2. Growth of different plants like vegetables, flowers, etc. in off seasons and growth of summer plants in winter season are the results of artificial greenhouse.

Oasis School Science and Technology - 8 185 Reasonable Thinking Skill R T S 1. Why do striking stones give sparks? Some stones give sparks when struck together because of the friction between the metals in them. 2. It feels very cold to touch metal door knobs. Why? It feels very cold to touch metal door knobs because metal is a good conductor of heat. So they absorb heat faster from our bodies. 3. Ice melts faster on the metal plate than in the towel. Why? Ice melts faster on the metal plate than in the towel because the metal plate is a good conductor, it supplies heat to ice faster. The towel is an insulator so it won’t supply heat to the ice as fast as metal. 4. Differentiate between conduction and convection. The differences between conduction and convection are: SN Conduction SN Convection 1 The process of transfer of heat from one particle to another without actual movement of the particles is called conduction. 1. The process of transmission of heat by the actual movement of molecules of a medium is called convection. 2. Solids transfer heat by conduction. 2. Fluids transfer heat by convections. 5. Chickens hide under their mother’s wing. Why? Chickens hide under their mother’s wing because it is warm inside the wings due to the air trapped in it. The air acts as an insulator and prevents the loss of heat. 6. Rama Kumari is at the beach. She noticed that the cool air blows from the sea to the land during the day. Why does air move from sea to land during the day? What is it called? Rama Kumari is at the beach. She noticed that the cool air blows from the sea to the land during the day. The air on the land becomes hotter faster than in the ocean. The hot air in the land will rise up leaving a low-pressure area. Then the cold air at a higher pressure on the ocean will move to fill the low-pressure zone at land. It is the convection of air. This wind is called sea breeze. 7. Food remains warm when wrapped in an aluminium foil. Give reason. The food remains warm when wrapped in an aluminium foil because the aluminium foil has a shiny smooth surface that reflects heat back to the food. 8. Ramesh is a farmer. He has constructed a tunnel of plastics in his field. What is it? How does it work? Why has he made it? Ramesh is a farmer. He has constructed a tunnel of plastics in his field. The tunnel is a kind of artificial greenhouse. It works by trapping the solar energy within it. Whenever solar energy passes inside the plastic tunnel, its wavelength increases. It cannot escape from the greenhouse. It

186 Oasis School Science and Technology - 8 is trapped within the greenhouse. The trapping of solar energy increases the room temperature. He made it so that he can grow off seasonal vegetables in them. Exercises 1. Multiple Choice Questions a. What is the SI unit of heat? i. kelvin ii. calorie iii. meter iv. Celsius b. Which of the following state of matter, transfers heat by conduction? i. solid ii. liquid iii. gas iv. plasma c. What is the method called in which heat transmits without a medium? i. conduction ii. convection iii. radiation iv. thermal expansion d. What is the function of the vacuum in the thermos flask? i. conduction of heat ii. insulation of heat iii. reflection of heat iv. to make flask light e. Which electromagnetic wave is used CT scan in the hospital? i. X-ray ii. visible light iii. microwave iv. gamma rays 2. Define the following. a. heat b. transmission of heat c. conduction d. good conductor e. convections f. bad conductor g. thermos flask h. wave i. mechanical wave j. electromagnetic wave k. greenhouse effect l. global warming 3. Very short questions a. In which type of transmission of heat, molecules of the substance travel to transfer heat? b. By which method does the sunlight travel from the sun to the earth? c. What is the disturbance travelling through a medium called? d. What is the type of wave that needs a medium to travel? Give an example. e. Give any two examples of greenhouse gases.

Oasis School Science and Technology - 8 187 4. Give reasons. a. Convection cannot take place in solids. b. A balloon filled with hot air moves upward. c. Solar energy can reach us. d. Water remains warm in a thermos flask for a long. e. We cannot hear sound from exploding stars. f. It is warm inside the greenhouse. g. The roof of the greenhouse should be slanted. h. Greenhouse gases can cause climate change. 5. Differentiate between : a. Conduction and Convection b. Convection and Radiation c. Thermos flask and Thermometer d. Heat and Temperature e. Artificial greenhouse and natural greenhouse 6. Short question answers a. Write down four effects of the heat. b. How does the opposite end of the spoon become hot when it is dipped in hot soup? c. How do liquid molecules warm up? d. Draw the diagram of the thermos flask. e. How does artificial greenhouse work? Explain. f. Write down the properties of electromagnetic waves. g. What are the sources of greenhouse gases? h. Discuss the importance of the greenhouse effect. i. How does the greenhouse effect harm the animals? j. How are plants affected by greenhouse effects? 7. Long question answers a. Describe an experiment to demonstrate the transmission of heat in solids. b. Describe the structure of a thermos flask in brief. c. How is wind created due to heat? Describe. d. Write down the advantages of the artificial greenhouse. e. Enlist the characteristics of the wave. f. Write any four uses of electromagnetic waves. g. Greenhouse effect is bad for the climate. Enlist the harmful effects of over greenhouse effect on climate. h. What can be done to prevent over greenhouse effect?

188 Oasis School Science and Technology - 8 Key terms and terminologies 1. Light : Light is a form of energy which produces the sensation of vision. 2. Ray of light : The narrow path of light is called a ray of light. 3. Beam of light : A collection of rays of light in a certain pattern is called a beam of light. 4. Parallel beam : A beam of light in which the rays are parallel to each other is called a parallel beam of light. 5. Convergent beam : A beam of light in which all rays meet at a point is called the convergent beam of light. 6. Divergent beam : A beam of light is called divergent if the rays of light scatter or diverge from a single point. 7. Regular reflection of light: When a parallel beam of light, coming from a source, strikes a surface and reflects in parallel way, such type of reflection is called regular reflection of light. UNIT 7.2 LIGHT Estimated teaching periods Theory 6 Practical 1  Introduction of concave and convex mirrors  Technical terms related to the reflection of light through spherical mirrors  Illustration of reflection of light through concave and convex mirrors  Real and virtual image  Ray diagram of reflection of light through concave and convex mirrors  Application of spherical mirrors 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

Oasis School Science and Technology - 8 189 8. Irregular Reflection : When a parallel beam of light strikes a surface and reflects in different directions, such type of reflection is called an irregular reflection of light. 9. Mirror : A smooth surface which forms an image due to the reflection of light is called a mirror. 10. Plane mirror : The mirror in which the reflection of light occurs on a smooth plane is called a plane mirror. 11. Spherical mirror : The mirror which is part of a sphere is called a spherical mirror. 12. Convex mirror : If the spherical part is polished from inner surface, it is called convex mirror. 13. Concave mirror : If the spherical part is polished from outer surface, it is called concave mirror. 14. Real Image : The image which can be obtained on the screen is called a real image. 15. Virtual image : The image which cannot be obtained on the screen is called a virtual image. 16. Real focus : The focus in which reflected rays of light converge and meet is called the real focus. 17. Virtual focus : The focus from which reflected rays appears to be diverging is called the virtual focus. Introduction Light is a form of energy which produces the sensation of vision. We can see different things around us due to the presence of light. So, light makes the things visible. If we keep an object in a dark room, we cannot see it. Various sources of light help us to see things around us. Generally, extremely hot body emits light. When the filament of a bulb is heated, it produces light. Light travels at a speed of 3 × 108 m/s in vacuum. Ray and Beam of Light Light travels in a straight path. So, it is easy to represent a ray of light by a straight line with an arrow. The arrow in the line gives the direction of light. The narrow path of the light which is represented by a straight line with an arrow is called a ray of light. A collection of rays of light in a certain pattern is called a beam of light. There are three types of beam of light: a. Parallel beam of light b. Convergent beam of light c. Divergent beam of light Fig. 7.2.1 A ray of light Fact File Luminous object emit light but nonluminous does not.

190 Oasis School Science and Technology - 8 a. Parallel beam of light: A beam of light in which the rays are parallel to each other is called parallel beam of light. The light emitting from a source at a distant place is represented by the parallel beam. b. Convergent beam of light: A beam of light in which all rays meet at a point is called the convergent beam of light. The light reflecting from a concave mirror is the convergent beam of light. Similarly, convex lens also produces convergent beam when it refracts light. c. Divergent beam of light: A beam of light is called divergent if the rays of light scatter or diverge from a single point. The light reflecting from a convex mirror is divergent beam of light. Divergent beam is obtained in a short distance from a source of light. Activity 1 Get a torch and observe the beams of light it makes. Identify the type of beam. Reflection of Light When light strikes on a surface, a part of it is absorbed by the surface. Some part of the light is reflected by it and some part may be transmitted through it (if the surface is transparent). The ray of light which returns to the same medium after striking a surface is a reflected ray which makes the things visible. The process of returning light to the same medium after striking a surface is called reflection of light. Types of Reflection of Light There are two types of reflection of light, viz. regular reflection and irregular reflection. i. Regular Reflection When a parallel beam of light, coming from a source, strikes a surface and reflects in parallel way, such type of reflection is called regular reflection of light. An example of regular reflection of light is the reflection by a plane mirror. (a) Regular reflection of light (b) Irregular reflection of light Fig. 7.2.6 Reflection of light Fig. 7.2.2 Parallel beam of light Fig. 7.3.3 Convergent beam of light Fig. 7.2.4 Divergent beam of light

Oasis School Science and Technology - 8 191 iii. Irregular Reflection When a parallel beam of light strikes a surface and reflects in different directions, such type of reflection is called irregular reflection of light. The reflection of light from a rough surface is an example of irregular reflection. We are able to see the object from different directions due to the irregular reflection of light. Mirror and Its Types A smooth surface which forms an image due to the reflection of light is called mirror. There are two types of mirror, viz. plane mirror and spherical mirror. Plane Mirror The mirror in which reflection of light occurs on a smooth plane is called plane mirror. It has a polished surface which reflects the rays of light coming from a source. Fig. 7.2.7 reflection from plane mirrors Polished surface Reflecting surface (a) (b) The image formed by a plane mirror is erect and of the same size to that of the object but laterally inverted. Real Image and Virtual Image The image which can be obtained on the screen is called real image. It is formed when any two rays of light meet each other at a point. Real image is always inverted. The concave mirror forms a real image. The image which cannot be obtained on the screen is called virtual image. It is formed at a point where the rays of light appear to meet when they are produced back. Virtual image is always erect. The plane mirror and convex mirror form a virtual image. Fact File Real image is formed when two rays of light meet each other at a point after reflection. Fact File Our eyes also make real and inverted images. Brain rotates the image and we can see erect images.

192 Oasis School Science and Technology - 8 Difference between real image and virtual image Real image Virtual image 1. It is formed at the point where the reflected rays meet. 1. It is formed at the point where the reflected rays appear to meet after diverging. 2. It is always inverted. 2. It is always erect. 3. It is always formed in front of the mirror. 3. It is always formed behind the mirror. 4. This image can be obtained on the screen. 4. This image can not be obtained on the screen. Spherical Mirrors The mirror which is a part of a sphere is called spherical mirror. If the spherical part is polished from inner surface, it is called convex mirror. Similarly, if the spherical part is polished from outer surface, it is called concave mirror. Fig. 7.2.10 spherical mirrors P C Reflecting surface (a) Convex mirror (b) Concave mirror P Polished surface C Reflecting surface Polished surface Let'sobserve a spoon. Its shallow bowl acts as a concave as well as a convex mirror. Convex surface Concave surface Shallow bowl Fig. 7.2.11 Spoon as a convex as well as well as a concave mirror Reasonable Facts Why is concave mirror called converging mirror? Concave mirror is called converging mirror because it converges the light rays falling on it at a point after reflection.

Oasis School Science and Technology - 8 193 Reasonable Facts Why is convex mirror called a diverging mirror? Convex mirror is called a diverging mirror because it diverges the light rays incident on its reflecting surface. Terminology a. Pole of the mirror: The geometrical centre of a spherical mirror is called pole of the mirror. In the fig. 7.2.12, P is the pole of the mirror. All the distances should be measured from the pole of the mirror. b. Centre of curvature: The centre of the sphere of which the mirror is a part is called centre of curvature. It is denoted by C. c. Radius of curvature: The radius of the sphere of which the mirror is a part is called radius of curvature. In the fig. 7.2.12, PC is the radius of curvature. d. Principal axis: The line passing through the centre of curvature and the pole of the mirror is called principal axis. e. Principal focus: The point at which all rays parallel to the principal axis strike a mirror meet or appear to meet after reflection is called principal focus. In a convex mirror, reflected rays appear to meet at focus so it is called virtual focus. But in a concave mirror, reflected rays meet at focus, so the focus is called real focus. The rays of light parallel to the principal axis are converged at a point by a concave mirror. So, it is called converging mirror. In case of convex mirror, the rays which are parallel to the principal axis are diverged from a point. So, it is called diverging mirror. f. Aperture: The total reflecting surface of a mirror is called aperture of the mirror. g. Focal length: The distance between focus and pole of the mirror is called focal length. Focal length is equal to half of the radius of curvature, i.e. F P C Fig. 7.2.12 Real focus of a concave mirror F P Fig .7.2.13 Virtual focus of a convex mirror Focal length (f) = Radius of curvature (R) 2

194 Oasis School Science and Technology - 8 Activity 2 • Take a concave mirror. Focus the image of any object on a screen or a paper. Observe the image by changing the object distance. What do you observe? • Different types of images can be observed at different position. Activity 3 • Take a concave mirror and produce a clear image of an object present outside the room on a paper. Measure the distance between the paper and the mirror. The distance is called focal length of the mirror. Rules for Drawing Ray Diagrams in a Concave Mirror Rule 1 : A ray of light parallel to the principal axis passes through the principal focus after reflection. Rule 2 : A ray of light passing through the centre of curvature reflects along the same path and is perpendicular to the surface of the mirror. Rule 3 : A ray of light passing through the principal focus reflects parallel to the principal axis. P Fig. 7.2.15 F C P Fig. 7.2.14 F C P Fig. 7.2.16 C F Images Formed by a Concave Mirror Now let us draw the ray diagrams for a concave mirror when object is placed at different positions. a. Object at infinity (distant object) When an object is at infinity, parallel beam of light falls on the mirror and image is formed at the focus after reflection. The image is real, inverted and highly diminished. Fig. 7.2.17 When object is at infinity C F P Concave mirror Image Rays of light from distant object

Oasis School Science and Technology - 8 195 b. Object beyond the centre of curvature (2F or C) When the object is kept beyond the centre of curvature, its image is formed between C and F. The image is real, inverted and diminished. c. Object at centre of curvature (C) When the object is kept at the centre of curvature (C), image is also formed at the same point. The image is real, inverted and of the same size as the object. d. Object between C and F When the object is kept between C and F, image is formed beyond C. The image is real, inverted and magnified. e. Object at F When the object is kept at F, image is formed at infinity. The image is real, inverted and highly magnified. f. Object between F and P When the object is kept between F and P, image is formed beyond the mirror. The image is virtual, erect and magnified. Fig. 7.2.18 When object is beyond 2F P F A' B' C A B Image Object Fig. 7.2.19 When object is at C P C F B' A Object Image B Fig. 7.2.20 When object is between C and F B P C A Object Image F P F Object Image at infinity C Fig. 7.2.21 When object is at F Fig. 7.2.22 When object is between F and P A F Object Image C A1 P B B1

196 Oasis School Science and Technology - 8 Summary of the images formed by a concave mirror S. N. Position of an object Position of the image Nature of the image formed 1. At infinity At F Real, inverted, diminished 2. Beyond C or 2F Between F and C Real, inverted, diminished 3. At C or 2F At C or 2F Real, inverted and of the same size as the object 4. Between C and F Beyond C or 2F Real, inverted enlarged 5. At F At infinity Real, inverted, enlarged 6. Between F and P Behind the mirror Virtual, erect, enlarged How to draw mirror, centre of curvature, pole and principal axis? i. Draw a circular arc AB with a compass taking C as a centre. ii. The centre C is the centre of curvature. iii. Locate the mid-point of arc AB. Let it be 'P' which is called pole of the mirror. iv. Now, join the points P and C. The line passing through P and C is called principal axis. v. The mid-point of PC is called focus. Let it be F. vi. According to the necessity, draw the object. Now, follow the rules to draw ray diagrams. Rules to Draw Ray Diagram for Convex Mirror Rule 1: A ray travelling parallel to the principal axis appears to diverge from the principal focus. Rule 2: A ray travelling through centre of curvature is reflected back along its own path. Focus (F) P A B C Mid point of PC Mid point of AB Fig. 7.2.23 P F C Fig. 7.2.24 P F C Fig. 7.2.25