Modern Concept Science and Technology 8 New

Modern Concept Science and Technology – 8 147 b) Second class lever The lever in which load lies in between fulcrum and effort is called a second class lever. Examples: wheel barrow, nutcracker, lemon squeezer,bottle-opener, etc. Wheelbarrow Nutcracker Bottle opener Fig: Second class lever In this lever, load lies at any point in between effort and fulcrum. Mechanical advantage of second class lever From the principle of lever, Effort (E) × Effort arm (Ed) = Load (L) × Load arm (Ld) or, L E = Ed Ld or, MA = Ed Ld In case of second class lever, effort distance (Ed) is always greater than the load distance (Ld). i.e. Ed > Ld ∴ MA > 1 Thus, by using the second class lever, a greater load can be lifted with lesser effort, i.e. the second class levers are used as force multipliers. FACTS WITH REASONS Cracking of nut shells becomes easier with the help of a nutcracker. Nutcracker is a second class lever in which the effort distance is longer than the load distance. So, effort gets multiplied. As a result, the nut can be cracked with lesser effort with the help a nutcracker. Solved Numerical 3.1 The efficiency of a wheelbarrow shown in the given figure is 80%. Calculate the effort required to carry sand with the help of this wheelbarrow. Solution: Load (L) = 450 N Load distance (Ld) = 0.4m Effort distance (Ed) = 1.5m Efficiency of the wheelbarrow (η) = 80% According to the formula, Load Pivot Effort Effort Pivot Weight of sand 450N 1.5m 0.4m Sand

148 Simple Machine Efficiency (η) = Output work Input work × 100% or, 80% = 450 × 0.4 E × 1.5 × 100% or, E = 180 80 × 1.5 × 100% = 180 120 × 100% = 150 The effort required to carry sand with the help of the wheelbarrow is 150 N. c) Third class lever The lever in which effort lies in between load and fulcrum is called third class lever. Examples, shovel, fishing rod, fire tongs, broom, etc. Shovel Fishing rod Fire tongs Broom Fig: Third class lever Mechanical advantage of third class lever From the principle of lever, Effort (E) × Effort arm (Ed) = Load (L) × Load arm (Ld) or, L E = Ed Ld or, MA = Ed Ld In case of third class lever, effort distance (Ed) is always less than the load distance (Ld). i.e. Ed < Ld ∴ MA < 1 Thus, by using a third class lever, a greater load cannot be lifted with lesser effort, i.e. the third class levers cannot be used as force multipliers. Instead, the third class levers are used as a speed multiplier. FACTS WITH REASONS Cleaning becomes faster by using a broom. In case of a broom, the effort distance is less than the load distance. It does not multiply effort. But a small movement of our arm pushes the dust a longer distance. So, cleaning becomes faster by using a broom.

Modern Concept Science and Technology – 8 149 Solved Numerical 3.2 Calculate the force with which the dust is pushed by a broom shown in the given figure. Solution: Effort applied (E) = 25 N Force on dust, i.e. load (L) = ? Effort distance (Ed) = 30 cm = 0.3 m Load distance (Ld) = 1.2 m Now, from principle of lever, Effort (E) × Effort arm (Ed) = Load (L) × Load arm (Ld) or, 25 × 0.3 = L × 1.2 or, L = 7.5 1.2 = 6.25 The dust is pushed by a force of 6.25 N. HOT SKILL HIGHER ORDER THINKING SKILL 1. Third class lever cannot multiply force because it has shorter effort arm. It has mechanical disadvantage. But also it is widely used in daily life. For example, spoon, broom and fire tong. Why is it used despite such disadvantages? Discuss. Although third class lever cannot multiply force and has mechanical disadvantage, it is widely used in daily life because it has a very long load distance. It can transfer the force to the longer distances so that we can use it to work faster from safe distances. For example, we can sweep floor without sitting, which is difficult. We can work with fire from safe distance without getting burnt. We can stir or scoop hot foods with spoon without burning hands. 2. No machine has equal amount of output work to the input work (100%efficiency), why? In a machine, some amount of input work is lost due to friction and weight of the simple machine. So, no machine has equal amount of output work to the input work. 3. The cutting edge of the cloth cutting scissors is much longer than the cutting edge of the metal sheet cutting scissors, why? Metal is harder than clothes. It needs more effort to cut metals. So, to increase effort to cut metals easily, the cutting edge of the cloth cutting scissors is much longer than the cutting edge of the metal sheet cutting scissor. 4. Write any two differences between Mechanical advantage and Velocity ratio. S.N. Mechanical advantage S.N. Velocity ratio 1 The ratio of load lifted to the effort applied in a simple machine is called mechanical advantage. 1 The ratio of the distance moved by effort to the distance moved by load in a simple machine is called velocity ratio. 2 It is affected by friction. 2 It is not affected by friction. Fulcrum Output force Input force 25N 1.2m 30cm

150 Simple Machine 5. A machine has MA 4. What does it mean? MA of a machine is 4 means that while using this machine, it means the effort is multiplied 4 times by the machine. 6. It becomes easier to carry a load in a wheelbarrow when the load is shifted towards its wheel, how? A wheelbarrow is a second class lever. It has load in between effort and fulcrum. The wheel works as a fulcrum. If we shift the load towards wheel, the load distance decreases and effort gets multiplied. So, it becomes easier to carry a load in a wheelbarrow when the load is shifted towards its wheel. 7. An effort of 100 N is used to lift a load of 400 N by using a lever. If the load is at a distance of 20 cm from the fulcrum then find the effort distance. Solution: Effort applied (E) = 100 N Load lifted (L) = 400 N Load distance (Ld) = 20 cm = 0.2 m Effort distance (Ed) = ? Now, from the principle of lever, Effort (E) × Effort arm (Ed) = Load (L) × Load arm (Ld) or, 100 × Ed = 400 × 0.2 ∴ Ed = 400 × 0.2 100 = 0.8 m = 80 cm 8. A worker lifts a load of 200 N with the help of a lever by applying an effort of 50 N. The load is kept at a distance of 20 cm and the effort is applied at a distance of 1m from the fulcrum. Find the mechanical advantage, velocity ratio, and efficiency of the lever used. Solution: Load lifted (L) = 200 N Effort (E) = 50 N Effort distance (Ed) = 1 m Load distance (Ld) = 20 cm = 20 100 = 0.2 m Now, MA = Load (L) Effort (E) = 200 50 = 4 VR = Effort distance (Ed) Load distance (Ld) = 1 0.2 = 5 Efficiency = MA VR × 100% = 4 5 × 100 = 80 % Therefore, the mechanical advantage is 4, velocity ratio is 5 and efficiency is 80 percent.

Modern Concept Science and Technology – 8 151 33 STEPS STEPS EXERCISE EXERCISE STEP1 1. Select the best answer from the given alternatives. a) Which class lever has fulcrum in between load and effort? i) first class lever ii) second class lever iii) third class lever iv) fourth class lever b) A lever which always uses more effort to lift a lighter load is: i) first class lever ii) second class lever iii) third class lever iv) fourth class lever c) If effort arm is longer than the load arm, then: i) MA < 1 ii) MA = 1 iii) MA > 1 iv) none of them d) A lemon squeezer is an example of: i) third class lever ii) second class lever iii) wedge iv) inclined plane e) If 20% energy supplied to a machine is lost due to friction, then the efficiency of the machine is: i) 20 % ii) 80 % iii) 10 % iv) 90 % f) For a real simple machine: i) MA < VR ii) MA = VR iii) MA > VR iv) MA > VR g) In an ideal simple machine the output work is: i) more than input work ii) equal to the input work iii) less than input work iv) double of the input work 2. Write True for the correct and False for the incorrect statements. a) Sewing machine is an example of a simple machine b) Output work is the useful work done by a machine. c) The ratio of output work to the input work is called velocity ratio. d) Mechanical advantage is a ratio of effort arm to the load arm. e) A crowbar makes our work easier by multiplying effort. f) A nut-cracker is an example of second class lever. g) Friction increases the efficiency of a machine. 3. Fill in the blanks with appropriate words. a) A ……. machine is made from the several simple machines working together. b) In a …… class lever, the effort distance is always smaller than the load distance. c) A nail cutter is an example of ……. class lever.

152 Simple Machine d) In third class lever, mechanical advantage is always……. than one. e) The ratio of the load lifted to the effort applied is called …….. f) An …….. machine must be frictionless and weightless. g) Efficiency of a simple machine is reduced by ............ 4. Answer the following questions in one word. a) How many types of lever are there? b) What is called to the point about which a lever rotates? c) What is the SI unit of input work? d) Which class lever has VR always less than one? e) 15% input work is lost due to friction, then what is the efficiency of machine? STEP2 5. Give reasons. a) Mechanical advantage has no unit. b) MA of a first class lever can be one or greater than one or less than one. c) MA of a second class lever is always greater than one. d) MA of a third class lever is always less than one. e) The cutting edge of the scissors used for cutting cloth is much longer than the cutting edge of the shears used for cutting metals. f) A practical machine cannot be 100 % efficient. g) It becomes easier to carry a load in a wheelbarrow when the load is shifted towards its wheel. 6. Write any two differences between: a) A simple machine and a complex machine b) Mechanical advantage and velocity ratio c) an ideal machine and a practical machine d) second class lever and third class lever 7. Answer the following questions a) What is a simple machine? b) Define the following terms related to the simple machines: i) load ii) effort iii) mechanical advantage iv) velocity ratio v) efficiency vi) input work vii) output work c) What is an ideal simple machine? d) What is a lever? Write down its types. e) State the principle of lever. f) Define first class lever, second class lever and third class lever.

Modern Concept Science and Technology – 8 153 g) Which kind of lever are the followings: i) Nutcracker ii) Pliers iii) A hockey stick iv) Fire tong v) A broom vi) Nail cutter vii) Human-arm vii) Wheelbarrow ix) Fishing rod STEP3 8. Answer the following questions. a) State four ways in which the machines are useful to us. b) Mechanical advantage of a simple machine is 4, what does it mean? c) Write any three methods to increase efficiency of a simple machine. d) Two persons are not equally strong. One is weaker than another. They are going to carry a wooden pole. Suggest them a way to carry the wooden pole in such a way that the stronger person needs to apply more effort and the weaker person needs to apply less force. Also explain the scientific reason behind your answer. e) Answer the questions with the help of the given figures. i) Label the parts A, B, and C in the given figures. ii) Copy the given diagram of forearm and indicate the position of load, effort and fulcrum. f) Draw a diagram to show the position of fulcrum, load and effort in a lever when it is used as a force multiplier. g) Draw a diagram to show the position of fulcrum, load and effort in a lever when it is used as a speed multiplier. 9. Numerical problems a) An effort of 200 N is used to lift a load of 800 N by using a lever. If the load is at a distance of 40 cm from the fulcrum then find the effort distance. [Ans: 160 cm] b) Ram with the weight of 400 N and Shyam with the weight of 500N are willing to play on a see-saw. If Ram is at a distance of 2m from the fulcrum then how far from the fulcrum should Shyam sit to balance Ram? [Ans: 1.6 m] c) The mechanical advantage of a simple machine is 3 and its velocity ratio is 4. Find its efficiency. [Ans: 75%]

154 Simple Machine d) A person lifts a load of 400 N with the help of a lever by applying an effort of 100N. The load is kept at a distance of 40 cm and the effort is applied at a distance of 2m from the fulcrum. Find the value of MA, VR and efficiency? [Ans: MA = 4, VR = 10, efficiency = 40%] e) A man uses a long crowbar to lift a load of 1000 N as shown in the figure. Find i) the effort needed to lift the load ii) the mechanical advantage of the crowbar [Ans: 250 N, 4] f) Find the MA, VR and efficiency of the lever shown in the given figure. [Ans: 1.25, 2, 62.5%] g) A force of 50 N applied in a simple machine displaces it downward by 0.25 m in rising a load of 100 N through 10 cm. Calculate MA, VR, and efficiency. [Ans: 2, 2.5, 80%] Project Work Project Work 1. Make a list of different types of simple machines used in our daily life. Name of the simple machine Type Use Fulcrum Crow bar Load Effort 100 N 80 N 10 m 5 m

Modern Concept Science and Technology – 8 155 Key terms and terminologies of the unit 1. Pressure : The force acting perpendicularly on a unit area of a surface is called pressure. 2. One pascal pressure : When a force of 1N is applied normally on an area of 1 m2 , then the pressure acting on the surface is called 1 pascal pressure. 3. Liquid pressure : The force applied by a liquid on per unit area of the walls of the container is called liquid pressure. 4. Atmospheric pressure : The force exerted by the weight of the atmosphere on per unit area of the earth’s surface is known as atmospheric pressure. 5. Standard atmospheric pressure : The atmospheric pressure at the sea level is called standard atmospheric pressure which is about 101300 N/m2 or 760 mm Hg. 6. Density : The mass in per unit volume of a substance is called density. 7. Relative density : Relative density is the ratio of density of a substance to the density of water at 4o C. 8. Barometer : An instrument that measures atmospheric pressure is called a barometer. 9. Compressed air : Compressed air is a gas or mixture of gases stored under greater pressure than normal atmospheric pressure. 10. Pressure gauge : The pressure gauge is a device that measures the pressure exerted by a fluid on the walls of its container. 11. Manometer : A device that uses the surface area and weight of a liquid column to measure and indicate pressure is known as a manometer. Sequence of Curriculum Issued by CDC  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 UNIT Pressure 6.3 Estimated teaching periods Theory Practical 4 1 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. Blaise Pascal About the Scientist

156 Pressure 6.3.1 Introduction to Pressure In our daily life, we prefer sharp and pointed objects for cutting and piercing. It is our common experience that we feel easy to cut vegetables and fruits with the help of a sharp knife but different with the blunt one. It is because the sharp knife has less area in the cutting edge. The force exerted per unit area is more in case of the sharp objects and less in case of the blunt one. Thus, the force acting perpendicularly on a unit area of a surface is called pressure. Pressure (P) = Force (F) Area (A) Pressure has a magnitude but no direction associated with it. So, pressure is a scalar quantity. Relation among Force, Area and Pressure The same force can produce different pressure depending on the area. For example, when we stand on one foot, more pressure is exerted on the ground. But, if we stand on both feet, less pressure is exerted on the ground. Therefore, pressure depends upon following two factors: Force applied Pressure increases with increasing force and decreases with decreasing force. In other words, pressure exerted is directly proportional to the force applied. i.e. P α F Surface area Pressure decreases with increasing area and increases with decreasing area. In other words, the pressure exerted is inversely proportional to the surface area. i.e. P α 1 A ACTIVITY 1 Objective : To observe that pressure is directly proportional to the surface area. Methods : 1. Put your hand with open palm on a table. 2. With the help of another hand, gently put a brick over your palm in two different positions as shown in the figure. 3. What do you observe? Do you feel more pain when the brick is kept vertically? Explanation : The weight of the brick is same in both positions. But, in vertical position, the weight acts on less area and exerts more pressure. Force Area Pressure

Modern Concept Science and Technology – 8 157 Measurement of Pressure Units of pressure The SI unit of pressure is newton per square metre (Nm-2), or pascal (Pa). The CGS unit of pressure is dyne per square centimetre (dyne cm−2), or barye (Ba). Relation between the SI unit and CGS unit of pressure: 1 Pa = 10 Ba One pascal pressure We know that, Pressure (P) = Force (F) Area (A) or, 1Pa = 1 N 1 m2 When 1N force is applied normally on an area of 1 m2 , then the pressure acting on the surface is called 1 pascal. Differences between force and pressure S.N. Force S.N. Pressure 1 Force is a push or pull which changes or tries to change the state of a body. 1 Force acting normally on per unit area is called pressure. 2 The SI unit of force is newton (N). 2 The SI unit of pressure is pascal (Pa). Solved Numerical 4.1 A plywood cupboard of 400N weight occupies 2m² surface area. Calculate the pressure exerted. Solution: Given, Force (F) = 400N Area (A) = 2m² Pressure (P) = ? According to the formula, pressure (P) = Force (F) Area (A) = 400 2 = 200 ∴ The pressure exerted (P) = 200 Pa. Application of Pressure in Daily Life A. Increasing pressure by reducing the area i. Studs are made on the boot of football players The studs on the football boot have small area that comes in contact with the ground. That results a high pressure. As a result, it gives extra grip and football players do not slip while running on the ground. ii. Tip of a needle is made sharp The sharp tip of a needle covers very small area of the surface to be penetrated. Due to this reason, it creates large pressure and penetration of a surface becomes easy. iii. A sharp knife cuts better than a blunt one A sharp knife has a very thin edge to its blade which decreases surface area. Due to this reason the force of our hand falls over a very small area of the object. This creates large pressure and the knife cuts the objects easily.

158 Pressure iv. Heel shoes apply more pressure on the ground than the flat slippers Flat slippers occupy more area than the heel shoes. Due to this, the flat slippers exert less pressure and heel shoes exert more pressure. B. Reducing pressure by increasing the area i. Tractors have broad tyres The wide tyres of a tractor spread its weight over a large area of the ground. Due to this reason, there is less pressure on the ground and does not dig in more. If the tyres are made thin, they occupy less area and dig in more due to more pressure. ii. More wheels are used in pair on heavy trucks Heavy trucks exert more pressure on the ground. So, to decrease pressure on the ground the contact area of the truck on the ground should be increased by using more wheels in pair. As a result of this, the pressure exerted on the ground decreases and truck can carry heavy loads easily. iii. The railway tracks are laid on large sized wooden, iron or cement sleepers. When large sized wooden, iron or cement sleepers are kept below the iron rails of a railway track, the thrust due to heavy weight of a train spreads over a large area. Due to this reason, railway line does not sink into the ground. iv. The school bag is provided with wide straps. Wide straps of a bag fall over larger area of the shoulder of a child. It produces less pressure on the shoulder. So, with the help of wide straps, it is comfortable to carry the heavy school bag. 6.3.2 Liquid Pressure A liquid does not have its own shape. It gains the shape of the container in which it is kept. A Liquid applies thrust on the wall of the container. This thrust causes pressure on the wall of the container. Thus, the force applied by liquid on per unit area of the walls of the container is called liquid pressure. Liquid also exerts pressure on any object in the liquid. Expression for liquid pressure Let us consider a regular liquid container having area of cross-section ‘A’. If the container contains a liquid of density ‘d’ up to the height ‘h’, then the force at the bottom of the container is the same as the weight of the liquid. Therefore, the liquid pressure is P = F A = Weight of liquid (W) A = mg A P = Vdg A [Since, mass of liquid (m) = volume of liquid × density] P = Ahdg A [Since, volume of liquid (V) = Areas of cross section × height] P = hdg d h A

Modern Concept Science and Technology – 8 159 Factors affecting liquid pressure Liquid pressure depends on three factors. They are: a) The depth of the liquid column Liquid pressure at a point is directly proportional to the depth of the point from the free surface of the liquid. FACTS WITH REASONS The base of a dam is made thicker. Water is collected in the dam. According to the formula P = hdg, the water pressure increases with depth. Therefore,the wall of dam is made thicker at the bottom to withstand the higher pressure of water. The deep sea divers wear a special suit. Water pressure increases with depth. Deep sea divers feel more pressure on their body while swimming deeper. This may cause bursting of the blood vessels resulting bleeding. So, to overcome the high water pressure, the deep sea divers wear a special suit. In case of the water supplies in buildings, the pressure of water on the ground floor is higher than that in upstairs. Water tanks are kept on the roof of the buildings. According to the formula P = hdg, the water pressure on a tap increases with the depth of water column in the pipe. So, the pressure of water on the ground floor is higher than that in upstairs. b) The density of the liquid Liquid pressure at a point is directly proportional to the density of the liquid. High density liquid exerts more pressure at bottom of the container. c) The acceleration due to gravity The liquid pressure at a point is directly proportional to the value of acceleration due to gravity at that place. Solved Numerical 4.3 Calculate water pressure at the bottom of a swimming pool of 2 m depth. (Density of water = 1000 kg/m3 ) Solution: Depth of swimming pool (h) = 2 m Density of water = 1000 kg/m3 Acceleration due to gravity (g) = 9.8 m/s2 Pressure at the bottom (P) = ? According to the formula, P = hdg = 2 × 1000 × 9.8 = 19,600 Pa ∴ The pressure at bottom of the swimming pool is 19, 600 Pa. Properties of liquid pressure a) Liquid applies pressure in all directions Liquids molecules have weak force of attraction and they push the walls of the container or surfaces in contact. If liquid is kept in a close container, it applies

160 Pressure pressure in all direction.Thus, liquid pressure is transmitted equally in all directions, if pressure is applied to the liquid kept in a closed vessel. ACTIVITY 4 Objective : To show that liquid pressure is transmitted equally in all direction. Methods : 1. Take a polythene bag or balloon and make holes on it with a pin. 2. Fill it with water. 3. Now, hold the balloon tightly on its mouth. 4. Press water down with your hand and observe the out flow of water from these holes. 5. Observe the effect of pressure on the flowing of water through all holes? Explanation : When pressure is applied in a liquid at a point in a closed container, the pressure transmits equally in all direction. This is Pascal’s law of liquid pressure. b) Liquid pressure increases with the depth of liquid column Liquid pressure increases with increasing depth of the liquid column. This results into high pressure. For example, water pressure on the ground floor tap is more than that on the top floor tap, to withstand high pressure deep inside the sea, the sea divers wear a special suit, etc. ACTIVITY 5 Objective : To show that liquid pressure increases with the depth of liquid column. Methods : 1. Take a cylinder and make holes on a side of the bottle as shown in the given figure. 2. Fill the bottle completely with water and observe the outflow of water from different holes. 3. Do you observe the maximum pressure of water on the bottom hole? Explanation : In case of the cylinder, the hole present at the bottom has maximum depth. So, the water pressure is also maximum at the bottom hole. A B C D c) Liquid pressure depends on the density of liquid Liquid pressure increases with increasing density of the liquid. For example, if oil and water are collected in two different containers and a hole is made at the same depth on the both containers then the water with high density comes out with more pressure. Water density = 1g/cm3 h Oil density = 0.93g/cm3 h Pressure depends on the density of liquid

Modern Concept Science and Technology – 8 161 d) Liquid pressure does not depend on the shape of the container. Whatever the shape or width, the liquid pressure at a particular depth is the same. 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 hydraluic lift. iii. The wall of dam is made thicker in 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. Similarity and Differences between the Liquid Pressure and Atmospheric Pressure Similarities i) Air pressure applies in all direction like the liquid pressure. ii) Air pressure becomes less with increasing altitude similarly liquid pressure decreases with decreasing depth. Differences S.N. Liquid pressure S.N. Atmospheric pressure 1 Liquid pressure cannot compress the liquid in lower layers. 1 Atmospheric pressure compresses the air in lower layers 2 Liquid is denser than air and applies more pressure. 2 Gas is very less dense than liquid and applies less pressure. 6.3.3 Compressed air Compressed air is an alternative to fossil fuel. The normal air is collected and compressed into a smaller size. 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.

162 Pressure 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. Fig: Air hammer Fig: Compressed air engine 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. 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, milibar or mmHg. ACTIVITY 1 Make a list of appliances used in our community that works with compressed air. MEMORY TIPS Compressed air and atmospheric gas have same composition.

Modern Concept Science and Technology – 8 163 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 gauge is 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 gauge can either be mechanical or digital. A pressure gauge is also known as a pressure meter and vacuum gauge. Most pressure gauge calculate the pressure relative to atmospheric pressure as the zero point. Most pressure gauge are filled with liquid glycerin and silicone. Manometer A Manometer is a device to measure fluid pressures. 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. FACTS WITH REASONS Some pressure gauge are filled with liquids. Some pressure gauge are filled with liquids to reduce the vibration so that readings can be more reliable. ACTIVITY 2 Make a model of a pressure gauge. 6.3.4 Atmospheric Pressure The earth has its own force of attraction called gravity. Due to this force of gravity, a thick layer of air is present around the earth’s surface called atmosphere. The huge mass of the atmosphere exerts force on the earth’s surface and creates a large amount of pressure. Thus, the force exerted by the weight of the atmosphere on per unit area of the earth’s surface is known as atmospheric pressure. Fig: pressure gauge Fig: Manometer MEMORY TIPS A pressure gauge is a fluid intensity measurement device. Fig: Earth and its atmosphere

164 Pressure ACTIVITY 3 Objective : To show that atmosphere exerts pressure. Methods : 1. Take a tin can and pour some water in it. 2. Heat the tin can until water starts to boil. 3. Allow water to boil until the steam pushes out all the air form the tin can. 4. Close the lid and stop heating. 5. Pour cold water over the hot tin can. Do you notice that the tin can gets crushed? Explanation : When air inside the tin can comes out, there is formation of a partial vacuum. So, the high atmospheric pressure from outside exerts a great force to crush the can. ACTIVITY 4 Objective : To show that atmosphere exerts pressure. Methods : 1. Take a glass tumbler and fill it completely with water. 2. Cover the tumbler with a thick card-board. 3. Use a hand to invert the glass tumbler pressing the card-board by the palm of another hand. 4. Remove your palm from the card-board. Does it fall down? Explanation : Card-board is pressed up by the atmosphere. The force caused by the atmospheric pressure acts on the surface of the cardboard is greater than the weight of the water in the glass tumbler. This shows that the atmosphere exerts pressure. Atmospheric pressure FACTS WITH REASONS We are not crushed by atmospheric pressure. At sea level, atmospheric pressure is about 105 Pa. The internal pressure produced inside our body due to air in lungs, intestine and other organs and blood pressure in vessels is slightly greater than the atmospheric pressure. So, we are not crushed by atmospheric pressure. Units of atmospheric pressure i) The SI unit of atmospheric pressure is N/m2 or pascal (Pa). ii) Atmospheric pressure is generally expressed in millimeter of mercury column. At sea level, 1 atm = 760 millimeters of mercury (mmHg) = 1.013 ×105 Pa iii) Another unit used for atmospheric pressure is torr. 1 torr = 1 mmHg = 133.3 Pa and 1 atm = 760 torr

Modern Concept Science and Technology – 8 165 iv) The unit of pressure used in meteorology is bar. 1 bar = 105 Pa and 1 atm = 1.013 bar 1 milibar (mb) = 1 1000 bar = 100000 1000 Pa = 100 Pa Standard atmospheric pressure The atmospheric pressure is more at sea level and decreases as we go up. Thus, the atmospheric pressure at the sea level is called standard atmospheric pressure which is about 101300 Pa, or 760 mmHg. Solved Numerical 4.2 A wall is of height 3 meters and length 7 meters. Find the force exerted by the atmosphere on its sides if standard atmospheric pressure is 1.013 × 105 Pa. Solution: Here, standard atmospheric pressure (P) = 1.013 ×105 Pa Height of wall (h) = 3 m Length of wall (l) = 7 m Area of the wall (A) = l × h = 3 m × 7 m = 21m2 Now, the force on the wall is given by the formula ∴ F = PA = 1.013 × 105 × 21 = 2.13 × 106 N 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. While flying aeroplanes and jet planes at high altitude, the air pressure inside them is maintained for comfortable breathing of the passengers. FACTS WITH REASONS At high altitudes, it is a common experience to have nose bleeding. At high altitude, the external atmospheric pressure is lower than the pressure within our body. This results bursting of the thin capillaries in the nose and bleeding occurs from the nose. So, at high altitudes, it is common experience to have nose bleeding. Astronauts wear specially designed pressure suits in space. There is no atmosphere in the space. The internal blood pressure of the astronauts becomes higher than the external pressure. It may cause blood vessel to rupture. The continuous bleeding may lead to the death. So, astronauts wear specially designed pressure suits in space. MEMORY TIPS The mercury barometer was invented in 1643 by Evangelista Torricelli (1608-1647), who was a student of Galileo. MEMORY TIPS Altimeter is an instrument in air craft to measure altitude. It is based on the principle that the atmospheric pressure decreases with the height above the sea level. Very low Low High fig: variation of atmospheric pressure with altitude

166 Pressure Importance of atmospheric pressure i) Atmospheric pressure is used for filling ink in a fountain pen. ii) Atmospheric pressure is used for filling medicine in a syringe. iii) It is used for filling air in a bicycle tube, or tube of vehicles. iv) It is used for lifting water by using a water pump. v) It is used to draw soft drinks through a straw. Some application of atmospheric pressure a) Drinking through a straw For sucking soft drink through a straw, the lower end of the drinking straw is dipped into the soft drink. When we suck through the upper end of the straw, it reduces the air pressure inside the straw. As a result, the high atmospheric pressure on the surface of the liquid pushes the liquid up through the straw. FACTS WITH REASONS Atmospheric pressure helps in sipping soft drinks from the bottle. One end of the straw is kept in the soft drink and air is sucked through another end. Sucking removes air from the straw tube and the air pressure inside it is reduced. The air pressure acting on the surface of the soft drink forces it into the straw tube and the drink rises into the mouth. b) Filling medicine in a syringe When we dip needle of the syringe in medicine and its piston is withdrawn, the pressure inside the syringe becomes low. The high atmospheric pressure acting on the surface of the medicine pushes the medicine up into the syringe. c) Lift pump A lift pump has a cylindrical barrel with a spout near the top. An air tight piston moves up and down the barrel with the help of a handle. A valve ‘V1 ‘ is at the bottom of the barrel, called foot valve. It is made up of rubber or leather flap. Another valve ‘V2 ‘, called piston valve which moves up and down with the help of handle as shown in the given figure. Working i) When we push the handle down, the valve ‘V1 ‘ opens and the valve ‘V2 ‘ remains closed. Water enters into the MEMORY TIPS A fan inside the vacuum cleaner lowers the air pressure inside it. The air rushes inside the vacuum cleaner carrying dust and dirt with it. The filter bag in the cleaner filters the dust and dirt from air which is cleaned regularly. Atmospheric Pressure Pressure less than atmospheric pressure Liquid being pushed up in straw Fig: Drinking through a straw Piston Low pressure Atmospheric Pressure Filling medicine in a syringe Down H H B B Up Atmospheric pressure Lift pump

Modern Concept Science and Technology – 8 167 barrel due to the atmospheric pressure. ii) When we pull the handle up, the valve ‘V2 ‘ opens and the valve ‘V1 ‘ remains closed. Water comes out through the spout. d) Filling ink in a pen A pen has a rubber tube in it. When the rubber is pressed by pushing the clip, the air inside the tube comes out and pressure becomes less. On releasing the clip, the greater atmospheric pressure on the surface of the ink pushes the ink up into the tube of the pen. HOT SKILL HIGHER ORDER THINKING SKILL 1. Water boils at temperature lesser than 100°C in the mountain region. The water receives heat energy from the fire. The water molecules gain kinetic energy and start to move faster. Since atmospheric is less at the mountain region, water molecules are more likely to move faster. So, water boils at lesser temperature than 100 °C in mountain region. 2. A balloon filled with more air bursts. If air pressure inside the balloon is more than the atmospheric pressure, it gets stretched beyond its limit and bursts. So, a balloon filled with more air bursts. 3. Binita was looking how the dozer works. Dozer was digging the land to make huge foundations for house. Binita asked the contractor if why they need to make so huge foundations. What might the contractor explain to her about constructing huge foundations. Binita was observing the construction of house. The dozer was digging room for construction of the huge foundation. She wondered and asked the contractor that why they need to make such a huge foundation. The reason for making such a huge foundation is to withstand the greater pressure exerted by the weight of the building. As we know pressure is inversely proportional to the area. So if the foundation of base is bigger and wider, it will significantly reduce pressure from the building. So it can prevent sinking of building. 4. Rama wants to install washing machine in her house in the top floor. The plumber suggested that she should install it in the ground floor? What might be the reason. Rama wants to install washing machine in her house in the top floor but plumber is suggesting her to install it in the ground floor. The reason is washing machine can easily collect sufficient water if it is in the ground floor than the top floor. The physics here is liquid pressure increases with the height of liquid column as given by the formula P = hdg. We know that liquid pressure (P) is directly proportional to the height (h) of the liquid column(i.e. P ∝ h). So, at more depth there is more pressure of liquid and at less depth there is less pressure of liquid. Water flows faster due to higher pressure at ground floor. Metal bar Filling ink in a pen

168 Pressure 5. Nose bleeding occurs at high altitude. At high altitude, there is less atmospheric pressure compared to the blood pressure. This results in burst of the thin capillaries in the nose and bleeding occurs from the nose. 6. A water tank has its height 1 m. Calculate the pressure at its bottom when it is completely filled with water. (g = 9.8m/s².) Solution: Height of water column (h) = 1m Density of water (d) = 1000kg/m³ Acceleration due to gravity (g) = 9.8m/s² Liquid pressure (P) = ? Now, the pressure at the bottom of the tank (P) = hdg = 1×1000×9.8 = 9800 Pa Thus, the pressure at bottom of the water tank is 9800 Pa. 7. Compressed air is used as source of energy instead of batteries. Compressed air is used as source of energy instead of batteries because it is safer, renewable and clean. 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answer from the given alternatives. a) What is the direction of the liquid pressure? i) upward direction ii) sideways iii) downward direction iv) in all direction b) Which equation helps to calculate the liquid pressure? i) P= dgh ii) P= hdg iii) P = ghd iv) all of the above c) What is the value of atmospheric pressure at sea level? i) 76 cm of Hg column ii) 0.76 cm of Hg column iii) 76 mm of Hg column iv) 76 m of Hg column d) What is the density of water at 4°C? i) 1000 g/cm3 ii) 1000 kg/cm3 iii) 1000 kg/m3 iv) 1000 g/m3 e) What is an air stored at greater pressure than the normal atmospheric air called? i) standard air ii) natural air iii) normal air iv) compressed air 2. Write True for the correct and False for the incorrect statements. a) Pressure is directly proportional to the surface area. b) The cutting edges are made sharp to increase pressure. c) The foundation of a building is made wider to increase pressure on the ground. d) A liquid in a container exerts pressure in all directions. e) Air exerts pressure because it occupies space and has weight.

Modern Concept Science and Technology – 8 169 f) The atmospheric pressure is minimum at the sea-level. g) Inside a bottle filled with water, pressure is maximum at the surface. 3. Fill in the blanks with appropriate words. a) The force acting perpendicularly on a unit area of a surface is called ……. b) The SI unit of pressure is …… . c) The instrument used to measure atmospheric pressure is called…… . d) Liquid pressure increases on ……….. depth of liquid. e) At higher altitude, atmospheric pressure is………….. 4. Answer the following questions in one word. a) What is the instrument used to measure atmospheric pressure? b) What is the layer of air above the earth surface called? c) In which condition an object floats in water? d) What is the value of normal atmospheric pressure at sea level? e) What is the CGS unit of pressure? STEP2 5. Give reasons. a) A sharp knife cuts objects more effectively than a blunt knife. b) Camel can walk easily in the desert than the horses. c) Blood pressure in human body is greater at the feet than at the brain. d) Deep sea divers have to wear specially designed air filled suits for their protection. e) Air exerts pressure. f) Air pressure is more near the earth’s surface and it goes on decreasing as we go up. g) Mountaineers usually suffer from nose bleeding at high altitudes. h) Compressed air can be used as alternative to fossil fuel. i) Water tank is kept at the top of house. j) Water dams are made thicker at bottom. 6. Write any two differences between: a) Force and pressure b) Liquid pressure and atmospheric pressure 7. Answer the following questions. a) Define the following terms with required examples. i) pressure ii) 1 pascal pressure iii) liquid pressure iv) atmospheric pressure v) standard atmospheric pressure b) What is the cause of atmospheric pressure? c) What makes a balloon get inflated when air is filled in it? d) What is compressed air? Where is it used? e) Write about pressure gauge. f) Draw the diagram to show the variation of liquid pressure with depth.

170 Pressure STEP3 8. Answer the following questions. a) Explain the relationship among force, area and the pressure. b) How would pressure change if: i) area is doubled by keeping force constant ii) force is doubled by keeping area constant. c) Write the properties of liquid pressure. d) Explain the factors that affect liquid pressure. e) Derive the expression for liquid pressure, P= hdg. Where the terms have their usual meaning. f) Explain any four importance of atmospheric pressure. g) Enlist the importance of compressed air. h) Three tiny holes are made in an empty can at different levels, one over the other. The can is filled completely with water. i) Which property of liquid is shown in the given figure? ii) Why does the water flowing from the lowest hole goes farthest and that from the uppermost hole nearest to the can? i) The piece of a post card does not fall off the glass though the glass is full of water and its whole weight is exerting pressure on the card, why? j) The tin can with water in its bottom is heated to boil water and the stem is allowed to escape for some time. The open mouth is sealed with air-tight cap and cooled under tap-water. The tin can gets crushed, why? 9. Numericals: a) A ground water tank has its height 2m. Calculate the pressure at its bottom when it is completely filled with water. ( g = 9.8 m/s2 ) [Ans: 19600 Pa] b) A liquid of density 1200 kg/m3 is filled in a beaker. The pressure at the bottom of the beaker is 5880 Pa. Find the height of the liquid column. (g= 9.8 m/s2 )[Ans: 0.5m] c) The pressure at the bottom of a drum in which a liquid is filled upto the height of 2m is 9800 Pa. Find density of the liquid filled in that drum. [Ans: 500 kg/m3 ] Project Work Project Work Fill one balloon with hydrogen and another with air. Release them in air. Which one flies and which one does not? What happens when you blow air continuously in the balloon? Write the conclusion of your finding.

Modern Concept Science and Technology – 8 171 Energy in Dailly Life Unit 7 Introduction Energy is the ability to do work. The river can carry away logs. The wind can carry away leaves. Food can keep us alive. The battery can produce electricity. The electricity can run electrical equipment. River, wind, food and electricity can do those work because they have energy. The energy exists in various forms and can be interconverted into one another. Some of the common forms of energy are heat energy, light energy, sound energy, magnetic energy, chemical energy, mechanical energy and electrical energy. The heat energy increases the kinetic energy of the molecules of the substances. It changes the temperature of an object. It can transmit from one place to another by conduction, convection or radiation. It is responsible for weather, wind and climate. It keeps us alive on the earth. Heat radiations are absorbed a lot by black and darker colours but reflected a lot by white and bright colours. Thermos is a device that is used to keep liquid warm for a longer duration. The artificial greenhouse traps solar energy and keeps the inside warm to preserve plants. The earth acts as a natural greenhouse. The over-greenhouse effect is harmful. The light energy helps us to see. It is used by plants for photosynthesis. It can be reflected by mirrors. The spherical mirrors reflect light to form real or virtual images. They are used as side mirrors in vehicles, beauty salons and at the turning of roads. The sound energy helps us in hearing. We can only hear audible sounds. Infrasound and ultrasound cannot be heard by human ears. Music is a pleasant sound. The unwanted sound is called noise. Heavy traffic and loudspeakers are the sources of noise. Hence we know that energy is everywhere around us and it is very useful for us. Radiation Conduction Convection

172 Heat Key terms and terminologies of the unit 1. Heat: Heat is a form of energy which gives us sensation of the warmth. It is the sum of the kinetic energy of all the molecules in a substance. Heat is the flow of thermal energy from hotter objects to colder objects. 2. Transmission of heat: The process of transfer of heat from one place to another is called transmission of heat. 3. Conduction: The process of transfer of heat in solids from one molecule to the neighboring molecule without their actual movement is called conduction. 4. Conductor of heat: Materials which allow heat to pass through them are called conductors of heat. 5. Good conductor of heat: The materials which allow heat to flow through them easily are called good conductors of heat. 6. Insulator of heat: Materials which do not allow heat to pass through them easily are called insulators of heat. 7. Convection: The process of transfer of heat by actual movement of molecules in liquid and gas is called convection. 8. Sea breeze : The cold air above the sea that blows towards the land during the day is called sea breeze. Heat Sequence of Curriculum Issued by CDC  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 UNIT Heat 7.1 Estimated teaching periods Theory Practical 5 1 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. James Prescott Joule About the Scientist

Modern Concept Science and Technology – 8 173 9. Land breeze : The cold air above the land that blows towards the sea at night is called land breeze. 10. Radiation : The process of transmission of heat without any medium is called radiation. We receive heat from the sun by the way of radiation. 11. Wave : A wave is the periodic disturbance in a medium that carries energy from one place to another. 12. Mechanical wave : The wave that requires a material medium for its propagation is called a mechanical wave. 13. Electromagnetic Waves: The waves that travel by forming electric fields and magnetic fields are called electromagnetic waves. 14. Thermos flask : The thermos flask is a special kind of bottle which prevents all the possible ways of flow of heat to and from the bottle. 15. Green house gases : Gases like carbon dioxide, methane, water vapour, oxides of nitrogen, etc. that absorb large amount of heat and determine the climate are called greenhouse gases. 16. Green house effect : The phenomenon of trapping of solar heat of longer wavelength inside the earth’s atmosphere is called greenhouse effect. 17. Artificial green house : An artificial greenhouse is a house made up of glass or plastic that can trap solar heat in the same way as the earth does. 7.1.1 Introduction Heat is a form of energy which gives us sensation of warmth. A body is made up of a large number of particles called molecules. The arrangements of molecules in solids, liquids and gases are different. Such molecules may possess translational, vibrational and rotational motions. Due to the motion of molecules in a substance, they have some kinetic energy which is called heat. Thus, the sum of kinetic energy of all the molecules in a substance is called heat. Thus, everybody contains some amount of heat. The greater the kinetic energy of the molecules of a body, the greater is the heat. The average kinetic energy of the molecules in a substance determines its degree of hotness or coldness, i.e. temperature. When we rub or strike a substance, the kinetic energy of the molecules increases. This results in the increase in heat as well as temperature. In this unit we will learn about the flow of heat, applications of heat, different modes of transmission of heat, temperature and measurement of temperature. FACTS WITH REASONS A match stick produces flame when its head is rubbed sharply against the side of a matchbox. When the head of a match stick is rubbed sharply against the rough surface on the side of a matchbox, the kinetic energy of the molecules on the surface increases due to friction. The surface gets heated and produces a tiny spark. This spark ignites the chemical substance on the head of the match stick. Thus a match stick produces flame when its head is rubbed sharply against the side of a matchbox. Calorimeter Thermometer

174 Heat Sources of Heat The substance or medium which gives us heat is called a source of heat. Some important sources of heat are sun, bio-fuel, fossil fuel and electricity. It is widely used in industries, metallurgy and domestic uses such as cooking, boiling water etc. Effects of Heat When an object is heated various changes take place in the object. i) Heat changes state of matter: On heating, a substance changes its state from a solid to a liquid or from a liquid to a gas. ii) Heat changes temperature of a body: On heating a substance, the average kinetic energy of its molecules increases and this causes an increase in its temperature. iii) Thermal Expansion: Matters expand on heating and contract on cooling. FACTS WITH REASONS A metal becomes hot when it is struck with a hammer. When a metal surface is struck with a hammer, it causes the molecules in the metal to vibrate faster. Such vibrations increase the kinetic energy of the molecules, that is heat. Units of Heat Calorie and joule are the two commonly used units of heat. For example, energy of food is specified in kilocalories (kcal). Transfer of Heat The process of transfer of heat from one place to another is called the transmission of heat. Heat always flows from a body at higher temperature to a body at lower temperature. 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 nonmetals and insulators. ACTIVITY 1 To show heat flows from a body at a higher temperature to one at a lower temperature. 1. Take some hot water in a tumbler. 2. Dip a metal spoon in that water for some time. Notice the time taken for the spoon to become hot. 3. Leave the tumbler for 20 minutes. Do you feel the water cooling down? It becomes uncomfortable to hold the hot spoon due to the heat transferred from the hot water in the tumbler to the spoon. This is due to the flow of heat from a body at a higher temperature to one at a lower temperature. Have you ever put some ice on your palm? Also, what do you feel when your finger is in a very warm water even for one second? In such cases, our sense of touch estimates the degree MEMORY TIPS 1 calorie heat is equal to 4.2 joules.

Modern Concept Science and Technology – 8 175 of coldness and hotness of a body on the basis of the flow of heat. We feel cold if the heat flows out of our body. Similarly, We feel hot if the heat flows into our body. If the heat energy flows in or out of the body very rapidly, we have a very hot or a very cold sensation accordingly. Thus, i) Heat can be transferred from one body to another body. ii) Heat flows from a body at a higher temperature to one at a lower temperature. FACTS WITH REASONS The heat flows from a hotter object to a colder objects. Heat flows from a hotter object to a colder object in order to balance the temperature between them. Heat continues flowing until both objects have equal temperature. We feel cool when we put our legs in cold water. We feel cool while putting our legs in cold water because our legs lose heat continuously. Modes of Transmission of Heat We feel warm while sitting in the sunlight. A cup of tea left open for some time gets cold. A steel spoon becomes hot while using it to stir the food being cooked on a stove. These all happen because of the heat transfer taking place through a medium. Heat transfers through solids, liquids and gases. The mode of transmission of heat depends on a medium. Heat can be transferred from one place to another by three different ways. They are conduction, convection and radiation. Mode of Transmission Medium Conduction Solid Convection Liquid and gas Radiation Vacuum Conduction ACTIVITY 2 To show the process of conduction. 1. Take an iron rod of about 25cm length. 2. Fix thumb pins along the length of the rod, using wax, as shown in the figure. 3. Fix one end of the rod on a stand and heat another end of the rod as shown in the given figure. Do you observe that the pins start to fall off after some time? As the rod gets heated, the heat gets conducted through the rod to the wax. The wax melts and the thumb pins fall off. The thumb pin closest to the end of the rod being heated falls first. The farthest thumb pin falls at last. Direction in which heat travels Radiation Conduction Convection Transmission of heat

176 Heat Heat transmits along the length of the rod from the hot end to the cold end without actual movement of the molecules in it. The process of transfer of heat in solids from one molecule to the neighboring molecule without their actual movement is called conduction. Mechanism of Conduction When a metallic rod is heated then its molecules start vibrating faster. These molecules at the heated end hit the nearby molecules and make them vibrate too. During this process they give heat to the nearby molecules. These molecules again give heat to the next closest molecules. In this way the heat gets conducted from the hot end to the cold end. Heat Heat Heat FACTS WITH REASONS The conduction is common in metal. Conduction is common in metal because molecules of metal are closely packed and cannot move easily. A steel cup becomes hot on pouring hot water in it. Molecules of steel cup transfer heat to the neighboring molecules when hot water is added in it. In this way heat transfers throughout the steel cup and it becomes hot. Conditions for Conduction i) The temperature of the objects should not be the same. ii) Two objects at different temperatures must be in contact. Conduction capacity of various objects The property of an object to transfer heat is 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 nonmetals and insulators. Conductors and Insulators of Heat ACTIVITY 3 To show that some materials are good conductor of heat and some are bad conductor of heat. 1. Take a steel spoon, a glass rod, a plastic scale, a wooden stick, etc. and dip one end of each of them in hot water. 2. Then touch the other end of each article with your finger. Do you feel some of them get heated and some do not get heated?

Modern Concept Science and Technology – 8 177 Conductors The materials which allow heat to flow through them are called conductors of heat. The rate of transfer of heat energy is different in different conductors. On the basis of the rate of transfer of heat, conductors are classified as good conductors and bad conductors. i) Good Conductors of Heat The materials which allow heat to flow through them easily are called good conductors of heat. For example, metals like iron, copper, silver and aluminum are good conductors of heat. Most metals are good conductors of heat. Cooking utensils are made of good conductors of heat. FACTS WITH REASONS Iron is good conductor of heat. Iron is good conductor of heat because iron allows heat to flow through it easily when comes in contact with a hot body. A chair made of metal is very cold to touch in winter. Metals are good conductor of heat. In winter, a metal chair is at lower temperature. When we touch it, the heat from our body quickly transmits into it. So, a chair made of metal is very cold to touch in winter. ii) Bad Conductors of Heat Some substances conduct little or no heat. They are called bad conductors of heat. Water, glass, wood, rubber, clay and all gases including air, are bad conductors of heat. Insulators Those materials which do not allow heat to flow through them are called insulators of heat. For example, ebonite, asbestos, etc. are insulators. Handles of cooking utensils are made of insulators. FACTS WITH REASONS Handles of cooking utensils are made of hard plastics. Hard plastics do not allow heat to flow through them as they are insulators. Therefore, handles of cooking utensils are made of insulators. Applications of Thermal Conductivity i) Quilts are filled with fluffy cotton : Air enters inside 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 a 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 hand. MEMORY TIPS An ice cube will soon melt if we hold it in our hand. This is because, the heat is being conducted from our hand into the ice cube.

178 Heat iv) Very hot water poured in a thick glass tumbler cracks it : Glass is a bad conductor of heat. When very hot water is poured in 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 surrounding to the rooms. vi) To prevent ice from melting, it is covered with jute and sawdust : Saw dust 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 to gain heat from the surroundings. vii) Woolen clothes are warm : Wool and air are bad conductors of heat. The fine pores in woolen clothes are filled with air. Therefore, the woolen 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. FACTS WITH REASONS Birds sometimes fluff out their feathers on a cold winter morning. Air is a poor conductor of heat. Birds trap air between feathers by fluffing them up. The trapped air provides insulation. It does not let the heat to go out of their body. So, birds sometimes fluff out their feathers on a cold winter morning. 7.1.2 Convection In conduction, the heated molecules do not leave their mean position. But in convention, there is an actual movement of the heated molecules. The process of transfer of heat by the actual movement of molecules in liquids and gases is called convection. For example, hot lighter air moves up and cold air settles down. There are two types of convection. They are: 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. ACTIVITY 4 1. Take a rectangular glass box with two openings ‘A’ and ‘B’, as shown in the given figure. 2. A lighted candle is placed below the opening ‘A’. 3. Take a source of smoke like joss stick and place it above the opening ‘B’. Do you observe the smoke moving in through ‘B’ and moving out through ‘A’? This is due to convection. The lighted candle warms the air in contact with its flame. Which becomes light and goes out of the box through ‘A’. The place of hot light air is occupied by the cold smoke air. This causes the smoke to move in through the opening ‘B’ and move out through the opening ‘A’. Cold air falling down chimney Hot air rising up chimney A B

Modern Concept Science and Technology – 8 179 a) Mechanism of Convection Consider water being heated in a pot as shown in the given figure. The density of water at the bottom of the pot decreases when it gets heated first. These hot molecules with high kinetic energy rise upward while the cold molecules come down to take their place. This causes a flow of water molecules in the pot and the flow is known as the convection current. This convection current transfers heat to the entire mass of water. b) Cause of Wind, Weather and Rain During the day time, air around the earth gets heated by the solar radiation. Hot lighter air moves up creating less pressure region. The colder air from the surrounding occupies the space of hot air. This phenomenon causes the wind and weather. Thus, the movement of wind from more density, more pressure and less temperature area region to less density, less pressure and more temperature region is responsible for wind, weather, thunder and rain. Applications of Convection 1. Ventilators in a Room Ventilators are made on ceiling or near the ceiling on the wall of a room. A ventilator in a room maintains a fresh flow of the 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 into 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. 2. Chimneys In factories, chimneys are fitted to remove undesirable smoke and fumes. The light smoke, fumes and hot gases rise up through the chimneys. 3. 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. 4. 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 the 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. Ventilation Cold door Hot radiator Cold air falling Warm air rising Room heater

180 Heat 5. 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 at 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 convection current and keeps all parts of the refrigerator cool. 6. Land Breeze and Sea Breeze 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 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 the sea. Thus, during the day, cold air above the sea blows towards the land which is called a sea breeze. Fig: Sea breeze Fig: Land breeze Warm air Cold sea breeze day land warmer Sea cooler warm air Sea warmer Cool land breeze night land cooler During the night, the land becomes cold and 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. FACTS WITH REASONS Land breeze occurs at night. Land breeze occurs at night because land is colder than ocean during nights and air always blows from colder place to hotter place. 7.1.3 Radiation In winter, we feel warm when we sit in the sunlight. How does the heat travel from the sun to our body? Such a transmission is neither conduction nor convection. There is nothing in the space between the sun and out of the earth’s atmosphere. The heat from the sun travels through a vacuum in the form of rays called radiation. Thus, the process of transmission of heat without any medium is called radiation. Hence we receive heat from the sun by the way of radiation. Refregirator Radiation

Modern Concept Science and Technology – 8 181 FACTS WITH REASONS Even without any medium, heat from the sun reaches to the earth. Heat from the sun reaches us even without a medium because sunlight is an electromagnetic wave. It does not need medium to travel. Applications of Radiation 1. 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. 2. Choice of clothes White clothes absorb less heat radiation than clothes of other colors. So, we wear white or light-color clothes in summer. On the other hand, we wear black and dark-color clothes in winter. Such clothes absorb more heat and keep us warm. 3. Solar heaters and solar cookers Heat from the sun is used to heat water in a solar water heater. The inner surface of solar cooker is polished with black. The black surface absorbs the heat. 4. Radiators of cars and air conditioners: The black coating on radiators of cars and air-conditioners absorbs maximum heat radiation and produces the cooling effect. FACTS WITH REASONS The people wear white clothes in summer season. People wear white clothes in summer season because white clothes reflect heat and keep us cool. Conduction, Convection and Radiation Parameter Conduction Convection Radiation Medium Needs a medium Needs a medium Does not need a medium Particles movement Particles in the medium do not leave their mean position. Particles of the medium leave their mean position. Particles of the medium are not involved Speed It is a slow process. It is faster than conduction It takes place at a speed of light. MEMORY TIPS Black bodies are good absorber and good radiator of heat. Shining bodies are poor absorber and poor radiator of heat.

182 Heat 7.1.4 Waves What do you see on a still water surface when a stone is dropped in it? Do you see the ripples on the surface of water? When waves propagate through water, the molecules of water vibrate up and down at their respective positions. The ripples carry energy from stone to all parts without the actual movement of the water molecules from one point to another. A wave is the periodic disturbance in a medium that carries energy from one place to another. Waves transport energy but not matter from one region to another. When a stone is dropped in water, its kinetic energy displaces the water molecules around it. Each molecule of water displaces the next molecule near it. This process continues until all the energy gets transferred. So waves transport energy but not matter from one region to another. FACTS WITH REASONS Types of Waves a) Classification Based on the Requirement of a Medium for the Wave Propagation Mechanical wave The wave that requires a material medium for its propagation is called a mechanical wave. This wave is also called an elastic wave because it depends on the elastic nature of the medium. For example, sound waves, waves set on a stretched spring, etc. The velocity of sound is 332 m/s in air, 1451 m/s in water and 5120 m/s in iron. Sound wave is a mechanical wave. Sound wave carries energy by sharing vibration of molecules in a medium like air, water, steel, etc. Sound needs a material medium for its propagation. It cannot travel through vacuum. So sound wave is a mechanical wave. FACTS WITH REASONS A wave has a frequency 100Hz. What does it mean? A wave has a frequency 100Hz, it means that the wave makes 100 complete cycles in the one second. FACTS WITH REASONS Electromagnetic Waves A wave is a fine path or disturbance that carries energy from one place to another. Waves are of two types, viz. longitudinal wave and transverse wave. Light energy propagates in the form of a transverse wave. It is an electromagnetic wave. The radio wave that comes from a radio station is also an electromagnetic wave. Light wave does not need material medium to propagate. It can travel in vacuum too. Thus, the waves that do not need a material medium Ripples produced in a pond

Modern Concept Science and Technology – 8 183 for their propagation and can travel even through a vacuum are called electromagnetic waves. The gamma ray, X-ray, ultraviolet ray, infrared radiation, microwaves and radio waves are all electromagnetic waves. 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. Properties of Electromagnetic Waves i) The electromagnetic waves are transverse waves. ii) They travel with the speed of light but differ in their frequency and wavelength. iii) These waves have oscillating electric and magnetic fields. iv) They are not affected by electric and magnetic fields. v) They obey the laws of reflection and refraction. 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 reflect almost all of the radiation. So, people wear white or bright shirts in summer. 7.1.5 Thermos Flask Thermos flask is a special kind of bottle which prevents all possible ways for the flow of heat to and from the bottle. In a thermos flask, hot liquids like tea, milk, etc. remain hot and cold items like ice cubes, cold water, etc. remain cold for a long period of time. A thermos flask is also called a vacuum flask. Construction of a Thermos Flask A thermos flask consists of a double-walled glass bottle. The air between the two layers is evacuated and the two layers are sealed. This double walled bottle is fixed inside a plastic or metallic casing for its protection. Walls of the bottle are polished to look silvery. These shining polished surfaces reduce the heat loss by radiation. The glass walls are bad conductors of heat. Hence the flow of heat is reduced by conduction. Also the vacuum in between glass walls reduces the heat losses due to conduction and convection. Thermos without reflective lining Thermos with reflective lining Conduction Conduction Convection Radiation Radiation Reflective lining Convection Thermos flask MEMORY TIPS Sir James Dewar first invented a thermos flask in 1892. That is why a thermos flask is also called Dewar flask.

184 Heat Body Parts of A Thermos Flask Reduced Mode of Heat Exchange Reason Plastic cap Convection, evaporation It prevents hot air from leaving the thermos flask Double walled bottle with vacuum Conduction, convection There is no matter to exchange heat in a vacuum. Silvery walls Radiation These walls reflect the heat radiation passing through the vacuum Advantages of a Thermos Flask In a thermos flask, all ways of heat loss viz. conduction, convection and radiation are highly reduced. So a thermos flask keeps the hot object hot and the cold object cold inside it for a long time. It is used to store tea, coffee, soup, warm water etc. The ice box have double wall. Ice box have double wall because air trapped inside the double wall, acts as insulator and stops heat from reaching to ice. Ice will not melt sooner. FACTS WITH REASONS 7.1.6 Greenhouse Effect The atmosphere of the earth is composed of different gases like nitrogen, oxygen, carbon dioxide, water vapour, inert gases, etc. Among these gases, some gases absorb large amount of sun’s heat while some absorb less. Gases like carbon dioxide, methane, water vapour, oxides of nitrogen, etc. that absorb large amount of heat are called greenhouse gases. These gases allow the short wavelength of solar radiation to enter into the earth’s atmosphere. But, when they reflect back from the surface, they lose energy and are emitted in the form of long wavelength. The greenhouse gases trap most of the heat and do not allow them to escape from the earth back to the sky. As a result, the trapped heat increases the temperature of the earth. This phenomenon of trapping of solar heat of longer wavelength inside the earth’s atmosphere is called greenhouse effect. Ozone layer also prevents solar energy from escaping. Hence, the earth works as a natural greenhouse. Due to the greenhouse effect, the overall temperature of the earth is increasing. This is called global warming. Global warming has caused the melting of polar ice, rise in sea level, increase in the temperature of Himalayan region and other numerous environmental effects. According to a report of Ministry of Science, Technology and Environment of Nepal, the temperature of Nepal is increasing by 0.6 degree Celsius every decade. MEMORY TIPS Vacuum flasks are often used as insulated shipping containers. Greenhouse effect

Modern Concept Science and Technology – 8 185 7.1.7 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. Importance of greenhouse effect Greenhouse effect is advantageous to the earth and life on it. It maintains the temperature of the earth’s surface to an average of about 150 C. In the absence of greenhouse gases, the temperature of the earth would be very low i.e. up to -180 C. There would be no life in such cold environment. But, excessive greenhouse effect is bad. The human activities have increased the levels of greenhouse gases higher than normal. This has resulted in the warming of the earth’s surface and oceans. Greenhouse effect is important for us. Greenhouse effect is important for us because it keeps the earth warm, prevents global freezing and supports life. FACTS WITH REASONS Adverse impacts of greenhouse effect Greenhouse effect itself does not adversely affect the environment. It is mostly advantageous. But, when the trapping of the solar rays is higher than the usual, it causes rapid global warming, that affects the environment. Some of its major impacts are listed below: i) It causes global warming and climate change. ii) Rate of melting of polar ice, ice-capped mountains and glaciers increases. iii) Sensitive organisms like the fishes and amphibians are affected due to global warming. iv) It forces native animals to change their natural habitat and migrate to new habitats. v) It also results in the increase in sea level and submerges the low lying lands, islands and sea shores. vi) It creates favorable environment for the growth and development of disease causing germs and insects. Ways of controlling greenhouse effect i) Discouraging the use of refrigerators, air coolers, body sprays, etc. that contains greenhouses gases like CFCs can reduce greenhouse effect. ii) Afforestation should be done on a regular basis. The trees absorb a large amount of CO2 from the atmosphere. Carbon dioxide is a major greenhouse gas. iii) The conventional sources of energy like coal and fossil fuels should be replaced by cleaner sources of energy like solar power, hydro-power, wind energy, tidal energy etc. Artificial greenhouse

186 Heat iv) Using improved cooking stoves (ICS) instead of traditional stoves, bio-gas instead of natural gas, hydro-power instead of coal plants, etc. v) Following energy friendly habits like use of public vehicles, mass transport system, short distance walking, cycling, etc. which decreases the consumption of fossil fuels and finally reduce greenhouse gas emission. Long wave radiation does not escape easily from the earth’s surface. Long wave radiation does not escape easily from the earth’s surface because they are absorbed by the greenhouse gases of the atmosphere but released only in a low quantity out in the sky. Most of the radiation is transferred back to the earth’s surface. FACTS WITH REASONS HOT SKILL HIGHER ORDER THINKING SKILL 1. Sun is very far away but its heat can reach to us. Even though the sun is far away, heat from sun can reach to us by radiation. 2. We should wear white dresses in summer. We should wear white dresses in the summer because white can reflect most of sunlight and heat. 3. What are the differences between mechanical wave and electromagnetic wave? The differences between mechanical waves and electromagnetic waves are: SN. Mechanical waves SN Electromagnetic waves 1 The wave that requires a material medium for its propagation is called a mechanical wave. 1 The waves that do not need a material medium for their propagation and can travel even through a vacuum are called electromagnetic waves. 2 They do not cast shadow. 2 They cast shadow. 4. Miss Sabina put some coffee in the Thermos flask and started to do homework. After an hour she poured some coffee in the cup and found that it is still warm. She is wondering why is it still warm after an hour? Clarify it for her. She put some coffee in the Thermos flask and was surprised to see it still warm after an hour. First of all thermos flask is a cylinder which has two walls. The inner wall is so smooth in the inside that it reflects all the heat into the coffee. The vacuum between the glass layers prevents convection. Its smooth surface reduces heat loss by radiation, convection and conduction. So any liquid stored in the thermos flask can remain warm for many hours. 5. The days are hot in summer so are nights. Mr. Kailash is wondering why is it so hot even at night although there is no sun light. Explain the reasons to him in simple words. Most people wonder why is it hotter at nights even though there is no sun. Mr. Kailash is also wondering about it. The short answer to the curiosity is green house effect.

Modern Concept Science and Technology – 8 187 The atmosphere is made up of gases. It contains green house gases too. Carbon dioxide, water vapour and methane are some common green house gases. They can trap solar heat energy during the day and release at night. Therefore the nights remains warm even if there is no sun. 6. Manisha was making tea. She dissolved some sugar with the spoon. She left spoon in the tea and went to get more sugar cubes. When she hold the spoon again, it was warm. Why did it get warm? Manisha used spoon to dissolve sugar in the warm tea and left it there and went to find more sugar. Since spoon is a metal, it can absorb heat and transfer it to the upper parts by the conduction. So the stem of spoon gets warmer. Thats why it was warm when she held it later. 7. Dengue virus and its vector aedes aegypti mosquito lives in warm tropical climate. However, dengue pandemic is frequent in sub-tropical regions of Nepal? What could be the cause? Mention other problems created by that cause. Dengue is a tropical disease. Its cases are growing in sub-tropical regions of Nepal. It means the hilly regions of Nepal are getting warmer. The only cause could be climate change caused by over green house effect. Some other problems to Nepal due to over green house effect and climate change are listed below: i. The snow from the mountains are melting faster. As a result there is lack of water in Himalayas and flood in Terai. ii. Insects, germs, animals and crops from tropical regions are spreading to the colder regions. iii. There is disturbance in water cycle, weather pattern and climate. iv. Agriculture is disturbed. Production has decreased. 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answer from the given alternatives. a) By which mode does heat transmit through an iron rod? i) conduction ii) convection iii) radiation iv) conduction and convection b) What is the term for the mode of transmission of heat by movement of molecules? i) conduction ii) convection iii) radiation iv) conduction and convection c) What is the transfer of heat from molecule to molecule, without the movement of molecules from one place to another called? i) conduction ii) convection iii) radiation iv) conduction and convection d) What is a thermos flask?

188 Heat i) a device to reduce heat loss ii) a device to produce heat iii) a device to transfer heat iv) a device to lose heat e) Which one of the following is a green house gas? i) oxygen ii) nitrogen iii) carbon dioxide iv) helium 2. Write True for the correct and False for the incorrect statements. a) Heat is a form of energy. b) Heat always transfers from a body at a low temperature to a body at a high temperature. c) Heat transmits by conduction in a solid substance. d) We should increase the use of fossil fuel to minimize the green house effect. e) Heat transmits by radiation in vacuum. 3. Fill in the blanks using a suitable word. a) Heat transmits by convection in …… and ……. b) The heat energy from the sun reaches the earth due to the process of ……. . c) Mercury is the metal which gets heated by the process of ……. d) The process of increase in the temperature of the earth due to trapping of solar energy is called ............ e) A thermos flask is also called ............ 4. Answer in one word. a) What happens to the volume of a liquid on heating it? b) In how many ways does heat transmit? c) What is the cause of wind in the earth? d) What are the substances that can transfer heat called? e) What is used to trap heat to maintain suitable temperature for growing off seasonal vegetables? STEP2 5. Give reasons: a) On touching an object we feel cold in winter season and hot in summer season. b) Handle of a metal pot becomes hot during cooking. c) Air blows from one place to another. d) Birds sometimes fluff out their feathers on a cold winter morning. e) Convection is not possible in solids. f) Ice box has a double wall. g) Water remains warm for longer duration in thermos flask. h) Earth do not freeze even in winters. i) It is warm inside the artificial green house than the surrounding outside. j) We should reduce the use of fossil fuel.

Modern Concept Science and Technology – 8 189 6. Differentiate between a) Heat and temperature b) Conduction and convection c) Natural green house and artificial green house d) Electromagnetic waves and mechanical waves 7. Answer the following in short. a) What is heat? Write its SI unit. b) What are the sources of heat? Give examples. c) What is transmission of heat? What are its modes? d) Define conduction, convection and radiation with examples. e) What are conductors and insulators? Give examples. f) What are green house gases? Give examples. g) Define green house effect. With its types. STEP3 8. Answer the following a) Explain the mechanism of convection of heat. b) How do convection currents in air help in ventilation? c) Explain the construction of a thermos flask with its use. d) Explain how heat loss due to conduction, convection and radiation are minimized in a thermos flask? e) Discuss the application of conduction, convection and radiation. f) Draw the diagram to show the land breeze and sea breeze. g) Describe the internal structure of a thermos flask with a diagram. h) What is artificial green house? Discuss its importance. i) What kind of harmful impacts of over green house effect are observed in our country and what can we do to tackle this problem? Discuss. j) Which mode of transmission of heat is shown in the given figure? Direction in which heat travels

190 Light Key terms and terminologies of the unit 1. Light : Light is a form of energy which makes objects visible to us. 2. Ray of light : The narrow path of the light which is represented by a straight line with an arrow 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. Reflection of light : The process of returning light to the same medium after striking a surface is called reflection of light. 5. Regular reflection of light : When a parallel beam of light, coming from a source, strikes a surface and reflects in a parallel way, such type of reflection is called regular reflection of light. 6. Irregular reflection of light : 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. 7. Image : When a number of rays, starting from a point after reflection or refraction, meet together, then image is formed. 8. Real image : The image which is formed by actual meeting of two or more rays after reflection or refraction is called a real image. 9. Virtual image : The image which forms without actual meeting of two or more rays after reflection or refraction is called virtual image. 10. Mirror : A smooth polished surface from which a regular reflection takes place is called mirror. Sequence of Curriculum Issued by CDC  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 UNIT Light 7.2 Estimated teaching periods Theory Practical 6 1 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. Willebrord Snellius About the Scientist

Modern Concept Science and Technology – 8 191 11. Plane mirror : The mirror which has flat and smooth reflective surface is called a plane mirror. 12. Lateral inversion : The phenomenon due to which the image of an object turns through an angle of 1800 about a vertical axis is called lateral inversion. 13. Spherical mirror : The mirror which is a part of a glass sphere is called a spherical mirror. 14. Concave mirror : In concave mirror, the polishing or silvering is done on the outer surface of the cut part of a hollow sphere. 15. Convex mirror : In convex mirror, the polishing or silvering is done on the inner surface of the cut part of a hollow sphere. 16. Pole : The geometric center of a spherical mirror is called pole. 17. Center of curvature : The center of the hollow glass sphere of which the spherical mirror is a part is called center of curvature. 18. Radius of curvature : The radius of a hollow glass sphere of which the spherical mirror is a part is called radius of curvature. 19. Principal axis : The line passing through the center of curvature and pole of the mirror is called principal axis. 20. Principal focus : The point on the principal axis at which all rays, parallel to the principal axis meet or appear to meet after reflection from a mirror is called principal focus. 21. Focal length : The distance between pole and principal focus of a spherical mirror is called focal length. Introduction We cannot see objects in a dark room. However, if we switch on a bulb, the objects become visible. When light falls on the objects, it bounces from their surfaces and enters into our eyes. As a result, image of the objects forms on the retina of the eyes and we can see objects present in the room. Thus, light is a form of energy which makes objects visible to us. It means that objects are not visible without light. The study of light and vision is called optics. Light travels at a speed of 3 × 108 m/s in vacuum. In this unit we will discuss about the reflection of light from a plane reflective surface and spherical reflective surface. We will also discuss about the phenomenon of bending of light when it passes from one optical medium to another optical medium. 7.2.1 Source of Light Those bodies which emit light are called sources of light. For example, the sun, electric bulb, lamp, etc. The sources of light are of two types. They are self-luminous and non-luminous. Light that enters into our eyes can come directly from a self-luminous object or it may be the reflected light from a non-luminous object. Selfluminous sources of light emit their own light. For example, the sun, stars, firefly, etc. On the other hand, objects like moon, rocks, plastics, glass, etc. do not emit their own light. They are called non-luminous objects. Source of light

192 Light 7.2.2 Ray of Light The narrow path of the light which is represented by a straight line with an arrow is called a ray of light. The arrow head of a ray gives its direction. Rays are produced when light passes through a small hole. 7.2.3 Beam of Light You might have seen a beam of sunlight when it enters into a room through a narrow opening or a hole. A collection of rays of light in a certain pattern is called a beam of light. A beam of light is produced when light passes through a larger collecting hole. There are three types of beam of light. They are: a) Convergent beam b) Divergent beam c) Parallel beam 7.2.6 Image When a number of rays, starting from a point after reflection or refraction, meet together or appear to diverge from a point then image is formed. There are two types of images. They are: Real Image The image which is formed by actual meeting of two or more rays after reflection or refraction is called a real image. A real image is inverted. It can be obtained on a screen. Virtual Image The image which is not formed by actual meeting of two or more rays after reflection or refraction is called a virtual image. When reflected or refracted rays appear to meet, a virtual image is formed. A virtual image is erect and it cannot be obtained on the screen. Differences between real image and virtual image. S.N. Real image S.N. Virtual image 1 Real image is formed when the reflected or refracted rays intersect at a point. 1 Virtual image is formed when the reflected or refracted rays appear to meet. 2 Real image is always inverted. 2 Virtual image is always erect. 3 It can be obtained on a screen. 3 It cannot be obtained on a screen. ACTIVITY 1 Put a lighted candle in front of a flat mirror. You will see the similar candle in the mirror. The candle which appears in the mirror is the image of the candle which is kept in front of the mirror. Is that image erect? Can you bring that image on a paper behind the mirror? A ray of light Parallel beam of light Convergent beam of light Divergent beam of light

Modern Concept Science and Technology – 8 193 7.2.7 Mirror A smooth polished surface from which a regular reflection takes place is called mirror. Characteristics of mirror a) The surface of mirror shines. b) It forms an image due to regular reflection. Types of Mirror There are two types of mirror. They are plane mirror and spherical mirror. 1. Plane Mirror We look our face on a plane reflective surface. It is called a plane mirror. Thus, the mirror which has flat and smooth reflective surface is called a plane mirror. Polished surface Reflective surface Mirror Reflection on a mirror ACTIVITY 2 Stand in front of a plane mirror and look at your image. Raise your right hand. Which hand does your image raise? Move away from the mirror and observe the movement of your image. Does the image move backward? Is there any difference in the shape and size of your image? Observe clearly. Observation : In the plane mirror, you will find that 'right' appears left'. Object distance and image distance are equal. The image formed by a plane mirror has the same size to the object. Characteristics of the image formed by a plane mirror i) The image formed by a plane mirror is laterally inverted. ii) Image is virtual. iii) It is erect. iv) Its size is equal to the object. v) The distance between image and mirror is equal to the distance between object and mirror. Image formed by a plane mirror

194 Light Applications of a plane mirror i) A plane mirror is commonly used as a looking glass in our daily life. ii) Plane mirrors are used as reflectors in optical instruments like periscope, kaleidoscope, etc. 2. Spherical Mirror Two curved pieces are obtained when a hollow glass sphere is cut as shown in the given figure. If the silver is polished on the outer side then the reflective surface is towards the hollow side and acts like a spherical mirror. Similarly, if the silver is polished on the inner side then the reflective surface is towards the outer side and acts like a spherical mirror. The nature of image formed by these spherical mirrors is different. Thus, the curved mirror made by polishing either the inner curved side or the outer curved side of a cut portion of the hollow glass sphere is called a spherical mirror. Types of spherical mirrors Spherical mirrors are of two types. They are concave mirror and convex mirror. i) Concave mirror : In a concave mirror, the polishing or silvering is done on the outer surface of the cut part of a hollow sphere. In this mirror, the inner surface is reflective surface. It is also called a converging mirror. Concave mirror is also called a converging mirror. After reflection, the reflected light rays from a concave mirror meet at a point. So, the concave mirror is also called a converging mirror. FACTS WITH REASONS ACTIVITY 3 Take a concave mirror. Hold it facing the sun. Focus the reflected light on the paper. Adjust the distance between the paper and the mirror until you get a sharp bright spot. ii) Convex mirror : In convex mirror, the polishing or silvering is done on the inner surface of the cut part of a hollow sphere. In this mirror, the outer surface is a reflective surface. It is also called a diverging mirror. MEMORY TIPS The phenomenon due to which the image of an object turns through an angle of 180° about a vertical axis is called lateral inversion. Due to lateral inversion, the left side of an object looks right in the image and vice C versa. onvex Concave Hollow sphere Reflecting surface B B P P C A A Spherical Mirrors MEMORY TIPS The mirror which is a part of a glass sphere is called spherical mirror. P Polished surface C Reflecting surface Concave Mirrors Convex Mirrors P C Reflecting surface Polished surface

Modern Concept Science and Technology – 8 195 Convex mirror is also called a diverging mirror. After reflection, the reflected light rays from a convex mirror diverse in different direction. So, the convex mirror is also called a diverging mirror. FACTS WITH REASONS 7.2.8 General Terms Used in Spherical Mirrors Pole The geometric center of a spherical mirror is called pole. It is denoted by the letter 'P'. Center of curvature The center of the hollow glass sphere of which the spherical mirror is a part is called center of curvature. It is denoted by the letter 'C'. Radius of curvature The radius of a hollow glass sphere of which the spherical mirror is a part is called radius of curvature. It is denoted by the letter 'R'. Principal axis The line passing through the center of curvature and pole of the mirror is called principal axis. Principal Focus a) Principal focus of a concave mirror A point on the principal axis of a concave mirror at which a parallel beam of light with principal axis meet after reflection is called the principal focus of a concave mirror. It is denoted by the letter 'F'. b) Principal focus of a convex mirror A point on the principal axis of a convex mirror from which a parallel beam of light with principal axis appear to diverge is called principal focus of a convex mirror. It is denoted by the letter 'F'. Focal length The distance between pole and principal focus of a spherical mirror is called focal length. It is denoted by 'f'. Relation between radius of curvature and focal length of a spherical mirror Focal length (f) = Radius of curvature 2 = R 2 MEMORY TIPS The point on the principal axis at which all rays parallel to the principal axis meet or appear to meet after reflection from a mirror is called principal focus. F P C Principal focus of a concave mirror F P Principal focus of a convex mirror MEMORY TIPS Focal length in mirror = half of its radius of curvature

196 Light 7.2.9 Image Formation by the Spherical Mirrors Image formation by a concave mirror ACTIVITY 4 Take a stainless steel spoon. Look your image in the inner side of the spoon. Do you find your image inverted? ACTIVITY 5 Fix a concave mirror on a stand and place it on a table. Keep a lighted candle on the table in front of the mirror. Try to obtain the image of the flame on a paper as shown in the given figure. Move the position of the candle towards and away from the mirror and obtain the corresponding images. Observe the nature of image in each case. Rules to draw the image formed by a concave mirror i) A ray of light which is parallel to the principal axis of a concave mirror passes through its focus after reflection. ii) A ray of light passing through the center of curvature of a concave mirror is reflected back along the same path. iii) A ray of light passing through the focus of a concave mirror becomes parallel to the principal axis after reflection. P F C P F C P C F i ii iii Rules to draw the image formed by a concave mirror Images formed by a concave mirror a) When the object is at infinity (distant object) i) The image is formed at F ii) It is real iii) It is inverted iv) It is highly diminished b) When the object is beyond centre of curvature i) The image is formed between F and C ii) It is real iii) It inverted iv) It is diminished