Modern Concept Science and Technology 10 New

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 197 upthrust (U) weight (W) upthrust (U) weight (W) upthrust (U) weight (W) a) b) c) a. When W> U: The body sinks down b. When W = U: The body floats freely c. When W < U: The body floats in the condition of being partially immersed. Verification of the Law of Flotation According to the law of flotation, "The weight of the liquid displaced is equal to the weight of a floating body". The above statement can be verified with the help of a given experiment. Take an overflow can and pour water upto the level of spout. Place a beaker under the spout. Similarly, take a wooden block and measure its weight. Let it be W1 Now, immerse the block in the overflow can. It displaces water which overflows through the spout and collect in the empty beaker. Now, weigh the displaced water. Let it be W2 . Finally, you will get that weight of the floating body = weight of the liquid displaced ∴ Wt. of the floating body = Wt. of the liquid displaced Some Applications of the Principle of Floatation FACTS WITH REASONS A balloon filled with hydrogen flies up to the sky. Why? Hydrogen is the lightest gas. In case of hydrogen filled balloon, the weight of the displaced air is more than its own weight. The upthrust of the air on the balloon is more than the weight of the hydrogen filled balloon. So, the hydrogen filled balloon flies up to the sky. i) Rising of a hot-air balloon A hot air balloon consists of a large bag called envelope, with a wicker basket suspended underneath. In a hot air balloon, the air inside the envelope is heated with the help of a high pressure stove. The density of hot air in the balloon is less than the cold air outside it. When hot air is filled in a balloon, then the weight of the air displaced by the balloon becomes more than its own weight (i.e upthrust > weight). The net upward force acting on the balloon is more than its own weight, and hence the balloon rises up. Hot air balloon

198 PReS SuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur As the balloon rises up, the density of air decreases. Here, the weight of the air displaced by the balloon also becomes less. Ultimately, when the weight of the air displaced becomes equal to the weight of the balloon, it does not rise any further. Hence, the balloon becomes stationary at a certain height. FACTS WITH REASONS A balloon filled with hydrogen flies up to the sky. Why? Hydrogen is the lightest gas. In case of hydrogen filled balloon, the weight of the displaced air is more than its own weight. The upthrust of the air on the balloon is more than the weight of the hydrogen filled balloon. So, the hydrogen filled balloon flies up to the sky. ii) Floating and sinking a submarine Submarine is a watercraft which can be made to float or sink in water. Floating and sinking of a submarine is controlled by upthrust on it. a) When water is admitted in the tanks of a submarine, its weight becomes more than the upthrust and the submarine sinks in water. b) When water is ejected out from the tanks of a submarine, its weight becomes less than the upthrust and the submarine floats on the surface of water. iii) An iron ship keeps floating but an iron nail sinks in water A ship made from iron has a concave shape. It is capable of displacing more volume of water. In this condition, the weight of water displaced is equal to the weight of the ship. So, a ship made from iron floats in water but a solid piece of iron sinks in water. vi) Floating iceberg Ice has less density than water. A big piece of ice can displace water equal to its weight. So, ice floats on sea water with a small portion exposed. v) Life belt Life belt is a big hollow tube of rubber. When air is filled in it, its volume increases. If a man wearing life belt jumps in water, then the weight of the water displaced becomes more than the weight of the man and belt. Thus, the man remains safe from being drowned. iv) While swimming While swimming, a good swimmer can displace water nearly equal to his weight. Because of this, a swimmer can swim easily in water. If a person is unable to displace water equal to his weight, he/she cannot swim and drowns in water. v) Hydrometer Hydrometer is an instrument which is used to measure relative density of the liquid. It Submarine Floating iceberg Life belt

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 199 has a heavy bulb filled with mercury or lead shot and narrow and long stem with calibration. It works with the principle of law of floatation. Its bulb sinks more in low density liquid and sinks less in high density liquid. Due to heavy bulb and light stem, the hydrometer remains vertically upright in the liquid. Lactometer is a special type of hydrometer which is used to measure purity of milk. FACTS WITH REASONS An iceberg made up of water floats in water, why? The density of ice (0.917 g/cm3 ) is less than that of water (1 g/cm3 ). Iceberg is able to displace water equal to its weight. Therefore, iceberg floats in water. A ship can carry more loads on sea water than on the river water, why? The density of salty water (1.026 g/cm3 ) is more than that of fresh water (1.00 g/cm3 ). So, sea water offers more upthrust to the ship than that of river water. As a result, a ship can carry more loads on sea water than on river water. Differences between Archimedes’ Principle and Law of Flotation. S.N. Archimedes’ Principle S.N. Law of Flotation 1. When a body is partially or wholly immersed in a liquid, the upthrust on the body is equal to the weight of the liquid displaced by it. 1. A floating body displaces liquid equal to its weight. 2. It applies equally to floating as well as sinking bodies. 2. It applies only to the floating bodies. HOT SKILL HIGHER ORDER THINKING SKILL 1. It is easier to swim in ocean water than in river water. It’s easier to swim in ocean water than in river water because ocean water has more upthrust due to its higher density because of the presence of salt which is absent in river water. 2. Air offers upthrust similar to the water, but a piece of wood cannot float in air but floats in water. Air offers upthrust similar to water but a piece of wood cannot float in air but floats in water because upthurst is directly proportional to the density of the fluid. Density of air is less than density of wood but density of water is more than density of wood. So, air cannot produce sufficient upthrust to carry wood due to lesser density but water can due to its higher density. 3. Study the given diagram and answer the following questions. i. 2 kg of water is displaced when 5 kg mass was introduced in the water. How much is the upthrust of the water acting on the 5 kg mass? Mass of the liquid displaced = 2 kg. Acceleration due to gravity (g) = 9.8 m/s2 Lactometer

200 PReS SuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Weight of the liquid displaced = mass of the liquid displaced x acceleration due to gravity = 2 × 9.8 = 19.6 N Upthrust acting on 5 kg mass =? Now, Upthrust= weight of the liquid displaced. = 19.6 N ii. What will be the weight of the solid inside the water and in the air? The mass of the object is 5 kg. Acceleration due to gravity (g) = 9.8m/s2 Upthrust of water = 19.6 N Weight of the object in air =? Weight if the object in water =? Now Weight of the object in air = mass of the object x acceleration due to gravity = 5 kg × 9.8 m/s2 = 49 N Again Weight of an object in water = weight in the air – upthrust = 49 N – 19.6 N = 29.4 N iii. Which law is the above experiment based upon? This experiment is based on Archimedes’ principle. 4. It is very difficult to submerge a football in water. It is very difficult to submerge a football in water because it is filled with air. Its density is too less than the density of the water. When we push it under water it can push very large weight of water than its own weight. It experiences a greater upthrust. So, its difficult for us to push it into the water. 5. Is it possible for objects denser than water to float? Describe the conditions required for that case. Yes, it is possible for the objects denser than water to float. For example, a steel plate, an iron pin, and a ship made of iron float in water. A pin carefully put in water floats because of surface tension. A steel plate put flat on water floats because it can displace water equal to its own weight. A ship made from iron can float in water because it is hollow inside, which decreases its apparent density below the density of water. It is streamlined so it can displace water equal to its weight. So, it can easily float in water. 6. A steel plate floats in water but spoon sinks. What could be the reason? According to the law of floatation, an object can float only if it can displace liquid equal to its total weight. So, a steel plate can float on water because it is shaped in such a way that it can displace water equal to its. However, steel spoon sinks easily because it is narrow and cannot displace the water equal to its weight. 7. How do hot air pilot do to ascend or descend the hot air balloon? Hot air balloon flies by changing the density of the air in it. The hot air balloon pilot can change the size of flame to change the temperature of the air inside the balloon. If flames are bigger, temperature of air rises. Density of hot air decreases. Then the balloon ascends. If flames are smaller, temperature of air decreases. Density of cold air increases. Then the balloon descends.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 201 8. Compare between ascending hot air balloon and descending hot air balloon. The comparison between ascending hot air balloon and descending hot air balloon are: SN Ascending hot air balloon SN Descending hot air balloon 1 It has higher temperature. 1 It has lower temperature. 2 It has lower density of air. 2 It has higher density of air. 3 The flame is made bigger for ascending. 3 The flame is made smaller for descending. 9. What is hydrometer? What does it mean if it sinks more in a liquid? Hydrometer is an instrument which is used to check relative density of a liquid. It has a heavy bulb and narrow and long stem.If its stem has sunken more in a liquid, it means the density of liquid is less. 10. What happens to the weight of water displaced, if more load is added in the ship? 59 people died in Philippines in 2015 when ship capsize carrying 200 persons instead of its capacity of 141 people. Why does probability of a ship capsizing increase if it is overloaded? A ship can float by displacing water equal to its weight. So, if more load is added in the ship, it sinks more and displaces greater weight of liquid so that it can keep floating. If too much load is added, the ship keeps sinking and eventually drown. The probability of ship capsizing increases if it is overloaded because ship has to displace liquid equal to its total weight to float. If it is overloaded it has to sink more. So, there is a chance that it will drown and capsize. 3 STEPS 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answers from the given alternatives a. Why are there studs under the football shoes? i. to make shoe fashionable ii. to prevent slipping iii. to increase surface area iv. to decrease friction and pressure b. Which principle can describe submarines? i. Pascal’s law ii. principle of simple machine iii. Law of inertia iv. Archimedes’ principle c. Why does a hydrometer float more in seawater than in freshwater? i. seawater has a lesser density than freshwater ii. freshwater has lesser mass iii. seawater has more density than freshwater iv. the hydrometer floats in water d. On which principle hydraulic garage works? i. Law of floatation ii. Archimedes’ principle iii. pascal’s law iv. Boyles law e. If a 20 N stone, displaces 5 N of water when it is placed in water, what is the upthrust? i. 5 N ii. 15 N iii. 25 N iv. 4 N

202 PReS SuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur f. If area of piston A is 6 units and the area of piston B is 2 units, how much load on piston A can be balanced by 5 N force on piston B? i. 5 N ii. 2.5 N iii. 10 N iv. 15 N 2. Define the following terms with required examples. a. Pressure b. Liquid pressure c. Upthrust d. Archimedes' principle e. Hydraulic press f. 1 pascal pressure g. Hydraulic machines h. Pascal's law i. Law of floatation j. Thrust 3. Answer the following questions in very short. a. Write the SI unit of the following: i. Upthrust ii. Pressure iii. Thrust b. Write down the factors on which the liquid pressure at a point in a liquid depends. c. State: i. Pascal's law ii. Archimedes' principle iii. Law of flotation d. What is a hydraulic machine? e. What are the two forces acting on a body inside a liquid? f. What is the resultant force acting on floating object called? In which direction does it act? g. If a diver displaces water with a weight of 500 N, what is the upthrust on the diver? h. A ship enters the river from the sea. Where does its hull sink more? What does it mean? i. Where does an object have the lowest weight if it is weighed in the air, water and salt water? j. If two spheres of a glass and steel having same volume are put in water, what will be the difference in upthrust experienced by them? STEP2 4. Give reasons. a. The tip of a sewing needle is made sharp. b. School bags are provided with wide straps. c. Nails have a pointed end. d. Deep sea divers have to wear specially designed air-filled suits for their protection. e. Dams are made with a broad base. f. Water comes out more slowly from an upstairs tap than from a similar downstairs tap. g. We feel our palms being pushed up when we dip an open hand in a bucket full of water. h. When we push a volleyball into the water, it comes back on the water's surface. i. Ice floats in water. j. Iron nail sinks in the water while a ship made of iron-steel floats in it.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 203 k. Giant hot-air balloons or hydrogen filled balloons rise up. l. Hot air balloon can ascend or descend by changing the size of flame. m. A boat sinks more in river water than in sea water. n. A hydrogen-filled balloon bursts when it attains a certain height from the earth's surface. o. Brake oil is used in hydraulic brakes of vehicles. p. It is easy to lift a bucket of water only when it is inside the water but gets harder when it is out of the water. q. It is difficult to put football under water than a table tennis ball. 5. Differentiate between the following. a. Force and pressure b. Upthrust and pressure c. Archimedes' principle and law of floatation d. Sinking of iron nail and floatation of iron ship e. Air filled balloon and hydrogen filled balloon released in air f. Flying of balloon when air temperature rise and flying of balloon when air temperature fall 6. Answer the following questions in short. a. Write the properties of liquid pressure. b. Discuss the applications of Pascal's law. c. Write any three applications of a hydraulic press. d. How does a submarine float and sink in water? e. Write down the application of Archimedes principle. f. When a rubber ball having small holes is filled with water and pressed after squeezing it with our hand, it is observed that water comes out with equal force in all directions, why? g. Do you agree with the statement, 'all heavy or denser object sink in water'? Explain why you agree or disagree. h. Write the relation between: i. applied force and pressure ii surface area of contact and pressure iii. upthrust and density of the fluid iv. upthrust and volume of the fluid displaced i. When is the pressure on the ground higher, when a man is standing on a single leg or both legs? Explain. j. If we put an egg in a glass of water ‘A’ and another in a glass of salt solution ‘B’, what will be observed? What happens to the egg if we keep adding salt in glass A? Why? k. An object weighs 30 N. It displaces a volume of water that weighs 25 N. i. What is the upthrust on the object? ii. Will this object float or sink? Explain your answer.

204 PReS SuRe Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur l. Write the conditions for a body which i. sinks ii. floats just below the surface iii. floats partially m. How does a hydrogen-filled balloon fly up in the sky? n. Write down the properties of hydraulic fluid. o. If the weight of an object in air is 750 N and its weight in water is 658 N, what will be the upthrust acting on the body? p. An iron nail sinks in water. Does upthrust act on it? Explain. STEP3 7. Answer the following long questions. a. Explain the basic construction of a hydraulic machine. How does it multiply effort? b. Describe with a diagram the conditions in which a body sinks, floats freely and floats partially immersed in a fluid. c. Explain an experiment to prove Archimedes' principle. d. Observe the given figure and answer the following questions. (Use g = 9.8 m/s2 ) i. How much is the weight of the body in the air? ii. Find out the mass of the stone. iii. How much is the upthrust exerted on the body immersed in water? iv. Calculate the mass of the water displaced. v. In which principle is the experiment based? State the principle. vi. In which condition the weight of the object will be more and why? vii. What will happen in the weight of object if water of the ureka can is replaced by salt water? e. Weight of a stone in the air, water, and salt solution are given in the table shown below. Answer the following questions. i. Identify the mediums A, B and C. ii. Identify the mediums with the highest and lowest density. Give reason. iii. Calculate the upthrust when the stone is immersed in the medium C. iv. Find the mass of the stone (use g = 9.8 m/ s2 . v. Find the mass of the water displaced. [Ans: iii. 4 N, iv. 2.041 kg, v. 0.41 kg] Spring balance Stone Water 5N Beaker Ureka can 20N Medium Weight A 20N B 14N C 16N

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 205 f. What is the resultant pressure exerted upon surfaces A and B in the given diagram and why? Out of surfaces A and B, which surface experiences more force and why? [Ans: 25,000 Pa] 8. Numerical a. Calculate the pressure exerted by a mercury column 76 cm high at its bottom, given that the density of mercury is 13600kg/m³ and g = 9.8m/sec². [Ans: 1.01×105 Pa] b. A ground water tank has the dimension of 5m × 3m × 2m. Calculate the pressure at its bottom when it is completely filled. ( g= 9.8 m/s2 ) [19600 Pa] c. In a hydraulic machine, the cross-sectional area of a narrow cylinder is 80 cm2 and that of the wide cylinder is 8m2 . What load is necessary on the wider piston to balance the 800 N force kept on the narrow piston? [Ans: 8 × 105 N.] d. The area of the cross-section of the large piston in a hydraulic machine is 1 m2 and that of the small piston is 0.5 m2 . Calculate the force that must be applied on the small piston to lift a load of 1000 N on the large piston. [Ans: 500 N] e. If pistons A, B and C of the apparatus given in the diagram are supposed to be frictionless, what is the area of piston B? What force is exerted on piston C? [ Ans: 30 cm2 ,125 N] f. In the given figure, the cross-sectional area of piston ‘A' is 2×10-3 m2 and the cross-sectional area of the other pistons (B, C and D) are 5×10–3 m2 each. If piston ‘A' is pushed inside by 10 N force, what is the pressure exerted in liquid by piston ‘A'? According to Pascal's law, how much force is needed to have the same pressure in the other pistons (B, C and D)? [Ans: 5×103 Pa, 25 N] g. If the weight of a body in the air is 20 N and in a liquid is 10N, find the weight of the displaced liquid and the mass of that liquid. [Ans: 10 N , 1.02 kg] h. Calculate the upthrust on an object immersed in water, if the object displaces water of mass 2 kg. (g = 9.8 m/s2 ) [Ans: 19.6 N] i. A body weights 50N in air and 45N when wholly immersed in water. Calculate i. loss in weight of the body in water. [Ans: 5 N] ii. mass of the water displaced. (g = 9.8 m/s2 ) [Ans: 0.51kg] iii. volume of the body. [Ans: 5.1×10-4m3 ] 20N F2 A B 8 cm2 320cm2 C = 10 cm2 B = 375N 250N A=20 cm2 D B A C

206 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Key terms and terminologies of the unit 1. Heat: The flow of thermal energy from hotter object to colder object is called heat. 2. Thermal energy: The sum of kinetic energy of all the atoms or molecules in a substance is called thermal energy. 3. Temperature: The degree of hotness or coldness of a body is called temperature. 4. Temperature based on kinetic energy:The average kinetic energy of all the atoms or molecules in a substance is called temperature. 5. One calorie heat: The amount of heat required to raise the temperature of 1 gram of water by 10 C is called one calorie heat. 6. Flow of heat: Heat flows from a body at high temperature to a body at low temperature. 7. Thermometer: The instrument which is used to measure the temperature is called thermometer. 8. Principle of a bulb thermometer: Principle of a bulb thermometer states "matter expands on heating and contracts on cooling." 9. Calibration: The process of making a number of equal divisions in between the upper fixed point and lower fixed point of a thermometer is called its calibration. Sequence of Curriculum Issued by CDC  Introduction to thermal energy, temperature and heat energy  Effect of heat on the volume of an object based on the kinetic energy of the molecules.  Introduction to anomalous expansion of water and its application in daily life.  Introduction to specific heat capacity  Simple calculations using the heat equation  Specific heat capacity in daily life  Introduction to thermometer and its types (liquid thermometer, digital thermometer, radiation thermometer)  Working principle and calibration of thermometer UNIT Heat Energy 9 Estimated teaching periods Theory Practical 8 2 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

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 207 10. Clinical thermometer: A thermometer which is used to measure our body temperature is called clinical thermometer. 11. Digital Thermometer: Digital thermometer is a mercury free thermometer which contains thermistor inside the tip to measure temperature. 12. Laboratory thermometer: A thermometer which is used to measure temperature of the different substances in the laboratory is called laboratory thermometer. 13. Specific heat capacity: The specific heat capacity of a substance is defined as the amount of heat required to raise the temperature of 1kg mass of that substance by 1°C. 14. Heat equation: According to heat equation, the amount of heat gained or lost by a body (Q) is equal to the product of its mass (m), specific heat capacity (s) and change in its temperature (dt), i.e. Q = m s dt 15. Calorimeter: A calorimeter is a highly polished cylindrical pot of copper or aluminium which is used to measure the quantity of heat. 16. Calorimetry: The branch of physics which deals with the measurement of heat energy is called calorimetry. 17. IR thermometer: An IR thermometer is an instrument that measures the temperature of a body through the detection of the intensity of thermal radiation. 18. Principle of calorimetry: The principle of calorimetry states, "When a hot body is mixed with a cold body, the heat lost by the hot body is equal to the heat gained by the cold body, provided that no heat escapes to the surroundings". Introduction We use the words 'warm' and 'cold' frequently in our daily life. Summer days are hot and winter nights are cold. We often prefer a cup of hot tea in winter. Similarly, we like to wear woolen clothes and bask in the sun in winter days. As a result of these activities we can make our body warm. Thus, the form of energy which gives us a sensation of warmth is called heat. Heat affects the degree of hotness or coldness of a body. The quantity by which we compare the degree of hotness or coldness of a body is called temperature. In this unit, we will learn the basic concept of thermal energy, heat and temperature. Thermal energy, heat and temperature in Terms of Molecular Motion Thermal energy All kinds of matter are made up of atoms or molecules. These atoms or molecules are in random motion. The random motion of the atoms or molecules is responsible for the kinetic energy of the body. Whenever the kinetic energy of the atoms or molecules in a substance increases, it becomes warmer. So, thermal energy is a form of energy which is due to the motion of the atoms and molecules of the substance. The sum of kinetic energy of all the atoms or molecules in a substance is called thermal energy. For example, a bucket of hot water has more thermal energy than that in a cup of the same water. This is because more molecules means more motion, more kinetic energy and more thermal energy. Temperature The degree of hotness or coldness is proportional to the average kinetic energy of the molecules in a substance. The change in average kinetic energy of the particles causes an effect of change

208 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur in degree of hotness or coldness of a body. Whenever the average kinetic energy of the particles in a substance increases, its degree of hotness also increases. So, the average kinetic energy of all the atoms or molecules in a substance is called temperature. The average speed of molecules in a hot body is faster and the average speed in a cold body is slower. That is why, the higher the temperature of a substance, the faster the motion of its molecules. Heat Take 10 litre of water at 800 C and mix it with 20 litre of water at 200 C. The thermal energy transfers from hot water to the cold water. This transferred thermal energy is called heat. Thus, the resultant thermal energy which is transferred from the body of high temperature to the body of low temperature is called heat. Its SI unit is joule(J) and measured by calorimeter. Heat does not store in a body but it transfers from one body to another body. When two objects are in thermal contact, heat flows from object at higher temperature to the object at lower object until they are at thermal equilibrium (equal temperature). The body where heat transfers will have more thermal energy. Similarly, the body from where heat transfers will have less thermal energy. Factors Affecting Thermal Energy Contained in a Body Molecules in a substance are in constant motion. The liquid molecules have more freedom of movement than those in a solid. Molecules in a gas have the greatest degree of motion. Hence, thermal energy of a body depends upon: i. average kinetic energy of the atoms or molecules in a substance. ii. the number of atoms or molecules of the substance. (i.e., the size or the mass of the body) The total quantity of thermal energy contained by a body is directly proportional to the average kinetic energy (K.E.) of the molecules and mass of the body. i.e. Quantity of thermal energy (Q) ∝ average kinetic energy × mass of the body FACTS WITH REASONS If equal amount of heat is given to two different substances, the change in their temperature is not same, why? Quantity of thermal energy (Q) ∝ average kinetic energy × mass of the body The type of particles is different in different substances. The average kinetic energy is not same when equal heat is given to them. So, if equal amount of heat is given to two different substances, the change in their temperature is the not same. An iron nail becomes hot on frequently hammering it, why? Striking an iron nail with a hammer causes the molecules in the iron nail vibrate faster. It increases the kinetic energy of the atoms of iron nail. So, an iron nail becomes hot when hammered frequently. The air inside the air pump becomes warmer on rapidly compressing, why? The sum of kinetic energy of all molecules in a body is called thermal energy. When air is compressed rapidly, its molecules move faster. As motion of the air molecules increases, the kinetic energy also increases. Increase in kinetic energy results in the increase in temperature. So, the air inside the air pump becomes warmer on rapidly compressing it. When matter gets warmer, the atoms or molecules in the matter move faster. When you warm up by a fire on a cold winter night, you are increasing the molecular kinetic energy in your body. Similarly, steam has more kinetic energy than boil water. MEMORY TIPS

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 209 A case in which more thermal energy but low temperature There are more molecules in a bucketful of lukewarm water than that in a cupful of boiling water. Each molecule has some thermal energy. So, there is more thermal energy in the bucketful of lukewarm water. However, the average kinetic energy of the molecules in bucketful of lukewarm water is less than that in cupful of boiling water. The cupful of boiling water has fewer molecules but moving faster. So, the temperature of bucketful of lukewarm water is less than the cupful of boiling water. Flow of thermal energy: Heat How does an ice cube melt when we put it on our palm? Does it get thermal energy from our palm? Yes, thermal energy goes out of our body and gets added into the ice cube. Thermal energy flows from a body at high temperature to a body at low temperature. This flow continues until two bodies attain a common temperature. The bigger the difference in temperature, the faster the energy flows. ACTIVITY 1 Take three beakers. Fill cold water in a beaker and hot water in another. Measure temperature of water in these beakers separately. Now, mix both in the third beaker and measure final temperature. Do you find the final temperature of water less than that of the hot water and more than that of the cold water? This is due to the flow of thermal energy from the water at high temperature to the water at low temperature. FACTS WITH REASONS We feel hot when we touch a hot body and cold when we touch a cold body, why? We feel hot when we touch a hot body because the thermal energy from the hot body transfers to our body. Similarly, when we touch a cold body thermal energy from our body transfers to the cold body. Differences between Heat and Temperature. Heat Temperature 1. Heat is the resultant thermal energy which is transferred from the body of high temperature to the body of low temperature. 1. Temperature is the average kinetic energy of all the molecules in a substance. 2. Heat is measured by a calorimeter. 2. Temperature is measured by a thermometer. Measurement of Heat: Units of Heat Heat is measured by a calorimeter in the same unit as energy. The SI unit of heat is joule (J). The more common unit of heat is the calorie. More thermal energy but less temperature Less thermal energy but more temperature If you add one calorie heat to one gram of water, you will raise its temperature by 10 C. MEMORY TIPS

210 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur One calorie heat The amount of heat required to raise the temperature of 1 gram of water by 1°C is called one calorie heat. Similarly, 4200 J heat is required to change the temperature of 1 kg pure water by 1°C. Here, 1000 gram water requires 4200 joule heat to change its temperature by 1°C. So, 1 gram water requires 4.2 joule heat to change its temperature by 1°C. That is, 1 calorie = 4.2 joules One kilocalorie Kilocalorie (kcal) is the bigger unit of heat. One kilocalorie is the heat required to raise the temperature of 1 kg water by 1°C. So, one kilocalorie = 1000 calorie = 4200 joules The branch of physics which deals with the measurement of heat energy is called calorimetry. A calorimeter is a highly polished cylindrical pot of copper or aluminium which is used to measure the quantity of heat. Its outer surface is kept shining which prevents the exchange of heat between the pot and the surrounding. Such arrangement is kept in a hollow wooden box. The gap between the pot and the box is filled with non-conductor of heat like, wool, cotton, etc. The stirrer kept in the pot is used to stir the mixture for uniform distribution of heat. MEMORY TIPS Thermometer Stirrer calorimeter Effects of Heat Heat can show the following effects: i. Heat changes the volume of a body. It is also called expansion due to heat. ii. Heat changes the temperature of a body. iii. Heat changes the physical state of a body. iv. Heat changes the solubility of a substance. v. Heat brings about a chemical change. Relation between heat and volume of the substance When we heat a substance, the kinetic energy of the molecules increases. As a result, it makes the molecules move faster and spread out a little more, taking up more space. This makes the substance less dense which is called expansion due to heat. Similarly, when we cool a substance, the kinetic energy of the molecules decreases. As a result, it makes the molecules slow down and move closer together, taking up less space. This makes the substance more denser. Applications of Expansion i. Thermometer is based on the principle that matter expands on heating and contracts on cooling. ii. A jammed glass bottle can be loosened by heating its neck. iii. Gaps are left between two successive rails.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 211 FACTS WITH REASONS Gaps are left between two successive railway tracks. To allow the railway tracks to expand during their heating, a small gap is left between two successive railway tracks while laying on the surface. If it is not done, rails curve on heating which causes accident. In the middle of a day or a hot day in summer, the electric wires sag more, why? Electricity transmission line, and overhead telephone lines suspended from the poles expand on heating. So, in the middle of a day or a hot day in summer, the wires sag more. Some cases where expansion can trouble us i. A thick glass vessel cracks when boiling water is poured into it. ii. Soda glass bottle bursts in very hot weather. iii. Time measured by an ordinary pendulum clock fluctuates in summer and winter. iv. In cold countries, sometimes water pipes burst during winter. FACTS WITH REASONS A thick glass vessel cracks when boiling water is poured into it, why? When boiling water is poured in a thick glass vessel, its inner walls suddenly expand while the outer walls do not. This uneven expansion breaks the glass vessel. Time measured by an ordinary pendulum clock fluctuates in summer and winter, why? In summer and winter the length of pendulum changes due to expansion and contraction. So, time measured by an ordinary pendulum clock fluctuates in summer and winter. Anomalous Expansion of Water Mostly, we see that liquids expand on heating and contract on cooling. But water behaves in a different way. If the water at 0°C is heated, it contracts until the temperature becomes 4°C. But the expansion is observed after 4°C only. The unusual behavior shown by water between 0°C and 4°C is called anomalous expansion of water. Application of anomalous behaiviour of water At 4°C, water has its highest density and lowest volume. This property of water is good for aquatic animals in cold countries. During winter, the temperature of the water in cold countries drops to 4°C. As the top layer of water cools down more, it gets lighter and remains on the surface. The water on the surface slowly cools to 0°C and then freezes, but the water in the pond is always between 0°C and 4°C. At 4°C, water has its highest density. So, the layer of water having 4°C remains at the bottom and above this, the layers of water have 3°C, 2°C, 1°C and 0°C are formed. Once the ice is formed at the top, it helps to trap Fig. Pond in very cold countries

212 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur the heat since it is a bad conductor of heat. As a result of the unusual behaviour of the water expands, aquatic animals can survive in ponds in very cold regions even if the surface of the pond has frozen over. Graphical plot of Anomalous expansion of water Graph of Volume-temperature curve Graph of Density-temperature curve FACTS WITH REASONS In winter the water pipes burst. In winter the water pipes burst because the temperature of water goes below 4°C. Due to anomalous expansion of water, the volume of water increases creating high pressure on the wall of the pipes. If water freezes into ice it will have more volume, exerts greater pressure and the pipe will burst. Measurement of Temperature The instrument which is used to measure temperature is called thermometer. Thermometers are based on the principle that matter expands on heating and contracts on cooling. Thermometer Structure of a liquid thermometer The structure of a thermometer can be described under the following headings. a. Capillary tube : The thermometer consists of a very fine glass tube having a very small bore called capillary tube. b. Stem : The capillary tube is protected by a thick glass tube called stem. c. Bulb : At one end of the capillary tube, a very thin walled glass is provided, called bulb. It is filled with thermometric liquid (mercury or alcohol). d. Calibration (Standard Points): Steam point (i.e. upper fixed point) and ice point (i.e. lower fixed point) are the two standard points fixed on the stem of a thermometer

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 213 to make a thermometric scale. It is called calibration. Thus, the process of making a number of equal divisions in between the upper fixed point and lower fixed point of a thermometer is called its calibration. The temperature at which water boils at standard atmospheric pressure is called the upper fixed point and the temperature at which water freezes at standard atmospheric pressure is called the lower fixed point. The different types of thermometers have different fixed points. For example, in Celsius scale the lower fixed point is 00 C and upper fixed point is 1000 C; in Fahrenheit scale the lower fixed point is 320 F and upper fixed point is 2120 F; similarly, in Kelvin scale the lower fixed point is 273K and upper fixed point is 373K. FACTS WITH REASONS The bulb of a thermometer is made thin-walled, why? The bulb of a thermometer consists of thermometric liquid, either mercury or alcohol. A thin walled bulb helps to conduct heat from outside to the thermometric liquid. So, the bulb of a thermometer is made thin. Types of thermometers based on thermometric liquids On the basis of the thermometric liquid used, there are two types of thermometers. They are mercury thermometer and alcohol thermometer. a) Mercury Thermometer The thermometer in which we use mercury as a thermometric liquid is called mercury thermometer. Advantages of mercury as a thermometric liquid i. Mercury is a good conductor of heat. It needs very little heat to expand. ii. It does not wet the glass. Thus, it helps in recording accurate temperature. iii. Expansion of mercury is uniform over a wide range of temperature. iv. It is opaque and silvery in colour. Thus, it can be clearly seen in the capillary tube. v. Boiling temperature of mercury is 357°C. So, a temperature as high as 357°C can be recorded with the help of a mercury thermometer. Disadvantage of a mercury thermometer i. Mercury is a very expensive liquid metal. ii. Freezing temperature of mercury is -39°C. So, a mercury thermometer can record only upto -39°C temperature. iii. Mercury is a poisonous substance. It becomes dangerous if the thermometer tube is broken. FACTS WITH REASONS A mercury thermometer cannot measure very low temperature, why? The freezing temperature of mercury is -39ºC. It means that it remains in liquid state only upto -39ºC. So, a mercury thermometer cannot measure very low temperature, such as temperature in Arctic and Antarctic region. Thermometric liquid Freezing point Boiling point Mercury -39°C 357°C Alcohol -117°C 78°C By using a mercury thermometer, you can measure a very high temperature but not a very low temperature. MEMORY TIPS

214 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur c) Alcohol Thermometer The thermometer in which we use alcohol as a thermometric liquid is called alcohol thermometer. Advantages of alcohol thermometer i. Melting point of alcohol is -117°C. So, an alcohol thermometer can record a temperature as low as -117°C. Alcohol thermometer can record the temperature in Arctic and Antarctic region. ii. Alcohol expands more than mercury for the same rise or fall in temperature. Thus, an alcohol thermometer can measure temperature more accurately than mercury. Disadvantages of an alcohol thermometer: i. Alcohol should be brightly coloured to use it in thermometer. ii. Boiling temperature of alcohol is 78°C. So, alcohol thermometer cannot measure high temperatures. FACTS WITH REASONS An alcohol thermometer cannot measure the boiling temperature of water, why? Boiling temperature of water is 100°C. Alcohol remains in liquid state only upto 78°C. So, an alcohol thermometer cannot measure the boiling temperature of water. Types of thermometers on the basis of uses On the basis of uses, there are different types of thermometers. They are clinical thermometer, laboratory thermometer, digital thermometer, radiation thermometer, maximum and minimum thermometers, etc. Clinical Thermometer A thermometer which is used to measure our body temperature is called clinical thermometer. It is also called doctor's thermometer. i) Clinical thermometer containing mercury Mercury is used in this clinical thermometer. It has a prismatic shape which helps to magnify the mercury thread inside the capillary tube. There is a bend near the bulb of the clinical thermometer called kink. It prevents the back-flow of mercury. Clinical thermometer is jerked before its use. As a result, mercury returns into the bulb. It ranges from 35°C to 43°C or 94°F to 108°F. FACTS WITH REASONS There is a vacuum in the capillary tube inside the stem of a thermometer, why? The capillary tube in a thermometer is fixed over the bulb filled with thermometric liquid like mercury. When the temperature increases, the level of mercury rises in the capillary tube. The vacuum inside the capillary tube makes possible the rise of mercury level inside the capillary tube. So, there is a vacuum in the capillary tube inside the stem of a thermometer. By using an alcohol thermometer, you can measure a very low temperature but not a very high temperature. MEMORY TIPS Clinical thermometer

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 215 ii) Digital Thermometer Digital Thermometer is a mercury free thermometer which contains thermistor inside the tip to measure temperature. Nowadays, digital clinical thermometers are used widely to measure our body temperature. They provide highly accurate result of the body temperature. It reads temperature when the built-in sensor is kept in contact with the human body. They are equipped with beep alarm and memory function. This helps to record a wide range of temperature. A digital thermometer has following advantages over the mercury-used clinical thermometer: iii. It is more accurate than a bulb thermometer. iv. It shows the temperature directly into digits. v. It is toxic-free thermometer. If a mercury thermometer breaks, then the disposal of mercury creates a problem. iii) Infrared Radiation (IR) thermometer Every hot object radiates thermal energy in the form of infrared radiation. The intensity of infrared radiation can be measured by an instrument such as an infrared radiation thermometer. IR gun is not a thermometer like that we used traditionally. It is actually a detector based on the principle to measure the intensity of thermal radiation from hot objects. Thus, an IR thermometer is an instrument that measures the temperature of a body through the detection of the intensity of thermal radiation. Working mechanism of radiation thermometer We simply hold an IR gun, press the trigger, and point the laser towards the target to measure the surface temperature of the body. The lenses will converge radiations into the thermopile. Thermopile generates heat which is converted into electricity and sent to the detector. The higher the amount of current detected higher will be the temperature of the object. The reading is displayed in the monitor. FACTS WITH REASONS Infrared radiation thermometer was widely used during corona pandemic. The infrared radiation thermometer was widely used during corona pandemic because it is a non-contact thermometer that can measure temperature from far away. ACTIVITY 2 Make a model of radiation thermometer and explain how it work. Digital thermometer Radiation thermometer Radiation thermometer was mostly used during corona pandemic to check temperature of people, students and office staffs. MEMORY TIPS

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

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 217 It can absorb or release more heat. It can absorb or release less heat. It is a poor conductor of heat. It is a good conductor of heat. Values of specific heat capacities of different substances S.N. Substances Specific heat capacity S.N. Substances Specific heat capacity 1. Lead 130 J/kg°C 10. Aluminium 900 J/kg°C 2. Mercury 140 J/kg°C 11. Air 1010 J/kg°C 3. Silver 234 J/kg°C 12. Petrol 1670 J/kg°C 4. Brass 380 J/kg°C 13. Wood 1755 J/kg°C 5. Copper 400 J/kg°C 14. Vegetable oil 2000 J/kg°C 6. Steel 447 J/kg°C 15. Ice 2100 J/kg°C 7. Iron 460 J/kg°C 16. Kerosene 2200 J/kg°C 8. Glass 670 J/kg°C 17. Alcohol 2400 J/kg°C 9. Sand 800 J/kg°C 18. Water 4200 J/kg°C Meaning of specific heat capacity of water is 4200 J/kg°C Specific heat capacity of water is 4200 J/kgo C. It means that 4200 J heat is required to raise the temperature of 1 kg mass of water by 1°C. Advantage of high specific heat capacity of water i. Water is used in cooling systems Water has high specific heat capacity i.e. 4200 J/ kg°C. It means that it can store more amount of heat. Therefore, water is used to cool the engines of automobiles and other machines. ii. Water is used in heating systems Water has high specific heat capacity i.e. 4200 J/kg°C. It means that it can store more amount of heat and can radiate more amount of heat for longer period of time. So, it is used in hot water bottles to warm the bed, in a central heating system, etc. FACTS WITH REASONS Water is used in hot water bag, why? Water has high specific heat capacity (4200 J/kg0 C). It can radiate large amount of heat slowly per degree celsius fall in temperature. So, water is used in hot water bag. Water is used as a coolant to cool hot engines of the vehicles, why? Water has high specific heat capacity (4200 J/kg0 C). It can absorb large amount of heat from the hot objects per degree celsius fall in temperature. So, water is used as a coolant to cool hot engines of the vehicles. A wet handkerchief is kept on the forehead of a patient suffering from fever, why? A wet handkerchief contains water which has high specific heat capacity (4200 J/kg0 C). It can absorb large amount of heat from the body of patient per degree celsius rise in temperature and lowers his/ her body temperature. So, a wet handkerchief is kept on the forehead of a patient suffering from fever. If you supply 4200 J of heat to 1 kg of water, you will raise its temperature by 1°C. MEMORY TIPS

218 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Mercury is used as a thermometric liquid, why? Mercury has low specific heat capacity, i.e., 140 J/kg0 C. So, it's temperature will change faster. It is a good conductor and highly sensitive to heat. So, it is used as a thermometric liquid. Fluctuation in temperature during day and night is very less around the coastal areas, why? Due to high specific heat capacity of water i.e. 4200 J/kg0 C, the sea water gets heated slowly during the day and cools during the night. Whereas the costal land having less specific heat capacity gets heated fast and cools down fast. So, the high density cold air from the sea blows towards the land during the day (sea breeze) and high density cold air from the land blows towards the sea during the night (land breeze). This makes lass fluctuation in temperature during day and night around the coastal areas. During the winter morning well water feels warmer, why? The specific heat capacity of soil is much less than that of water. So, comparatively during the winter night water cools less than soil. As a result, heat flows from well water to our body when we touch well water in the winter morning. Therefore, well water feels warmer in the winter morning. In a desert, it is very hot during the day and very cold during the night, why? Desert means sandy area. It has low specific heat capacity (i.e. 800 J/kg0 C). It's temperature will rise or fall faster. Due to this, the sand becomes very hot during the day as it absorbs heat of the sun. Whereas the hot sand loses its heat very soon during the night. Therefore, desert is very hot during the day and very cold during the night. Factors Affecting Amount of Heat Contained The amount of heat contained in a body depends upon the following factors. i) Change in temperature : The heat energy contained in a body is directly proportional to the change in temperature, i.e Q α dt. ii) Amount of mass : The heat energy contained in a body is directly proportional to the mass of the substance, i.e. Q α m. iii) Nature of the body : Specific heat capacity of the different substances is different. So, the heat contained in a body depends upon the specific heat capacity of the body. ACTIVITY 4 To show that amount of heat contained in a body depends upon change in temperature and mass 1. Fill 100 g of water in a beaker and immerse a thermometer into the water to measure its temperature. 2. Supply heat for some time to raise its temperature upto 200 C. Again heat the water for some more time. Does the temperature increase with the increase in amount of heat supplied? 3. Next time add 200 g of water in another beaker and heat both of them equally for the same time. 4. Note the change in temperature in these two beakers. Do you observe the change in temperature more in case of 100 g water? 5. Take another beaker and partially fill it with 200 g of alcohol. 6. Heat both 200 g of water and 200 g of alcohol equally for the same time and observe the change in temperature. 7. Do you observe the rise in temperature of alcohol more?

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 219 Heat equation When a cold body is heated, it absorbs heat and its temperature will rise. Similarly, when a hot body is cooled, it releases heat and its temperature will fall. But it has been proved that heat gained or lost (Q) by a body is directly proportional to the mass of the body and change in temperature. From the above statement, The heat gained or lost (Q) is directly proportional to its mass (m), i.e. Q α m ………………. (i) The heat gained or lost (Q) is directly proportional to the change in temperature (dt), i.e. Q α dt ………………. (ii) Combining relation (i) and relation (ii) Q α m dt Q = m s dt (Where 's' is a proportionality constant called specific heat capacity.) This is heat equation. Thus, according to heat equation, the amount of heat gained or lost by a body (Q) is equal to the product of its mass (m), specific heat capacity (s) and change in its temperature (dt), i.e. Q = m s dt. The heat equation shows that, the amount of heat lost or gained by a body depends upon: i. Mass of the body (m). ii. Specific heat capacity of the body (s). iii. Change in temperature (dt). From the heat equation, Q = m s dt, when 'Q' and 'm' are kept constant, dt α 1 s From the above relation, it is clear that on supplying equal amounts of heat, the rise in temperature will be more in the substances having low specific heat capacity, whereas the rise in temperature will be less in the substances having high specific heat capacity. MEMORY TIPS Calorimetry and its Principle In winter season, people mix hot water with cold water for bathing. The temperature of the mixed water is less than the temperature of the hot water and more than the temperature of the cold water. In this process, the hot water loses heat and the cold water gains heat. The amount of heat lost or gained by an object is measured by using calorimetry. The principle of calorimetry states, "When a hot body is mixed with a cold body, the heat lost by the hot body is equal to the heat gained by the cold body, provided that no heat escapes to the surroundings". From the above statement, it is clear that Heat lost = Heat gained m1 s1 (t1 – t) = m2 s2 (t–t2 ) Here, t = Final temperature of the mixture Heat lost or gained is measured by calorimeter. MEMORY TIPS

220 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur m1 = Mass of the hot body m2 = Mass of the cold body s1 = Specific heat capacity of the hot body s2 = Specific heat capacity of the cold body t1 = Temperature of the hot body t 2 = Temperature of the cold body Solved Numerical 9.1 Calculate the amount of heat required to change the temperature of 2 kg of iron from 30°C to 90°C. The specific heat capacity of iron is 460 J/kg °C. Solution: Given, Mass of iron (m) = 2.0 kg Specific heat capacity (s) = 460 J/kg°C Change in temperature (dt) = 90 ° C – 30 °C = 60 °C Amount of heat (Q) = ? We have, Q = m s dt = 2 × 460 × 60 = 55200 J ∴ The required heat is 55,200 J. Solved Numerical 9.2 Specific heat capacity of water is 4200 J kg-1 o C-1. Calculate the amount of heat energy released when 5 kg of water cools down from 80o C to 30o C. Solution: Given, Specific heat capacity of water (s) = 4200 J kg-1 o C-1 Mass of water (m) = 5 kg Initial temperature (t1 ) = 80o C Final temperature (t2 ) = 30o C Change in temperature (dt) = t1 - t2 = 80o C - 30o C =50o C We have, Q = m s dt = 5 × 4200 × 50 = 1050000 = 1.05 × 106 J ∴ The heat released is 1.05 × 106 J. Solved Numerical 9.3 100 g of water at 1000 C is mixed with 200 g of water. If the mixture temperature becomes 500 C, find the initial temperature of the cold water. Solution: Here, Mass of hot water (m1 ) = 100 g = 100 1000 = 0.1kg Initial temperature of hot water (t1 ) = 100°C Mass of the cold water (m2 ) = 200 g = 0.2 kg Final temperature of the mixture (t) = 50°C

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 221 Initial temperature of cold water (t2 ) =? From the principle of calorimetry Heat lost by hot water = Heat gained by cold water m1 s1 (t1 – t) = m2 s2 (t–t2 ) or, 0.1 × (100 – 50) = 0.2 × (50 – t2 ) or, 0.1 × 50 0.2 = 50 – t2 or, t 2 = 50 – 25 = 25°C ∴ The initial temperature of cold water is 25°C. HOT SKILL HIGHER ORDER THINKING SKILL 1. The clinical thermometer is not cylindrical but prismatic. Clinical thermometer is made prismatic instead of cylindrical because it helps to refract more light to the mercury. So, that mercury can shine and it will be easy to take a reading. 2. In winter we can warm water and keep it in a hot water bag. Then it is kept with us in bed. It keeps us warm all night long. How does it work? Explain. Usually, people boil water and fill it in a hot water rubber bag to keep them warm on cold nights. It is possible only because the water has a high specific heat capacity. When boiled water can absorb a large quantity of heat per degree Celsius rise in temperature and later it will lose same quantity of heat per each degree Celsius fall in temperature. So, it can keep us warm for a very long time. 3. The pond was frozen. When we cut open a hole in the ice and saw there was water below. How is it possible? Describe. The pond was frozen. When we cut open a hole in the ice and saw, there was water under the ice along with aquatic animals and plants. It is possible only because of the anomalous expansion of water. Materials expand on heating but water contracts when heated from 0°C to 4°C. As result, water at 4°C has the highest density and sinks to the bottom. The temperature falls below 0°C at the top. Then the water starts freezing from the top. The layer of ice at the top acts as an insulator and prevents further cooling of water below. As a result, liquid water at 4°C remains at the bottom of the pond where aquatic animals and plants can survive. 4. Study the given graph where the relation between the volume of the water and the change in water temperature is shown graphically. i. What does the arc from 0°C to 4°C represent? The curve from 0°C to 4°C represents the anomalous expansion of water. ii. At what temperature water has the minimum volume? Why? Water has a minimum volume at 4°C.

222 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur It happens because of anomalous expansion of water. The density of water increases and volume decreases when heated from 0°C to 4°C. iii. What happens if the water at O°C is filled in glass and warmed up to 6°C? If the water at 0°C is filled in glass and warmed up to 6°C, its density increases and volume will decrease from 0°C to 4°C. Then its density will decrease and volume will again start to increase if heated above 4°C. 5. Why does sugar dissolve faster in hot water than the cold one? The sugar dissolves faster in the hot water than the cold water because the molecules of the water are moving faster, they have more kinetic energy, less intermolecular force and large intermolecular distances than the cold water. So, sugar molecules can enter in them. 6. Compare between thermal energy and heat energy. The differences between thermal energy and heat energy are: SN Thermal energy SN Heat energy 1 The total kinetic energy possessed by the molecules of the object is called thermal energy. 1 The flow of the thermal energy from hotter objects to colder objects on thermal contact is called heat energy. 2 It is the stored form of kinetic energy in an object. 2 It is the form of energy which is transferring. 3 Geothermal energy, nuclear energy, heating effect of electricity produces thermal energy. 3 Burning fuels such as firewood, petroleum etc. produces heat energy. 7. Average temperature during the day and average temperature during the night is almost same in coastal areas. So weather is pleasant there. What is the role of specific heat capacity for such climate? Specific heat capacity is the nature of a substance to retain or lose heat per unit mass per unit change in temperature. The specific heat capacity of water in the sea is very high compared to the land nearby in coast. The water absorbs larger quantity of heat from surrounding during day but is still at lower temperature than the land. It creates sea breeze. So, the coast remains cooler. During nights lands are very cold due to low specific heat capacity but the ocean is warm because it cools down slowly. It releases huge quantity of heat into the surrounding. So, the coast gets warmer. Hence the average temperature during the day and average temperature during the night is almost same in coastal areas. 8. Wooden or plastic spoon is used to eat ice-cream than a steel spoon. Why? Steel spoon is a very good conductor of the heat. It can transfer heat into the ice cream faster and melt the ice cream. However, wood is a bad conductor of heat. It won’t transfer heat into the ice cream and ice cream won’t melt. Therefore, wooden or plastic spoon is used to eat ice cream than a steel spoon. 9. During winter, in cold countries, water pipes burst in many places. What could be the reason? How do people deal with this problem? Water pipes usually burst in winters in cold countries. The reason is anomalous expansion of water. Due to anomalous expansion of water, it has highest volume at 0°C. When it turns to ice it has even greater volume. It exerts greater pressure on the walls of the water pipes and breaks them if left unchecked for long time. People usually leave the tap running, to prevent water from freezing in the pipes. It helps to prevent pipe from bursting.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 223 3 STEPS 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answers from the given alternatives a. What is thermal energy? i. energy responsible for the temperature of an object ii. average kinetic energy iii. flow of kinetic energy iv. heat energy b. Why heated copper ball unable to pass through the copper ring? i. its volume has increased due to heating ii. ring has shrunk due to cold iii. copper ball has rust and cannot pass iv. its volume has decreased due to heat c. Why is mercury used in liquid thermometers? i. it is shiny and has a uniform rate of expansion. ii. it is heavy iii. it is cheap and easily available iv. it can work in cold places d. Why does a clinical thermometer have a short range? i. the thermometer is short ii. human body temperature does not vary much iii. mercury in it cannot measure more than that iv. our temperature is constant e. Why does water start to freeze from the top? i. anomalous expansion of water ii. fall in temperature iii. ice is lighter iv. specific heat capacity of water 2. Define the following terms the required examples. a. Heat b. Temperature c. One calorie heat d. Thermometer e. Calibration f. Digital Thermometer g. Digital clinical thermometer h. Heat equation

224 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur i. Calorimetry j. Thermal energy k. Temperature based on kinetic energy l. Principle of a thermometer m. Radiation thermometer n. Specific heat capacity o. Principle of calorimetry p. Anomalous expansion of water 3. Answer the following questions in very short. a. Write the SI unit of the following: i. heat ii. temperature iii. specific heat capacity b. What is the sum of kinetic energy of the molecules called? c. Write the relation between vibrations of the molecules with heat and temperature. d. Which physical quantity of heat equation depends upon nature of the substance? e. How much heat energy should be supplied to increase the 1°C temperature of 1kg water? f. What is the lower fixed point and upper fixed point of thermometer in different scales? g. What is used in digital thermometers to measure temperature? h. Do water and ice have the same specific heat capacity? STEP2 4. Give reasons. a. If an equal amount of heat is given to two different substances, the change in their temperature is not the same. b. An iron nail becomes hot on frequently hammering it. c. If a cup full of water at 4°C is cooled further water overflow from cup. d. A clinical thermometer has a prismatic shape. e. A mercury-used clinical thermometer has a kink near its bulb. f. In winter, we feel the room cold but when we stroll outside for some time and enter the same room then we feel the air warmer. g. In the day, the air blows towards land from the sea, and at night, the air blows towards the sea from the land in a coastal area. h. During the day, the desert is very hot and at night it is very cold. i. There is not a big fluctuation in day temperature and night temperature around the coastal areas. j. Water is used to cool car radiators. k. Water boils below 100°C in Himalaya region. l. Handkerchief soaked in water is kept on the forehead of a patient suffering from fever. m. In cold countries, hot water bags are used in bed in the winter season. n. The burn caused by steam is more severe than that caused by boiling water.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 225 o. Rock break into sand due to the effect of heat. p. A cup of tea has less thermal energy than a kettle of tea, even if they are at same temperature. q. Icebergs at ocean have more heat than a cup of hot water. r. It’s easy to open tight corks and bottle caps by dipping them in warm water. s. Aquatic animals can survive even if the pond is frozen in winters. 5. Differentiate between the following. a. Heat and temperature c. Upper fixed point and lower fixed point d. Theremal energy and heat e. Mercury used clinical thermometer and a digital thermometer f. Digital thermometer and radiation thermometer 6. Answer the following questions in short. a. Based on molecular movement, what are the factors which affect the quantity of heat in a substance? b. List any four effects of heat. c. Write two advantages and two disadvantages of mercury as a thermometric liquid. d. Write the advantages of a digital thermometer over the mercury-used clinical thermometer. e. The specific heat capacity of copper is 400 J / kg°C. What does it mean? f. What is the relation between the specific heat capacity of a substance and temperature change? g. If temperature of two different masses, where A is heavy and B is light, are heated to same temperature, which one absorbs more heat? Why? h. Water has the highest specific heat capacity, write down any two applications of it. i. Prove that Q = m s dt, where 's' is the specific heat capacity of the substance. j. Explain the structure and working mechanism of a clinical thermometer with the help of a diagram. k. Write down the relation of the heat gained or heat lost (Q) by a body with its mass (m) and change in temperature (dt). l. What is a radiation thermometer? How does it work? m. It feels very cold when touching metal chair than wooden chair in winters. Explain based on kinetic molecular theory of heat. n. Ramesh was warming water for bathing. He noticed that water at the bottom of bucket was less colder than the water at top. What could be the cause? Explain how does this happen. o. How is expansion of water upon heating different from other liquids? p. How is a thermometer calibrated?

226 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur STEP3 7. Answer the following long questions. a. Enlist the applications of anomalous expansion of water. The ghee starts to solidify from bottom but water starts to solidify from the top. Explain this mechanism based on temperature, density and volume. b. Children should wear multiple clothes than adult despite all of us are at 37°C. Explain the reason. How do wearing multiple clothes keep children warm? Explain. c. The specific heat capacity of four different metals A, B, C and D is shown in the given table. With the help of this table answer the following questions. (i) What is meant by the statement, “The specific heat capacity of B is 380 J/kg°C?” (ii) If an equal amount of heat is given to an equal mass of these substances, which one will have the highest and which one will have the lowest temperature? Why? (iii) If an equal mass of these substances at 120°C temperature is kept on a wax slab, which one will make the deepest hole in the wax slab? Why? d. If three liquids A, B and C having equal masses are taken in three same-sized beakers and exposed to sunlight. After 10 minutes, the increased temperature in each is given in the table. i) Which liquid has the highest specific heat capacity? Write with a reason. ii) If all three liquids are heated up to 90°C and allowed to cool down, which one cools faster? Why? e. An iron ball can easily pass through a ring but after heating, that ball cannot pass through the same ring, why? Support your answer with the help of an activity. f. We feel cold when we touch ice but we feel hot when we touch hot water, why? g. We keep glass bottle in hot water to open its lid, why? h. Draw the diagram to show construction of the following instruments. i. Clinical thermometer ii. Radiation thermometer iii. Digital thermometer 8. Numerical a. How much heat energy is required to change the temperature of 5 kg water by 20°C? [Ans: 4.2 × 105 J] Metals Specific heat capacity A 900 J/kgºC B 380 J/kgºC C 500 J/kgºC D 258 J/kgºC Liquids Temperature A 10°C B 8°C C 6°C

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 227 b. What calorie of heat is required to increase the temperature of 20 litres of water by 16°C? (1 litre of water =1 kg.) [Ans : 3.2 × 105 cal] c. What calorie of heat is released when 15 litres of water is cooled from 20°C to 10°C? (1 litre of water =1 kg) [Ans: 1.5 × 105 cal] d. Calculate the amount of heat required to raise the temperature of 800 g of iron from 40°C to 240° C. (Specific heat capacity of iron = 460 J kg-1oC-1) [Ans: 7.04 × 104 J] e. What will be the final temperature of the water if 5 litres of water at 35°C is mixed with 10 litres of water at 20°C? f. 1kg paraffin requires 44000 J heat energy to increase its temperature by 20°C, then how much heat is required for 5 kg paraffin to increase its temperature by 10°C? [ Ans: 1.1 × 105 J] g. 5kg of the copper ball and 1 kg of water are at the same temperature of 31°C. Which substance loses more heat energy, when both of them are cooled up to 30°C? (Specific heat capacity of copper = 400 J/kg °C and water = 4200 J /kg°C) [Ans: Qcopper = 2000J, Qwater= 4200 J] h. A heater is connected to 220 V a.c. to heat 2 kg water at 40°C. If the water absorbs 3.6 × 105 J heat energy, find the final temperature of the water. [Ans: 46.8°C] i. The temperature of an iron ball of mass 4 kg changes by 10°C when 18.4 kJ heat energy is supplied to it. What is the specific heat capacity of the iron ball? [Ans: 460 J/kg°C] j. It takes 2 minutes to heat a substance of 10 kg mass from 25 °C to 30° C on heating by an electric heater of power 1000W. Find the specific heat capacity of that substance. Hint: 1000W= 1000 J of heat energy is supplied by the heater in 1s [Ans: 2400 J/kg °C] k. 200 ml of milk at 90°C is mixed with 10 ml of water at 15°C. Find the final temperature of the mixture. (Suppose the specific heat capacity of water = specific heat capacity of milk, 200 ml = 200 g) [Ans: 86.4 °C] l. A hot iron sphere of 200 g is kept in 500 g of water at 10°C. As a result, the final temperature of the water becomes 30°C. Find the initial temperature of the iron sphere. (Specific heat capacity of iron = 470 J/kg°C) [Ans: 476.8°C]

228 Heat en eRgy Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Wave Unit 10 I Air N R r i O Glass Refracted ray Refraction of light Newton's color disc Refraction during sun set Endoscopy

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 229 Key terms and terminologies of the unit 1. Light : Light is a form of energy that produces the sensation of vision. 2. Optical medium : Optical medium is a material medium through which the electromagnetic waves propagate. 3. Rarer medium : The medium in which light travels faster is called an optically rarer medium. 4. Denser medium : The medium in which light travels relatively slower is called an optically denser medium. 5. Refraction of light : The phenomenon of bending of light as it passes obliquely from one optical medium to another is called the refraction of light. 6. Cause of refraction : The change in speed of the light wave upon crossing the boundary of two optical media is the cause of refraction. 7. Refractive index : The refractive index of a material medium can be defined as the ratio of the speed of light in vacuum (c) to the speed of light in the medium (v). 8. Absolute refractive index :The refractive index of a medium, with respect to a vacuum, is called the absolute refractive index of the medium. 9. Real depth : The real position of the bottom or a point inside water is called a real depth. Sequence of Curriculum Issued by CDC  Refraction of the light  Laws of refraction of the light  Total internal reflection of light: introduction, critical angle, condition for total internal reflection, application of total internal reflection of light(mirage and shining of the diamond)  Introduction to light pipe, working mechanism of light pipe, application of light pipe on telecommunication, endoscopy, keyhole surgery etc.  Dispersion of light: introduction and dispersion of light from a prism UNIT Light 10.1 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 demised 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

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

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 231 Refraction of light According to the ray model of light, it travels through space along straight lines. Light does not travel in the same direction in all media. When light enters obliquely from one transparent medium to another, the direction of propagation of light in the second medium changes. This phenomenon is called refraction of light. Thus, the phenomenon of bending of light as it passes obliquely from one optical medium to another is called the refraction of light. Cause of Refraction Light travels more slowly when it enters into an optically denser medium from an optically rarer medium. On the other hand, when light travels from an optically denser to an optically rarer medium, it speeds up. The change in speed of the light wave upon crossing the boundary of two optical media is the cause of refraction. Refraction through Glass Slab General Terms Used in Refraction of Light i. Incident Ray : A ray which strikes the surface of separation of two optical media is known as the incident ray. In the given figure, AO is the incident ray. ii. Refracted Ray : The ray which travels in the second optical medium, with a change in direction, is called the refracted ray. In the given figure, OB is the refracted ray. iii. Normal : A perpendicular line drawn at the point of incidence is called the normal. In the given figure, NN' is a normal line. iv. Angle of Incidence : The angle made by an incident ray with the normal, at the point of incidence, is known as the angle of incidence. In the given figure 'i' is the angle of incidence. v. Angle of Refraction : The angle made by refracted ray with the normal, at the point of incidence, is known as the angle of refraction. In the given figure, 'r' is the angle of refraction. vi. Emergent Ray : The ray of light which comes out from the denser medium to the same rarer medium after its refraction is called emergent ray. vii. Emergent Angle : The angle made by emergent ray with normal is called emergent angle. vii. Lateral Shift: When light travels from denser medium to the rarer medium, it bends away from normal towards the direction of incident ray. The net distance shift by the emergent ray is called lateral shift. Rules for the Bending of Rays at the Interface of Two Optical Media i. When a ray of light travels from an optically rarer medium (air) to an optically denser medium (glass), it bends towards the normal at the interface of the two media. I Air N R r i O Glass Refracted ray Refraction of light Refraction through glass slab i A P S Q Emergent ray Incident ray Refracted ray R C D B q N N' N1 N'1 AIR O Glass Rectangular glass slab Direction of original ray Lateral displacement AIR Normal Normal

232 ligHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur FACTS WITH REASONS For a ray of light travelling from an optically rarer to an optically denser medium, the angle of incidence is always greater than the angle of refraction, why? A ray of light travelling from a rarer medium to a denser medium slows down and bends towards the normal at the point of incidence. So, for this ray of light, the angle of incidence is always greater than the angle of refraction. Normal r i Denser (glass) Rarer (air) ii. When a ray of light travels from an optically denser medium (glass) to an optically rarer medium (air), it bends away from the normal at the interface of the two media. FACTS WITH REASONS For a ray of light travelling from an optically denser to an optically rarer medium, the angle of incidence is always less than the angle of refraction, why? A ray of light travelling from a denser medium to a rarer medium goes faster and bends away from the normal at the point of incidence. So, for this ray of light, the angle of incidence is always less than the angle of refraction. iii. When a ray of light strikes the interface of two media at an angle of 90°, it does not bend from its path. Denser (glass) Rarer air If a ray of light travels from air to water along a certain path, it retraces the path, when light travels from water to air. The path of light is reversible. MEMORY TIPS Light passing through normal ACTIVITY 1 Objective: To show refraction through glass slab 1. Fix a sheet of white paper on a drawing board using drawing pins. 2. Place a rectangular glass slab over the sheet in the middle. Draw the outline of the slab with a pencil and name the outline as ABCD. 3. Draw a line XO as an incident ray on the face AB. 4. Take four identical pins. Fix two pins, say P1 and P2 , vertically on the line XO. 5. Look for the images of the pins P1 and P2 through the opposite edge. Fix two other pins, say P3 and P4 , such that these pins and the images of P1 and P2 lie on a straight line. 6. Remove the pins and the slab. Join the positions of the tip of the pins P3 and P4 and produce it up to the edge DC with the help of a ruler. Let P3 P4 meets DC at O2 . B M N M1 N1 e O2 P3 P4 r i P1 P2 D C A O X Y xy

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 233 7. It is found that the light ray has changed its direction at points O on the surface AB and at a point O2 on the surface DC. When light enters into the glass slab from air, it bends towards normal and when light emerges out of the glass slab, it bends away from the normal. 8. When light travels from denser medium to the rarer medium, it bends away from normal. The net distance shift by the emergent ray is called lateral shift. Here xy is lateral shift. FACTS WITH REASONS When a ray of light passes from a denser medium to a rarer medium, it bends away from the normal, why? A ray of light travelling from a rarer medium to a denser medium slows down, causing it to bend towards the normal. But when it travels from a denser medium to a rarer medium, it goes faster, causing it bend away from the normal. Laws of Refraction i) The incident ray, the normal, and the refracted ray all lie in the same plane at the point of incident. ii) For a given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. This law is also known as Snell’s law of refraction. If 'i' is the angle of incidence and 'r' is the angle of refraction, then, sin i sin r = constant (µ) This constant value is called the refractive index of the second medium with respect to the first. Refractive Index The degree of the change in direction that takes place in a given pair of media is expressed in terms of the refractive index. It refers to the degree to which a medium manages to slow down the wave that is transmitted through it. Thus the higher the refractive index of a material medium, the slower a wave passes through it. The refractive index indicates how much slower the electromagnetic wave passes through the medium with respect to its speed in vacuum (3 × 108 m/s). There is a relationship between the refractive index of a material and the speed of light in that material. The refractive index of a material medium can be defined as the ratio of the speed of light in vacuum (c) to the speed of light in the medium (v). µ = Speed of light in vacuum Speed of light in the medium = c v The refractive index of a medium, considered with respect to vacuum, is called the absolute refractive index of the medium. The absolute refractive index of a medium is simply called its refractive index. The refractive index of several media is given in the table. Snell's law is not applicable when the angle of incidence is equal to zero, i.e., when the ray is incident along the normal. MEMORY TIPS

234 ligHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Medium Absolute Refractive Index Vacuum 1 Ice 1.31 Water 1.33 Ethyl Alcohol 1.36 Kerosene 1.44 Glass 1.50 Crown Glass 1.52 Diamond 2.42 Solved Numerical 10.1.1 The refractive index of diamond is 2.4. What is the speed of light in diamond if its speed in a vacuum is 3 × 108 m/s? Solution: Given, Speed of light in vacuum (c) = 3 × 108 m/s Refractive index = 2.4 From the formula, µ = Speed of light in vacuum (c) Speed of light in the medium (v) or, 2.4 = 3 × 108 Speed of light in the medium or, Speed of light in diamond = 3 × 108 2.4 = 1.25 × 108 m/s Real Depth and Apparent Depth Light ray bends away from the normal when it passes from water to air. This bending of light gives us a false impression of depth. When light rays pass from water to air, the light rays from the point at the bottom of a pond bend away from the air-water interface. The observer outside sees the point to be at a higher position above the actual position. The real position of the bottom or a point inside water is called a real depth. Whereas, the position of the bottom or a point inside water which is observed from outside is called an apparent depth. Apparent depth is due to the refraction of light that occurs at the water-air interface. The real depth and apparent depth are related to the refractive index of water by the given equation. µ = Real depth Apparent depth In comparing two media, the one with the larger refractive index is an optically denser medium than the other. The other medium of lower refractive index is optically rarer. MEMORY TIPS An optically denser medium may not possess a greater mass density. For example, kerosene, which has a higher refractive index than that of water, is optically denser than water but mass density of kerosene is less than that of water and it floats on the surface of water. MEMORY TIPS

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 235 Solved Numerical 10.1.2 An observer looks at a water tank. According to him, half of the tank is filled with water. If the height of the tank is 180 cm, find the real height of the water in the tank. (Refractive index of water = 1.33) Solution: Given, Height of the tank = 180 cm Apparent depth = 180 2 = 90 cm Refractive index of the water = 1.33 From the formula, µ = Real depth Apparent depth or, 1.33 = real depth 90 or, real depth = 90 × 1.33 = 119.7 cm The real height of the water in the tank is 119.7 cm. FACTS WITH REASONS A spear hit by a fisherman does not hurt the fish inside water, why? When the rays of light refracted from the fish inside water pass from the water to the air, they bend away from the normal. Hence the fish appears at a position above its actual position. A person hitting at the apparent position of the fish inside water misses the target. So, a spear hit by the fisherman where fish is seen won't hurt the fish inside the water. Effects of Refraction i) A stick immersed obliquely in water appears to be bent. ii) A coin in a cup, just below the eye sight, becomes visible when the cup is filled with water. iii) A spear hit by a fisherman does not hurt the fish inside water. iv) When a thick glass slab is placed over a book page, the letters appear raised while viewing through the glass slab v) Twinkling of stars in the sky vi) The sun is visible to us about two minutes before the sunrise (i.e. advanced sunrise) and about two minutes after the sunset (delayed sunset). vii) The sun near the horizon appears flattened at the sunset and sunrise. FACTS WITH REASONS A stick immersed obliquely in water appears to be bent. Why? When rays of light from a part of the stick inside water pass from water to air, they bend away from the normal. The image of immersed part of stick is formed at an apparent depth. Hence, the stick appears to be bent at the air-water interface. Water Apparent bend Apparent position of stick When you look at the depth of a swimming pool, the bending of light can give you a false impression of depth. MEMORY TIPS

236 ligHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur ACTIVITY 2 1. Take a beaker and fill ¾ th of it with water. 2. Place a pencil obliquely in the beaker and observe it from a side of the beaker. 3. Does the pencil appear bent at the air-water interface? Real position Apparent position ACTIVITY 3 1. Place a coin at the bottom of a glass tumbler filled with water. 2. With your eye to a side above water, make an attempt to touch the coin with the help of a pencil. Did the pencil touch the surface of the coin exactly? Water Coin (real position) Apparent position of the coin ACTIVITY 4 1. Place a cup on a table in your classroom and put a coin in it. 2. Move away slowly from the cup and stop to move when the coin just disappears from your sight. 3. Ask a friend to pour water gently into the cup without disturbing the coin. 4. Keep looking for the coin from your position. Does the coin become visible again from your position? FACTS WITH REASONS A coin in a cup, just below the eye sight, becomes visible when the cup is filled with water, why? The direction of light reaching our eyes from the coin inside water through water (optically denser medium) to air (optically rarer medium) bends away from the air-water interface. Due to this, the coin seems just above of its actual position inside water in the cup. So a coin in a cup, just below the eye sight, becomes visible when the cup is filled with water. ACTIVITY 5 1. Place a glass slab over a page of a book on your table. 2. Look at the letters under the slab from the sides. Do you see the letters raised? FACTS WITH REASONS When a thick glass slab is placed over a book page, the letters appear raised while viewing through the glass slab, why? The direction of light reaching our eyes from the letters under the glass slab through glass (optically denser medium) to air (optically rarer medium) bends away from the air-glass interface. Due to this, the letters seem just above of their actual position under the slab. So when a thick glass slab is placed over a book page, the letters appear raised while viewing through the glass slab.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 237 Twinkling of Stars Twinkling of stars is due to the atmospheric refraction. Light from distant stars undergoes a continuous refraction through layers of atmosphere. During the refraction in the atmosphere, the refracted rays bend towards the normal and the stars appear slightly higher than where their actual position is. The apparent position of stars fluctuates continuously due to the change in the earth's atmosphere. Stars appear brighter some times and fainter at the other times. So twinkling of stars is due to the fluctuation of the apparent positions of the stars during their refraction. FACTS WITH REASONS Stars twinkle but planets do not, why? Compared to the planets, stars are very far. Light from the distant stars undergoes a continuous refraction through layers of atmosphere. It also fluctuates the apparent position of the stars and twinkle. But, such continuous refraction does not occur in planets. Hence planets do not twinkle. The sun is visible to us about two minutes before the sunrise (i.e. advanced sunrise) The actual sunrise means the actual crossing of the horizon by the sun. Light from the sun travels through layers of the atmosphere. When the sun is just below the horizon, its rays enter the earth's atmosphere from rarer to the denser medium and get refracted. But the observer on the earth thinks that the light is coming straight from the sun. Refracted rays that reach us make the sun appear on the horizon. This is the apparent sunrise. But the actual sunrise occurs when the sun reaches horizon a little later. Sun set occurs about two minutes after the sunset (delayed sunset) The apparent sunset occurs slightly later than the actual sun set. The light from the sun is already below the horizon. It gets refracted towards the earth. While travelling through rarer to denser medium of the atmosphere, the sunlight from apparent position of the sun reaches the observer on the earth. This enables us to see the apparent sunset for a while after the sun has already set. So, we can see the sun about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset because of atmospheric refraction. The sun near the horizon appears flattened at the sunset and sunrise When the light rays from the sun enter the earth's atmosphere, they get refracted towards the earth. Due to the atmospheric refraction, the rays from the top and bottom portions of the sun on horizon are refracted by different degrees. This causes the apparent flattening of the sun. But the rays from the sides of the sun on a horizontal plane are generally refracted by the same amount. So the sun still appears circular along the sides. Earth Observer Horizon Apparent sun Rising sun

238 ligHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Critical Angle When light travels from a denser medium to a rarer medium (e.g. glass to air as shown in figure), it bends away from the normal. At this condition, the angle of refraction (r) becomes greater than the angle of incidence (i). If the angle of incidence in denser medium is increased then the corresponding angle of refraction also increases. At some stage, the angle of refraction becomes 900 . At this condition, the angle of incident is also called critical angle. Thus, the angle of incidence in a denser medium for which the corresponding angle of refraction in the rarer medium is 90° is called the critical angle. It is denoted by C. In the given figure (ii) angle of incidence is equal to critical angle (i.e. i = C). c r i i i r r = 900 Rarer Rarer Rarer Denser Denser Denser Critical angle and total internal reflection Fig (i) Fig (ii) Fig (iii) FACTS WITH REASONS Critical angle of glass is 42°. What does it mean? Critical angle of glass is 42°. It means that, when light passes from glass to air, at the incident angle 42° the corresponding angle of refraction in air will be 90°. Critical angles for some substances with respect to air Substances Critical angle Substances Critical angle Ice 50° Turpentine 43° Water 49° Glycerine 43° Alcohol 48° Glass 42° Paraffin 44° Diamond 24° Total Internal Reflection (TIR) When the angle of incidence is further increased beyond the critical angle, the ray of light reflects back to the same medium instead of refraction. This phenomenon is called total internal reflection of light. Thus, the phenomenon of light returning into the same medium when it passes from an optically denser to an optically rarer medium with an angle of incidence greater than the critical angle (i > C) is called the total internal reflection. i r Rarer Denser Total internal reflection

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 239 ACTIVITY 6 To demonstrate the total internal reflection of light 1. Immerse an empty test tube in a beaker containing water as shown in the given figure. 2. Gradually tilt the test tube and look at it from outside the beaker. 3. At a certain angle of incidence on the water-air interface, the test tube shines brightly. Explain: The test tube shines due to the total internal reflection. The small thickness of the glass wall of the test tube in between the air-water interface is neglected in this case. Conditions for Total Internal Reflection In order to have total internal reflection, two conditions must be fulfilled. They are: i) Light must travel from a denser medium to a rarer medium. ii) The angle of incidence in the denser medium must be greater than the critical angle. Effects of Total Internal Reflection Total Reflecting Prism A total reflecting prism is a right-angled isosceles prism. It rotates light through 90° or 180° based on the total internal reflection. It is used to obtain a correct image of objects. Uses of total reflecting prism The ability of total internal reflection by a prism is used to make optical devices such as periscopes, binoculars, microscopes, single-lens reflex cameras etc. The prism does not absorb or refract or reflect any rays of light. Every ray of light suffers total internal reflection and travels to another point. So it makes a clear image. Prism also helps to convert inverted images into erect ones so that it will be easy to use binoculars. R Q P 450 450 450 450 450 Q R R 450 450 900 450 450 P Q 900 450 R 450 450 450 Inverted image Erect image Effects of total internal reflection Total Internal Reflection in water Water is a transparent liquid in which light travels at the velocity of 225, 000 kilometres per second. The refractive index of water is 1.33 and the critical angle of the water-air interface is 48.6⁰. Therefore, a ray of light travelling from water (denser medium) to air (rarer medium) and incident on the water-air interface at an angle greater than 48.6⁰, suffers total internal reflection.

240 ligHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Medium 1 (Air) Medium 2 (Water) Refracted ray Total internal reflection Critical angle Incident ray n n n a 1. 2. 3. b c d e Fig: Total internal reflection in water Due to the multiple total internal reflections of light, laser light can pass through a stream of water as shown in the diagram. Fig: Multiple total internal reflections in a stream of water Fibre Optics and Light Pipe Fibre optics and light pipes are the devices used to transmit light in any desired path. They work because of the total internal reflection. When a ray of light enters a fibre, it suffers a series of total internal reflections and finally emerges out from the other end. During this process, the light energy is not absorbed and it gets transmitted over a long distance. Optical fibre Optical fibre is a very thin, transparent tube slightly thicker than human hair made from silicon or plastic. Optical fibres are based on the total internal reflection of light. Usually, an optical fibre consists of three parts. They are the core, cladding and jacket. Fig: structure of the optical fibre

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 241 Core: The innermost flexible transparent thin cylinder of glass is called the core. It is the path through which light can travel from one end to another by multiple total internal reflections of light. It has a higher refractive index and acts as a denser medium. Cladding: It is the second layer of the optical fibre. It is coated around the core. It is a transparent medium made of glass. Its refractive index is lesser than that of the core. It acts as a rarer medium. Jacket: The jacket is the outermost, flexible protective cover of the optical fibre. Working mechanism of optical fibre: When light is passed from one end, it tries to pass through the denser core to a rarer cladding medium. If the angle of incidence is greater than the critical angle, the light suffers total internal reflection. The process repeats multiple times so that light can come out from another end. Optical fibres are widely used in medicine and telecommunication. i) Use of fibre optics in communication: Optical fibre is used extensively as infrastructure for long distance telephone and internet. For this purpose, communication signals like sound are converted into electrical energy and finally into light energy. The light energy confines itself inside the material by total internal reflection and the signal is transmitted over a long distance. By using fibre optics, we can see HD videos, to send and receive large data through internet. FACTS WITH REASONS Optical fibres can transfer data from source to receiver. The optical fibres can transfer data because the internal surface of the fibre is reflective. The light suffers total internal reflection many times and reaches the destination. ACTIVITY 7 Observe the optical fibre if available and discuss its findings in the class. ii) Use of light pipe for endoscopy and colonoscopy: A light pipe is a flexible fibre that confines a beam of light by total internal reflection. It is used to examine internal organ of human body like stomach, intestine, etc. by physicians. If the light pipe is passed through mouth to observe stomach, food pipe, etc., it is called endoscopy and if light pipe is passed through anus to observe small intestine, large intestine, etc. it is called colonoscopy. Data travels faster through fibre optical cables. MEMORY TIPS Fig: optical fibres Fig: endoscopy and colonoscopy

242 ligHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Light pipe Endoscope is a device used by physicians to examine the internal organs of the human body passing through mouth. MEMORY TIPS Keyhole surgery Keyhole surgery is a surgical procedure in which the surgeon can access our abdomen and pelvis with the help of a laparoscope. A laparoscope is a modified light pipe. It contains a small and thin tube, a light source and a camera. It works on the principle of multiple reflections of light. It helps to perform surgeries without large cuts. A laparoscope, a small surgical tool and a tube are passed into the abdomen through a tiny hole at abdomen up to the required place by controlling it with a controller. It is used to remove stone from kidney, stone from gallbladder, to takeout tissue for biopsy, etc Figure of keyhole surgery using laparoscope FACTS WITH REASONS Patients recover faster after keyhole surgery. Patients recover faster after a keyhole surgery because smaller cuts cause less pain, less bleeding, and faster healing. ACTIVITY 8 Discuss the advantages of keyhole surgery. Make a chart and paste in the classroom. Mirage A mirage is generated by layers of air at different temperatures. This effect can commonly be seen on asphalt roads and deserts during the summer. It is an optical illusion in which there is an appearance of water mainly on roads, deserts, etc. during the peak summer. The advantage of keyhole surgery is shorter recovery time and lesser chance of infection. MEMORY TIPS Virtual image Rarer layer of air Denser layer of air Tree Mirage

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 243 Formation of a Mirage During peak summer days, the black-topped road and the sand on desert absorb solar radiations and make the air above them hot very quickly. This makes the lower layers of air above the road hotter than the upper layers. Cold air is denser than hot air. At the boundary of the denser upper layer and rarer lower layer, light from the blue sky and surroundings bends more and more away from the normal. After several refractions, the angle of incidence becomes greater than the critical angle, which is the condition for the total internal reflection. Hence, light is reflected. The viewer no longer sees the road or desert floor. But the light reflected from the blue sky and surroundings, gives him to an illusion of water. FACTS WITH REASONS Why does mirage occur? Mirage occurs due to total internal reflection of light on the asphalt roads and deserts during the summer. Sparkling of Diamond Diamond has a special cut of the faces along with its high refractive index of 2.42. Its critical angle is only of 240 , which allows total internal reflection over a wide range of angles. Therefore, a lot of incident light undergoes several total internal reflections inside the diamond before exiting through it and causes the “sparkling” effect. This effect makes diamonds so appealing. A well-cut diamond sparkles more than a poorly cut diamond. FACTS WITH REASONS An air bubble inside water shines. Why? Water is an optically denser medium and air is a rarer medium. When light rays passing through water fall on air bubble inside water at an angle greater than the critical angle then they suffer total internal reflection. These reflected rays enter an observer's eyes. Due to this, the air bubble inside water shines. Sound is not heard at a distance during the day. During the day, land becomes warm and it makes air above it warm. The warmer air rises. As a result, a layer of air near the ground becomes the rarer medium and the upper layers become denser. When we speak, our sound, similar to light, has to pass from a warm rarer medium. So, it bends toward the normal. As a result, the sound waves will travel to higher altitudes and cannot be heard by the person on the ground a few meters away. Sound can be heard at a distance during the night. At night, the air near the land is colder and acts as a denser medium. The upper layers of the air act as a rarer medium. When we speak the sound has to travel from denser to rarer medium. If sound travels at an angle of incidence greater than the critical angle, it will bend away from the normal and reflect back to the ground. So, sound can be heard at a distance during the night. Sparkling of diamond

244 ligHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Differences between Reflection and Total Internal Reflection S.N. Reflection S.N. Total Internal Reflection 1. A smooth surface like mirror is required for reflection of light. 1. An optically denser and an optically rarer media are required for total internal reflection. 2. It takes place at any angle of incidence. 2. It occurs only when the angle of incidence in denser medium is greater than its critical angle. 3. Some percentage of light is absorbed by the reflecting surface. 3. The entire incident light reflects back totally without absorption at the interface of two media. Dispersion A prism is a piece of transparent material like glass, which is bounded by two triangular surfaces and three rectangular surfaces. The rectangular surfaces are called the refracting faces and the angle between two refracting faces is called angle of prism. The white light consists of seven colors in which the violet color has the least wavelength and red color has the maximum wavelength. Dispersion causes the spreading of all the colors. When white light falls at the first face of an equilateral prism, the red color bends least and the violet color bends maximum. Other colors also bend in their respective order. Thus, when white light falls on the refracting face of a prism it splits up into seven colors and gets dispersed. While passing out from the opposite face of the prism, these seven colors suffer further refraction and separate out further. Thus, the phenomenon due to which white light splits into seven colors (VIBGYOR), when passed through a prism, is called dispersion. ACTIVITY 9 1. Take a prism and keep it on the table. Ensure that the room is considerably dark for the light to be obvious. 2. When light travels through the prism, it splits the white light into 7 colors. Observe the colors and check whether you find them in the order of Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR) or not. Cause of Dispersion of White Light by a Prism When light touches the surface of the prism, refraction takes place at the boundary of the prism i.e. boundary between air and glass. Then the white light is separated into its component colors, viz. red, orange, yellow, green, blue, indigo, and violet. The seven colors in white light constitute seven different wavelengths. In a glass prism, the speed of the shortest wavelength (the violet color) is slower than the longest wavelength (the red color). The shortest wavelength (the violet color) color bends more than the longest White light beam Glass prism Dispersion of light through glass prismVIBGYOR

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 245 wavelength (the red color). Due to this, the violet color comes at the bottom and the red color comes at top on the screen. Therefore, the difference in speed of seven different wavelength colors inside a prism is the cause of the dispersion of light. FACTS WITH REASONS How is Rainbow formed? The colors of the rainbow are formed by the dispersion of light in small water droplets. A ray of white light from the sun is refracted as it enters a spherical raindrop and dispersion occurs. The dispersed light rays undergo total internal reflection. These reflected rays refracted again as they leave the water drop. The emergent rays that come out are now travelling in different directions, depending on their frequencies. So, they appear to come from different parts of the sky. Recombination of White Light: Newton's Experiments a) Two-prism experiment In this experiment, Newton took two prisms P1 and P2 of the same material and having the same refracting angle. He allowed a fine beam of white light to fall on the prism P1 . He obtained a spectrum VIBGYOR on the screen. He removed the screen and placed another prism P2 in an inverted position as shown in the given figure. It was observed that the light coming out from the prism P2 is white again. The prism P1 that disperses a white light into its constituent colors is called a dispersing prism. The second prism P2 that recombines the seven constituent colors to form a white light is called a recombination prism. White light beam Glass prism VIBGYOR White spot Coloured Beam of light white light A D E C B F Two-prism experiment b) Newton's color disc It is a circular metallic disc divided into seven sectors. These sectors are painted with the colors of the solar spectrum. The disc is made capable of rotating about an axis passing through its center. When the disc is rotated at high speed with the help of the handle, the colored sectors are no longer visible. The whole disc appears dull white in color. It proves that white color of sunlight has seven colors. HOT SKILL HIGHER ORDER THINKING SKILL 1. The 6-foot-deep swimming pool does not appear that deep. Even though the pools are deep they do not seem that way. If a pool is actually 6 feet deep, it looks like four feet. The reason for this is the refraction of light. To see an object, light reflected by it must reach to our eye. When the light reflected by the bottom of the Newton's color disc

246 ligHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur pond has to pass from water into the air, it bends away from the normal and reaches our eye. But our eye will perceive reflected light as a straight line. So, the image of the bottom of the pond is formed slightly above the actual position. 2. Ramesh has had gastric problems for many years. He has acute pain in the abdomen. He was admitted to the hospital. The doctor inserted a tube through his mouth and they were able to locate an ulcer in his intestine. What was the device used? On what principle does it work? How does it work? Describe. As informed, we learnt that a long tube was inserted through his mouth to observe any injury in the intestine. The device is an endoscope. It is a modified light pipe. It works on the principle of total internal reflection of the light. The working mechanism of the endoscope: It has a long tube that can be directed and moved with the help of the computer. It has a tiny camera that can take videos and photos and show them on the computer. It is gently inserted down the throat. Doctors control it remotely and navigate the internal parts of our body. 3. A glass jewel cut in the same shape as a diamond does not shine much. Explain. A glass jewel cut in the same shape as a diamond does not shine much because the critical angle of glass is 42° whereas that of the diamond is 24° only. So, there is too less total internal reflection of light in glass compared to the diamond. As a result, the glass jewel does not shine much even though it has the same shape as a diamond jewel. 4. I was told that the sun we see during the sunset has actually, already set, a few minutes earlier. How was already set a few minutes ago, if I can see it right now? Clarify the statement for me. Indeed, the sun we see during the sunset has actually set, a few minutes earlier. Even though the sun is set, we are still able to see it because of atmospheric refraction of light. The earth is surrounded by the atmosphere. When the sun sets behind the horizon, its parallel beam of light has to pass through the atmosphere. The upper layers are rarer and the lower layers are denser. So based on the laws of refraction, when a beam of light passes through a rarer to the denser medium they bend toward the normal. So, the sunlight bends downward and reaches us even when the sun has already set. 5. Differentiate between rarer medium and denser medium. The differences between rarer medium and denser medium are: SN Rarer medium SN Denser medium 1 The optical medium in which light travels at a greater velocity than another medium is called a rarer medium. 1 The optical medium in which light travels slower than in another medium is called a denser medium. 2 The refractive index of a rarer medium is smaller than that of a denser medium. 2 The refractive index of a denser medium is greater than that of a rarer medium. 3 Light bends away from the normal when entering rarer medium from denser. 3 Light bends toward the normal when entering denser medium from rarer. 6. Elle is a 5-year-old child. She is trying to hit a fish with coins but failed many times even if she shoots coins where the fish is seen. What could be the reason? What physical phenomenon cause this? Explain the phenomenon. Elle is a 5-year-old child who is trying to hit a fish with coins but failed every time. The reason behind it is the fish is seen above its real position. The physical phenomenon responsible for this is the refraction of light. For a fish to be seen, the reflected rays of light from fish must pass from a denser medium, water, into the