CHP # 8 HERMA PROPERIES OF MAER Q.1 Differentiate between heat and temperature? (Ans) Heat It can be defined as "the sum of kinetic energy of the molecules present in a substance is called heat". Heat is a form of energy. It is denoted by "Q". Its SI unit is joule "j". emperature he degree of hotness or coldness of a body is called temperature. OR It can be defined as "the average kinetic energy of the molecules present in a substance is called temperature". It is denoted by "". Its SI unit is Kelvin "K". Q.2 Explain the principle, calibration and various scales of temperature. (Ans) Measurement of temperature emperature is measured by an instrument called thermometer. hermometer is based on the following principle. Principle Any property of a substance that increases or decreases uniformly with temperature can be used for measurement of temperature. Mercury and Alcohol thermometer are based on this principle. Calibration It involves two steps; (i) wo standard reference temperatures are decided. (a) Freezing point (b) Boiling point (ii) A scale of temperature is established by dividing the interval of emperature into a number of equal sub-intervals. C 100 212 373 F K 100 Equal parts 180 Equal parts 100 Equal parts 0 32 273
Scales of temperature here are three scales of temperature. (1) Celsius or Centigrade scale his scale was introduced by a Swedish Astronomer Celsius. It is denoted by C. Its ice point is 0 C and steam point is 100 C. he interval between ice point and steam point is divided into 100 equal parts, each part is called a degree centigrade. (2) Fahrenheit scale his scale was introduced by Fahrenheit. It is denoted by F. Its ice point is 32 F and steam point is 212 F. he interval between ice point and steam point is divided into 180 equal parts, each part is called a degree Fahrenheit. (3) Kelvin or Absolute scale his scale was introduced by ord Kelvin. It is denoted by K. Its ice point is 273K and steam point is 373K. he interval between ice point and steam point is divided into 100 equal parts, each part is called one Kelvin. Relationship between different scales of temperature. 5 C ( F 32) 9 9 F ( C) 32 5 K C 273 C K 273 Q.3 What is thermometer? On what principle does it work. (Ans) hermometer An instrument used for measuring temperature is called thermometer. Construction A thermometer consists of a narrow glass tube having one end closed and the other contains a metallic bulb. he tube is filled with a specific liquid. Calibrations are placed on the glass tube on suitable scale which can be read easily.
Working It works on the principle that matter expands on heating and contract on cooling. hus the degree of expansion and contraction of matter can be recorded by the calibration on the tube. Q.4 What is meant by thermal expansion? Describe the thermal expansion of solids. (Ans) hermal expansion he process of increase in physical structure (length, area, volume) of a body due to rise in temperature is called thermal expansion. hermal expansion of solids he process of increase in physical structure (length, area, volume) of a solid body due to rise in temperature is called thermal expansion. Explanation When heat is supplied to a body the K-E of molecules of the body increases due to which amplitude of vibrations of molecules increases. As a result the body expands. Q.5 What do you meant by inear thermal expansion? Derive an expression for linear thermal expansion. (Ans) inear thermal expansion he increase in length of a solid material due to rise in temperature is called linear thermal expansion. Mathematical derivation Consider a rod of length O having temperature 1. After heating the temperature of the rod is raised to 2 and its length becomes. O Δ Change in length = Δ = - o Change in temperature = Δ = 2 1 As from linear thermal expansion the change in length is directly proportional to the original length and change in temperature. Mathematically
Const ) ( Where is constant of proportionality and are called coefficient of linear thermal expansion. Mathematically o find the final length of the rod we have; ) 1 ( Q.6 What do you meant by olumetric thermal expansion? Derive an expression for volumetric thermal expansion. (Ans) olumetric thermal expansion he increase in volume of a solid material due to rise in temperature is called volumetric thermal expansion. Mathematical derivation Consider the original olume of a solid is O having temperature 1. After heating the temperature of the rod is raised to 2 and its olume becomes. Change in olume = Δ = - o Change in temperature = Δ = 2 1 As from olumetric thermal expansion the change in olume is directly proportional to the original olume and change in temperature. Mathematically Const ) (
Where is constant of proportionality and are called coefficient of volumetric thermal expansion. Mathematically 3 o find the final volume of the rod we have; ( 1 ) Q.7 Write some practical applications of thermal expansion of solids. (Ans) Practical applications of solids are as under. (i) Railway lines We know that solids expand when its temperature rises. So a gap is left between the rails of a railway lines. In order to avoid bending of railway line because in summer the temperature of the railway line rises. he gap provides room for expansion, otherwise it will bend. (ii) (iii) (iv) (v) o remove a tight glass stopper from a bottle When a bottle top is too tight to open, we can loosen the top by placing it in hot water for a while. he top expands and can be opened easily by doing so. Fixing of an iron ring to a cart wheel he diameter of the iron rim is slightly lesser at room temperature than the diameter of the wooden wheel. he rim is heated, so it expands on heating and can be placed around the wooden wheel. When the ring comes to room temperature it contracts and produces a tight fit. Gaps between the roof girders and steel bridges Gaps are left at the ends of metallic roof girders and bridges to give room for expansion. Usually one end of the metallic structure is fixed and the other end is allowed to expand in summer season. Bimetallic strip wo metallic strips whose co-efficient of expansion are different welded together to form a bimetallic strip. In a brass, iron strip brass expands more rapidly than iron. hus the strip bends as shown below.
Q.8 What is meant by a thermostat? (Ans) thermostat it is actually a bimetallic strip which may be used to open and close electrical circuits. One end of the strip is open and the other end is fixed. he moveable end of the strip changes its position with the change in temperature. Hence the circuit is automatically opened or closed. Figure Q.9 State and explain thermal expansion of liquids. (Ans) hermal expansion of liquids he liquids also expand on heating. As the liquids have no definite shape, they acquire the shape of the container. So we consider only volumetric expansion in case of liquids. he volumetric expansion of liquids is greater than the volumetric expansion of solids, because in liquids the volumetric expansion occur in two ways. (i) Real expansion (ii) Apparent expansion in case of real expansion we take the expansion of container as well as the expansion of liquid.
While in case of apparent expansion we take only the expansion of liquid. he real and apparent expansion of liquid can be demonstrated with the help of the following experiment. Experiment Consider a flask which is filled with coloured water as shown in the following figure. Flask is fitted with a cork and a glass tube is passed through the tube. et the water rises to point A before heating. Now if we supply heat to the flask, then the heat is first absorbed by the flask due to which the flask expands and its volume increases. As a result the level of water falls from A to B. On further heating, the heat is transferred to the liquid. Now the liquid expands and rises to point A and then C. he expansion between point A and C is apparent expansion. While the expansion between points B and C is known as real expansion. Mathematically Real expansion = apparent expansion + expansion of the flask Apparent expansion = real expansion expansion of the flask Q.10 Define heat capacity and specific heat capacity? (Ans) Heat capacity he amount of heat required to raise the temperature of entire mass of the substance by one Kelvin is called heat capacity. It is denoted by C m. Mathematically Q C m he SI unit of heat capacity is joule/kelvin (JK -1 ). Specific heat capacity he amount of heat required to raise the temperature of one kilogram mass of the substance by one Kelvin is called specific heat capacity. It is denoted by C. Mathematically Q C m he SI unit of specific heat capacity is (JK -1 Kg -1 ). Q.11 What are the different effects of high specific heat of water? (Ans) Water has particularly high specific heat. Its value is 4200Jk -1 Kg -1. his makes water a very useful material for storing and carrying energy. Water is used for the following purposes due to high specific heat.
(i) (ii) (iii) Hot water bottles A hot water bottle remains warm for more than a hour and can be used for therapeutic and other purposes. Water as coolant in radiators Because of high specific heat of water, it carries unwanted heat from the engine of a car to the radiator. Internal heating of buildings In cold winter, water is used for internal heating. he hot water can carry a large amount of water from the furnace to the room and keep them at a moderate temperature. Q.12 Define and explain atent heat of Fusion of solids? (Ans) atent heat of fusion he quantity of heat required to melt a unit mass (1Kg) of solid without changing its temperature is called atent heat of fusion of solid. It is denoted by f. Mathematically Q f m he SI unit of atent heat of fusion is joule/kilogram (j/kg). Example he amount of heat required to transfer one kilogram ice into water at 0 C is known as latent heat of fusion of ice. Its value has been found to be 5 3 3410 J / Kg. Explanation When heat is supplied to a solid body, then the K-E of the molecules of the body increases and the molecules begin to separate from each other. he solid begins to melt. his process continues until the whole solid is converted into liquid. During melting process the temperature of the solid remains constant. his heat is called latent heat of fusion of the solid. Q.13 Define and explain atent heat of vaporization of liquids? (Ans) atent heat of vaporization he quantity of heat required to boil a unit mass (1Kg) of liquid without changing its temperature is called atent heat of vaporization of liquid. It is denoted by v. Mathematically Q m he SI unit of atent heat of vaporization is joule/kilogram (j/kg).
Example he amount of heat required to transfer one kilogram water into steam at 100 C is known as latent heat of vaporization of water. Its value has been 5 found to be 2 2610 J / Kg. Explanation When heat is supplied to a liquid, then the K-E of the molecules of the body increases and the intermolecular attractive forces between the molecules of the liquid becomes weaker. As a result the liquid begins to boil. his process continues until the whole liquid is converted into steam. During boiling process the temperature of the liquid remains constant. his heat is called latent heat of vaporization of the liquid. Q.14 Define the evaporation of liquids. On what factors does it depend? (Ans) Evaporation he process in which a liquid slowly change to vapours at any temperature (below its boiling point) is called evaporation. (i) (ii) (iii) (iv) (v) Evaporation of liquids depends upon the following factors. Nature of liquid iquids with low boiling points evaporate quickly than those of high boiling points. Example Rate of evaporation of alcohol is higher than water. emperature of liquid and the surrounding iquids with high temperature evaporate quickly than liquids with low temperature. Higher the temperature of the surrounding higher the rate of evaporation and vice versa. Example Under a hot iron wet clothes dry out quickly as the water evaporates. Wet clothes dry more quickly in hot summer season as compare to winter. Presence of water vapours in air Higher the amount of water vapours in air, smaller will be the rate of evaporation and vice versa. Example Wet clothes dry slowly in rainy season as a lot of water vapours are present in air. Dryness of air drier the air, the rapid will be the process of evaporation and vice versa. Example Wet clothes dry quickly on a dry day and slowly on a humid day. Air pressure on the surface of liquid Greater the air pressure on the surface of the liquid, lesser will be the rate of evaporation.
Q.15 How evaporation produce cooling? Also write some applications of cooling by evaporation. (Ans) Cooling effect of evaporation during evaporation more energetic molecules escape from the liquid surface. Molecules that remain in the liquid have less K-E. as a result cooling effect produced due to evaporation. Applications of cooling by evaporation (i) Cooling by fans We use fans in the hot summer season because the moving air increases the rate of evaporation or perspiration from our bodies. (ii) Fever control Usually a wet towel is placed on the forehead of a person having high fever. It is because the water evaporates which take heat away from the head. hus the temperature of the head remains within the safe limits and the patient does not suffer any brain damage. (iii) Refrigerators the cooling effect in many refrigerators is produced by evaporation of a volatile liquid called Freon. he liquid Freon evaporates in the pipes of freezer compartment. As the Freon evaporates, it draws the necessary latent heat from the food inside the refrigerators.
CONCEPUA QUESIONS 1. Why are small gaps left behind the girders mounted in walls? A. See questions No.7 part (iv) 2. Why liquids have two co-efficient of expansion? A. iquids have no particular shape. So we can supply heat to a liquid in a container. he liquid as well as container expands with heat. herefore liquids have two co-efficient of expansion. 3. Water melts to form ice at 0 C. At what temperature does water freezes to form ice? A. he freezing or ice point of water is 0 C, same is the melting point of ice. Because during melting the temperature remain constant and the heat is absorbed by the ice to change the state of matter. Similarly liquid is change into solid during freezing and the energy is losses by the water. 4. Why is water used as coolant in radiators of automobile engines? A. See question No.11 part (ii) 5. Why do we sweat in summer? A. In summer the temperature of surrounding becomes more than the temperature of our body. his high temperature affects our body. o keep our body at suitable temperature the extra heat comes out in the form of sweat from the pores of our body and evaporates from the skin. 6. What is the effect of high specific heat of water on the climate of coastal areas? A. During the day the shines equally on the land and sea. he land heats up more quickly than the sea, because of high specific heat of water. the hot air over the land rises and the cold air from the sea blows to replace it. hus there is a sea breeze during the day. At night the process is reversed, the land cools quickly than the sea. As a result the cold air blows from the land and the hot air rises from the sea to replace it. herefore the temperature of the coastal areas remains at moderate level. 7. Why steam at 100 C produce more severe burns than boiling water at 100 C? A. When the temperature of water reaches to 100 C, it is converted to steam. At this time the temperature remains constant. Although heat is being given to water which is absorbed as latent heat of vaporization 5 2 2610 J / Kg. herefore steam at 100 C produce more severe burns than boiling water at 100 C.
8. Why does the temperature not change during a change of state? A. he temperature does not change during change of state of matter, because in this process the heat supplied is busy in breaking the bonds of the molecules. 9. An iron rim is fixed around a wooden wheel is heated before fixture. Explain A. See questions No.7 part (iii) 10. If a hot piece of thick glass is dipped in cold water, it breaks. Give reason A. he K-E of the molecules of the hot thick glass is greater due to which the molecules expands. When we dipped it in cold water it contracts quickly. his sudden change in temperature causes cracks in the thick glass as a result it breaks. 11. Why do soda water bottles often burst in summer? How can the bursting be minimized. A. Soda water bottles contain gases and it often burst in summer. Because in summer the temperature rises which also affect the K-E of the gases inside the bottled. his bursting of the bottle can be minimized by leaving some space at the top of soda water bottle. 12. Why is the water at the bottom of a water fall is warmer than water at the top of water fall? A. We know that temperature is directly proportional to the K-E. he K-E of the molecule at the bottom is greater than the K-E of the molecules at the top of the water fall. herefore the water at the bottom of water fall is warmer than the top water. 13. Does the land cools at slower rate than sea water? Give one reason for your answer. A. and cools at faster rate than sea water. Reason It is because of high specific heat of water. Sea water takes a lot of time to lose heat which is absorbed throughout the day. 14. Why we feel cool after perspiration? A. See question No.15 part (i)
15. Why is ice at 0 C a better coolant of soft drink than water at 0 C? A. Because for ice to transform into water it must absorb heat, called the latent heat of melting. his heat is required to break down the crystalline structure of the ice. he heat is taken from the drinks/food and thus makes them colder than they would be if kept in the same amount of cold water. Numerical Problems 1. A person running fever has a temperature of 104 F. What is his temperature in degree centigrade ( C)? Given data = 104 F =? C Solution 5 C = ( F 32) 9 5 C (104 32) 9 5 C (72) 9 C = 40 C 2. A railway line 1200Km long is laid at 25 C. By how much will it contract in winter when the temperature falls to 15 C? By how much will it expand when the temperature rises to 40 C in summer? 6 1 (he coefficient of linear expansion is 1210 C ) Given data 1200Km 0 0 0 W S 12001000m 1200000m 25 C 15 C 40 C W? S? Solution o calculate the contraction in winter W 25 15 10 C
W W 1200000 1210 6 10 6 W 144000000 10 144m o calculate the expansion in summer W 40 25 15 C S 6 1200000 1210 15 S 6 S 216000000 10 216m 3. he volume of a brass ball is 800cm 3 at 20 C. Find out the new volume of the ball if the temperature is raised to 52 C? he coefficient of volumetric expansion of brass is 57 10 6 1 C. Given data 3 800cm 20 C 52 C 57 10 6 1 C.? Solution 52 20 32 C 6 8005710 32 6 3 1459200 10 1 46cm 3 New volume of ball = 800+1 46=801 46cm 4. A brass disc at 293K has a diameter of 030m and a hole of diameter 010m is cut in the centre. Calculate the diameter of the hole when the temperature of the disc is raised to 323K? Coefficient of linear 6 1 expansion of brass is1910 K. Given data Diameter of disc = D = 0 30m Diameter of hole = D = 0 10m 293K 323K 6 1 1910 K Diameter of the hole =? 323 293 30K Solution
D D D 0 101910 D 5710 6 6 30 D 0 000057m Diameter of the hole = 0 10 0000057 0100057m 5. 05 Kg of copper needs 1950J of heat to raise its temperature through 10 C. Calculate the heat capacity of sample? Given data m = 0 5Kg Q = 1950J 10 C C? Solution Q C 1950 C 10 C 195JK -1