Page 1 :
CHAPTER – 11, , Constant volume gas thermometer, , THERMAL PROPERTIES OF, MATTER, , Principle: -, , Heat and Temperature, Heat is the form of energy, which is, transferred from one body to, another because of their, temperature difference., , Temperature is the degree of hotness, of a body., SI unit of heat is joule (J), SI unit of temperature is Kelvin (K), Measurement of temperature, The device used to measure, temperature is called thermometer., There are two types of thermometers, , We have PV  nRT , for a low density, gas., , If volume is kept constant P α T, I.e., when temperature increases,, pressure also increases., In constant volume gas thermometers, temperature is read in terms of, pressure., Common units of temperature, The commonly used units are Degree, Celsius, Kelvin and Fahrenheit., Formulae for unit conversion, , tK = tC+ 273, , i. Liquid – in –glass type, thermometers., ii. Constant volume gas thermometers., Liquid – in –glass type, thermometers, Principle: - Thermal expansion of, liquids. I.e., when temperature, increases, volume of liquids increases., , PV α T, , i.e.,, , 9, t F  t C  32, 5, Problem1: -Convert the ice point, (00C) and the steam point (1000C) into, Kelvin and Fahrenheit scales., Soln: Ice point (00 C), , Steam point (1000C), , Commonly used liquids: - Mercury,, Alcohol., , tC  0, , tC  1000 C, , t K  0  273  273K, , tK  100  273  373K, , Question1: - Why mercury is used in, thermometer?, , 9, t F   0  32  320 F, 5, , 9, t F  100  32, 5, =180  32, , Ans: - Mercury is used because of, the following properties of it., i. It has high thermal expansivity., ii. It has a shining surface., iii. It does not wet glass., ,  2120 F, , Problem2: Temperature of a normal, human body is 98.60F. What is the, corresponding temperature shown in the, Celsius scale?, Ans:, , SAJU K JOHN, M.Sc. Physics, NET, Doctoral Research Fellow at NIT Calicut, , 1

Page 2 :
A low density gas obeys the, following two equations :, 1, V  and V  T, P, So if we combine these, , Boyle’s law, Boyle’s law states that ‘at constant, temperature, the volume of a given, mass of gas is inversely, proportional to pressure’., , 1, V, P, , equations,, T, V   PV  T, P,  PV  nRT,, PV,  Cons tan t, or, T, P1V1 P2 V2, , T1, T2, Where n is the no. of moles (n ) and, ,  PV  cons tan t, P1 V1  P2 V2, , Charles’ Law, Charles’s law states that ‘at constant, pressure, the volume of a given, mass of gas is directly proportional, to absolute temperature’., V,  cons tan t, T, V1 V2, , T1 T2, , R is the universa l g as cons tan t., R  8.31 Jmol 1K 1 ., The equation PV  nRT is known as, idea l g as equation and is obeyed by, low density gases., , Problem3: The volume temperature, graph of certain amount of perfect gas, at two pressures P1 and P2 are shown., Which pressure is larger P1 or P2?, , V T , , Ideal gas equation, , Ans:, , SAJU K JOHN, M.Sc. Physics, NET, Doctoral Research Fellow at NIT Calicut, , 2

Page 3 :
Absolute zero (O0K), , A,  T, A, A, , =  A T, A, , αA is called coefficient of area, expansion., 3. Volume expansion, , When temperature decreases, the, pressure of a low density gas, decreases. At -273.150C (or O0K) the, pressure become zero. This, temperature is known as absolute, zero., , Thermal expansion, When temperature increases, volume, of substances (solids, liquids and, gases) increases. This is called, thermal expansion., 1. Linear expansion, When temperature increases, length, of a solid (rod like structure), increases. This is called linear, expansion., ,  T, , , , =   T, , , αl is a constant called coefficient of, linear expansion., 2. Area Expansion (or superficial, expansion), When temperature increases area of a, solid substance increases. This is, called area expansion., , When temperature increases, the, volume of a substance (solid, liquid or, gas) increases. This called volume, expansion., , V,  T, V, V, = v T, V, But we can prove that, A  2 and V  3, Anomalous expansion of water, Water contracts on heating from, 00C to 40C. This is called abnormal, expansion of water., , On cooling below room temperature, up to 40C the density of water, increases. Below 40C the density of, water decreases., Water has highest density at 40C., Important environmental effect of, anomalous expansion of water., Water bodies, such as lakes and, ponds, freeze at the top first. As a lake, cools towards 4 0C, water near the, surface become denser and sinks; the, warmer, less dense water near the, bottom rises. However, once the, colder water on top reaches a, temperature below 40C, it becomes, , SAJU K JOHN, M.Sc. Physics, NET, Doctoral Research Fellow at NIT Calicut, , 3

Page 4 :
less dense and remains at the surface, itself. So a water body will not freeze, from top to bottom. If water did not, have this property, lakes and ponds, would freeze from top to bottom., , Specific heat capacity,, , Problem4: Railway lines are laid with, gaps to allow for expansion. If the gap, between steel rails 66m long is, 3.63cm at 100C, then at what, temperature will the lines just touch?, Coefficient of linear expansion for, steel is 11x10 -6/0C, , or  Q  ms  T, , Ans:, , Q, mT,  T  C hange i n t emperature, m  M ass of the subs tan ce, , s, , SI unit of specific heat capacity, is JKg -1 K  1 ., , Heat capacity (S), It is the amount of heat required to, raise the temperature of a, substance by 10C or 1K., , S, , Q, T, , SI unit is J/K, , Molar specific heat capacity, It is the amount of heat required to, raise the temperature of 1mole of a, substance by 10C or 1K., , SI unit J/mol. K, Q, nT, n  no. of moles, , C, , Molar specific heat capacity is of two, types for gases:, i. Specific heat capacity at constant, volume (CV), It is the amount of heat required to, raise the temperature of one mole, of a gas at constant volume by 10C or, 1K., , Specific heat capacity (s), It is the amount of heat required, to raise the temperature of 1kg, substance by 10C or 1K., , ii. Specific heat capacity at constant, pressure (CP), It is the amount of heat required to, raise the temperature of one mole, , SAJU K JOHN, M.Sc. Physics, NET, Doctoral Research Fellow at NIT Calicut, , 4

Page 5 :
of a gas at constant pressure by 10C, or 1K., , Mayer’s Relation, , body can be calculated by finding the, heat gained by the materials of the, calorimeter., , Cp - Cv = R, Note: Water has highest specific heat, capacity compared to other, substances., Specific heat capacity of water =, 4186JKg-1K-1, Advantages of high specific heat, capacity of water, , Change of state, Change of State of Ice on Heating, , i. Water is used to as a coolant in, automobile radiators., ii., Sea breeze & land breeze., , Calorimetry, Calorimetry is the measurement of, heat., Calorimeter: Calorimeter is a device used to, measure heat., Principle of calorimeter, When a hot body and cold body, come in contact, the heat lost by the, hot body is equal to the heat gained by, the cold body., Calorimeter consists of a metallic, vessel and stirrer both of same, material like copper or aluminium., The vessel is kept inside a wooden, jacket which contains heat insulating, materials like glass wool etc. There is, an opening in the outer jacket through, which a mercury thermometer can be, inserted into the calorimeter. When a, solid hot body is inserted in the, calorimeter, the heat lost by the hot, , Melting: - The change of state from, solid to liquid., Fusion: - The change of state from, liquid to solid., Melting point: - The temperature at, which a solid is converted into liquid., Regelation: -, , When pressure is applied, ice melts, at low temperature. If pressure is, removed, water refreezes. This, refreezing is called regelation., , SAJU K JOHN, M.Sc. Physics, NET, Doctoral Research Fellow at NIT Calicut, , 5

Page 6 :
Application of regelation: - Skating, is possible on snow due to the, formation of water below the skates., Water is formed due to the increase of, pressure and it acts as a lubricant., Vaporisation: The change of state from liquid to gas, is called vaporisation., Boiling point: - It is the temperature, at which a liquid is converted into gas., Sublimation: The change from solid state to vapour, state without passing through the, liquid state is called sublimation., Eg: - dry ice (solid CO2), iodine,, Camphor, naphthalene tablets etc., , Questions2: Water kept in earthern, pots gets cooled. Why?, Ans: Water molecules moves out, through the minute pores in the, earthern pot. When these water, molecules come in contact with air,, they evaporate by absorbing heat from, the water in the pot. So water in, earthern pots get cooled., Problem5: Find out the work done to, convert 10g of ice at -5 0C to steam at, 100 0. Specific heat capacity of ice is, 2100J/kg K, specific latent heat of, fusion of ice is 336x103J/kg, Latent, heat of vaporisation of steam is, 2250x103J/kg and specific heat, capacity of water is 4200J/kg K., Soln:, , Latent heat of fusion (Lf), It is the amount of heat required, to convert one kilogram solid, substance completely into liquid at, its melting point, without any, change in temperature., , Q,  Q = mLf., m, Latent heat of Vaporisation (Lv), It is the amount of heat required, Lf , , to convert one kilogram liquid, substance completely into gas at its, boiling point, without any change, in the temperature., , LV , , Q,  Q = mL V ., m, , SAJU K JOHN, M.Sc. Physics, NET, Doctoral Research Fellow at NIT Calicut, , 6

Page 7 :
cross section A and is inversely, proportional to the length L:,  T  TC , H  KA  H, ,  L , K  Thermal conductivity of material., SI unit of thermal conductivity is JS1M 1K 1, , Modes of Heat transfer, There are three distinct modes of heat, transfer., i. Conduction, ii. Convection, iii. Radiation, , Question3: Which is the best thermal, conductor?, Ans: - Silver, Question4: Why cooking pots have, copper coating on the bottom?, Ans: - Copper is a good conductor of, heat. So heat is uniformly distributed, over the bottom of the pot., Question5: Why people usually, prefer to give a layer of earth, insulation on the ceiling during hot, summer days?, , Conduction, Conduction is the transfer of heat, between two adjacent parts of a, body because of their temperature, difference., , T H  T e m p e ra tu re o f h o t re s e rv io r, T C  T e m p e ra tu re o f c o ld re s e rv io r, , The rate of flow of heat (H) is directly, proportional to the temperature, difference (TH-TC) and the area of, , Ans: - Concrete roofs get very hot, during summer days. Layer of earth, has very low thermal conductivity, than concrete. Therefore layer of earth, reduce heat transfer and keeps the, room cooler., Question6: A brass tumbler feels, much colder than a wooden tray on a, chilly day. Why?, Ans: Brass has greater conductivity, than wood. So when we touch a brass, tumbler, it absorbs more heat from our, body. So it feels much colder than, wood., Questions7: All thermal conductors, are electrical conductors also.” Do, you agree with this statement? If your, answer is No, clarify it., Ans:, , SAJU K JOHN, M.Sc. Physics, NET, Doctoral Research Fellow at NIT Calicut, , 7

Page 8 :
No. Mica is a good conductor of heat, but it is a bad conductor of electricity., , Heat radiation can propagate through, vacuum with a speed of 3×108m/s., , Convection, , Note: -, , Convection is the mode of heat, transfer by actual motion of matter., , Black bodies absorb and emit, radiation better than bodies of lighter, colours., , Convection is possible only in fluids., Eg: - When water in a vessel is heated, from the bottom, the hot water moves, up., Wind is another example for, convection., Convection takes place in stars., , Applications: 1. We wear white or light coloured, clothes in summer so that they, absorb least heat from the sun., 2. During winter we use dark, coloured clothes which absorb, more heat from the sun and keep, our body warm., 3. The bottoms of the utensils for, cooking food are blackened so that, they absorb maximum heat from, the fire and give it to the food, materials., Newton’s Law of cooling, According to Newton’s law of, cooling, the rate of loss of heat,, dQ, of a body is directly, dt, proportional to the temperature, difference ΔT = T2 - T1 between the, body and the surroundings., ,  dQ,  K(T2  T1 ), dt, K  is a positive cons tan t, depending on the area and, nature of the surface of the body., , Radiation, Radiation is the mode of heat, transfer without the need of any, material medium., , SAJU K JOHN, M.Sc. Physics, NET, Doctoral Research Fellow at NIT Calicut, , 8

Page 9 :
From the graph we can see that, cooling is faster initially,, then the speed of heat loss decreases., , Question8: (a) You are in a, restaurant waiting for your friend and, ordered coffee. It has arrived. Do you, add sugar in your friend’s coffee and, then wait for him or do you add sugar, after he arrives? Explain with respect, to the concept of cooling., Ans: For the coffee to be hotter when, the friend arrives, the better option is:, first mix sugar and then wait for the, friend. When we mix sugar with, coffee the temperature of the coffee, decreases. So according to Newton’s, law of cooling the rate of loss of heat, decreases., , SAJU K JOHN, M.Sc. Physics, NET, Doctoral Research Fellow at NIT Calicut, , 9