HAPTER LEVEN - NCERT

1 CHAPTER ELEVENTHERMAL PROPERTIES OF INTRODUCTIONWe all have common sense notions of heat and is a measure of hotness of a body. A kettlewith boiling water is hotter than a box containing ice. Inphysics, we need to define the notion of heat, temperature,etc., more carefully. In this chapter, you will learn what heatis and how it is measured, and study the various proceses bywhich heat flows from one body to another. Along the way,you will find out why blacksmiths heat the iron ring beforefitting on the rim of a wooden wheel of a horse cart and whythe wind at the beach often reverses direction after the sungoes down. You will also learn what happens when water boilsor freezes, and its temperature does not change during theseprocesses even though a great deal of heat is flowing into orout of TEMPERATURE AND HEATWe can begin studying thermal properties of matter withdefinitions of temperature and heat.

2 Temperature is a relativemeasure, or indication of hotness or coldness. A hot utensilis said to have a high temperature, and ice cube to have alow temperature. An object that has a higher temperaturethan another object is said to be hotter. Note that hot andcold are relative terms, like tall and short. We can perceivetemperature by touch. However, this temperature sense issomewhat unreliable and its range is too limited to be usefulfor scientific know from experience that a glass of ice-cold water lefton a table on a hot summer day eventually warms up whereasa cup of hot tea on the same table cools down. It means thatwhen the temperature of body, ice-cold water or hot tea inthis case, and its surrounding medium are different, heattransfer takes place between the system and the surroundingmedium, until the body and the surrounding medium are atthe same temperature.

3 We also know that in the case of glasstumbler of ice-cold water, heat flows from the environment and equation andabsolute heat of s law of coolingSummaryPoints to ponderExercisesAdditional Exercises2022-23the glass tumbler, whereas in the case of hottea, it flows from the cup of hot tea to theenvironment. So, we can say that heat is theform of energy transferred between two (ormore) systems or a system and itssurroundings by virtue of temperaturedifference. The SI unit of heat energytransferred is expressed in joule (J) while SI unitof temperature is Kelvin (K), and degree Celsius(oC) is a commonly used unit of an object is heated, many changes maytake place. Its temperature may rise, it mayexpand or change state. We will study the effectof heat on different bodies in later MEASUREMENT OF TEMPERATUREA measure of temperature is obtained using athermometer.

4 Many physical properties ofmaterials change sufficiently with such properties are used as the basis forconstructing thermometers. The commonly usedproperty is variation of the volume of a liquidwith temperature. For example, in commonliquid in glass thermometers, mercury, alcoholetc., are used whose volume varies linearly withtemperature over a wide range. Thermometers are calibrated so that anumerical value may be assigned to a giventemperature in an appropriate scale. For thedefinition of any standard scale, two fixedreference points are needed. Since allsubstances change dimensions withtemperature, an absolute reference forexpansion is not available. However, thenecessary fixed points may be correlated to thephysical phenomena that always occur at thesame temperature.

5 The ice point and the steampoint of water are two convenient fixed pointsand are known as the freezing and boilingpoints, respectively. These two points are thetemperatures at which pure water freezes andboils under standard pressure. The two familiartemperature scales are the Fahrenheittemperature scale and the Celsius temperaturescale. The ice and steam point have values32 F and 212 F, respectively, on the Fahrenheitscale and 0 C and 100 C on the Celsius the Fahrenheit scale, there are 180 equalintervals between two reference points, and onthe Celsius scale, there are plot of Fahrenheit temperature (tF) versusCelsius temperature (tc). A relationship for converting between the twoscales may be obtained from a graph ofFahrenheit temperature (tF) versus celsiustemperature (tC) in a straight line (Fig.)

6 ,whose equation isttFC 32180100=( ) EQUATION ANDABSOLUTE TEMPERATUREL iquid-in-glass thermometers show differentreadings for temperatures other than the fixedpoints because of differing expansion thermometer that uses a gas, however, givesthe same readings regardless of which gas isused. Experiments show that all gases at lowdensities exhibit same expansion behaviour. Thevariables that describe the behaviour of a givenquantity (mass) of gas are pressure, volume, andtemperature (P, V, and T)(where T = t + ;t is the temperature in C). When temperatureis held constant, the pressure and volume of aquantity of gas are related as PV = relationship is known as Boyle s law, afterRobert Boyle (1627 1691), the English Chemistwho discovered it. When the pressure is heldconstant, the volume of a quantity of the gas isrelated to the temperature as V/T = relationship is known as Charles law,after French scientist Jacques Charles (1747 1823).

7 Low-density gases obey theselaws, which may be combined into a singleTHERMAL PROPERTIES OF MATTER2792022-23280 PHYSICS relationship. Notice that since PV = constantand V/T = constant for a given quantity of gas,then PV/T should also be a constant. Thisrelationship is known as ideal gas law. It can bewritten in a more general form that applies notjust to a given quantity of a single gas but to anyquantity of any low-density gas and is known asideal-gas equation:PVRT =or PV = RT( )where, is the number of moles in the sampleof gas and R is called universal gas constant:R = J mol 1 K 1In Eq. , we have learnt that the pressureand volume are directly proportional totemperature : PV T. This relationship allows agas to be used to measure temperature in aconstant volume gas thermometer.

8 Holding thevolume of a gas constant, it gives P T. Thus,with a constant-volume gas thermometer,temperature is read in terms of pressure. A plotof pressure versus temperature gives a straightline in this case, as shown in Fig. , measurements on real gases deviatefrom the values predicted by the ideal gas lawat low temperature. But the relationship is linearover a large temperature range, and it looks asthough the pressure might reach zero withdecreasing temperature if the gas continued tobe a gas. The absolute minimum temperaturefor an ideal gas, therefore, inferred byextrapolating the straight line to the axis, as inFig. This temperature is found to be C and is designated as absolute zero is the foundation of the Kelvintemperature scale or absolute scale temperaturenamed after the British scientist Lord Kelvin.

9 Onthis scale, C is taken as the zero point,that is 0 K (Fig. ).The size of unit in Kelvin and Celsiustemperature scales is the same. So, temperatureon these scales are related byT = tC + ( ) THERMAL EXPANSIONYou may have observed that sometimes sealedbottles with metallic lids are so tightly screwedthat one has to put the lid in hot water for sometime to open it. This would allow the metallic lidto expand, thereby loosening it to unscreweasily. In case of liquids, you may have observedthat mercury in a thermometer rises, when thethermometer is put in slightly warm water. Ifwe take out the thermometer from the warmFig. versus temperature of a lowdensity gas kept at constant plot of pressure versus temperature andextrapolation of lines for low density gasesindicates the same absolute of the Kelvin, Celsius andFahrenheit temperature PROPERTIES OF MATTER281water the level of mercury falls again.

10 Similarly,in case of gases, a balloon partially inflated in acool room may expand to full size when placedin warm water. On the other hand, a fullyinflated balloon when immersed in cold waterwould start shrinking due to contraction of theair is our common experience that mostsubstances expand on heating and contract oncooling. A change in the temperature of a bodycauses change in its dimensions. The increasein the dimensions of a body due to the increasein its temperature is called thermal expansion in length is called linearexpansion. The expansion in area is called areaexpansion. The expansion in volume is calledvolume expansion (Fig. ).Fig. Thermal the substance is in the form of a long rod,then for small change in temperature, T, thefractional change in length, l/l, is directlyproportional to T.