Heat And Thermodynamics

S A N J AY C H O P R A

C H A P T E R 1

Thermal Properties Of Matter

Heat Heat is a form of energy which appears when two bodies at different temperature are placed into thermal contact. It can flow from high temperature to low tempera-ture till temperature of the two bodies becomes same. Thus, we can say that heat is the energy in transit. Heat is not property of system, a system can give out heat or can absorb heat but it does not contain heat.

The energy associated with the random motion and configuration of the atoms and molecules within the body is called internal energy and a part of this internal energy which is transferred from one body to another due to temperature difference is called heat energy.

Heat is a form of energy which can be transformed into other energies and vice versa. For e.g in thermocouple heat energy is converted into electrical energy or resistor can convert electrical energy into heat energy when current flows through it.

Temperature: Temperature is defined by zeroth law of thermodynamics which states that when two bodies are separately in thermal equilibrium with the third system or body then they are in thermal equilibrium with each other also. Temperature is a scalar quantity which is a property of thermodynamic systems such that equality of temperature is necessary and sufficient con-dition for thermal equilibrium. According to kinetic theory of gases, tempera-ture is measure of average translational kinetic energy of a molecule. Temperature of the body can be raised without limit but theoretically limiting low temperature is taken as zero of the kelvin scale.

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As temperature is measured by the value of thermodynamic property which varies linearly with temperature, two fixed points are needed to define the tempera-ture scale.

These two fixed points in modern thermometry are

1.Triple point of water i.e. The state of water where solid , liquid and gaseous phases of water co exist in equilibrium. It is characterized by unique value of temperature and pressure. Triple point of water is 273.16

2.On this scale the other fixed point may be taken as absolute zero.

Thermometry Instrument used to measure the temperature is called thermometer. The

branch of physics which deals with the measurement of temperature is called ther-mometry.

The linear variation of some physical property with the change in temperature is the basic principle of thermometry and these properties are defined as thermo-metric properties of the substance [x]. The thermometric property can be

1.Length of liquid in capillary

2. Pressure of gas at constant volume

3.Volume of gas at constant pressure

4.Resistance of given platinum wire.

Earlier, two arbitrary fixed points ice point and steam point are used to define the temperature scale. In Celsius scale the freezing point of water is 0 and boiling point or steam point as 100C and the temperature interval between these points is divided into 100 equal parts.

So if thermometric property at 0,100 and T is

0 = ax0 + b

100 = ax100 + b

Tc = ax + b

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From these equations

Tc 0100 0 =

x x0x100 x0

Tc =x x0x100 x0

1000C

Thus, if we know the value of thermometric property at ice point, steam point and at any given temperature, we can find that temperature.

In modern thermometry, instead of two fixed points only one reference point is taken which is triple point of water or 273.16K at which ice water and steam coex-ist, the other point we use is 0K where value of thermometric property is zero.

So if the values of thermometric property at 0K, 273.16K and TkK are 0, xTr, and x respectively, then

0=a[0] +b

273.16= axtr + b

Tk = ax + b

Solving

Tk = 273.16[ xxtr]Temperature Scale Conversion: Temperature in one scale can be converted

into another using

C 0100 =

K 273100 =

F 32180 =

x FPBP FP

Latent HeatWhen substance changes from one state to another [say solid to liquid or liquid to gas ] then energy is either absorbed or liberated. The energy absorbed or liberated to change the state of one gram of substance is called its latent heat. There is no change in temperature when subsonic changes the state. This means that phase

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transformation is an isothermal process. Latent heat is also called heat of transfor-mation.Units: The units in SI are J/kg and in cgs calorie/gram.Latent Heat of FusionThe latent heat of fusion is the heat energy required to change 1kg of material in solid state at its melting point to 1 kg of material in liquid state.. It is also amount of energy released when 1 kg of liquid at freezing point changes into 1kg os solid. For water at normal freezing point of 00 C the latent heat of fusion is 80 cal/gm or 336KJ/kg.

Latent Heat Of Vaporization:The latent heat of vaporization is the heat energy required to change 1 kg of

liquid at its boiling point to 1kg os material in its gaseous state. It is also amount of energy released when 1 kg of material changes into 1 kg of material in liquid state at its condensation temperature. For water, the latent heat of vaporization is 540cal/gm or 2260KJ/kg.

Important Points [based on Latent Heat] In the process of melting or boiling, heat supplied is used to increase the inter-

nal potential energy of the substance and also in doing work against the external pressure while internal kinetic energy remains constant. This is the reason that internal energy of steam of 1000C is more than internal energy of water at 1000C

Steam burns are more painful than water burns because when steam a1000C is converted into water at 1000C it gives out 536 cal of heat.

Latent heat of vaporization is more than the latent heat of fusion because when the state of substance changes from liquid to gas there is large increase in volume also. Thus, heat is required for change in volume also. But if solid changes to liquid then change in volume is negligible and heat it required mainly to change the state only. Thus, latent heat of vaporization is more than latent heat of fusion.

After snow fall the temperature of atmosphere becomes very low. This is be-cause the snow absorbs heat from from the atmosphere to melt down. So, in the mountains, when snow falls, one does not feel cold but when ice melts tempera-ture will fall.

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There is more shivering effect of ice cream on teeth as compared to that of water [obtained] from ice because when ice melts down, it absorbss large amount of heat from the body.

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