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Heat And Thermodynamics
S A N JAY C H O P R A
CHAPTER 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 temperature 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 condition for thermal equilibrium. According to kinetic theory of gases, temperature 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 temperature 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 thermometry.
The linear variation of some physical property with the change in temperature
is the basic principle of thermometry and these properties are defined as thermometric 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 = a x0 + b
100 = a x100 + b
Tc = a x + b
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From these equations
x − x0
Tc − 0
=
100 − 0 x100 − x0
Tc =
x − x0
× 1000C
x100 − x0
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 coexist, 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
x
Tk = 273.16
[ xtr ]
Temperature Scale Conversion’: Temperature in one scale can be converted
into another using
C − 0 K − 273 F − 32
x − FP
=
=
=
100
100
180
BP − FP
Latent Heat
When 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 transformation.
Units: The units in SI are J/kg and in cgs calorie/gram.
Latent Heat of Fusion
The 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 internal 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 because 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 temperature 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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