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11.HEAT AND THERMODYNAMICS eLearn.Punjab

Thus if the gas expands and does external work, it is
done at the expense of the internal energy of its mol-
ecules and, hence, the temperature of the gas falls.
Conversely an adiabatic compression causes the tem-
perature of the gas to rise because of the work done
on the gas.

Adiabatic change occurs when the gas expands or is
compressed rapidly, particularly when the gas is con-
tained in an insulated cylinder. The examples of adia-
batic processes are

Fig. 11.6(b)

(i) The rapid escape of air from a burst tire.

(ii) The rapid expansion and compression of air through which a sound wave is passing.

(iii) Cloud formation in the atmosphere.

In case of adiabatic changes it has been seen that

PV g = Constant

where, g is the ratio of the molar specific heat of the gas at constant pressure to molar specific heat

at constant volume. The curve representing an adiabatic process is called an adiabat (Fig. 11.6 b).

11.5 MOLAR SPECIFIC HEATS OF A GAS

One kilogram of different substances contains different number of molecules. Sometimes it is
preferred to consider a quantity called a mole, since one mole of any substance contains the same
number of molecules. The molar specific heat of the substance is defined as the heat required to
raise the temperature of one mole of the substance through 1 K. In case of solids and liquids the
change of volume and hence work done against external pressure during a change of temperature
is negligibly small. But same cannot be said about gases which suffer variation in pressure as well
as in volume with the rise in temperature. Hence, to study, the effect of heating the gases, either
pressure or volume is kept constant.

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