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11.HEAT AND THERMODYNAMICS eLearn.Punjab
Isothermal Process

It is a process which is carried out at constant temperature and hence the condition for the application

of Boyle’s Law on the gas is fulfilled. Therefore, when gas expands or compresses isothermally,

the product of its pressure and volume during the process remains constant. If P1, V1are initial
pressure and volume whereas P2, V2are pressure and volume after the isothermal change takes
place (Fig.11.6 a), then

P1V1 = P2V2

In case of an ideal gas, the P.E. associated with its molecules is zero, hence, the internal energy of
an ideal gas depends only on its temperature, which

in this case remains constant, therefore, 3U = 0.

Hence, the first law of thermodynamics reduces to

Q=W

Thus if gas expands and does external work W, an
amount of heat Q has to be supplied to the gas
in order to produce an isothermal change. Since
transfer of heat from one place to another requires
time, hence, to keep the temperature of the gas
constant, the expansion or compression must take
place slowly. The curve representing an isothermal
process is called an isotherm (Fig. 11.6a).

Fig. 11.6(a)

Adiabatic Process
An adiabatic process is the one in which no heat enters or leaves the system. Therefore, Q = 0 and
the first law of thermodynamics gives

W = −∆U

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