Page 92 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

Note that after expansion, p2 is smaller than p1. In the p – V diagram, the shaded area
under the process represents the amount of work transfer.

As this is an expansion process (i.e. increase in volume) the work is done by the system.

In other words, the system produces work output and this is shown by the direction of

the arrow representing W (as shown in Fig 3.2.3-2).

Heat transfer:

In an adiabatic process, Q = 0.
Applying an energy balance to this case (Fig3.2.3-2):

U1 - W = U2

W = U1 – U2

Thus, in an adiabatic expansion the work output is equal to the decrease in internal

energy. In other words, because of the work output the internal energy of the system
decreases by a corresponding amount.

For a perfect gas, U1 = mcvT1 and U1 = mcvT1

On substitution

W = mcv(T1-T2) [larger T- smaller T] (3.10)

We know

cp- cv = R
or

R
cv =
 − 1

Substituting in equation 3.10

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