Page 102 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

Work transfer:

Referring to the process represented on the p-V diagram (Fig. 3.2.3-3) it is noted that the

volume increases during the process.

In other words the fluid is expands and the expansion work is given by

2
W =  pdV
1
2 c
n
=  n dV (since pV = C, a constant)
1 V
2 dV
= c  n
1 V
p V − p V
= 1 1 2 2 [larger pV- small pV] (3.17)
n − 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.
Since 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. 3.2.3-3

Heat transfer:

Energy balance is applied to this case (Fig.5.1) as:

U1 – Qloss - W = U2

Qloss = (U1 – U2) – W

or

W = (U1 – U2) - Qloss

Thus, in a polytropic expansion the work output is reduced because of the heat loses.

Referring to the process represented on the p–V diagram (Fig. 3.2.3-3) it is noted that

during this process the volume increases and the pressure decreases. For a perfect gas,
equation 5.4 tells us that a decrease in pressure will result in a temperature drop.

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