Page 110 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

Work transfer:

Work transfer (pdV) must be zero because the change in volume, dV, during the process

is zero. However, work in the form of paddle-wheel work may be transferred.

Heat transfer:

Applying the non flow energy equation

Q – W = U2 – U1

gives Q – 0 = U2 – U1
i.e. Q = U2 – U1 (3.19)

This result, which is important and should be remembered, shows that the nett amount
of heat energy supplied to or taken from a fluid during a constant volume process is equal

to the change in the internal energy of the fluid.

e) Constant pressure process

If the change in pressure during a process is very small then that process may be

approximated as a constant pressure process. For example, heating or cooling a liquid at
atmospheric pressure may be analysed by assuming that the pressure remains constant.

P

W

1 2

W

v

Q v1
v2

Figure 3.2.3-5 Constant pressure process

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