For any process we can use the combined Boyle’s and Charles’ laws expression,
p1V1/T1 = p2V2/ T2

If we apply this to a constant pressure, constant volume or isothermal (constant temperature) process, all we need do is strike out the constant term before we start calculating.

The use of other expressions will depend upon the particular process. We will consider the following non-flow processes, e.g. gas in an engine cylinder.

Constant pressure
The gas is held at constant pressure as the volume changes. This would require the addition or extraction of heat energy. For instance, if the piston is moved up the cylinder, the heat energy produced would need to be taken away if the pressure was to remain constant

Constant volume
The volume remains constant, i.e. the piston is fixed. Clearly, the only process which can occur is heating or cooling of the gas. It is important to remember that these are the only two processes which are straight lines on the p/V diagram.

Adiabatic compression and expansion
During an adiabatic process, no heat transfer occurs to or from the gas during the process. This would require the perfect insulation of the cylinder, which is not possible, and it is worth noting that even the insulation itself will absorb some heat energy.

Polytropic expansion and compression
This is the practical process in which the temperature, pressure and volume of the gas all change. All gas processes in the real world are polytropic – think of the gas expanding and compressing in an engine cylinder.

Isothermal process
In this case, which is Boyle’s law, the temperature of the gas remains constant during the process. Like the adiabatic process, this cannot be achieved in practice.

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