Internal vs External Pressure

[005917072]

When calculating the work done in an expansion, are there any cases in which the internal pressure will be relevant, or do you only ever need to know the external pressure for the calculation?

[ClaireN3B]

The internal pressure is important in problems where the volume is not given. In this course there are many problems where the moles of gas, temperature, and internal pressure are given, and you need to use them to calculate the volume of the gas with PV=nRT. If you are given the volume in the problem though, then the external pressure is what will be used in calculations, and you do not need to worry about the internal pressure of the species.

[alexia11]

When calculating the work done in expansion, the external pressure is necessary when given the change in volume. This is because the external pressure is what causes this change in volume and is therefore necessary in calculating the work done. Internal pressure is necessary for problems in which this change is not given, therefore you need the internal pressure to calculate W. This is because there will be no external pressure to cause a change in the volume.