Change in Pressure

[Emmet Asker]

Why is it that only a change in volume results in a change in work? Is this understanding incorrect? And if not could someone explain why changes in pressure mean we don't calculate work?

[Michelle Fong 3F]

Hello! Work is defined as a force acting over a distance. When all other factors are held constant, we know that a change in pressure will equal an opposite change in volume (ex. decreasing the volume of a container of gas increases the pressure of the gas on the container) from the ideal gas law PV = nRT. For example, if a piston compresses a system then the energy of the system has increased by an amount equal to the energy used as work of expansion. This is because the piston is acting as an external pressure that the system must push against, therefore the system is doing work. Hope this helps!

[Roni Nissim 1D]

I'm confused as to why -p delta h is negative.

[Rebecca Liu 2E]

The negative in front of the P in the work equation is because the system is losing energy while pushing against external pressure from the surroundings. For example, in the work of expansion, change in volume is positive, but the work is negative due to that negative sign in front of the formula.