3 Ways of Changing Energy in a System

[narek kurkjian 2I]

I know that in lecture we learned that there are 3 ways to change the energy in a system, including adding or removing a substance, heating or cooling a system, or doing work/ letting the system do work on surroundings, but I was wondering what it means to do "work" on a system and what examples there are besides a piston.

[Andy Sun 2I]

Yes, 3 ways of changing energy in a system are:
1. adding/removing substance (open system)
2. heating/cooling (closed system)
3. Do work

Work refers to the changes to the internal energy of a system caused by exerting some force over some distance. A system that does work on the surroundings loses energy so its internal energy decreases, while a system that has work done on it gains that internal energy. ]

W= Force x distance
= pressure x area x distance
= pressure x (change in Volume)
= change in n(moles)RT ---> due to PV=nRT

Therefore, anything that changes the pressure, volume, or number of moles of substances inside the system will be doing work on the that system. This includes pistons, reactions, heating/cooling with constant pressure (volume changes), etc.