## Expansion done by/on the system

lilyhui3I
Posts: 8
Joined: Tue Nov 25, 2014 3:00 am

### Expansion done by/on the system

How can we identify if expansion is done by/on the system? Is there an equation to examine how the variables change(q and w)?
For ex #7.15 from the book:

Each of the pictures below shows a molecular view of a system undergoing a change at constant temp. In each case, indicate whether heat is absorbed off by the system, whether expansion work is done on or by the system and predict the signs of q and w for the process.

Neil DSilva 1L
Posts: 70
Joined: Fri Sep 26, 2014 2:02 pm

### Re: Expansion done by/on the system

Skatwal posted a good solution to this problem in response to another student's question here: viewtopic.php?f=130&t=5018

We can determine if work is done by or on a system by looking at the sign of w. If w is negative, the system is losing energy and expansion work is being done by the system. If w is positive, the system is gaining energy because expansion work is done on the system.

The two pictures in the problem depict phase changes at constant temperature. For isothermal processes, $\Delta U=0$, so you can assume that q and w are equal and opposite ($q = -w$). The signs of q and w are then determined by the flow of heat.