# Homework Help: Temperature change in gasses

1. Feb 18, 2014

### Dassinia

Hello,
I just have a question.
When a gaz expands against a piston (that is insulated), the system is doing work so W>0 and we know that the volume will increase and the pressure decrease.

But what I want to know, is how about the temperature, I'd say that it will decrease since the pressure also decreases , but is there any other argument ?

I know that if we want to maintain a constant temperature in such a system we have to "connect" the system to a tank , so that the heat transfer will compensate the work done by the system.

Thanks !

2. Feb 18, 2014

### vela

Staff Emeritus
As you noted, a pressure decrease can occur while the temperature remains constant, so saying decreasing pressure implies decreasing temperature isn't a good argument. Think instead about what happens to the internal energy of the gas if Q=0 and W>0.

3. Feb 19, 2014

### Dassinia

We have that dE=-W
Ef-Ei = -W <0
We must have Ef<Ei that means that the kinetic energy in the final state is <
So the temperature decreases

Last edited: Feb 19, 2014
4. Feb 19, 2014

### vela

Staff Emeritus
Yup.