# Change in entropy at constant temperature

1. Nov 21, 2009

### ricof

1. The problem statement, all variables and given/known data

Calculate the entropy change when 0.011m3 of a perfect gas at a constant temperature of 273K is compressed from an initial pressure 1x10^5 N/m/m to a pressure of 1x10^6 N/m/m.

I know the equation but cannot work out Q!! Please help!

2. Relevant equations

dS=Q/T

3. The attempt at a solution

2. Nov 21, 2009

### Mapes

Hi ricof, welcome to PF. Can you combine the First Law with what you know about the energy of an ideal gas at constant temperature?

3. Nov 21, 2009

### cavalier

Some more relevant equations.
The first law
$$\Delta E=Q+W$$
Work for isothermal expansion
$$W=-nRTln\frac{V_{f}}{V_{i}}$$

If you combine this with what you know about change in internal energy at constant temperature, like Mapes said, and that integral of yours, you can figure out the answer.

4. Nov 21, 2009

### ricof

Because T is constant, there is no change in internal energy right?

And also, in the above equation for isothermal expansion, how is n calculated?

Last edited: Nov 21, 2009
5. Nov 21, 2009

### Mapes

Agreed on the first question. For the second: apply the ideal gas law.

6. Nov 21, 2009

### ricof

Ok, so as dE = 0,

Q = -W

and as PV = nRT,

-W=PV ln (v1/v2) which is Q

Thanks guys, my answer is correct. Thanks for helping!