Internal energy of a substance.

1. Nov 9, 2013

shenjie

Hi all, I was taught that internal energy of an ideal gas in depending on the temperature which mean during isothermal process, the change in internal energy is equal to zero. Is this statement true?

How about the change in the internal energy of a substance when it is condensed from saturated vapour to saturated liquid isothermally?

Sincerely thanks to those who help me :)

2. Nov 10, 2013

Staff: Mentor

Yes.
No. During the condensation, it is not an ideal gas. It's part vapor and part liquid.

3. Nov 10, 2013

hilbert2

That's true for an ideal gas, but in real gases there's a slight dependence of internal energy on volume, i.e. the partial derivative $\left(\frac{\partial U}{\partial V}\right)_{T,n}$ is not exactly zero. This is related to the Joule-Thomson effect.

4. Nov 10, 2013

BruceW

yeah, for a 'true' ideal gas, the internal energy only depends on the temperature (assuming the number of molecules is constant). I think some people call this 'true' ideal gas a 'perfect' gas.