internal energy for isothermal expansion

$\Delta U=q+w$

camrynpatterson3C
Posts: 44
Joined: Fri Sep 29, 2017 7:07 am

internal energy for isothermal expansion

Why is it that the change in internal energy for the isothermal expansion of an ideal gas is always 0? I know q will be 0 because it's isothermal, but isn't there still work being done if it's expanding or compressing?

Jeremiah Samaniego 2C
Posts: 33
Joined: Fri Sep 29, 2017 7:05 am

Re: internal energy for isothermal expansion

I think this is because internal energy is a state function dependent on temperature, and since the temperature stays the same, there should be no change in internal energy. We can also relate this relationship to q = -w. If the change internal energy is 0, then 0 = q + w and thus, q = -w.

Shawn Patel 1I
Posts: 54
Joined: Thu Jul 27, 2017 3:01 am

Re: internal energy for isothermal expansion

Hey,
The book explains it best on page 274, but basically the molecules of an ideal gas move at the same average speed as it expands, meaning the total kinetic energy is the same, and since there is nothing acting against the molecules, the potential energy remains the same. Because both of these energies don't change, neither does internal energy.