## isothermal expansion

$\Delta U=q+w$

Emma Li 2C
Posts: 51
Joined: Fri Sep 29, 2017 7:07 am
Been upvoted: 1 time

### isothermal expansion

Why is delta U=0 for the isothermal expansion of an ideal gas? (pg 323)

Thank you!

Luke Bricca 1H
Posts: 30
Joined: Fri Sep 29, 2017 7:06 am

### Re: isothermal expansion

In an isothermal process, the internal temperature remains constant and the internal energy of an ideal gas is independent of its volume and pressure. Therefore, the internal energy of an ideal gas is wholly reliant on its temperature which is unchanging, so the delta U is unchanging as well, equalling 0.

Garret G 2F
Posts: 24
Joined: Fri Sep 29, 2017 7:06 am

### Re: isothermal expansion

Here is how the book answers that on page 274:

"For instance, when an ideal gas
expands isothermally, its molecules continue to move at the same average speed, so
their total kinetic energy remains the same. Because there are no forces between the
molecules, their total potential energy also remains the same even though their
average separation has increased. Because neither the total kinetic energy nor the
total potential energy changes, the internal energy of the gas is unchanged too. That
is, changeU=0 for the isothermal expansion (or compression) of an ideal gas. It follows
that when the volume of a sample of an ideal gas changes by any path between two
states, then, provided the initial and final states have the same temperature, we
know at once that changeU=0."