Internal Energy of Isothermal Processes

[Chiara R 1K]

I'm still a bit confused conceptually as to why U is equal to zero in an isothermal process, since U=q+w and work can still be done on or by the system. Thanks :)

[Irena Roy 4H]

The textbook has a nice passage that explains it quite well:

"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, so deltaU=0 for isothermal expansion of an ideal gas."

[Alex Nguyen 3I]

I think the heat cancels the work. I think of it as temperature being the only thing that can change internal energy, and there's no change in temperature in an isothermal process.