internal energy of an ideal gas

[Michael Du 1E]

Can anyone please explain why the delta internal energy for am isothermal expansion or compression of an ideal gas is zero? Thank you.

[Skyllar Kuppinger 1F]

When the gas is expanding it is losing energy in the form of work. However if the expansion is reversible, then this is happening slowly. This means there is time for heat from the surroundings to enter the system. So basically the energy lost by work is replaced by heat.
Here is the math:
Delta u= q + w
Delta u = 0
0=q+w
-q=w

The q and the w offset each other to make zero. Hope this helps !! :)

[Nick Fiorentino 1E]

In addition to the previous poster, since the reaction is isothermal, the temperature is constant, so the heat can slowly enter the system and offset the energy lost by work. It's important to note that this happens when the reaction is isothermal