Why doesn't internal energy change when an ideal gas is compressed isothermally?

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TFM
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[SOLVED] Internal Energy of an ideal gas

Homework Statement



The temperature of 0.158 mol of an ideal gas is held constant at 67.0 degrees Celsius while its volume is reduced to a fraction of 20.0 % of its initial volume. The initial pressure of the gas is 1.19 atm.

What is the change in its internal energy?

Homework Equations



[tex]U = \frac{1}{2}nRT[/tex] (Per degrees of Freedom)

The Attempt at a Solution



I tried putting in the values to get U, but it doesn't say how many deggress of Freedom, so I triued using three (Monatomic) But this is wrong. Is there another Formula for the Internal Energy, becasue there are several similar questions, but none seem to be used with the releveant formula above?

So any help/idea will be very greatly appreciated,

TFM
 
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Discussion

hmm U depends only on T, as you can see from the equation.
And In the question, it is mentioned that T is kept constant. (n doesn't change anyway and R is a constant)

so change in U = ??
 
As there is no change intemprature, would there be no change in U, then?

TFM
 
Yes, if I'm not missing something.

This is valid for ideal gases only, though.
 
I just put 0 into MasteringPhysics, and it is the right answer. Thanksm, Raze2Dust! :smile:

Technically, the should the formula actually be:

[tex]U = \frac{1}{2}nR\Delta T[/tex] per degree of Freedom?

Thanks,

TFM