Change in internal energy for an isobaric process?

[erik-the-red]

Question:

When a quantity of monatomic ideal gas expands at a constant pressure of 4.00 \times 10^{4} {\rm Pa}, the volume of the gas increases from 2.00 \times 10^{ - 3} {\rm m}^{3} to 8.00 \times 10^{ - 3} {\rm m}^{3}.

A.

What is the change in the internal energy of the gas?

It's isobaric, so the pressure is constant.

I know the work is P\Delta V = (4.00 * 10^4)(6.00 * 10^{-3}).

But, I don't know how to get \Delta U from this.

[Physics Monkey]

What variables does the internal energy of the ideal gas depend on? How do these variables change in the aforementioned process?

[erik-the-red]

OK, the internal energy depends only on temperature.

For a monatomic gas, \Delta U = \frac{3}{2}nR\Delta T.

I don't know the number of moles or the change in temperature.

[Physics Monkey]

Progress! Ok, so now you need to know the change in temperature times n R , right? You know the pressure and volume of the gas at two different points in P,V space. Can you use this information to find the unknown? Hint: ideal gas law.

[mezarashi]

This relationship should be helpful as well.

\Delta U = Q - W

Apparently you have the equation for the W right. Now use the ideal gas law and a bit of calorimetry.

[erik-the-red]

Thanks a lot!