# Thermal Physics > Air and Internal Energy

1. Jun 24, 2005

### Ivegottheskill

This Q has me stumped, I'm still flicking through some web pages and my text book, but been unable to find a useful formula to work it out yet:

2. Jun 24, 2005

### jdstokes

For each molecule, the kinetic energy associated with each degree of freedom is $\frac{1}{2}kT$, so the kinetic energy of each molecule in the O2, N2 mixture is $\frac{5}{2}kT$. Summing over the entire gas gives the internal energy $U = N\frac{5}{2}kT = nN{_\mathrm{A}} \frac{5}{2}kT = \frac{5}{2}nRT$.

P.S. Not PHYS1901 by any chance?

3. Jun 24, 2005

### Ivegottheskill

No. PHYS 1001 actually
I just found this forum, looks like a useful resource all round.

Thanks for clearing that up. My main problem I think is remembering all the letters and where they come from.

BTW, you have U = N*5/2*k*T = n*N_a*5/2*k*T

The only thing that changes there is N --> n*N_a

What is "N" if N=n*N_A

(Haven't worked out how to use the "Latex" code yet)

4. Jun 24, 2005

### jdstokes

$k$ is defined as $\frac{R}{N_\mathrm{A}}$, N is the number of gas molecules.