# Calculate the rms speed of free electrons

1. Oct 6, 2007

### Chele

Hi and thanks for checking this out!! :)

1. The problem statement, all variables and given/known data
The temperature and pressure in a star's atmosphere are 1.20E6 K and 0.0400 Pa. Calculate the rms speed of free electrons (mass = 9.11 10-31 kg) there.

2. Relevant equations

What am I doing wrong and why do they give the pressure if that's not needed in the formula!?

3. The attempt at a solution

I'm using this formula:

v=$$\sqrt{kT/m}$$

k=1.380658E-23 J/K
T=1.2E6 K
m=9.11E-31 kg

Seems like a simple "plug and chug" type problem. My answer was 4.3E6 m/s. This is not the correct answer.

2. Oct 6, 2007

### Staff: Mentor

That formula is incorrect. The average KE equals (3/2)kT (you must include all three translational degrees of freedom).

3. Oct 6, 2007

### Astronuc

Staff Emeritus
4. Oct 6, 2007

### Chele

That's what I get for trying to do homework at 330am. Thanks, I can't believe I missed that.

Thanks for the information, I'll check those out!