# Magnetism problem

1. Feb 20, 2006

### jakeowens

An electron of mass 9.11 x 10-31 (9.11e-31) kg travels in a circular orbit within a large evacuated chamber. The orbit has a 2.0 mm radius and is perpendicular to a B-field of 0.030 T. What's the electron's speed?

For this problem, the only equation i can think to apply is F=qvB.

I know B, i know q, i need to solve for V, but how do i get F?

F =ma, but that still leaves me stuck because i dont know the acceleration of the electron...

I understand this problem and know what to do, i'm just missing something somewhere because i don't seem to have enough to solve the problem, although i know its in there. I'm just not seeing it, and need a slight prod in the right direction.

2. Feb 20, 2006

### Raza

I am not sure but,
F=qvb and
F=ma
so qvb=ma?
I am confuse as you are.

3. Feb 20, 2006

### phucnv87

Remember that the electron travles in a circular orbit with a constant velocity, so you also have this equation

$$a=\frac{v^2}{R}$$