# Magnetic Field Strength

1. Aug 7, 2007

### Hellphish

1. The problem statement, all variables and given/known data

A magnetron is a microwave source in which a "bunch" of electrons travel on a circular orbit in a uniform magnetic field. As the electrons pass the electrodes, a high-frequency alternating voltage difference occurs. Suppose the resonant frequency is f = 2.82×10^10 s-1; that is, the electron period of revolution is T = 3.55×10^-11 s. What is the corresponding strength of the magnetic field?
The electron charge is -1.6E-19 C and the electron mass is 9.1E-31 kg.

2. Relevant equations

f=qv * b
where q is the charge, v is the velocity, and b is the magnetic field

3. The attempt at a solution

I think I need to set a centripetal force equal to the magnetic force. The only equation I know for centripetal force is f=m(v^2 / r) and I'm not sure how to solve that with the period. I'm really just not sure if I'm even going about this right or how I can solve for v and r. Any help would be appreciated.

2. Aug 7, 2007

### nrqed

You are only missing one equation (which is quite simple). Consider a particel going in a circle of radius r. If it takes a time T to go around once, what is the speed of the particle?

3. Aug 7, 2007

### Hellphish

Oh wow, I didn't even think of that. Thanks.