# How to calculate magnitude of magnetic field

1. Aug 29, 2010

### Aquam

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

Ill try to translate as good as possible.

Electrons are sent through potential difference of 2500V. Then they get into transverse magnetic field where they move in circular arc with 22cm radius. What is the magnitude of magnetic field?

So the radius is= 22cm
and the potential difference is 2500V.

2. Relevant equations

B= u0 I / 2 Pi r

I dont know how to calculate I out of this formula, so i can calculate B (magnitude of m.f.)

Or i have missed the solution to this problem completely? Please help

2. Aug 29, 2010

### boyu

Your equation is for calculating B from current, but in this question the B field is already there before the electrons went into, and this B field also accounts for the centripetal force which pulls the electrons into circular motion, provided the fact that the electrons have velocities after accelerated by the potential, and the B field is transverse.

So I guess the solving strategy is like this: get the kinetic energy of electrons from the 2500V potential difference, and together with the radius you can have the acceleration, just like we did in classical mechanics. Then the force F is obvious and B can be derived from F = Bqv.

I haven't touched EM for years, so cannot promise this answer is right.

Good luck.