What is the radius of a charged ion in a magnetic field?

[TbbZz]

Homework Statement

A singly charged positive ion has a mass of 2.50 x 10^-26kg. After being accelerated through a potential difference of 250 V, the ion enters a magnetic field of 0.500 T, in a direction perpendicular to the field.

Homework Equations

Calculate the radius of the path of the ion in the field.

The Attempt at a Solution

Firstly, I drew this picture to try to help me with the problem:
http://img411.imageshack.us/img411/4197/picxc7.jpg

I've tried to set Fm = Fe, therefore coming up with the derived equation of V=E/B, but I don't know E.

I then tried to find the electric field by using the voltage, but I don't know the distance. (V=ED)

I also tried setting Fm = Fc, getting Bqv = mv^2/r, and finally resulting in Bqr/v=m. I had B, q, and m, but I didn't have r and v.

Thanks for reading.

 

[turdferguson]

Try energy conservation

 

[TbbZz]

Try energy conservation

You're a lifesaver.