# Magnetic Field Problem!

1. Nov 19, 2012

### Kennedy111

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

An electron (m= 9.11 x 10^-31 kg) enters a downward magnetic field of 5.00 x 10^-1 T with a velocity of 6.50 x 10^6 m/s West. Calculate the radius of the circular path it will follow when it is travelling within the magnetic field.

m = 9.11 x 10^-31 kg
v = 6.50 x 10^6 m/s West
B = 5.00 x 10^-1 T
q = -1.60 x 10^-19 C

2. Relevant equations

Fm = qvB
Fc = mv^2/r
Fm = Fc

3. The attempt at a solution

Fm = qvB
= (1.60 x 10^-19 C)(6.50 x 10^6 m/s)(5.00 x 10^-1 T)
= 5.2 x 10^-13 N

Fm = mv^2/r
r = mv^2/Fm
= (9.11 x 10^-31 kg)(6.50 x 10^6 m/s)^2 / (5.2 x 10^-13 N)
= 7.40 x 10^-5 m

I feel like I may have done this question wrong...

2. Nov 19, 2012

### Delphi51

It looks good!
May I offer a suggestion?
It is a little more efficient and quite a bit clearer if you write it up this way:
The centripetal force is provided by the magnetic force
Fm = Fc
qvB = mv²/r
qB = mv/r
r = mv/(qB)
Put in the numbers and run it all through the calculator in one go, so no rounding takes place until you write the final answer.

3. Nov 20, 2012

### Kennedy111

Thank you very much!