# Magnitude of the field used for electron

1. Jul 8, 2010

### flynostrich08

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

a proton is projected perpendicularly into a magnetic field that has a magnitude of .5 T. the field is then adjusted so that an electron will follow a circular path of the same radius when it is projected perpendicularly into the field with the same velocity that the proton had. What is the magnitude of the field used for the electron?

2. Relevant equations
r=mv/qB

3. The attempt at a solution
i basically used the above equation to solve for r but left v=1 since it is the same in both then used B=mv/qr and plugged in r and realized i just did the same thing so can someone please help explain what i have to do in detail since i am lost. Thanks for any help

2. Jul 8, 2010

### Karmalo

It should help you knowing that the proton mass is 1836 times bigger than the electron mass (mp=1836me)

3. Jul 8, 2010

### flynostrich08

thanks i was looking at my book wrong so i ended up solving for r of the proton then took that and solved for B of the electron using the same equation and got 2.7x10^-4. did anyone else get that?

4. Jul 8, 2010

### Karmalo

I got the same result but it has to be negative because of the charges (qp=-qe). That result is for the same radius (distance from the center) but in the opposite direction.

5. Jul 8, 2010

### flynostrich08

thanks for pointing that appreciate the help