# Homework Help: Finding magnitude in a capacitor and finding velocity

1. Feb 23, 2010

### jrollz316

1. The problem statement, all variables and given/known data
the voltage across two plates is 500 volts, and they are 10 cm apart.

a. what is the magnitude of the electric field- it is directed downward

b. a proton is placed between the plates. what is the magnitude of the electric force on the proton?

c. A magnetic field of .05 T exists in the same space between the plates directed into the paper. it produces a magnetic force on the proton that balances the electric force. what is the velocity of the proton

2. Relevant equations
V/M

F=qE

V=qBr/m

V = √ ( - ((2)(q)(Vba)) / (m))

F = q(E + v x B)

not sure which of those last ones to use. is there anyway to make it so that last equation equals v? im not very good at doing that myself

3. The attempt at a solution

a. 500/.10m=5000 v/m up

b. F=qE
= (1.6x10^-19C)(5000v/m)
=8x10^-16 N/C

c.F = q(E + v x B)

i think you would use this

but does anything look wrong? thanks