# Finding magnetic field component

1. Jul 13, 2007

### wendo

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

An electron has a velocity of 1.63×104 m/s (in the positive x direction) and an acceleration of 1.98×1012 m/s2 (in the positive z direction) in uniform electric and magnetic feilds. If the electric field has a magnitude of 19.8 N/C (in the positive z direction), what is the y component of the magnetic field in the region?

2. Relevant equations
this I'm really not sure... but could lorentz force law be applied to this question?

F=q(E+vxB)

C^2= a^2+b^2??????

3. The attempt at a solution

I'm really sorry but I seem to really have no idea bout this one... heres what I thought but is wrong..

Since they give you the acceleration in the z direction, with it the force due to the movement of the electron can be found. (or does this not work because F=ma doesn't apply to electric forces?)

-then I thought maybe if I found the force then i can use Lorentz' equation to solve for B?

But because the question is asking for a y component i'm really stuck on how to relate the acceleration, velocity and E field into finding the magnetic field.

Can someone help push me towards the right direction for this question?? Thanks!!!

2. Jul 13, 2007

### Dick

Yes, lorentz force law. And F=ma applies to all forces. If you think about F=q*vxB you will realize that with v is the x direction that an x component of the magnetic field will contribute no acceleration and a z component will contribute only to the y acceleration. So the only two things contributing to z acceleration are y component of the magnetic field and the E field. Does that help?