# Lorentz Law

## Homework Statement

An electron has a velocity of 1.2 * 10^4 m/s ( in the positive x direction ), and an acceleration of 2 * 10 ^ 12 m/s^2 ( in the positive z direction ) in a uniform electric and magnetic field. If the electric field has a magnitude of 20 N/C ( in the positive z direction), what can you determind about the magnetic field in the region? what can you not determine?

F = qE + qv x B

## The Attempt at a Solution

I need the force quantity to figure out the problem, but am lost besides this. Can someone offer a little help. Thanks.

Pengwuino
Gold Member
You are given the acceleration and the mass of an electron is something you can easily look up.

Ok I was thinking I was going to be using Newton's second law, but wan't sure. I have the answers to the problem and for the x- vector it says it can be any negative value, can anyone explain why this is?

Pengwuino
Gold Member
Well you do need to use Newton's 2nd law since F = ma to use the left hand side of your equation. As far as the x-direction of the magnetic field is concerned, what happens when you cross a velocity, that is in the x-direction, with a field in the x-direction? Matter of fact, what happens when you cross any 2 parallel directions together?

It would be zero not any negative value correct?

Thanks for all of the help.

Pengwuino
Gold Member
Yes, any x-component, negative or positive, crossed into the velocity would give 0 here.