# Vector evaluation & Lorentz force law

## Homework Statement

An electron in a magnetic field B=2.0T(ex-ez) has velocity v=(2.5x107 ms-1 (ex-ey)
a) calculate magnetic force on electron at that instant
b) what is the magnitude of this force

## Homework Equations

I am using the Lorentz force law.
F = q(vXB)
& evaluating the directions by vector

My argument is that the ey term is negative, but another person evaluates this as positive. Please confirm whether I'm right or wrong.

This affects the outcome of magnitude of force, too.

## The Attempt at a Solution

F = -e 2(2.5x107) (ex-ez)* (ex-ey)

I ex ey ez I
I 1 . -1 . 0 I
I 1 . 0 . -1 I

= ex((-1x-1)-(0x0)) + ey((1X-1)-(0x1)) + ez ((1x0)-(-1x1))
= ex - ey + ez)

so
F = -8x10-12 (ex - ey + ez)

rude man
Homework Helper
Gold Member

Last edited:
Ah! Having looked at it, I found that I didn't change the sign for the second term.
So simple it's annoying.
Thank you.

rude man
Homework Helper
Gold Member
Ah! Having looked at it, I found that I didn't change the sign for the second term.
So simple it's annoying.

Welcome to the club!