# Homework Help: Lorentz force - e- in EM field

1. Apr 9, 2013

### Roodles01

1. The problem statement, all variables and given/known data
An electron in an electric field E = 3.0x107 ex NC-1 and a magnetic field B = 3.0 ez T has velocity v = 1.0x107 (2ex - ey) ms-1.

Calculate electromagnetic force on the electron.

2. Relevant equations
F = q (E + v X B)
where q = e- = -1.6x10-19 C

3. The attempt at a solution
I have attached a working out, but have difficulty with just one aspect.

Whilst working out the matrix vXB I have a negative sign where I think a positive sign should be.
I have attached the working as a pic.
Could someone show me why I am correct or incorrect.
Thank you.

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2. Apr 9, 2013

### Roodles01

Ah! Is this to do with ex X ey = -ey?
They're orthogonal?

3. Apr 9, 2013

### ap123

The negative sign comes from the definition of the cross product.
v x B = ex(...) - ey(...) + ez(...)

4. Apr 9, 2013

### Roodles01

Again, I have to go "Doh!"
Thank you.