# Particle moving in an electric field

1. Nov 7, 2013

### dwn

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

A particle with charge q = 5.0 nC and mass m = 3.0 µg moves in a region where
the magnetic field has components Bx= 2.0 mT, By=3.0 mT, and Bz= -4.0mT. At an instant when the speed of the particle is 5.0 km/s and the direction of its velocity is 120°relative to the magnetic field, what is the magnitude of the acceleration of the particle?

ans: 38.86 m/s2

2. Relevant equations

I assume it is: Fm= qv χ B

3. The attempt at a solution

5*10-6(5*103) χ (0.02i + 0.03j - 0.04k)sin(120)

this was my initial setup, but then I proceeded to do the following:

5*10-6(5*103)sin(120)(.02) = c
5*10-6(5*103)sin(120)(.03) = d
5*10-6(5*103)sin(120)(-.04) = e

√(c2+d2+e2)

and calculate the magnitude of the product --- then using this value in F=ma to determine the acceleration.

2. Nov 7, 2013

### voko

This formula makes no sense. You have a number to the left of the cross, and a vector to the right.

A cross product requires two vectors.

3. Nov 7, 2013

### dwn

That's what i thought, which left be quite perplexed as to how i need to solve this. I wasn't sure what to do given that we were presented with a vector in the question.

4. Nov 7, 2013

### voko

If you know the magnitudes of two vectors, and the angle between them, can you compute the cross product?

What prevents you from doing this here?

5. Nov 7, 2013

### dwn

I see where my error was, but my order of operations was not in the correct order. Thank you.