# Magnetic field

1. Feb 19, 2008

### arod2812

1. The problem statement, all variables and given/known data
A charged particle is moving in a uniform, constant magnetic field. Which one of the following statements concerning the magnetic force exerted on the particle is false?
It changes the velocity of the particle.
It increases the speed of the particle.
It does not change the kinetic energy of the particle.
It can act only on a particle in motion.
It does no work on the particle.

2. Relevant equations
Could you explain why it false?

3. The attempt at a solution
I am told that the answer is the last one.

2. Feb 19, 2008

### mgb_phys

Not really an attempt!

3. Feb 19, 2008

### blochwave

This isn't obvious but a static magnetic field, whose force on a charge particle is given by F=q(VxB), can not do work.

Remember that work in general depends on the direction between the applied force and the direction of motion. If they're in the same direction then you get the simple W=F*d, and slightly more generally, W=F*d*cos(theta) where theta is the angle between them.(because really it's a dot product)

So now look at the equation for force, and tell me, knowing the definition of a cross product, if the force vector will ever be anything but perpendicular to the velocity of the particle? What's the result of that?

4. Feb 19, 2008