# Magnetic field

1. Oct 2, 2008

### nikhilthunder

when a charge moves in magnetic field a force perpendicular to velocity and B acts on charge then why the charge do not move in the direction of that force

2. Oct 3, 2008

### granpa

it does. where did you get the idea that it doesnt?

well in one sense it doesnt move 'through' the force because the force is always at a right angle to the motion of the particle. so the particles direction changes but its speed doesnt change.

3. Oct 13, 2008

### tiny-tim

Welcome to PF!

Hi nikhilthunder! Welcome to PF!

("why does the charge not move in the direction of that force" )

From Newton's second law, force = mass times acceleration …

so the acceleration is always in the direction of the force …

but in circular motion, the acceleration is centripetal (towards the centre of the circle), so the charge carries on moving perpendicular to the force.

(and, as granpa says, the speed doesn't change … so the energy doesn't change … this is because of the work-energy theorem … work done = change in energy, and work done is zero because the force is perpendicular to the motion )

4. Oct 13, 2008

### Naty1

Nik....
There is a Force (vector) F= qV x B (where V and B are vectors) I seem to remember that from many years ago...anyway, the force IS "sideways" via the right hand rule.

Is that the relationship you are questioning???