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### force on a charge in magnetic field

Posted: Sat May 01, 2010 10:50 pm
Hi,
It is said that for a force to be experienced on a charged particle in a magnetic field, the particle must have a velocity:
ie by the formula: F = qvBsin@.

Why is it that the particle has to have a velocity?

In the similar scenario of a electric field, the formula F=qE, the charged particle experiences a force even if it is stationary!
Why is the magnetic field different and why does it need a velocity?

Any thoughts,
Thanks

### Re: force on a charge in magnetic field

Posted: Wed May 05, 2010 11:32 am
A simple answer to your question would be that different forces act on different quantities:
- gravity acts on mass
- colour force acts on colour charge
- electric force acts on charge
- magnetic force acts on the product of charge and velocity

A more subtle answer would observe that the magnetic force must refer to a particular frame of reference: in a frame moving with a charge, there is an electric force but no magnetic force. So electricity, magnetism and relative motion are all bound up together.

So it's no surprise that Einstein's relativity is developed from electromagnetism. See
http://www.phys.unsw.edu.au/einsteinlight

I give a more complete answer here
http://www.phys.unsw.edu.au/einsteinlig ... e2_FEB.htm

Joe