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Force on a stationary charge
QUESTION:
A positive electric charge of negligible weight is released from rest between the poles of
horseshoe magnet. What should be the direction of the acceleration of the charge caused by
the magnetic field?
Answer
You don’t say if the magnet is in a gravitational field or not.
However since the force on a charge is given by F = Bqv where B is the magnetic field
strength, q the charge and v the velocity of the charge your charge will not move. Since it has
no initial velocity (and we will assume that it does not acquire one since there is no
gravitational field) it does not experience a force and therefore will not accelerate.
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If the charge did have mass and there was a gravitational field the force on it will grow. As
soon as it starts to fall (accelerate) it will acquire a velocity and so there will be a force on it
die to the magnetic field. We would need to know the direction of the gravitational field
compared with the magnetic field to suggest which way it would accelerate.