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.
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 it will acquire a velocity and so there will be a force on it due to the
magnetic field. As it accelerates the force on it will increase. We would need to know the direction of the gravitational field compared with
the magnetic field to suggest which way it would accelerate.
A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS CD