A straight horizontal wire carries a current of 3.55 ?A. A proton is moving above the wire in a direction parallel to it but opposite to the direction of the conventional current in the wire, at a constant velocity of 8.00×106 m/s. Assume that the force of gravity acts on the proton and that the proton is moving parallel to the Earth's magnetic field, so that the only magnetic force on it comes from the field generated by the current in the wire.
(a) What is the magnitude of the upward force that the magnetic field must exert on the proton to keep it at a constant velocity?
(b) How far is the proton above the wire?