Consider an electron traveling horizontally in the positive direction, above, near, and parallel to a current-carrying wire. The current is traveling in the negative direction.

(a) Will the electron keep on going straight, move towards the wire or away from the wire?

(b) If we replace the electron with a proton and now the proton is coming from above, traveling in the perpendicular direction, moving towards the wire. Will the proton hit the wire, veer to the left? veer to the right? stop? go out of the page? go onto the page?

(a) The electron will move towards the wire.

The direction of the magnetic fields created by the wire can be found via right-hand rule. If you point your thumb towards the direction of the current, and if you curl your fingers, the direction of your four fingers will give the direction of the magnetic field. In this case, magnetic field is around the wire, and into the page just above the wire, where the electron is located.

According to the above formula, the direction of the force the wire applies to the electron can be found by right-hand rule.

Since the electron has a negative charge, the direction of the force is towards the wire.

(b) The proton will veer to the right.

The direction of the magnetic field is the same as the previous part. The proton has a positive charge, and coming from above. The direction of its velocity is downwards. The magnetic field above the wire is pointed into the page. Using the right-hand rule, the magnetic force on the proton is directed to the right, with respect to us.

d) the number of neutrons plus the number of protons