Proton motion

Question:

A proton is travelling south as it enters a region that contains a magnetic field. The Proton is deflected downward toward the Earth. What is the direction of the magnetic field?

Possible answers:
(a) downward toward the Earth
(b) west
(c) north
(d) east
(e) south.

Answer

Imagine you are looking down towards the Earth. That is you have a plan view of the initial motion of the proton.

The proton comes from the north travelling south. The magnetic field deflects it downwards towards the Earth.
In order for the field to do this it must be acting at right angles to the motion of the particle. In other words the field must either act E-W or W-E.

Using Fleming's left hand rule (and remembering that the moving proton is like a conventional current) the filed must act from East to West.

The correct answer is therefore (b)