Question:

A positively-charged particle is travelling southward in a downward-directed uniform magnetic field. The magnetic force is the merely force if any acting on the particle. Explain the trajectory of the particle assuming that it remains in the uniform magnetic field and that it's elevation does not change.

Answer:

Calling the direction in that the particle is travelling at any instant in time the forward direction the force will by the right-hand rule for the cross product of two vectors always be leftward. Therefore the particle will continually be in the process of making a left turn. For the reason that the force is and always will be perpendicular to the velocity the force will never change the velocity. A particle that is travelling at a constant speed and is continually in the process of making a left turn and indeed always has the same value of acceleration which is always directed leftward is moving in a circle. In the situation in question the particle is moving counter clockwise as viewed from above in a circle.