A proton and an electron are moving in the + x direction in a magnetic field in the + z

direction. What are the directions of the forces on the proton and electron?

Force on a moving charge is given by the following relation

F = q (v x B)

for proton

q = e, v = vi, B = Bk

F = e (vi x Bk)

= Bev - j

= - Bevj

The direction of force is along negative of y axis or - y - axis.

for electron

q = - e, v = vi, B = Bk

F = - e (vi x Bk)

= - Bev - j

= Bevj

The direction of force is along positive of y axis or + y - axis.