Question

Gold ions (Au+) are accelerate to high speeds using electric fields.

These ions are then passed through a region where uniform electric E and magnetic B fields are present.

If the ions are traveling to the right, which configuration of E and B will serve as a velocity selector?

A.Electric field E up; Magnetic field B into the page
B.Electric field E down; Magnetic field B out of the page
C.Electric field E up; Magnetic field B out of the page
D.Electric field E down; Magnetic field B into the page
E.Both A and B
F.Both C and D
G.All of the above

Answer 1

You want the forces of the E and B selectors to be equal and opposite at the chosen velocity, all other velocities, B and E are not equal, and the ions will deflect one way or the other.

So choose an E, either up, or down.

Say UP. Then the positive gold ions will deflect up due to E. Now, the idea is to have B deflect them down.
There are a number of right hand rules:
http://physicsed.buffalostate.edu/SeatExpts/resource/rhr/rhr.htm
So, B must be out of the paper.

But it could be reversed, E force down, then B would be...

Pick a right hand rule, memorize it. They all work.