I had the following problem:
A proton is moving to the right at constant speed v and enters a region with uniform magnetic and electric fields. It continues to move in a straight line. The magnetic field is directed toward the top of the page. What is the direction of the electric field?
Using the right hand rule, with fingers pointed to the right and curled fingers up, my thumb points out of the page, leading me to believe that would be the answer. The correct answer is actually that the electric field points into the page. (I have received the answer to this question but not the explanation). Why is this?