Question

The conventional current I in a long straight wire flows in the upward direction as shown in the figure. (Electron flow is downward.) At the instant a proton of charge +e is a distance R from the wire and heading directly toward it, the force on the proton is:

Answer 1

The correct option is: (E)

Explanation:

The B field at the location of the charge +e is created by the wire next to it and given by B = μ_oI / 2πR. Based on RHR curl the direction of that B field is into the page at that location. Then the force on that charge is given by F_b = qvB, with q=e and B from before so F_b = ev(μ_oI / 2πR). Using the RHR flat for the + charge, the force comes out as down.