1. An electron moves in a circle of radius R in a uniform magnetic field B. The field is into the page. (a) Does the electron move clockwise or counterclockwise? (b) How much time does the electron take to make one complete revolution? Derive an expression for the time, starting with the magnetic force on the electron. Your answer may include R, B, and any fundamental constants.
    (a) CW
    (b)
    F =  evB = mac = mv2/R and C = 2πR.
    Find v in the equation.
    mv2/R = evB
         v = eRB/m
    Find t
    t = 2πR/v = (2πR)/(eRB/m) = 2πm/eB.
  2. Determine the direction of B for each case, where F represents the force on a positively charged particle moving with velocity v.
    (a) down, (b) into the page, (c) right.
  3. An electron beam in vacuum moving at 1.8 × 107 m/s passes between the poles of an electromagnet (the figure below). The diameter of the magnet pole faces is 2.4 cm and the field between them is 0.20 × 10-2 T. How far and in what direction is the beam deflected when it hits the screen, which is 25 cm past the magnet? [Hint: The electron velocity changes relatively little while they are in the field, so you can approximate the magnetic force as a constant force.]

    4.