1. The problem statement, all variables and given/known data Determine the required magnetic field to give an electron deflection angle of θ=90deg. The other variables (such as Rd, Re, and the electron energy in keV) are currently dummy variables such that this problem will initially be solved symbolically. 2. Relevant equations (1) F=q(v x B) (2) F = qvbsinθ (3) QBR = p 3. The attempt at a solution Using the following figure (apologies for poor paint skills), I've taken the cross product of (v x B), recognizing that vx = 0, By = Bz = 0. This gives me that v x B = (vzBx)y - (vyBx)z where y and z are unit vectors. Edit: Apologies; I seem to be awful at remembering to attach photos, files in e-mail, etc. Here's the figure: http://i.imgur.com/V5TN9.png I am having trouble fitting the cross product to another equation to yield any kind of results. Let me know if I should do more work (I've been reading through Griffith's Intro to Electrodynamics for about two hours now trying to make any headway).