Consider a parallel-plate capacitor with charge density 7.5 10-7 C/m2 on the two plates and an electric field that points in the +z direction. What magnetic field is necessary to provide a velocity selector for 58 keV deuterons that move in the +y direction? A deuteron has a mass of 3.2 x 10-27 kg and a charge of 1.6 x 10-19 C; 1 keV = 1.6 x 10-16 J.
I will be glad to critique your work or thoughts. You are looking for a value of B such that.
q V B = q E
The charge q cancels out.
V = (B/E)
Use the deuteron energy to compute V