A beam of protons with various speeds is directed in the positive x direction. The beam enters a region with a uniform magnetic field of magnitude 0.45 T pointing into the page, as indicated in Figure 22-45. It is desired to use a uniform electric field (in addition to the magnetic field) to select from this beam only those protons with a speed of 1.60 105 m/s; that is, only these protons should be undeflected by the two fields. (a) Determine the electric field that yields the desired result. size: ____72000___ N/C dir: -y dir (b) Suppose the electric field is to be produced by a parallel-plate capacitor with a plate separation of 2.5 cm. What potential difference is required between the plates? ________ V for part B, I understand that C=EA/d and V=IR also, I can find I, by that I can use I=2pieRB/(4pie x 10^-7) but I am really not sure how to find this potential difference correctly, can anyone help?