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 0.46 T pointing in the negative z direction, as indicated in Figure. 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 1.40 x 105 m/s; that is, only these protons should be undeflected by the two fields.
![458_A beam of protons with various speeds.gif](https://secure.tutorsglobe.com/CMSImages/458_A%20beam%20of%20protons%20with%20various%20speeds.gif)
(a) Determine the electric field that yields the desired result.
(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?
(c) Which plate in Figure should be positively charged? Top or bottomtop. Explain.