A beam of protons with various speedsis directed in the positive x direction. The beam enters aregion with a uniform magnetic field of magnitude 0.43 T pointing in the negative zdirection, as indicated in the figure. It is desired to use auniform electric field (in addition to the magnetic field) toselect from this beam only those protons with a speed of1.62 105 m/s; that is, only these protons should beundeflected by the two fields.

(a) Determine the electric field that yieldsthe desired result.
( i + j + k)kV/m

(i means +x, j means +y, and k means +z)

(b) Suppose the electric field is to be produced by aparallel-plate capacitor with a plate separation of 2.5 cm. Whatpotential difference is required between the plates?

(c) Which plate in the figure should be positively charged?