(a) A velocity selector consists of electric and magnetic fields described by the expressions $\vec{E} = E \hat{k}$ and $\vec{B} = B \hat{j}$, with $B = 21.0$ mT. Find the value of $E$ (in kV/m) such that a 850 eV electron moving in the negative $x$-direction is undeflected.
How do you determine the speed of the electron if you know its kinetic energy? kV/m
(b) What If? For the value of $E$ found in part (a), what would the kinetic energy of a proton have to be (in MeV) for it to move undeflected in the negative $x$-direction?
MeV