physics question (help please!)?

Question

A beam of electron is accelerated through a potential difference of 10kV before entering a velocity selector. Of the B-field of the selector has a value of 10^-2 T, what value of the E-field is required of the particles are to be undeflected?

Answer 1

(1.6x10^-19 C)(10,000V) = (1/2) (9.11x10^-31) v^2.

That is qV = 1/2 m v^2. Solve for v to get the speed of the electron as it enters the velocity selector.

Then inside the velocity selector you want the magnetic force to equal the electric force on the electrons or

qvB = qE

q cancels out leaving

vB = E You are given B=10^-2 you found v in the first step. So solve for E.