A velocity selector consists of magnetic and electric fields described by the expressions E = E k and B = B j.

Question

also If B = 0.0110 T, find the value of E such that a 700 eV electron moving along the negative x axis is undeflected.

Answer 1

Just set the forces equal so they cancel and the electron experiences no net force.

Electrostatic force = magnetic force

qE = qv x B = qvB (since v and B are perpendicular)

Sove for your electric field strength:

E = vB

The kinetic energy of the electron is given:

T = 1/2 mv^2

Solve for speed:

v = sqrt (2T / m)

And plug in:

E = sqrt (2T / m) B

You are given T and B.  To convert eV to Joules, plug in the fundamental charge on the electron for e.  Look up the electron's mass.  Plugnchug.