Relativity: Initial velocity and e/m of Thomson's experiment

[Matt21]

Homework Statement

In an experiment similar to Thomson’s, we use deflecting plates 5 cm in length with an electric field of 1.0×10^4 V/m. Without the magnetic field, we find an angular deflection of 30°, and with a magnetic field of 8×10^-4 T we find no deflection. What is the initial velocity of the electron and its e/m?

Homework Equations

u = E/B
e/m = q/m = u/RB

The Attempt at a Solution

I'm not sure if I'm using the correct equations but given the equations above, initial velocity is
u = 1.0×10^4/8×10^-4 = 1.25×10^7, and thus e/m is equal to
q/m = 1.25×10^7/(0.05*8×10^-4) = 3.125×10^11
If necessary, can anyone explain what I am doing wrong and perhaps the correct formula to use?

 

[TSny]

u = 1.0×10^4/8×10^-4 = 1.25×10^7

OK. Units?

and thus e/m is equal to
q/m = 1.25×10^7/(0.05*8×10^-4) = 3.125×10^11

This is not correct. You will need to use kinematic equations to relate q/m to the angular deflection. Review the kinematics of the Thomson experiment when just the E field is present. It is similar to projectile motion.