Calculating Electron Velocity and Orbit in Crossed Electric and Magnetic Fields

[Angie K.]

Homework Statement

What is the velocity of a beam of electrons that go undeflected when passing through crossed electric and magnetic fields of magnitude 6.8 kV/m and 4.9 mT, respectively?

What would be the radius of the electron's orbit if the electric field were turned off?

Homework Equations

v = E/B

The Attempt at a Solution

I got the first part by using the equation above. (I converted the measurements because I'm looking for m/s). So 6.8 kV/m is 6800 V/m and 4.9 mT is .0049T.

v = E/B
v = (6800V/m)/(.0049T)
v = 1.3877*10^6 m/s

For the second part, I am a bit confused as to how to find the radius (which equation to use?).

 

[ehild]

What force acts on a charged particle in magnetic field? That force provides the centripetal force for the circular motion.