Answer:

Explanation:
When a charged particle moves perpendicularly to a magnetic field, the force it experiences is:

where
q is the charge
v is its velocity
B is the strength of the magnetic field
Moreover, the force acts in a direction perpendicular to the motion of the charge, so it acts as a centripetal force; therefore we can write:

where
m is the mass of the particle
r is the radius of the orbit of the particle
The equation can be re-arranges as

where in this problem we have:
is the magnitude of the charge of the electron
is the strength of the magnetic field
The beam penetrates 3.45 mm into the field region: therefore, this is the radius of the orbit,

is the mass of the electron
So, the electron's speed is
