An electron enters the vacuum between two oppositely charged plates with velocity \(v\). The electron is followed by an alpha particle moving with the same initial velocity as the electron. A uniform magnetic field is directed out of the plane of the paper.

The electron’s path is undeflected. The path of the alpha particle will be

A.     deflected out of the plane of the paper.

B.     undeflected.

C.     deflected upward.

D.     deflected downward.

B

A surprising number of students were confused by this question, with the evidence being that many were simply guessing. Crossing an electric and a magnetic field is the basis of selecting the velocity of charged particles as can be readily seen by equating \(Bqv\) to \(qE\). The charge cancels which means that if the electron is undeflected then the alpha particle will also be undeflected.