An electron enters a region of uniform perpendicular fields. It is observed that the velocity of the electron is unaffected. A possible explanation is:
A) is parallel to and has magnitude E/B
B) is parallel to
C) is perpendicular to both and has magnitude B/E
D) is perpendicular to both and has magnitude E/B
E) the given situation is impossible

Question

An electron enters a region of uniform perpendicular  fields. It is observed that the velocity  of the electron is unaffected. A possible explanation is:
A) is parallel to and has magnitude E/B 
B) is parallel to 
C) is perpendicular to both and has magnitude B/E 
D) is perpendicular to both and has magnitude E/B 
E) the given situation is impossible

Quizplus · Accepted Answer

Since the velocity of the electron is unaffected, it must be moving at a constant speed perpendicular to the magnetic field. This means that the magnetic force and the electric force are balanced, so the electric field must be perpendicular to the magnetic field. Furthermore, the ratio of the magnitude of the electric field to the magnitude of the magnetic field is equal to the velocity of the particle. Therefore, choice D is the best explanation, as it states that the electric field is perpendicular to the magnetic field and has a magnitude of E/B.

An Electron Enters a Region of Uniform Perpendicular Fields