A positive ion is shown midway between the two dees of a cyclotron; at this instant its velocity is in the positive x direction. Which of the five arrangements of the electric, , and magnetic, , fields shown is applicable to the situation depicted?
A) 1
B) 2
C) 3
D) 4
E) 5

Question

A positive ion is shown midway between the two dees of a cyclotron; at this instant its velocity is in the positive x direction. Which of the five arrangements of the electric,  , and magnetic,  , fields shown is applicable to the situation depicted?
A) 1 
B) 2 
C) 3 
D) 4 
E) 5

Quizplus · Accepted Answer

Since the velocity of the ion is in the positive x direction, it means that it is moving perpendicular to the magnetic field direction, which is pointed into the page. Thus, it experiences a force that is perpendicular to both its velocity and the magnetic field direction, which is directed upwards. In order for the ion to continue in a circular motion, this force needs to be balanced by the electric field, which is directed downwards. This situation is only depicted in choice 2, where the electric field is pointed downwards and the magnetic field is pointed into the page. Therefore, choice B is the correct answer.

A Positive Ion Is Shown Midway Between the Two Dees