SAT Subject Physics Practice Question 270: Answer and Explanation
The figure above shows a cation (a positive ion—that is, an atom that has lost one or more electrons) entering a mass spectrometer, which contains a region with a uniform magnetic field, B. Once in the magnetic field, the cation moves in a semicircular path in the direction indicated. What is the direction of B?
A. Upward in the plane of the page
B. To the left in the plane of the page
C. To the right in the plane of the page
D. Out of the plane of the page
E. Into the plane of the page
Correct Answer: D
At the top of the semicircle, the cation’s velocity, v, is to the right, and the net force it feels, F, must be downward in the plane of the page (that is, toward the center of the circle since it’s undergoing uniform circular motion).
For a positive charge moving to the right to feel a downward magnetic force in the plane of the page, the magnetic field must point out of the plane of the page (according to the right-hand rule).