SAT Subject Physics Practice Question 91: Answer and Explanation

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Question: 91

1. Which of the following is/are true concerning magnetic forces and fields?

I. The magnetic field lines due to a current-carrying wire radiate away from the wire.

II. The kinetic energy of a charged particle can be increased by a magnetic force.

III. A charged particle can move through a magnetic field without feeling a magnetic force.

A. I only
B. II and III only
C. I and II only
D. III only
E. I and III only

Correct Answer: D

Explanation:

I is false: The magnetic field lines due to a current-carrying wire encircle the wire in closed loops. II is also false: Since the magnetic force is always perpendicular to the charged particle’s velocity vector, it can do work on the charged particle; therefore, it cannot change the particle’s kinetic energy. III, however, is true: If the charged particle’s velocity is parallel (or antiparallel) to the magnetic field lines, then the particle will feel no magnetic force.

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