SAT Subject Physics Practice Question 251: Answer and Explanation

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Question: 251

22. Traveling at an initial speed of 1.5 × 106 m/s, a proton enters a region of constant magnetic field, B, of magnitude 1.0 tesla. If the proton’s initial velocity vector makes an angle of 30° with the direction of B, compute the proton’s speed 4 seconds after entering the magnetic field.

A. 5.0 × 105 m/s
B. 7.5 × 105 m/s
C. 1.5 × 106 m/s
D. 3.0 × 106 m/s
E. 6.0 × 106 m/s

Correct Answer: C


Since the magnetic force is always perpendicular to the object’s velocity, it does zero work on any charged particle. Zero work means zero change in kinetic energy (a direct consequence of the work–energy theorem), so the speed remains the same. Remember: The magnetic force can only change the direction of a charged particle’s velocity, not its speed.

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