SAT Subject Physics Practice Question 243: Answer and Explanation

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Question: 243

14. Each of the following particles is projected with the same speed into a uniform magnetic field B such that the particle’s initial velocity is perpendicular to B. Which one would move in a circular path with the largest radius?

A. Proton
B. Beta particle
C. Alpha particle
D. Electron
E. Positron

Correct Answer: C


When the particle enters the magnetic field, the magnetic force provides the centripetal force to cause the particle to execute uniform circular motion.

Since v and B are the same for all the particles, the largest r is found by maximizing the ratio m/|q|. The value of m/|q| for an alpha particle is about twice that for a proton and thousands of times greater than that of an electron or positron.

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