SAT Subject Physics Practice Question 243: Answer and Explanation
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?
B. Beta particle
C. Alpha particle
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.