SAT Subject Physics Practice Question 356: Answer and Explanation

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Question: 356

52. Two moving electrons enter a strong magnetic field at right angles to the field. The velocity of one of the electrons is four times greater then the velocity of the other electron. Which of the following best describes the ration of the circular radii the two electrons follow?

A. The faster electron has a radius two times larger than the slower electron.
B. The faster electron has a radius four times larger than the slower electron.
C. The faster electron has a radius eight times larger than the slower electron.
D. The faster electron has a radius sixteen times larger than the slower electron.
E. The faster electron has a radius sixty-four times larger than the slower electron.

Correct Answer: B

Explanation:

The correct answer is (B). The radius of the curved path the electron follows in the magnetic field is r=mv/Bq. Both particles are electrons, which have the same mass, and the force exerted by the magnetic field remains the same, so the only difference is the velocity. The ratio of the velocity of the fast electron compared to the slow electron is 4:1.The fast electron has a radius 4 times larger than the slow electron.

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