SAT Subject Physics Practice Question 525: Answer and Explanation

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Question: 525

11. An unknown particle is being studied in a magnetic field of variable intensity and direction. When the magnetic field is turned off, the particle is observed to move toward the earth. When the magnetic field is turned on, the particle is observed to continue to move toward the earth, no matter the strength or the direction of the magnetic field. Which of the particles listed below is most likely the unknown particle?

A. Beta particle
B. Alpha particle
C. Positron
D. Neutron
E. Gamma ray

Correct Answer: D


The correct answer is (D). The particle is completely unaffected by the field, which eliminates the charge-carrying particles. Thus the electron, the positron (a positive electron), and the proton are not the correct choice. The gamma ray is also unaffected by a magnetic field, but it is not a particle and would not be seen, so it, too, is eliminated from consideration. The only particle that is unaffected by the magnetic field is the neutron.

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