SAT Subject Physics Practice Question 74: Answer and Explanation

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Question: 74

1. True statements about the relationship between the electric field and electric potential include which of the following?

I. If the electric field at a certain point is zero, then the electric potential at the same point is also zero.

II. If the electric potential at a certain point is zero, then the electric field at the same point is also zero.

III. The electric potential is inversely proportional to the strength of the electric field.

A. I only
B. II only
C. I and II only
D. I and III only
E. None of the above

Correct Answer: E

Explanation:

1 E A counterexample for I is provided by two equal positive charges; at the point midway between the charges, the electric field is zero, but the potential is not. A counter-example for II is provided by an electric dipole (a pair of equal but opposite charges); at the point midway between the charges, the electric potential is zero, but the electric field is not. As for III, consider a single positive point charge +Q. Then at a distance r from this source charge, the electric field strength is E = and the potential is V = . Thus, V = rE, so V is not inversely proportional to E.

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