SAT Subject Physics Practice Question 225: Answer and Explanation

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Question: 225


How much work is done by the electric field created by the stationary charge +Q = +2.0 C to move a charge of +1.0 × 10–9 C from position X to position Z ? (Note: The value of Coulomb’s constant, k, is 9 × 109 N-m2/C2.)

A. 0 J
B. 150 J
C. 300 J
D. 560 J
E. 1,000 J

Correct Answer: A


The change in the electrical potential energy as a charge q moves from position X to position Z is equal to q times the difference in potential between the points: ΔPE = qV = qΔφ = q(φZφX). Because positions X and Z are equidistant from the source charge (+Q), the potentials at these locations are the same. Since Δφ = 0, there is no change in potential energy, so the work done by the electric field is also zero.

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