SAT Subject Physics Practice Question 220: Answer and Explanation

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Question: 220


Three point charges are arranged along a straight line. If k denotes Coulomb’s constant, what is the strength of the electrostatic force felt by the positive charge at the left end of the line?


Correct Answer: C


Label the charges 1, 2, and 3, in order from left to right. We’re asked to find the force on Charge 1 due to the other two charges. The electrostatic force on Charge 1 due to Charge 2 is a repulsive force, of magnitude , and the electrostatic force on Charge 1 due to Charge 3 is a weaker, attractive force, of magnitude . Since these individual forces act in opposite directions, the magnitude of the net force is found by subtracting the magnitudes of the individual forces. So, the total electric force on Charge 1 has magnitude

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