AP Physics 1 Question 165: Answer and Explanation

Test Information

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Question: 165

5.

Question below refer to the following information:

Two metal balls of equal mass (100 g) are separated by a distance (d). In state 1, shown in the figure, the left ball has a charge of +20 µC, while the right ball has a charge of –20 µC. In state 2, both balls have an identical charge of –40 µC.

An experimenter claims that he took the balls from state 1 to state 2 without causing them to contact anything other than each other. Which of the following statements provides correct evidence for the reasonability of this claim?

  • A. The claim is not reasonable, because there was more net charge in the two-ball system in state 2 than in state 1.
  • B. The claim is not reasonable, because there was identical charge on each ball in state 2.
  • C. The claim is reasonable, because in both states, each ball carried the same magnitude of charge as the other.
  • D. The claim is reasonable, because the positive charge in state 1 could have been canceled out by the available negative charge.

Correct Answer: A

Explanation:

A-Charge is conserved, meaning that while negative charge can neutralize positive charge, the net charge of a system cannot change. The net charge of state 1 is zero; thus, the net charge of state 2 must also be zero, regardless of whether the balls touched or not. State 2 has a net charge of –80 µC, not zero; thus, the claim is not reasonable due to this violation of charge conservation.