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# SAT Subject Physics Practice Question 181: Answer and Explanation

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Question: 181

27. Four point charges are labeled Charge 1, Charge 2, Charge 3, and Charge 4. It is known that Charge 1 attracts Charge 2, Charge 2 repels Charge 3, and Charge 3 attracts Charge 4. Which of the following must be true?

A. Charge 1 attracts Charge 4.
B. Charge 2 attracts Charge 3.
C. Charge 1 repels Charge 3.
D. Charge 2 repels Charge 4.
E. Charge 1 repels Charge 4.

Explanation:

We know that like charges repel and opposite charges attract. So, we can put Charges 1, 2, 3, and 4 into two “camps.” Because Charge 1 attracts Charge 2, these charges must be in opposite camps

1 2

Next, since Charge 2 repels charge 3, Charge 3 is in the same camp as Charge 2

1 2
3

And, finally, since Charge 3 attracts Charge 4, these charges are in opposite camps, giving us

1 2
4 3

We now see that only (E) can be correct.

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