SAT Subject Physics Practice Question 67: Answer and Explanation

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Question: 67

4.

The figure above shows two point charges, +Q and –Q. If the negative charge were absent, the electric field at point P due to +Q would have strength E. With –Q in place, what is the strength of the total electric field at P, which lies at the midpoint of the line segment joining the charges?

A. 0
B.
C.
D. E
E. 2E

Correct Answer: E

Explanation:

Since P is equidistant from the two charges, and the magnitudes of the charges are identical, the strength of the electric field at P due to +Q is the same as the strength of the electric field at P due to –Q. The electric field vector at P due to +Q points away from +Q, and the electric field vector at P due to –Q points toward –Q. Since these vectors point in the same direction, the net electric field at P is (E to the right) + (E to the right) = (2E to the right).

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