MCAT Physics Question 64: Answer and Explanation

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Question: 64

4. If the electric field at a distance r away from charge Q is what is the ratio of the electric fields at r, 2r, and 3r?

A. 9:03:01
B. 36:09:04
C. 36:18:09
D. 36:18:12

Correct Answer: B

Explanation:

The first step in answering this question is to remember that the magnitude of the electric field is inversely proportional to the square of the distance:

Therefore, if the electric field at radius r, E_r, is

then the electric field at radius 2r will be

Similarly, the electric field at radius 3r is equal to

Therefore, the ratio of E_r:E_2r:E_3r is 36:9:4.

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