SAT Subject Physics Practice Question 96: Answer and Explanation

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Question: 96

6. A straight wire of length 2 m carries a 10-amp current. How much stronger is the magnetic field at a distance of 2 cm from the wire than it is at 4 cm from the wire?

A. 2
B. 2
C. 4
D. 4
E. 8

Correct Answer: A


The strength of the magnetic field at a distance r from a long, straight wire carrying a current I is proportional to I /r; that is, B is inversely proportional to r. So, at the distance from the wire, the magnetic field will be twice as strong.

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