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SAT Subject Physics Practice Question 205: Answer and Explanation

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Question: 205

51.

The figure above shows a pair of long, straight current-carrying wires and four marked points. At which of these points is the net magnetic field zero?

A. Point 1 only
B. Points 1 and 2 only
C. Point 2 only
D. Points 3 and 4 only
E. Point 3 only

Correct Answer: A

Explanation:

Call the top wire (the one carrying a current I to the right) Wire 1, and call the bottom wire (carrying a current 2I to the left) Wire 2. In the region between the wires, the individual magnetic field vectors due to the wires are both directed into the plane of the page (use the right-hand rule with your right hand wrapped around the wire and your right thumb pointing in the direction of the current), so they could not cancel in this region. Therefore, the total magnetic field could not be zero at either Point 2 or Point 3. This eliminates B, C, D, and E, so the answer must be A. (Because the magnetic field created by a current-carrying wire is proportional to the current and inversely proportional to the distance from the wire, the fact that Point 1 is in a region where the individual magnetic field vectors created by the wires point in opposite directions and that Po

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