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A bar magnet freely suspended in the horizontal component of Earth’s magnetic field H has a time period of 4 s. If a second magnet is placed nearby and the time period becomes 2 s, then the magnetic field produced by the second magnet is:

1. 4H
2. 3H
3. 2H
4. √3 H

Correct answer: 3H

Solution

Since T is proportional to 1/sqrt(field), (T1/T2)^2 = 4 gives a net field of 4H. The added field of the second magnet is 4H - H = 3H.

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