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To measure the magnetic field between the poles of a loudspeaker magnet, a small flat search coil of area 2 cm² with 25 closely wound turns is placed normal to the field and then quickly removed (or turned 90 deg). A ballistic galvanometer records a total charge of 7.5 mC flowing through the coil. The total resistance of coil plus galvanometer is 0.50 ohm. Estimate the magnetic field strength.

1. 0.75 T
2. 1.5 T
3. 7.5 T
4. 15 T

Correct answer: 0.75 T

Solution

The charge through a coil equals the change in flux linkage divided by resistance, q = N*delta(BA)/R. With the field removed, delta flux = B*A per turn. Solving for B gives the field strength.

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