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Two coils A and B have a mutual inductance M = 2 H. The magnetic flux through coil A changes by 4 Wb in 10 s because of a changing current in coil B. Which statement is correct?

1. the change in current in B during this interval is 0.5 A
2. the change in current in B during this interval is 2 A
3. the change in current in B during this interval is 8 A
4. a change of 1 A in coil A's current changes the flux through B by 4 Wb

Correct answer: the change in current in B during this interval is 2 A

Solution

Mutual inductance relates the flux through one coil to the current in the other: phi_A = M I_B, so Delta(phi_A) = M Delta(I_B). Then Delta(I_B) = Delta(phi_A)/M = 4/2 = 2 A. Option 4 is wrong because a 1 A change in A's current would change flux through B by M*1 = 2 Wb, not 4 Wb.

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