Answer: 4L.
- A 2L
- B L/2
- C L/4
- D 4L
Correct answer: D. 4L
Explanation: Self-inductance of a solenoid L = μ0n²Al, which depends on the square of the number of turns per unit length, so doubling n makes L four times as large.
As a magnet approaches a coil, flux rises steadily and a constant non-zero EMF is induced (Faraday's law). Once the magnet stops moving, flux stays constant and the induced EMF drops to zero - EMF only exists while flux is actively changing.
Concept context
Faraday's and Lenz's laws, motional EMF, self/mutual inductance, eddy currents, and the AC generator - how a changing magnetic flux creates an electric current.