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A lamp P is in series with an iron-cored inductor L. When switch S is closed, the lamp reaches full brightness more slowly than it would without the inductor. The reason is

1. The low resistance of P
2. The induced (back) emf in L
3. The low resistance of L
4. The high voltage of the battery B

Correct answer: The induced (back) emf in L

Solution

As the switch is closed the current tries to rise suddenly; the inductor opposes this change by generating a back emf (Faraday/Lenz). This limits how fast the current can grow, so the lamp brightens gradually with time constant L/R rather than instantly.

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