First slide

Lenz's law and conservation of energy

Question

In the circuit shown, when a soft iron bar is quickly inserted into the inductor, the brightness of the bulb,

Moderate

A

first increases and then decreases to initial value.
B

first decreases and then increases to its initial value
C

remains unchanged
D

increases

Solution

When the soft iron bar is being inserted, emf is induced in the circuit and according to Lenz's law, this induced emf will try to decrease the current and proportional to current, brightness first decreases. When the relative motion between the soft iron becomes zero, induced emf also becomes zero and the brighness increases to its initial value.

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