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A light bulb and an open coil inductor are connected to an ac source through a key as shown in the figure. The switch is closed and after some time an iron rod is inserted into the interior of the inductor. The glow of the light bulb _______.

Asked in GSEB Board August 2020 · Choke coil and AC devices

Figure: Choke coil and AC devices
Answer: (2) Decreases

Step-by-step solution

The bulb and the coil are in series, so the same current flows through both, and the glow depends on that current.

Current: I=V/(√R²+X_L²), with X_L=ω L.

Soft iron has a much higher permeability than air, so pushing the rod into the coil increases its inductance L.

A larger L means a larger X_L, a larger impedance and a smaller current.

Less current means less power I²R in the filament, so the glow of the bulb decreases.

Why the other options are wrong

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