When does the filament of a bulb glow?

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When an electric current is passed through the filament.

When a nuclear fusion reaction happens in the filament.

When the filament is broken due to an excess electric current flowing through it.

When the filament starts reflecting all the light incident on its surface.

answer is A.

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Detailed Solution

Due to the heating effect of electric current, when electric current passes through a conductor with some resistance, the conductor heats up.
The more the resistance of the conductor, the hotter it gets.
A filament has a high electrical resistance.
As a result, when an electric current is passed through the filament, it becomes so hot that it begins to glow.
When the filament in a bulb breaks due to an excess electric current, the electric circuit breaks and the bulb stops glowing.

Therefore, we can say that bulb starts glowing when the filament heats up due to an electric current flowing through it.

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