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A light bulb is nothing more than a piece of wire inside an evacuated (gasless) glass envelope. Light is heat, and the wire is made of such materials to promote great heat to develop quickly. Removing all gasses from the glass envelope makes sure that the wire doesn't just burn - instead, it glows with bright light.

Now, it uses a LOT of energy, transforming it into heat and light. As such, it is an effective current limiter - in a conductor, heat equals resistance.

Calculate it and you will see - a 100W light bulb at 230V will have to use around 0.45A to generate that much power, and in order for it to allow exactly this much, it needs to be a resistor (a 500ohm resistor, to be precise).

230V / 500ohm = 0.46A, and 0.46A * 230V = 105.8W

And that is why a light bulb is an effective resistor.

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Q: Why does a light bulb add resistance to a circuit?
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