A lightbulb is made of a filament which has a much higher resistance than the rest of the “grid.” High resistance is kind of like squeezing through a tube that’s way too narrow: you’re going to rub against the walls a lot.
Similarly, when electrons (the stuff that flows through the grid) go through a filament that has high resistance, they “rub” a lot and this can generate either a) heat or b) light. Both of these are forms of energy.
Lightbulb filaments have been designed to produce as much light as possible, usually while trying to keep heat as low as possible.
So that’s what happens when current is flowing. When you flip the switch, you break the circuit which means electrons are no longer flowing. Since they’re no longer moving through the filament they’re no longer “rubbing” so there’s no more light being produced.
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