# eli5: When a circuit is switched on, why does the current increase all at once to the maximum before gradually decreasing?

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eli5: When a circuit is switched on, why does the current increase all at once to the maximum before gradually decreasing?

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ELI the ICE man.

That is, current (I) leads voltage (E) in a (C)apacitor. Or put another way, all of the capacitors in a circuit act as shorts until they are charged.

This isn’t necessarily true. Different devices respond differently.

The current rushes in (almost) instantly because there is almost no inertia. Electrons are light and a “lot” current is very few moving electrons.

What happens next just depends on the circuit. In incandescent light bulbs the bulb heats up and so won’t let electricity flow as well.

A lot of electrical circuits include a component called a capacitor. A capacitor can do a bunch of different jobs by storing a bunch of energy and then being slow about how it releases it back into the circuit. But capacitors can also be dangerous because they store a bunch of energy that can suddenly be released all at once when you short them out. So typically, they will have “bleed off resistors” to discharge them when the circuit is switched off. So typically, when you switch a circuit on, there will be an initial inrush of energy to the capacitors as they fill themselves up to operating levels.