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Comment by supertroop | Hacker News Reader

Dumb question but I though capacitors store energy and not power?

Power is just the time derivative of energy, like velocity is the time derivative of position. Both batteries and capacitors store energy, which they can release at some given peak and average power. Batteries tend to focus more on energy density (energy stored per weight), while capacitors tend to focus more on power density (rate of energy stored or released per weight).

I think that just reflects that the typical usage for a capacitor is smoothing out some very spiky load that runs at a high frequency, say a microcontroller running at 100MHz. The average power needs of the MCU are low, but it's drawing almost all of it in a big burst on every clock tick (10ns).

Power supplies handle that badly, and the pulses turn your supply traces in a big antenna and suddenly you're an unlicensed FM radio station.

So for the capacitor that's put in place to buffer that load, the total capacity is important, but what really matters is that the part can manage that 100MHz charge/discharge cycle.

You could have just said yes.

That wouldn't have contributed much to the discussion. Electrical insights often come from analogies. It's easier for most people to understand time derivatives in the context of an accelerating car than it is to understand them in the context of a capacitor or inductor charging.

Another cool insight is around "dual" relationships in circuits. You can often think of some components as roughly analogous to each other, with a switch of current and voltage. For example, if you connect a capacitor in parallel with a power supply and load, the voltage across it will increase as a function of time until it reaches the power supply voltage. Similarly, if you connect an inductor in series with a power supply and load, the current through it will increase as a function of time until it reaches the current that would otherwise pass through the load.

Like most good analogies, it's not 100% airtight, but it provides useful insights.

They store energy but there's a limit to how quickly you can get that energy out, hence a limit to power. And I suppose you can kinda talk about them "storing power" in the sense that power is what you might be interested in getting out of them.

At least, that's my understanding of it.

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