how does a capacitors works

by ezeanyim
August 14, 2021

Basically, it is made of two planar plates separated by an insulator.

The water analogy can help. Imagine two plastic bags filled with water, but constrained against each other inside a rigid can.

We assume that if the "can" with our bags is unconnected, the water in the bags cannot escape. Same for a capacitor, if left unconnected, nothing happens (ideally).

If you push water, with a pump, into ONE of the bag, it will try to expand. But the other bag will resist since water cannot leave from it and water is not compressible. So is a capacitor connected to just one end, even if we put a battery at that end, nothing happens. We need a complete circuit. A dandling capacitor is a strange antenna, not much anything more...

If we connect the two bags of water, externally, and one of the bag is connected to a pump pushing water in the bag, even if NOT a single water drop can travel from one bag to the other, inside the can, water will flow out of the second bag since the bag filled by the pump will be able to expand and so, crunch the second bag. In this mode, we "charge" de capacitor. When the other bag cannot expel water anymore, the flow stops. With capacitor, that occurs when the electrons are so dense on one plate that they repulse any incoming electrons, from the electrical source, for a given voltage.

Next, if we remove the pump or the electrical source, but still connect the "can" / capacitor inside a close loop, the bag with the most pressure will try to relax itself into supplying the other deflated bag. For the capacitor, that would appear as if the capacitor was a source of voltage or current, but "fastly" discharging itself.

That is just an analogy. Sometimes using air instead of uncompressible water would be more correct to visualize some non-linearities of the capacitor, or some capacitor recharging themselves, partially, without any external sources. But those are "advance" stuff.

That analogy hides some useful approximation, though, such as that we assume that, in total, there is no excess of electrons or protons if we consider the two plates of the capacitor for the total. And in the end, capacitors are concerned with electrons (protons don't move much inside an unmoving solid).

by vanderghast
August 15, 2021

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