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Name: Matthew
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Question:
I was recently reading about capacitors, and I was confused as to how they hold a charge. In the material I was reading, it said that there is some dielectric material between two conductors. Since that means it does not conduct electricity, how does it hold a charge, and transfer the charge?



Replies:
Mathew-

I like water-in-a-pipe analogies for electric components. Just as electrons can move around in a metal but cannot escape the metal, water in a pipe can move around but cannot get out. Definable water-pressure results, which is analogous to electrical voltage.

My water-in-a-pipe analogy for a capacitor is: a wide pipe with a stretchy rubber diaphragm stretched across inside, with one pipe going in on each side of the diaphragm.

The stretchy diaphragm acts to water-flow much like the dielectric film acts to electric current: It allows some motion, but builds up an increasing push-back force. Both are analogous to mechanical springs.

Of course, the diaphragm requires empty space in the "pipe" on either side to allow water to push on all parts of it. Likewise a capacitor's dielectric material must have a conductor on both sides to give electric current access to charge it up. This conductor is often metallization on both sides of a plastic or oxide film, but one side can be an ionic liquid (like salt-water) instead of a metal. The conductor is just as essential as the dielectric to make a capacitor.

The dielectric film and rubber diaphragm both block the flow of their respective fluids. The rubber diaphragm never lets any individual water molecules all the way through it. The dielectric film never lets any individual electrons all the way through it. It works only by allowing many of the electrons or ions that live in it to lean to one side a little. When they do so, that pushes some electrons out the metal wire on one side, and pulls some in through the metal wire on the other side. Charge_displacement = (Charge_moved) X (distance_moved) is the same at all points around the circuit: 1 million electrons moving one atom-width in a wide sheet of dielectric can force 1 electron to move 1 million atom-widths down a sufficiently skinny metal wire. Thus a large area of dielectric can allow significant though temporary current-flows. And if the sheet is very thin, the resulting push-back voltage will not be unreasonably high and the capacitor will seem useful.

A capacitor holding a charge is much like the diaphragm being stored in a bulging stretched state, with the pipe capped closed one or both sides to hold the water that keeps it stretched. If your rubber was primitively made, it would always have pinholes or pores through it, so the water would leak from the higher-pressure side to the lower-pressure side, and the stored stretch would gradually decline. Likewise most real capacitors have some tendency to self-discharge.

Jim Swenson


Matthew,

A standard capacitor is made of two metal plates that do NOT touch. The plates can be large steel disks or thin pieces of foil. There can be empty space between them. There can also be a dielectric material. A dielectric material does not conduct electric charge. Charge will not flow through it. A dielectric is special in that each molecule has a positive charge at one end and a negative charge at the other. These charges cannot leave the molecules, but the molecules can rotate.

When a battery is connected to a capacitor, electric charge is pulled through the battery from one plate to the other. One plate has a positive charge and one plate has a negative charge. The capacitor is "charged". Electric charge cannot cross the capacitor directly. If you then disconnect the battery, there is no way for the charge to get back. The capacitor remains charged. When something like a light bulb is connected across this charged capacitor, the electric charge moves through the light bulb, causing both plates of the capacitor to have zero charge. Now, the capacitor is discharged.

A dielectric increases the amount of charge that each volt of the battery can pull from one plate to the other. The dielectric itself doesn't hold the charge. When charge starts to build up in the capacitor, the dielectric molecules start to rotate. The positive ends point toward the plate with the negative charge. The dielectric surface next to the negative plate is positive, and the dielectric surface next to the positive plate is negative. These dielectric surfaces pull on the capacitor plate charges, allowing the battery to pull even more charge through itself. The dielectric cannot pull the charges off the plates, but it can help the battery hold them in place.

The capacitor plates hold charge. More volts means more charge. A dielectric just makes it possible for the capacitor plates to hold more charge per volt.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College



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