Does a capacitor store electrons

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Mooney117

Joined May 27, 2020
7
I know that a capacitor stores electric charge but I'm confused as to what that means? Does it the capacitance value refer to the number of electrons it can store or what?
 

Papabravo

Joined Feb 24, 2006
14,201
Capacitors store charge without respect to the sign of the charge. When there is no voltage difference across the capacitor, the charges on the plates are in balance or equilibrium. That is another way of saying that each plate has the same number of positive and negative charges so there is a balance on both plates. When a capacitor charges there can be either an excess or a deficit of any particular charge type. When there is a voltage difference it means that one plate has an excess and the other plate has a deficit. So in an AC circuit the charge moves back and forth between the plates via the external circuit.

In a piece of doped semiconductor we often refer to positive and negative charge carriers. Negative charge carriers are electrons as you might suspect, but the positive charge carriers are called "holes" and represent the absence of an electron or a negative charge carrier. Even though "holes" are a somewhat abstract concept, it turns out they behave just like actual particles. How weird is that?
 

crutschow

Joined Mar 14, 2008
25,247
The total net charge on a capacitor is always zero, but you can transfer charge from one electrode to the other, thus leaving a net excess of electrons on one plate, balanced by an equal net deficiency of electrons on the other plate.
Thus for every electron you add to one plate, an electron will be subtracted from the other plate.

The capacitance refers to the number of charges it can transfer for a given voltage applied across the plates.
The fomula is C = Q/V where C is the capacitance in farads, Q is the charge in coulombs, and V is the voltage across the capacitor terminals.
 

WBahn

Joined Mar 31, 2012
26,036
I know that a capacitor stores electric charge but I'm confused as to what that means? Does it the capacitance value refer to the number of electrons it can store or what?
As others have already noted, when a capacitor stores charge it is almost always the result of electrons being moved from one plate of the capacitor to the other. The total net charge on the capacitor as a whole is almost always very close to zero, but there is a separation of charge within the capacitor between the two plates and that is what the capacitance value is referring to.

Q = CV

is the defining (constitutive) equation for a linear capacitor. What this tells us is that for every volt of potential difference across the capacitor plates, one coulomb of charge is transferred from one plate to the other for every farad of capacitance.

If you charge a capacitor and then isolate it, it is theoretically possible to disassemble the capacitor and obtain two separate plates, one with excess electrons (negatively charged plate) and the other with a shortage of electrons (positively charged plate). In practice, this is very difficult to do because of how capacitors are generally constructed and the rather large forces trying to hold the plates together (against the dielectric). Also, in some capacitor types, the plates can't physically be separated because they are actually different parts of the same physical material. But, by the same token, you certainly can construct capacitors that are intended to be forcibly separated.
 

MrChips

Joined Oct 2, 2009
21,623
Here is an interesting experiment.
You can create a parallel plate capacitor consisting of two very large circular plates made of metal separated by a gap of air.

Charge the capacitor with a 9V battery.
Remove the battery.
Move the plates away from each other. Since doing so results in a lower capacitance, the voltage V = Q / C will increase to infinity as you increase the separation.

The problem here is you are unable to measure the voltage with any common voltmeter because the voltmeter will discharge the capacitor.
 
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