Whether capacitors generally have net charge

[diagopod]

Trying to learn the basics of capacitors, but I'm hung up on a conceptual issue. It seems that capacitors must be charged up, generally with the help of a current from outside the capacitor, such as a flow of electrons that pile up on one of the plates. So in that sense, the capacitor must be storing, in this case negative, charge, at least on one of the plates. Yet it also seems that capacitors, at least once charged, are described in terms of two conductors with equal opposite charge, thus having no net charge. My question is essentially: does the capacitor, taken as a closed system, have net charge? If not, then how is charge conserved when so much negative charge was added to the capacitor to charge it up?

Any help on this would be greatly appreciated, as I'm sure there is a flaw somewhere in my conception of a capacitor and how it works.

 

[Antiphon]

There is no net charge in the capacitor. From a circuit theory point of view the current is continuous though it so whatever charge enters the top also leaves the bottom.

Turning to physics, if you force electrons onto the upper electrode, they will repel an equal number of electrons from the opposing electrode thus satisfying the current continuity found in circuit theory.

 

[kmarinas86]

Not generally, but obviously materials can store excess charge.

Hair+Comb+Pieces of Paper=A common science experiment you can do yourself

Yet, a capacitor can still work without receiving additional charge. For example, you can have stored potential in a capacitor by forcing charge through a membrane (as is the case with a plasma membrane), but that charge doesn't have to leave the capacitor to make a potential difference.

Ultimately it is based on increased separation of opposite charges (such as by forcing them through a membrane) or reduced separation of like charges (such as by shoving in more electrons from the outside).

 

[kmarinas86]

There is no net charge in the capacitor. From a circuit theory point of view the current is continuous though it so whatever charge enters the top also leaves the bottom.

Turning to physics, if you force electrons onto the upper electrode, they will repel an equal number of electrons from the opposing electrode thus satisfying the current continuity found in circuit theory.

http://en.wikipedia.org/wiki/Electrets

Electret (formed of elektr- from "electricity" and -et from "magnet") is a dielectric material that has a quasi-permanent electric charge or dipole polarisation. An electret generates internal and external electric fields, and is the electrostatic equivalent of a permanent magnet. Oliver Heaviside coined this term in 1885. Materials with electret properties were, however, already studied since the early 18th century. One particular example is the electrophorus, a device consisting of a slab with electret properties and a separate metal plate. The electrophorus was originally invented by Johan Carl Wilcke in Sweden and again by Alessandro Volta in Italy.

[...]

Similarity to capacitors

There is a similarity between an electret and the dielectric layer used in capacitors; the difference is that dielectrics in capacitors possesses an induced polarization that is only transient, dependent on the potential applied on the dielectric, while dielectrics with electret properties exhibit quasi-permanent charge storage or dipole polarization in addition.

[...]

Electret types

There are two types of electrets:

* Real-charge electrets which contain excess charge of one or both polarities, either
on the dielectric's surfaces (a surface charge)
within the dielectric's volume (a space charge)
* Oriented-dipole electrets contain oriented (aligned) dipoles. Ferroelectric materials are one variant of these.

[...]

Materials

Electret materials are quite common in nature. Quartz and other forms of silicon dioxide, for example, are naturally occurring electrets. Today, most electrets are made from synthetic polymers, e.g. fluoropolymers, polypropylene, polyethyleneterephthalate, etc. Real-charge electrets contain either positive or negative excess charges or both, while oriented-dipole electrets contain oriented dipoles. The quasi-permanent internal or external electric fields created by electrets can be exploited in various applications.

 

[diagopod]

Thanks Antiphon. Thanks kmarinas86. I'll have a look at electrets. Overall, I'm starting to get the sense that there are few cases in which very high concentrations of net charge are stored. Van De Graaff generators, for example, seem to store a surprisingly small amount of charge, yet discharge it quickly at a high voltage. Is there a law governing maximum net charge density related to voltage breakdown and dialectric strength? In the case of a Van De Graaff globe, for example, would covering the aluminum globe in a material with a dialectric strength greater than the surrounding air increase its capacitance or net charge storage capacity?

 

[sophiecentaur]

A Van der Graff ball holds only a small charge because it is only 'half' of the sort of capacitor you'd find in a circuit. The capacity is between it and the Earth - only a few tens of picoFarads. (Q = CV, remember?) Hence, you need a massive voltage in order to store even a small charge - which is equal and opposite to the charge you have transferred to the Earth.
Your question about voltage breakdown: What counts is the electric field strength. If it's high enough, it can ionise the air molecules and allow conduction (a spark). The field strength depends upon the Potential and also the radius of the object in question. Pointed objects have high field strength around the points and will form 'corona' very easily. The VDGG uses a 20 to 30 cm ball with no sharp edges so it can reach 100kV or so without discharging. I imagine a coating of insulating material could increase the maximum voltage it could reach.

 

[diagopod]

Thanks sophiecentaur. Your description of the Van der Graaff as half of a capacitor, the other half being the Earth, is illuminating, and gives me a lot to ponder. Thanks again.