# Electronics/Static Electricity

## Static Electricity

This is a rough draft.

As this is a book, it should have historical stories and stuff to make it more interesting. Greeks and amber in this section, for instance.

When a solitary atom has the same number of electrons and protons, it has no overall charge. The individual charges cancel out. It is said to be neutral.

An atom that has more electrons than protons is negatively charged. The atom will have a tendency to attract other particles that have a positive charge, and repel particles with a negative charge.

An atom that has less electrons than protons is positively charged. The atom will attract negative particles (like solitary electrons or negatively charged atoms) and repel positive particles (like solitary protons or positively charged atoms).

If a solitary proton and electron are placed near each other (or even far away, as long as nothing else is nearby), they will move closer together, until the electron becomes "attached" to the proton and they form a hydrogen atom. An atom of metal that is positively charged (missing electrons) and a free electron will behave the same way.

The same goes for macroscopic objects. A metal sphere that has a significant amount of atoms that have the same number of electrons and protons is neutral. Also, if there are several atoms that are positively charged or negatively charged, it is said that the whole object is charged. This may not be noticeable if only a few atoms on the object are charged.

Charged metal spheres will attract and repel each other according to the same rules as individual particles.

A solitary metal sphere with an excess of electrons will tend to have the extra electrons spread out evenly across the surface of the sphere (not in the interior). This is because the electrons repel each other, and try to get as far apart as possible. On a positively charged sphere, the positive charge (absence of electrons) also spreads evenly across the surface due to its lack of electrons. (Talk about the neat shielding properties of Faraday cages) In the space inside to the sphere there is no charge.

In the case of insulating spheres charge does not move throughout the object, it just sits there. The electrons in this object are not constantly moving from atom to atom, so the empty holes have no way to be filled by their neighbors.

A Van de Graaff generator is basically a pump for electrons (or a "charge pump"). It uses the triboelectric effect to take electrons from one conductor and deposit them on another. The triboelectric effect is caused when two different materials are brought into contact and then separated. One material will have more of a tendency to attract extra electrons, and some of the electrons will transfer from one material to the other. The Van de Graaff generator accomplishes this with a silk or rubber belt held tightly between two rollers of different materials. As the belt comes into contact with one roller and then separates, it picks up charge from the roller. When it comes into contact with the other roller and then separates, it deposits the charge.

It thus makes one roller positively charged and the other negatively charged. The electrons are supplied to one roller and extracted from the other by metal brushes, connected to the large conductors. This creates a voltage between the conductors, due to their distance.

(Should we really be going into so much detail about it though?... I guess as an example of a very basic electric circuit, it is a good example. Maybe we could just link to Wikipedia triboelectric effect? That's the thing I don't like about wikibooks. You have to provide all of the background instead of just directing them to it.)