Static Cling and More

When different materials scrape against each other, some electrons go from one to the other. This will happen with any pair of different materials because they differ in their affinity (or desire, so to speak) for electrons. During the scrape, there is a tremendous number of electrons freed up, and most of them go to the material that most wants them. Talk to chemists about this.

The result is that one becomes positive and the other negative. (Electrons are negative, so the one that grabbed most of the electrons is negative.) Now they attract each other, and we have the dreaded static cling.

A charged body and a neutral body will attract each other, too, although a brief look into Coulomb's law would indicate that this cannot be. Coulomb's law: F = kq₁q₂/r² is the force between charges q₁ and q₂ separated a distance r. So if one charge is zero, this says that the force is zero, so what is going on?

The trouble is, that was too brief a look at Coulomb’s law. The sketch below shows a rough picture of a normal atom on the left compared to what it is like when a charged object is nearby: the electron cloud gets distorted (polarized), making the left side negative in this case. So when a charged object A is brought near a neutral object B, the atoms in B get distorted like that, creating a surface charge on B opposite in sign to the charge on A. So they attract. (There is a repulsion due to the fact that the other side of B is left with a charge of the same sign as A, but that is farther away and hence the repulsive force is weaker than the attractive force.)

normal atom……………..distorted atom (polarized) because it is near a + charge

Another somewhat tricky thing is using a charge to cause another object to acquire the opposite charge: charging by induction. Example: you have positive object A and you want object B, a conductor (allows charge to flow), to become negative. Put A and B near each other, then while standing away from A, touch B. B is now negative, because A pulled lots of electrons out of your body onto B. They want to get as near to A as they can. Touching B with a ground wire would be another good way to charge it.

Speaking of charging things, the Van de Graaff generator is a nifty device for generating high voltages. The original idea came from Faraday (look up Faraday's ice-pail experiment). The generator consists of an isolated conducting sphere with a hole in it, and a belt which carries charge to the inside of the sphere, where the charge is transferred to the sphere. The charge on the sphere does not repel additional charge added to it from the inside, so one can keep on adding charge (although there are practical limitations on the amount of charge it can acquire).

To really understand the Van de Graaff, you need to know about electric field E (force on a small charge divided by that charge) and that if there is E in a conductor, there will be current flow. So in statics, E = 0 in conductors, and if a conductor is charged, the charge will be on the surface, arranged in such a way as to make E = 0 inside. If you try to charge something from the outside, any charge it already has will oppose further charging. No problem charging a hollow conductor from the inside, but you need to bring charge into the interior mechanically. To do it electrically would require a higher voltage than the thing you are trying to charge.

If you have a lot of charges on your charge card, the bank might oppose further charging.

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