Electrostatic charging

Tuesday 7-2-96

The relevant section in the textbook is 15.2

Homework problems: 5, 6, 7, 12

Main concepts:

Materials can be divided into three main categories depending on how easily they allow charge to flow. These are conductors, which allow charge to flow easily; insulators, through which it is difficult to pass charge; and semiconductors, which are somewhere in between conductors and insulators.
insulators can be charged by friction - by rubbing them with something which will remove or add electrons.
conductors can be charged by connecting them to a source of charge, or by induction, where a charged object is brought close to, but not touching, the conductor.

In more detail:

Forces between two objects that are electrically charged can be extremely large; the electric force is roughly 40 orders of magnitude larger than the gravitational force. Fortunately, most things are electrically neutral, meaning that they have equal amounts of positive and negative charge. If this wasn't the case, the world we live in would be a much stranger place. It is also fortunate that we have a lot of control over how things get charged and what sort of charge they accumulate. This is because we can choose the appropriate material to use in a given situation.

The MBTA, for example, powers the trains on the Green Line with a metal cable that runs above the trains. A metal contact on top of each train touches the cable, and current flows into a motor in the train, turning the wheels and making the train go. Now, there's a reason why the MBTA doesn't use a rope instead of a metal cable, and doesn't use a plastic contact instead of a metal one on each train. Why is that?

The reason is that metals are good conductors, while plastics and typical rope materials are not. They're called insulators. The trains need this conductive material, because it allows charge to flow. Charge does not flow nearly as easily through insulators...this is why wires you plug into your wall socket are covered with a protective rubber insulating coating, to protect you from the charge flowing along the metal wire.

Materials are divided into three categories, depending on how easily they will allow charge (i.e., electrons) to flow along them. These are

conductors - metals, for example
semi-conductors - silicon is a good example
insulators - rubber, for example

Most materials are either conductors or insulators. The difference between these two types of material is that in conductors, the outermost electrons in the atoms are so loosely bound to their atoms that they're free to travel around the metal. In insulators, on the other hand, the electrons are much more tightly bound to the atoms, and are not free to flow. Semi-conductors are a very useful intermediate class, being not as conductive as metals but considerably more conductive than insulators. By adding certain impurities to semi-conductors in the appropriate concentrations the conductivity can be well controlled.

There are three main ways that objects may be given a net charge. These are:

Charging by friction - this is useful for charging materials that are insulators. If you rub one material with another (say, a plastic ruler with a piece of paper towel), electrons have a tendency to be transferred from one material to the other, depending on the material. For example, rubbing glass or nylon with silk or saran wrap generally leaves the glass with a positive charge; rubbing PVC rod with fur generally gives the rod with a negative charge.
Charging by conduction - useful for charging metals and other conductors. If a charged object is brought in contact with a conductor, the conductor will acquire a charge. If the object is negatively charged, electrons will flow off the object on to the conductor, giving the conductor a negative charge. If the object is positively charged, electrons will flow from the conductor on to the object, giving the conductor a positive charge.
Charging by induction - also useful for charging metals and other conductors. Again, a charged object is used, but this time it is only brought close to the conductor, and does not touch it. If the conductor is connected to ground (ground is basically anything neutral that can act as a source or repository for a large number of electrons) briefly, allowing electrons to flow on to it or away from it, then, after in turn removing the ground connection and then the charged object, the conductor will have a charge opposite to that of the charged object.

Going through the charging-by-induction process step by step, using a negatively charged object:

bring the negatively-charged object close to the isolated conductor, but not touching. Electrons on the conductor will be repelled away from the area nearest the charged object.
connect the conductor to ground. The electrons on the conductor want to get as far away from the negatively-charged object as possible, so some of them flow to ground.
remove the ground connection. This leaves the conductor with a deficit of electrons.
remove the charged object. The conductor is positively charged.

Try this at home: Using common household items, it's very easy to charge things by friction. Saran wrap, or something equivalent, is good. A paper towel works, too. Try rubbing a piece of hard plastic, or glass, and see if you can attract something metal, like a soda can.

Demonstrations that go with today's session:

Relevant web links: A photocopier is a good example of an application of electrostatic charging. There are a couple of good web pages which give nice descriptions of how a photocopier works: