Charging an Electrophorus and Electroscope by Induction

A conductor can be charged by induction, without contact.

Discussion

To understand what is involved in electrically charging or discharging a metal object, such as the electrophorus and electroscope used in this movie, we need to review a few basic facts about the structure of matter and the interactions of electrical charges.

Matter is made up of atoms. Each atom contains a nucleus that is surrounded by negatively charged electrons. The nucleus contains one or more protons (positively charged) and may contain neutrons (uncharged). The electrons in an atom are attracted to the oppositely charged protons in the nucleus.

Matter can be classified as either as an insulator, such as wood and plastic, or as a conductor, such as metals. In insulators, all electrons are tightly bound to individual atoms and molecules, and cannot move freely through the material. In conductors, such as metals, some of the outermost or valence electrons are not bound to individual atoms, but can move freely in the metal. These mobile or conduction electrons are responsible for all charge movement in such materials. (We should mention here that in some metals, including aluminum and zinc, and in "p-type" semiconductors, mobile charge carriers are not electrons, but positively charged "holes" that move in the direction opposite that in which electrons would move when current is flowing.)

An electroscope is used to detect electrical charge. The glass-walled metal body of the electroscope is electrically insulated from the metal post that passes through the top of the electroscope. Two metal foil leaves are attached to the bottom of this post, and if there is an excess of positive or negative charge on the foil leaves, repulsive electrical forces between the charges on each leaf will cause them to separate. At the beginning of this movie, the foil leaves of the electroscope are separated. This means that the post and foil leaves of the electroscope have an electric charge on them. When the demonstrator touches the electroscope, the post and leaves lose their electrical charge, and the leaves collapse (move together). This process (discharging by contact) is explained in the next movie.

There are two ways to put an electrical charge on an electroscope, by contact and by induction. In this movie charging by induction is demonstrated.

In charging by induction, mobile electrons are "induced" by electrical attraction or repulsion to move onto or off of a neutral conductor. An electrophorus consists of a metal disk or plate with a non-conducting handle and a base made of an insulator such as the plastic sheet used here.

Rubbing the plastic sheet with a piece of fur causes static electric charges to build up on the plastic sheet and on the piece of fur. The amount of charge on the fur is the same as that on the plastic sheet, but one has an excess of positive charge and the other an excess of negative charge. These charges are called "static" because they cannot move about, but are fixed in place on the surface of the insulator.

When the electrophorus disk is placed on the charged plastic sheet, the electric charges on the surface of the plastic sheet cannot move onto the metal disk, but they can polarize it. If the surface charge on the plastic sheet is positive, mobile electrons in the metal will be pulled toward the charges on the plastic sheet, or if the surface charge is negative, pushed away from them. In either case, the result is that the upper and lower surfaces of the metal disk now have equal but opposite charges.

At this point the metal disk is still electrically neutral, but if the demonstrator (an electrical conductor) now touches the disk, mobile electrons will be pulled from the demonstrator to the plate if the plastic sheet has a positive charge, or pushed from the plate to the demonstrator if the charge on the plastic sheet is negative. If the demonstrator now removes the finger touching the disk, the electrons that moved to or from the neutral electrophorus disk are trapped either on the disk or demonstrator, so the disk is now charged, negatively, if electrons moved onto it, or positively if they moved off. As we saw in the previous movie, our electrophorus plate is negatively charged, which means that the plastic base is positively charged.

When the charged electrophorus disk is brought near the neutral electroscope, the leaves move apart; when it is moved away, the leaves collapse. This happens because when the electrophorus disk is near, its electric charge pulls (if the charge on the disk is positive) mobile electrons away from the foil leaves, or pushes (if the charge is negative) mobile electrons to the foil leaves. While the electroscope is still electrically neutral overall, it is now polarized: the top of the metal post and the foil leaves at the bottom have equal but opposite electrical charges. As a result, the foil leaves push each other apart for as long as the charged disk is held near the electroscope.

If the demonstrator touches the post of the neutral electroscope while the charged electrophorus disk is held near the electroscope, mobile electrons move to or from the electroscope (and from or to the demonstrator). When the demonstrator stops touching the disk, the electroscope is left with a charge — negative if mobile electrons moved to it, and positive if they moved from it.

What can we say about the induced charge on the electroscope? We saw in the previous movie that the electrophorus disk used in these movies is negatively charged, so mobile electrons in the electroscope will move away from the disk, repelled by its negative charge, and the charge induced on electroscope will be positive. This is confirmed by the fact that when the electroscope has been charged by induction, its leaves collapse when the charged electrophorus disk is brought close, and move apart when the disk is removed, indicating opposite charges are on electrophorus disk (negative) and electroscope (positive).