Purpose:

To understand how the physics of the electrophorus and to illustrate the differences between free charge and stationary charge, and to demonstrate the effect of electric stored energy.

Discussion:

The electrophorus has historically been used as a charge dispensing device. It is simply a condensor (which is a capacitor) with dissectable parts. The electrophorus is charged not by an external voltage source, but rather the insulator separating the conductors is charged through triboelectrification.

The electrophorus was originally developed by Johannes Wilcke in 1762. The device was modified and later improved by Allessandro Volta in 1775. Volta atcually named the device an electrophorus, and soon thereafter become known as the perpetual electrophorus. It was so-called becase once the insulator was charged, the electrophorus could seemingly produce an endless quantity of electric charge. Wilke reported quite accurately the physics of the electrophorus. However, since they did not have an understanding of atomic theory at that time in history, they did not fully understand the triboelectrification of the insulator, nor why the insulator appeared to hold the charge for such a large period of time.

It has been said that the electrophorus is the electrical analog to a permanent magnet in the way that it permanently keeps its charge. Interestingly, keeping the top plate on the electrophorus preserves its power in an analogous manner as a keeper (or iron rod) placed across the poles of a permanent magnet. All in all, the electrophorus is a magnificant device that played a significant role in the early development of electrostatic theory.

Experiment I Description:

An electrophorus simply consists of two metal sheets separated by an insulator. In the figure below, the electrophorus is circular in shape, or disk shaped. The lower metal sheet is typically grounded for best performance and is attached to the insulating material. The insulator of the electrophorus used in our experiments is simply vinyl. The upper metallic disk has an insulating handle attached to its center such that the electrophorus can be dissected without grounding the top plate.

The operation of the electrophorus procedes as follows: Initially, the top of the insulating vinyl sheet is briskly rubbed with a piece of cloth (preferable wool, silk, or fur). This will create a static charge on the surface of the insulator through a process known as triboelectrification. In this process, the electrons are actually exchanged between the insulator and the cloth through a chemical reaction of the dissimilar materials. The rubbing actually increases the surface area over which the materials make contact, and hence increases the amount of charge distributed on the insulator. The sign of the charge induced will be dependent upon the insulator and the material used to rub it. For sake of argument, let us assume that a positive charge is now distributed on the surface of the insulator.

The metal disk with an insulating handle is then placed on top of the charged insulating sheet. Negative charges are attracted at the bottom surface of the metal disk by Coulomb Force, and positive charges are repelled to the top surface of the metal disk polarizing the disk, as shown in Figure 1. It is noted that the charges in the metal disk are "displaced" by the Coulomb force. The top surface of the metal disk is then grounded by either touching it with a finger or using a direct connection to ground with a conductor. The positive charges will be neutralized while the negative charge remains at the bottom of the metal disk, as shown in Figure 2.

By holding the insulating handle, the metal disk is then removed from the top of the insulator. Once it is removed and is sufficiently far from the vinyl sheet, the negative charge will distribute itself about the metal disk until an equilibrium is reached (recall equilibrium is established when the net force acting on each charge is zero). The charged disk is then touched to the electroscope. This causes the deflection arm to repel from its center, as shown in the Figure 1 of the Electroscope experiment.

The above procedure is then repeated, but without rubbing the insulator with the cloth a priori. Before repeating these steps, the electroscope and the metal disk are grounded such that they are charge neutral. Then, the disk is placed on top of the insulator, the top is grounded, the disk is removed, and then the disk is touched to the electroscope. It is observed that the electroscope is deflected the same distance as in the previous case. In fact, this can be repeated again any number of times, and the amount of charge induced on the metal disk remains the same. The reason for this is that the charge established on the top of the vinyl sheet by the process of triboelectrification is bound to the surface of the insulator. This charge is also stationary in that it cannot be redistributed on the insulator, nor can it conduct onto the the metal sheet. On the other hand, the charge in the metal disk is free to redistribute under the influence of Coulomb forces. When the disk is placed near the positive bound charge, the electrons are attracted to the positive bound charge leaving a net positive charge on the top surface. When grounded, electrons are actually conducted from ground to neutralize the top of the metal plate. This renders a net negative charge on the plate. Thus, it is a combination of the stationary charge on the insulator, free charge movement in the conductor, and ground supplying free charge that renders the electrophorus as a perpetually charged device.

Experiment II Description:

With a charged vinyl sheet, place the metal disk on top of the vinyl sheet using the insulating handle. Then ground the top of the metal disk. Slightly raised the metal disk up, there would be a large potential difference between the metal disk and the plastic plate. The potential difference can be determined by the equation:

As the disk is raised, the total charge (Q) is UNCHANGED since the stationary charge is bound to the insulator, and the free charge is confined to the disk. According to the Gauss's Law the net flux emanating from either the stationary charge or free charge is also constant. Thus, when the distance (d) is increased by raising the metal disk up, the capacitance (C) would be decreased. Therefore, by the equation, V=Q/C, the potential difference is increased. Thus, the store energy (W_e = 1/2 (Q*V)) would be increased. This makes sense because you are adding work to the system by applying an external force to lift the metal disk off the vinyl sheet. This force must be greater than the attractive Coulomb force between the opposing charges. Thus work is added to the system and the total energy is increased. As the disk is raised, and the potential difference increase, voltage breakdown can actually occur. This results in a "spark", which is due to a voltage breakdown of air that provides a path for the conduction of the free charge on the plate.

Interestingly, if a second dielectric media is inserted between the two plates, the electric field would be decreased because of the polarization due to the bound charges. Therefore, the store energy (W_e = 1/2 (Q*V)) would be decreased. Thus, it is concluded that the electrical forces actually pulls the dielectric into the charged capacitor. This can be seen if the charge in the electrophorus is large enough and a piece of polystyrene is held near the edge of the capacitor. It will actually be pulled into the center region of the electrophorus.

Results:

Briskly rubbed the vinyl sheet with fur	Placing the metal disk on the vinyl sheet and then grounded it by touching	Contacting the metal disk with an electroscope, the deflection arm is deflected due to Coulomb Force

Created by: Tang, wee-hua and S. Gedney ; University of Kentucky, EE Department Created on 25th June 1998, Last modified 10th September 1998