PURPOSE: To produce small amounts of negative or positive static electric charge.
DESCRIPTION: The electrophorus is a flat, 12 in. by 12 in. square piece of material which when rubbed with paper will produce a separation of charge. There are two types of electrophorus, one produces a net positive charge and the other produces a net negative charge. The PVC type electrophorus is dark grey in color and will produce a negative charge when rubbed. It is shown to the left in the photo above, with the paper towel marked with a circled "-" sign that is used to rub it. Acrylic is clear in color and will produce a net positive charge when rubbed. It is shown to the right in the above photo, with the paper towel marked with a circled "+" sign. A separate paper towel is used to rub each material. Do not rub the different electrophorus's with the same paper towel because the conservation of charge dictates that the opposite charge will remain on the paper towel after it rubs one of the electrophorus's.
To get the charge off of the electrophorus a large, round metal plate with a handle is used to perform charge by induction. Hold the handle and place the plate onto the electrophorus after it has been charged by rubbing. While holding the handle carefully the outermost pinky finger to the electrophorus to induce the charges onto the metal plate. The charge on the metal plate can then be transported and used to perform electrostatic experiments.
Some interesting features of the electrophorus: Negative charged is a net excess of electrons. Positive charge is a net excess of electron holes. Quantum tunneling is used to understand the triboelectric effect today, also known as CHARGING BY FRICTION. The "Triboelectric Series", that dates back to Ben Franklin and the 18th century, is an ordered list of materials that can be used to make an electrophorus.
REFERENCES TO TRIBOELECTRICITY - CHARGING BY FRICTION
Duane Roller and Duane H. D. Roller, The Prenatal History of Electrical Science, AJP 21,
343-356 (1953).
Robert C. Plumb, Triboelectricity, Journ. Chem. Ed. 48, 524, (1971).
D. S. Ainslie, What are the Essential conditions for Electrification by Rubbing?, AJP 35,
535-537, (1967).
Stanley Anderson, An Experiment to Demonstrate that "Friction" Electricity
Depends on Contact Potential, AJP
144, (1936).
Gilbert O. Spencer, Demonstrations in Electrostatics, AJP 26, 269, (1958).
EQUIPMENT: The electrophorus are stored in the electricity and magnetism cabinet.
SETUP TIME: None.