Demonstrations

Sliding Capacitors

Code Number :	5C10.20
Disclaimer:	Reprinted by permission of Dick Berg, University of Maryland, for use on this website. The demonstrations contained and referenced herein are listed for the purposes of cataloging and describing physics demonstrations which should be conducted only under the direction of a trained instructional support professional or physicist. These demonstrations are not presented for the purpose of being conducted by persons unconnected to this Facility and/or persons not consulting with or being supervised by the recognized instructional support professional or physicist and his/her staff. The University is responsible only for those demonstrations carried out using its own equipment using established safety and scheduling policies, and bears no responsibility for those choosing to use this source material for their own purposes. All demonstrations described and contained herein are public domain, and can also be found in reference materials in libraries, bookstores, and electronic sources. Further information regarding legal liability in use of demonstrations and labs will be found on the web site Injuries in School/College Laboratories in USA. The University of Iowa Disclaimers: University of Iowa Disclaimer All Rights Reserved..
Condition :	Good
Principle :	Capacitance Dependence on, Distance Between Plates
Area of Study :	Electricity & Magnetism
Equipment :	Sliding capacitors, Keithley Electrometer, Overhead DC voltmeter, 67.5 V battery, Dielectric plates, Sliding Capacitors, Electroscope.
Procedure :	Set the voltmeter to the 100 volt range. Assemble the apparatus as shown. Touch the positive battery cable to the positive plate and the meter voltage should read around 70 volts. Move the plates together and the voltage will drop to around 30 volts. Another twist on this is to place the plates close together first and then charge them. Now, pull them apart and the voltage will rise. Use the dielectrics as desired or a thick textbook will also act as a dielectric. The Plexiglas dielectric may have to be cleaned with alcohol to remove any built up static charge.

References

Marcos Alfredo Salami, Joao Bernardes da Rocha Filho, Claudio Galli, "Capacitors Made by Drawing with Graphite on Paper or Plastic", TPT, Vol. 45, # 6, Sept. 2007, p. 392.

Mustafa Baser, "Hydraulic Capacitor Analogy", TPT, Vol. 45, # 3, March 2007, p. 172.

Nathaniel R. Greene, "Energy Flow for a Variable-Gap Capacitor", TPT, Vol. 43, # 6, p. 340, Sept. 2005.

Wojciech Dindorf, "Parallel Plate Capacitor at Home", TPT, Vol. 42, # 4, April 2004, p. 250.

Walter Connolly, "Measuring Capacitance and the Dielectric Constant," TPT, Vol. 26, # 3, p. 169, March 1988.

John B. Kwasnoski, "Measuring the Charge Stored on a Capacitor", TPT, Vol. 25, # 8, Nov. 1987, p. 492.

Thomas B. Greenslade, Jr., Richard H. Howe, "A Modern Use of Volta's Electroscope", TPT, Vol. 19, # 9, Dec. 1981, p. 614.

Herbert H. Gottlieb, "Variable Capacitor", TPT, Vol. 5, # 2, Feb. 1967, p. 86.

G. W. Parker, "Electric Field Outside a Parallel Plate Capacitor", AJP, Vol. 70, # 5, May 2002, p. 502.

Ali Naini and Mark Green, "Fringing Fields in a Parallel-Plate Capacitor", AJP, Vol. 45, # 9, Sept. 1977, p. 877.

Ching Yao Fong and C. Kittel, "Induced Charge on Capacitor Plates", AJP, Vol. 35, # 11, Nov. 1967, p. 1091.

Ed-1, 2: Freier and Anderson, A Demonstration Handbook for Physics.

Mail Questions and Comments to: Dale Stille