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Name: Eric
Status: educator
Grade: 9-12
Country: USA
Date: Winter 2012-2013


Question:
I have been wondering -- how does the proximity of the conductors relate the effectiveness of a Leyden Jar? In the different jars I have seen on-line, the conductors are both flush with either the inside or outside surface of the insulator, but given that it has been demonstrated that the insulator itself does not contain the charge, is contact with the conductor important or necessary? Could a Leyden Jar actually be made of two conductors separated by distance rather than being separated by an insulator? If not, why would that not work?

Replies:
Hi, Eric

Thank you for your excellent question. I am an electrical engineer. A Leyden jar is actually a capacitor. I use capacitors in all of my designs.

While the charge (measured in Coulombs) resides on the metal conductors, the insulator has the electric field. While air can be used as the insulator it has a low "dielectric constant" or permittivity. Different insulator materials provide different permittivities; but glass and plastics for example have at least several times higher than (air or vacuum). The result is that for the same applied voltage you get more capacitance, thus more stored charge on the conductor plates if a high permittivity insulator material is used.

The other essential factor is to place the conductor plates as closely as possible. Because electric field strength is measured in Volts per meter, reducing the spacing in meters will give you a higher field strength for the same voltage. As a practical matter minimizing the spacing is easiest to do with a thin insulating film than if air only is used.

Finally all insulator materials will fail or "break down" if the applied Volts per meter exceeds the dielectric strength of the material. To store the greatest amount of charge, the dielectric thickness should be as thin as possible but must be adequate for the applied voltage.

I hope this answers your questions. Please feel free to reply if you have additional questions. Best regards, Robert M. Zwicker


Eric, The purpose of the insulator is to contain the pellets and to keep the two oppositely charged conductor plates from moving toward one another. If the insulator were gone, many of the pellets would quickly work their ways to the outward surfaces of the conducting plates. The insulator also prevents air currents in the room from blowing the extremely light pellets out of the jar. For a piece of exploded polystyrene coated with a very thin conducting surface, a light breeze could easily have much greater effect than the electric force.

Dr. Ken Mellendorf Physics Instructor Illinois Central College


A Leyden jar is just a capacitor, and the insulator of the jar is the capacitor?s dielectric. A jar would work with empty space as the dielectric instead of a physical jar. However, an insulator makes it easier to prevent contact between the two conductors, and it allows more charge to be stored at the same separation distance.

The closer the conductors, the more charge can be stored, but that also introduces practical concerns of accidental contact and arcing.

Richard E. Barrans Jr., Ph.D., M.Ed. Department of Physics and Astronomy University of Wyoming


Hi Eric,

Thanks for the question. Basically a Leyden jar is a cylindrical capacitor (or condenser, if you are "old-school"). The capacitance of a capacitor depends upon the geometry of the conductors. Here is a nice link for the cylindrical capacitor:

http://www.ajdesigner.com/phpcapacitor/cylindrical_capacitor_equation.php

(Also be sure to check Wikipedia.)

From the capacitance equation, if the two conductors become closer together, the capacitance increases. Another name for an insulator is a dielectric. The dielectric does not need to be in contact with the conductors in order for the Leyden jar to function.

Yes, a Leyden jar can be made of two conductors without any insulator. However, the capacitance will be lower by a factor of about 80. This factor is known as the relative permittivity.

I hope this helps. Please let me know if you have any more questions. Thanks Jeff Grell


Hi Eric,

Your idea will certainly work, if the distance is small. Basically, an increasing distance between the plates causes a decreased capacitance. Closeness and area are the keys for any particular dielectric to form high capacitance.

The math is straightforward: for a dielectric of permittivity, e; an area of the plates, A and the distance between the plates, d:

Capacitance = eA/d

A vacuum has e = 1.0, air = 1.0006, polyethylene = 2.25, glass = 4.7, sulfuric acid = about 90.

Air variable capacitors were used as tuners in most AM radios during the 1920's to early 1960's quite effectively. They are still being used today by historically inclined HAM radio, antique radio and crystal radio enthusiasts. A very good reference would be the people of the Xtal Set Society,

http://www.midnightscience.com/

Hoping this helps! Peter E. Hughes, Ph.D. Milford, NH


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