Dissectible Capacitor Apparatus

Thomas B. Jones
Professor of Electrical Engineering
University of Rochester



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Description

The dissectible capacitor consists of two planar electrodes separated by one or more sheets of insulating material of thickness ~100 mm. The electrodes can be made of solid metal, conductive plastic, or even aluminum foil taped to cardboard. In the photo above, the lower, fixed electrode (black in color) is made from a sheet of the antistatic plastic used to line the work surfaces in electronic assembly stations. The upper electrode is a polished aluminum plate with a long insulating handle. A good choice for the insulating layer, possessing good dielectric properties, is MylarTM. Another good choice is overhead transparency stock. The insulating sheet is laid down flat upon the lower electrode and then the upper electrode with its handle is placed on top. The insulating sheet must be large enough so that it extends well beyond the edge of the upper electrode.

The capacitor is charged to a relatively low voltage, open-circuited, and then "dissected" by lifting the upper electrode straight up. As the electrode separation is increased, the capacitance decreases dramatically -- by several orders of magnitude. The capacitance in fact varies essentially as the inverse of the separation distance x. The linear relationship between charge Q and voltage V is

Q = CV

The capacitance is C = eA / x , with area A and dielectric permittivity e . Thus, if the capacitor is first charged up and then the electrodes separated, the voltage must rise because of the constant charge constraint. A 9 V transistor radio battery will sometimes provide enough initial charge to achieve ~15 kV, but greater reliability for the demonstration can be achieved if a higher starting voltage is employed. Below is shown a handheld dc voltage supply made with a dc/dc converter (10 volt to 500 volts @ 5 Watts [model #SC-5 made by Standard Energy Products]), a pair of 9 volt batteries, a push-button switch, and a neon indicator light. Because of its very low current capability, this supply by itself is quite safe. Even so, when it is used to charge the dissectible capacitor, strong 1 to 2 cm sparks are obtainable after the electrode has been lifted up. Flammable vapors can be ignited readily with this apparatus.

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Irrespective of the initial charging voltage, decent sparks become much harder to obtain with the dissectible capacitor in humid weather. This is apparently due to moisture-induced surface conduction which forms a semi-conductive path for current flow on the surface of the insulating sheets. As the sheets are raised, current flows between the two electrodes and essentially shorts the capacitor. Nevertheless, the voltage multiplication effect can still be observed in humid weather using a simple leaf electroscope attachment. The lower electrode is usually grounded and the electroscope is mounted atop the upper electrode. This scheme, shown in the figure below, works reliably up to relative humidities of ~65%.

CLICK HERE to view an interactive, animated version of this demonstration that reveals the movement of charge as the steps of the demonstration are followed. Please be patient while the Java script loads!

A very interesting set of tests can be performed with the dissectible capacitor using different insulating sheet materials. In addition to MylarTM and overhead transparency stock, various coated and uncoated papers and card stock may be tried. Except when the humidity is high, plain copier paper works surprisingly well. Its good behavior illustrates nicely how, in an electrophotographic copy machine, charged dry ink (toner) particles can be transferred from the photoreceptor surface to the paper before fusing. The sensitivity of paper to humidity, a result of the tendency of all wood-fiber-based papers to absorb moisture and become more electrically conductive, is a leading cause of poor copier performance in summer time.

The dissectible capacitor described above is really quite similar to the condensing electroscope, an apparatus first used by Volta in his investigations over 200 years ago [Greenslade and Howe, 1981]. A rather similar demonstration can be performed using a dissectible Leyden jar. The dissectible Leyden jar has the advantage of being somewhat more compact than the parallel-plate apparatus described here, and is quite capable of producing large voltages and impressive sparks.

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Safety considerations


Voltage measurement

To monitor the voltage multiplication achieved as the electrode is raised up, a rotating vane electrostatic voltmeter may be employed. It should be understood that this voltmeter has a capacitance of ~10 pF and therefore influences the measurement. To understand this so-called loading effect, refer to the equivalent circuit shown below.


When such a voltmeter is used, its capacitance Co determines the highest voltage achievable.

Vfinal = Q / Co

The circuit shown above models the effect of the voltmeter's capacitance on the measurement. Capacitive and also resistive loading effects are very common in electrostatic and high-voltage measurements and almost always must be taken into account to achieve accurate results [Schwab, 1972].

Ignition of flammable vapors

The spark from the dissectible capacitor is capable of igniting hydrocarbon vapors, in just the way that the electrophorus does. Performing ignition demonstrations with both the dissectible capacitor and the electrophorus provides a good opportunity to contrast the electrostatic principles at work in the two apparatuses. Remember that the electrophorus electrode is charged by induction, while the dissectible capacitor is charged by conduction.

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Practical application

The practical significance of the dissectible capacitor becomes evident in the context of moving webs and other driven belt systems. The figure below shows that, as a charged, insulating belt moves away from a pulley, the effective constant charge constraint increases the voltage and leads to the possibility of electrical breakdown between the pulley and the belt [Cross, 1987; Schwab, 1972].



For a rapidly moving web of insulating material, these electrostatic discharges can be very powerful. They have been the cause of many fires in printing apparatus and other manufacturing equipment where flammable solvent vapors are present. These discharges are also capable of ruining entire batches of sensitized film and paper during roll-to-roll processing.

Library reference

J.A. Cross, Electrostatics: principles, problems, and applications, (Adam-Hilger, Bristol, UK), 1987, p. 370.

T.B. Greenslade and R.H. Howe, "A modern use of Volta's electroscope," Physics Teacher, Vol. 19, No. 9, December, 1981 p. 614-615.

A. Schwab, High-Voltage Measurement Techniques, (MIT Press, Cambridge, MA, USA), 1972, pp. 149-161.

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Last modified: Sunday, 28-Dec-2008 10:29:18 EST