Modern Electrostatic Apparatus for Demonstrations and the Teaching Laboratory

A Presentation at the National Meeting of the

American Association of Physics Teachers

Philadelphia, Pennsylvania

Presentation FA05, 2:00 PM, Wednesday, January 23, 2002

F. C. Peterson

Department of Physics and Astronomy

Iowa State University, Ames, IA 50011-3160

Voice: (515) 294-3875 FAX: (515) 294-6027 internet: fcp@iastate.edu

These notes available at: http://www.public.iastate.edu/~fcp

Abstract: Common electrostatic apparatus typically lacks reliability and quantitative accuracy. We have modified a number of classical instruments and activities and designed some new devices, so that experiments involving electrostatics can be done in humid as well as dry weather, and include activities that yield accurate and interesting numerical results. These instruments and activities will be described. Materials related to this paper will be available at http://www.public.iastate.edu/~fcp/ .

Materia ls available on the web site referenced above:

(note - some figures are absent from these files; I hope to have this deficiency corrected by 2/11/02)

1. Synopsis of presentation (without transparencies)

2. Equipment list and equipment notes for Experiment #15 of Physics 221, "Electric Charge and Capacitance" (PDF) (WORD)

3. Pre-lab problems, page 9 - 12 of Pre-lab #15, "Electric Charge and Capacitance" (PDF) (WORD)

4. Lab #15, pages 1 - 25, "Electric Charge and Capacitance" (PDF) (WORD)

5. Miscellaneous comments concerning electrostatic activities

Common electrostatic apparatus is well known for unreliability and a lack of quantitative accuracy. We have modified a number of classical instruments and activities and designed some new devices, so that student experiments involving electrostatics can be done in humid as well as dry weather, and include activities that yield accurate and interesting numerical results. The unreliability of electrostatic devices often results from surface leakage currents, which we have greatly reduced by the use of Teflon where appropriate. Accurate quantitative results are achieved by the use of an inexpensive auto-ranging digital multimeter which, in its most sensitive voltmeter range, has exceeding high input resistance. Successful modifications of classical apparatus and the design and use of an accurate coulombmeter are described below.

Over the past dozen years, annually some 1000 of our students have been making reliable qualitative and quantitative electrostatic measurements. The apparatus has required little maintenance, and its performance has been reliable, whether used in December or the often humid months of May and July.

The more traditional apparatus used includes a Braun electroscope¹, an electrophorus and plastic rods for the generation of static electric charge, conducting spheres mounted on insulating rods, and insulated wires. Many of the key modifications and design features on which their reliable functioning depends involve the use of virgin Teflon for crucial components, such as the insulator for the high-voltage portion of the electroscope, the handle for the electrophorus disk, and the rods which support the painted ping-pong balls that serve as conducting spheres. Modern plastics, namely gray PVC and clear acrylic, are also used for the friction rods and the charged plate of the electrophorus; these materials become negatively and positively charged, respectively, when rubbed with facial tissue.

The quantitative measurement of electric charge by our students centers on the use of an inexpensive digital multimeter². With the meter in its voltmeter mode, one terminal connected to ground, and a low-leakage capacitor across its terminals, this meter serves as a crude coulombmeter. Both the sign and magnitude of the charge transferred to the ungrounded terminal are determined by the meter reading and the value of the capacitance. Since erratic results in these types of measurements can occur when the students themselves become charged (electrically, that is), conductive wrist straps which are grounded (through a large resistance) are worn by the students for some of the activities.

To achieve more accurate results, our students use an assembled coulombmeter based upon a similar design. This instrument yields measurements of the net charge present on small objects lowered into a shielded cup mounted on its chassis to an accuracy of about 1%. The resolution of this instrument on its most sensitive scale is 10 pC; for comparison, a standard ping-pong ball coated with a conducting paint typically has a net charge of from 10 to 50 nC when charged by common electrostatic techniques.

Interesting measurements based on the use of this accurate coulombmeter abound, and include that of the charge carried by small spheres and disks, both before and after being charged or making contact with one another, the charge on insulating rods charged by friction, or that on an electrophorus disk charged by induction.

Another activity accessible to our students is to charge a small conducting sphere by touching it to the end of a stiff, straight wire maintained at a known potential, V, of several kilovolts. By then measuring the charge carried away on the sphere of radius, R, the students in effect determine (approximately) the capacitance of the sphere and can see a physical application for the ubiquitous relationship, V = Q / ( 4 p e₀ R ).

One can also measure the net static charge generated by rubbing various insulating rods and compare the measurements with the maximum charge expected, a result which is proportional to the dielectric strength of air.

As we have done, more advanced students can study the force of attraction between a charged sphere and a grounded plate mounted on the pan of an electronic balance, as a function of their mutual separation; of course, the force varies with separation as predicted by Coulomb's law, while the charge determined from the balance readings (by applying Coulomb's law) and that measured with the coulombmeter agree to a few percent.

1. Braun electroscope, Klinger model KE5235.

2. Soar, model ME-540, an auto-ranging meter with an input resistance greater that 10¹⁰ ohms on its most sensitive range as a voltmeter.

file: SYNOPSIS.htm printed 1/18/02 5:14 PM