ELECTROSTATIC CHARGE DISTRIBUTION
Objective:
To infer the spatial distribution of (invisible) electric charge by observing the behavior of charged
bodies.
Theory:
Electric charge is recognized by the force it causes upon the bodies on which it resides.
Electrification of bodies is the result of separation of unlike charge. The absence of the observed
long-range forces is the result of electrical neutrality caused by the local balance between the two
types of charge. Ben Franklin arbitrarily chose the charge on a glass rod rubbed with silk to be
positive. The opposite charge is thus negative. Adding positive and negative charges of equal
magnitude thus gives zero net charge as well as zero long-range forces. It is found that like
charges repel and unlike charges attract.
Electrified bodies differ in their degree of conductivity. Those with large conductivities are
called conductors and those with very small conductivities are called insulators. Charge is free to
move anywhere in a conductor in response to electrical forces, even weak ones, but in an
insulator, the charge has a fixed location unless subjected to relatively strong electric forces such
as accompany close contact of bodies.
In the present exercise, the observer is asked to diagram various distributions of electric charge.
To do so, schematic representations of rods, pith balls, ground wires and indicating conductors of
electroscopes will be required. Concentrations of positive and negative charge should be
indicated by + and – signs. Indicate the strength of the charges by showing a greater
concentration of charge signs.
Procedure:
(1) Obtain a pith ball for use in this experiment. It may be necessary to untangle one from the
large mess in the box. In consideration of others, don’t leave them in such a mess when you
are done.
(2) Present an unelectrified glass rod to an uncharged conducting pith ball. Describe the effect.
Repeat with an unelectrified hard rubber rod.
(3) Electrify the glass rod by rubbing with silk, and present the rod to an uncharged conducting
pith ball. Describe the effect before contact and diagram the corresponding charge
distribution. Describe the effect after contact and diagram the corresponding charge
distribution.
(4) Electrify the glass rod again by rubbing with silk, and present it to the charged pith ball (still
charged from the previous step). Describe the effect before contact and diagram the
corresponding charge distribution. Describe the effect after contact and diagram the
corresponding charge distribution.
(5) Electrify the rubber rod by rubbing with fur, and present it to the charged pith ball (still
charged from the previous step). Describe the effect before contact and diagram the
corresponding charge distribution.
(6) Present a ground conductor to the charged pith ball (use your hand). Describe the effect
before contact and diagram the corresponding charge distribution. Describe the effect after
contact.
(7) After momentarily contacting the indicating conductor of an electroscope with a ground wire
(touch it with your hand), present (but do not touch) an electrified glass rod to the indicating
conductor. Describe the effect and diagram the corresponding charge distribution.
Withdraw the rod without touching the electroscope.
(8) Present the rod again and hold it in position while contacting the indicating conductor of the
electroscope with the ground wire (your hand). Describe the effect and diagram the
corresponding charge distribution.
(9) After removing the ground wire (your hand), withdraw the rod. Describe the effect and
diagram the corresponding charge distribution. The electroscope has been charged by
induction. What does this mean?
(10) Attach the large cup upside down on the electroscope and momentarily contact the cup on
the electroscope with the ground wire (your hand). Rub the proof-plane (the thing that looks
like a wand with a flat piece of metal on the end) on an electrified glass rod to collect charge
and then introduce into the cup without touching the surface. Describe the effect and
diagram the charge distribution. It may be necessary to do this a few times before the
electroscope has acquired enough charge to “do its thing”…
(11) Withdraw the proof-plane and describe the effect. Then re-introduce the proof plane and
touch it to the interior surface. Describe the effect.
(12) Withdraw the proof-plane. Describe the effect and diagram the charge distribution.
(13) Discharge the proof-plane with the ground wire (your hand). Then touch the proof-plane to
the inside of the cup and then withdraw it. Describe the observed effect.
(14) Discharge the proof-plane with the ground wire (your hand). Then touch the proof-plane to
the outside of the cup. Describe the effect and explain what is going on.
(15) Experiment with the other odds and ends that are supplied with the electrostatic equipment.
See if you can come up with other experiments or exercises for next year’s students.