Electrophorus, Conductors and Insulators - Modeling Physics

Electric Charge Behavior and Interactions Model:
Electrophorus Activity
Introduction
The 18th century American scientist and political activist Benjamin Franklin invented the
electrophorus. He constructed his device with wood, sulfur, wax and pewter. Our electrophorus
is constructed from an aluminum pie pan, a foam plate and cup, some tape and a small neon
bulb.
Procedure
Construct an electrophorus by taping the foam cup
to the center of a pie pan. The neon bulb will
clearly glow when a spark passes through it.
1. Place the foam plate upside down on the table.
Charge the foam plate by rubbing it with a wool
sock. Using the foam cup taped to the pie pan as a
handle, hold the aluminum pan just above the foam
plate. Be careful not to touch the aluminum with
your hand. Gently separate the wire leads of the
neon bulb. Holding one of the bulb leads, touch the
other wire of the neon bulb to the rim of the
aluminum plate. You should feel a tiny shock and
see the bulb briefly light.
2. Now raise the aluminum pan by the handle, and again touch your finger to the outer wire of the
neon bulb. The light should again glow. Repeat the down-touch, up-touch cycle several times
with your partners.
©Modeling Workshop Project 2007 1 E1-Charge&Field Activity v3.1 JBS

Discussion & Further Exploration
1. The original source of charge for the electrophorus is the charged foam plate. How does this
charge come about
2. What kind of electric charge is on the foam plate
When Benjamin Franklin rubbed a glass rod with silk, he called the charge on the glass rod
positive, (+) so to be consistent with history, we will do the same.
Record your observations below as you use a glass rod, silk, and charged tapes to determine the
kind of charge on the foam.
3. The bulb can be lit when touched to the aluminum plate. Can the bulb be lit when touched to the
foam plate What is the difference between how the foam and the aluminum “hang on” to their
charge
4. Does charge move more easily in the foam or in the aluminum Why is a foam cup used as a
handle on the aluminum plate
5. Use the following set of diagrams to illustrate the distribution and movement of charge during an
up-and-down cycle of the electrophorus.
a. The uncharged foil plate is placed upon the charged foam plate. Draw the charges on the foam
plate. Draw the charges on the foil plate keeping in mind the forces exerted on them by the charges
from the foam plate.
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. The wire from bulb is touched and lights while the foil plate sits on the foam plate. Does the
charge on the foam plate change from the previous situation Draw the charges on the foam plate.
How will touching the bulb affect the charges on the foil plate Draw the charges on the foil plate.
What is the net charge of the pie pan after the bulb lights
c. The aluminum plate is lifted away from the foam plate using the foam cup handle. How will
lifting the foil plate away from the foam plate affect the charges on the foil plate Draw the
distribution of charge on the foil plate.
d. The wire from the light bulb is touched a second time and lights briefly. How will touching
the bulb affect the charges on the foil plate Draw the charges on the foil plate. What is the net
charge of the pie pan after the bulb lights
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6. Closely examine the Neon bulb during the down-touch, up-touch cycle. Only one side of the
Neon bulb will glow at a time, and this depends on whether the Aluminum plate is up or down.
Which side of the bulb (most positive or most negative) glows
7. The down-touch, up-touch cycle can be repeated almost indefinitely or until the charge on the
foam leaks away. Since the charge on the foam is not "used up," where does the energy to light
the bulb repeatedly come from in this device
8. Suppose the foam plate was replaced by an object that acquired a charge opposite to that of the
foam. Would the electrophorus still work What would be different
©Modeling Workshop Project 2007 4 E1-Charge&Field Activity v3.1 JBS