Pie Plate Electrostatics
Plexiglas is an insulator. Normally, it is electrically neutral with equal numbers of electrons
(negative charges) and protons (positive charges). Electrons can be placed on or removed from
an insulator, but they will stay put; they cannot move around easily. Large numbers of electrons
can be rubbed off the Plexiglas using quick, vigorous strokes with the felt cloth. This leaves a
net positive charge on the Plexiglas, and a net negative charge on the felt (also an insulator).
You can neutralize the felt by rubbing it slowly against a grounded, metallic outlet plate.
The aluminum pie plate is a good conductor. Normally it is neutral. Electrons can be removed
from or added to a conductor and can move freely within the metal. The electrons will move to
locations governed by their attraction to positive charge and repulsion from negative charge.
Due to the salts and fluids in its cells, your body is a good conductor. Thus, electrons travel
freely through your body. It is also large and thus a reservoir of charge, meaning it can give or
receive electrons from this experiment without itself becoming very affected by the loss or gain
in charge. Its voltage stays low when it gets charged. (Nonetheless, it may need to be grounded
by touching a finger to a wall outlet plate from time to time.) When a grounded, conducting
object is placed near a source of charge, opposite charges will collect in the grounded object.
Charge the Plexiglas by vigorously rubbing it with the felt cloth. Starting with the aluminum pie
plate uncharged, hold it by the Styrofoam cup and lower it onto the Plexiglas. Electrons in the
aluminum are attracted to the positive charge on the Plexiglas and collect at the bottom of the pie
plate. This leaves a net positive charge near the rim of pie plate. If the rim is now touched with
your finger (a ground), the rim charge runs off, but the negative charge on the bottom of the pie
plate remains. If you now lift the pie plate by the Styrofoam cup away from the Plexiglas, the
negative charge on the bottom moves to the rim as it repels away from itself. This process is
called charging by induction.
The audio cassette tape is a flexible conductor. It becomes charged as the rim charges up (with
the same sign of charge as the rim) and rises because like charges repel. Try wiggling your
finger near it while it is up in the air.
The neon bulb (available at Radio Shack for less than $1) requires over 70 volts to light. Of the
two electrodes inside, it is always the one on the negatively charged side that lights when the
unattached wire is discharged to a ground. See if you can light first the outside, then the inside
electrode.
Place your finger near the aluminum-covered piece of straw as shown in the figure. Try to
explain why it bounces back and forth as the pie plate becomes neutralized.
Pie Plate
Electrostatics
Demonstrator
neon
bulb
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negative side
lights
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audio
cassette
tape
Styrofoam
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plastic
straw
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aluminum
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pie plate
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thread
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covered straw piece
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Plexiglas
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