Physics 260 Calculus Physics II: E&M
Grist
Laboratory Exercise: Electrostatics
Object
In this lab you will get an initial exposure charges produced by friction. You will investigate
how the charges interact and get a feel for electrostatic force.
Introduction
We know that electrical phenomenon has been a point of curiosity and investigation since at
least the time of the ancient Greeks. For instance, elektron comes from the Greek word for
amber. We know that the Greeks found that when they rubbed amber with a cloth or fur they
could attract small metal filings. In the 1700’s Benjamin Franklin established a theory that there
was a characteristic of electrical phenomenon which he called charge, and that there were two
types of this charge. If you rub a rubber rod with fur, you leave one of the types of charge on
the rubber rod. This particular charge was arbitrarily called “negative”.
All matter is constructed of atoms, and all atoms are constructed of three basic particles:
electrons, protons and neutrons. These three basic particles are all much too small to see, and in
fact so is the atom. In Franklin’s time people had no way of knowing what particles were being
moved around, left behind, or added to an object. We now know that the negative charge left
on the rubber rod is due to an excess of electrons left there from the fur.
Electrostatics 1
Physics 260 Calculus Physics II: E&M
Grist
Procedure
Section I: Qualitative Exploration
Task A: The Rods
1) Take the rubber rod and vigorously rub it with the fur. Place the rod on the turn-stand.
2) Next, take the glass rod and vigorously rub it with the silk. Place the rod near the rubber rod
that is on the turn-stand, but don’t let them touch. When you are done place the glass rod on
the second turn-stand.
3) Write a couple of sentences below including the following: What happened? Were the rods
attracted to each other or did they repel one another? Does this verify the idea that there are two
kinds of charge?
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Task B: The Rods and a Balloon
1) Select three balloons that you can keep track of individually. Select different colors if you can
manage it, or mark them in some way with a felt pen.
2) Inflate the first balloon. Vigorously stroke it against your hair or with the fur. Place the
balloon near the rubber rod that is on the turn-stand, but don’t let them touch. What happens?
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3) Next, place the first balloon near the glass rod that is on the turn-stand, but don’t let them
touch. What happens?
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4) Is the first balloon like one of the rods? Which one? Does this verify the idea that there are
two kinds of charge? Could there be more than two kinds of charge here?
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Electrostatics 2
Physics 260 Calculus Physics II: E&M
Grist
Task C: The Solo Balloon
1) Next, vigorously stroke the first balloon against your hair or with the fur again. Try to pick
up some Styrofoam peanuts or puffed rice with the first balloon. What happens? What can you
say about the Styrofoam peanuts or puffed rice - what charge might they have, if any? Sketch
how the charges might be arranged on the balloon.
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2) Next, vigorously stroke the first balloon against your hair or with the fur yet again. Can you
cause the balloon to stick to the wall? Where and where not - find both! What is causing it to
stick? Sketch how the charges might be situated on the balloon and the wall.
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3) Stroke one side of the first balloon against your hair or with the fur yet again. Now place the
opposite side of the balloon against a wall where is stuck before. Let the balloon go. What
happened? Is the charge on the balloon the same or different?
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Electrostatics 3
Physics 260 Calculus Physics II: E&M
Grist
Task C: The Balloon Group
1) Inflate the second and third balloons. Do not rub them on your hair, clothing or the fur. If
you rub these two electrically neutral balloons together, can you create a ‘static electricity’?
Explain your results and what you think is happening.
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2) Now rub all three balloons against your hair or with the fur. Do the balloons attract or repel
each other? Where did the charges come from? What does this indicate about how many types
charge there are?
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Electrostatics 4
Physics 260 Calculus Physics II: E&M
Grist
Section II: Relative Measurements
Faraday Ice Pail
Setup
1)
Connect the electrometer input lead (red alligator clip) to the ice pail to create a system for
measuring charge. The electrometer ground lead (black alligator clip) attaches to the
shield. (Setup is shown in Figure 1.)
2)
Ground the ice pail (i.e., connect the inner pail to the shield) by touching the inner pail and
the outer shield at the same time with the finger of one hand. (See Figure 2). The
electrometer should read zero when grounded, indicating there is no charge in the ice pail.
Press the Zero button to completely remove all charge from the electrometer and the ice
pail. Note: While conducting the experiment it may be convenient to continually rest one
hand on the upper edge of the shield. This also grounds the experimenter, providing the
electrometer is connected to both ground and shield and it allows the ice pail to be easily
grounded whenever necessary.
3)
Always start with the voltage range in the higher setting (100 V) and adjust down if
needed. Adjust the sensitivity of the electrometer so that most readings appear in the
upper 2/3 of the scale. Analog meters are typically most accurate in the range of 1/3 to
2/3 of full scale.
4)
The proof plane will be used to sample charged objects. Here is a general procedure to
follow:
Figure 1
Figure 2
•
Always remove any stray charge on the neck and handle of the proof plane by
touching the neck and handle to the grounded shield. You must also be grounded
while doing this. It also helps if you breathe on the neck of the proof plane, so that
the moisture in your breath removes any residual charge on the neck.
•
Before inserting the charged disk of the proof plane in the ice pail, make sure
you’re touching the grounded shield.
Electrostatics 5
Physics 260 Calculus Physics II: E&M
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Task D: Charging by Induction vs. Charging by Contact
1)
Once again take the rubber rod and vigorously rub it with the fur.
2)
Touch the aluminum surface of the proof plane to the rubber rod. The proof plane is
conductive and will take on the charge of the object that it is in contact with. If the charged
object is significantly larger than the proof plane, then it will also have close to the same
charge density as the charged object.
3)
Carefully insert the proof plane into the ice pail, all the way to the lower half of the pail,
but without letting it touch the pail. Record the electrometer reading here:
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4)
Remove the proof plane and again note the electrometer reading. If the handle never
touched the pail, the reading must be zero.
Question: Why was there a potential difference between the pail and the shield only while the
proof plane was inside?
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5)
Push the Zero button to remove any residual charge. Now insert the proof plane again,
but this time let it touch the ice pail.
6)
Remove the proof plane.
Record the electrometer reading here:
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Questions: Why is there now a permanent potential difference between the ice pail and the
shield? Where did the charge on the ice pail come from?
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7)
To show that the charge gained by the ice pail was lost by the proof plane, ground the ice
pail to remove all charge. Press the ZERO button to remove residual charges from the
electrometer. Insert the wand again into the ice pail. Does any charge remain on it?
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8)
Repeat steps 1-7 with the glass rod as charged object. Do you see the same general results?
Is anything different?
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Electrostatics 6
Physics 260 Calculus Physics II: E&M
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Task E: Conservation of Charge
1)
Starting with initially uncharged charge producers, rub the blue and white materials
together. Note: You must keep both producers from touching anything else after
charging. Keep them in your hands, without letting them touch each other or the ice pail.
2)
Use the Faraday Ice Pail to measure the magnitude and polarity of each of the charged
wands by inserting them one at a time into the ice pail and noting the reading on the
electrometer.
Blue reading _______________
White reading _______________
Questions: What is the relation between the magnitude of the charges?
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What is the relation between the polarity of the charges?
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Was charge conserved?
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3)
Completely remove all charge from the charge producers by grounding them. Do not
forget to also remove any stray charge from the necks and handles.
4)
Insert both charge producers into the ice pail and rub them together inside the pail.
Note: Do not let the charge producers touch the pail.
Record the electrometer reading here:
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5)
Remove one charge producer and Record the electrometer reading here:
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6)
7)
Replace the charge producer and remove the other.
Record the electrometer reading here:
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Using the magnitude and polarity of the measurements, comment on conservation of
charge.
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Electrostatics 7