Physics Laboratory

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Investigation 11 - Electric Charge
In this investigation you will examine the interactions between charged objects, and between charged and
uncharged objects.
Materials:
I.
A.
roll of magic tape
small pieces of tissue paper
small aluminum foil ball on a thread
Electrical Interactions
Cut off a piece of magic tape, about 10-15 cm in length and press it onto the surface of your table.
(For ease in handling, make a “handle” by folding over the end of tape to form a portion that is not
sticky.) Quickly remove the tape from the surface and hang it from the edge of the table.
Describe the behavior of the tape as you bring objects toward it (e.g., a hand, a pen, etc.). Later on,
we will be able to explain why this occurs.
B.
Make another piece of tape as described above. Bring the second tape toward the first. Describe
Does the distance between the tapes affect the interaction (force) between them? How so?
C.
Again prepare pieces of tape as instructed above (section I, part A). Press two pieces of tape onto the
table and write “B” (for bottom) on them. Then press two more tapes on top of each B tape and label
them “T” (for top).
Remove a pair of tapes (B and T together) from the table as a unit, and then quickly separate the T
and B tapes. Hang each of them from the edge of the table.
Remove the second pair of tapes from the table, and then quickly separate the T and B tapes.
Describe the interaction between the following pairs of tape when they are brought near one
another.
two T tapes
(T-T Interaction)
two B tapes
(B-B Interaction)
one T and one B tape
(T-B or B-T Interaction)
Notice that the tapes exert a force on each other. We know this force is not a gravitational force, but
some “other” force. This “other” force is referred to as an “electrostatic force.” Just as mass is the
property responsible for the gravitational force, the property responsible for the electrostatic force is
called “charge.” We say that the tapes are electrically charged when they interact as you have
observed.
The two T tapes must have the same kind of charge because they were charged in the same way, and
likewise the two B tapes must have the same kind of charge. But the T and B tapes must have
different kinds of charge since the T-B interaction is different from the T-T and B-B interactions.
How many different kinds of charge do there appear to be?
There are two types of charge and they are referred to as “positive charge” and “negative charge.”
In our situation, the “T” tapes are found to be positively charged and the “B” tapes are found to be
negatively charged. (This was determined in earlier trials.)
II.
Electrical Interactions between a Charge and a Conductor
A.
A small ball with zero net charge (electrically
neutral) is positively charged on one side and
negatively charged on the other side. The
ball is placed near a positive point charge, as
shown.
Will the ball be attracted toward, repelled
from, or unaffected by the positive point
charge? Why?
+
positive
point charge
positively
charged
negatively
charged
-
+
+
+
small ball
with zero net
charge –
uncharged
B.
Hang the uncharged (zero net charge) light, aluminum foil ball from the insulating thread. Now
charge a piece of tape as in of section I, part A, and bring the tape toward the ball. Describe what
you observe.
C.
From what happened, can you deduce what happened to the charges on the aluminum foil ball? On
the diagrams below, sketch the charge distribution on the ball, both before and after the tape and
ball are brought close one another.
before (very, very far apart)
_
_
_
_
_
_
_
charged tape
after (close together)
_
_
_
_
_
_
_
uncharged
aluminum foil
ball
charged tape
uncharged
aluminum
foil ball
Why did the charges redistribute themselves on the ball?
III. Electrical Interactions between a Charge and a Nonconductor -- Polarization
A.
Tear off little pieces of tissue paper and place them on the table. Remember that paper is unlike the
aluminum foil ball -- the paper is a nonconductor (insulator).
Now charge a piece of tape as in of section I, part A and bring the tape toward the pieces of paper.
What happened to the pieces of paper?
Because the paper is attracted to the charged tape, the charge
distribution on the paper must be such as to cause the
attraction. In this situation, draw how the charges are
distributed on the piece of paper.
charged
tape
_
_
_
_
_
_
_
piece of
tissue paper
The phenomenon taking place in the paper is called charge
“polarization.” Remember polarization and how it occurs?
B.
Go to one of your sinks at home and turn on the water until a thin stream of water is flowing. Now
charge a piece of tape as in of section I, part A and bring the tape toward the stream of water. (Try
to keep the tape dry.)
What happens to the stream of water? (You may not notice anything because of the tape getting wet
or the charge on the tape not being strong enough.)
_
_
What should happen to the stream of water is
_
shown. Draw how the charges must be distributed
_
in the stream of water in order for the stream to be
stream of water
charged
_
attracted to the tape.
tape
_
_
What property of water molecules causes this to
occur?
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