About your report: Since you'll need to draw diagrams, make this a handwritten report to scan and submit to Brainhoney.
You must complete L153 before doing the following
investigations. When explaining observations, you're now expected to use scientific
terms introduced in the text. Include with your explanations
sketches and sequences of sketches showing how you think the positive and
negative charges are distributed over the objects as you perform the tests.
Small sketches will be difficult for the instructor to interpret.
Therefore, make each sketch at least a quarter page in size. For
examples of how your sketches should look, see these figures in your
textbook: 19.2, 19.4, 19.6 and 19.22.
Tear off about a square inch
of aluminum foil and wad it around the end of your nylon thread so
that the ball of foil won't slip off the thread. It's important to use nylon rather than cotton thread. Cotton thread can give poor results. Tape the loose end
of the thread from the edge of your desk or from an overhead support so
that the ball hangs freely in a pendulum arrangement. The ball must
be far from the table legs or other objects that it might touch.
Neutralize the ball by touching it with your finger.
This process is called grounding. If the ball has
excess negative charge, that excess is conducted through you to ground.
If the ball has excess positive charge, negative charge is conducted from
the ground through you to neutralize the excess. In either case,
negative charge is conducted. Why is negative rather than positive
charge conducted? (By the way, this process of grounding is what you
were doing in L153 when you passed the tape strips over your lips or a
metal object. You were removing any excess charge on the strips.
In this way, when you pulled the tape strips apart, they were given
approximately equal and opposite charges.)
Charge a balloon by rubbing it with fur. Then bring the charged
balloon close to but not touching the ball. (If the ball accidentally touches the
balloon, neutralize the ball and try again.) Describe what you observe.
Explain your observation in
step 3 in words and with a diagram. See Preparation and
Presentation above for information about how your diagrams should look
and what they should contain.
Ground the ball and bring the
charged balloon near it again. This time, however, let the ball and
balloon meet. Describe what you observe.
Explain your observation in
step 5 in words and with a diagram.
Ground the ball. While
holding the charged balloon near to but not touching the neutral ball,
bring your finger close to the ball on the side opposite the balloon until
your finger meets the ball. Be prepared to carefully observe
everything that happens. Ground the ball and repeat the process as
many times as necessary in order to be confident of your observations.
Then describe what you observe.
Now explain your observations
in step 7 in words and with a series of diagrams. Your diagrams must
show how the charge is distributed on the balloon and the ball and how the
distributions of charge change.
Tape the open end of a Styrofoam cup to the inside center of an aluminum pie plate
as shown in the diagram. In the following activities, use the Styrofoam cup as a handle.
Avoid touching the aluminum plate with your hands unless you have a reason
to do so. This simple device has the unusual name of electrophorus.
Prepare top and bottom test
strips as you did in L153.
Touch the pie plate to
neutralize it initially. How can you test for neutrality using the
test strips? Try it.
Use your flat piece of
Styrofoam as a surface on which to place the pie
plate. We'll call this the platform. Charge the platform by
rubbing it vigorously with the rabbit fur. Bring the platform near
the test strips. Is the platform positive or negative? How do
Now place the platform on a
table. Ground the pie plate once more to make sure it's neutral.
Then, while holding the cup only, place the pie plate flat on the
platform. Lift the pie plate immediately and test its charge with
the test strips. Is the pie plate positive, neutral, or negative?
How do you know?
Ground the pie plate and place
it on the platform once again. Briefly touch the pie plate itself
with a finger. Lift the pie plate from the platform and test its
charge. Is the pie plate positive, neutral, or negative? How
do you know?
Repeat your observations in
steps 4-6 as needed in order to be confident of them.
Explain your observations in words and with a sequence of