Honors Physics
2/16/16
Induction
Purpose
In this lab you will study how conductors can be charged via a process called “induction.”
Background
Insulators can be charged by friction – but not metals. Why not? What exactly is it about conductors that
makes friction not work? Induction is a method that can place a charge on a conductor. In this activity, you
will use several different methods to test for charges and you will follow the procedures outlined below to
transfer charge to a conductor. Finally you will explain how this procedure works.
Testing for Charge
Three procedures for testing electric charge:
1. Al foil bits: Cut a strip of aluminum foil the width of a piece of piece of scotch tape. Wrap a piece of
tape around a pencil with the sticky side facing out. Cut a 6 to 8 inch piece of silk thread and place
one end on the tape. Roll the strip of aluminum foil around the tape so that you have a small loop of
foil dangling on the end of a piece of thread. For convenience, the thread can be secured to a ruler or
straw, for something to hold. Touch the Al foil bit to a charged object (e.g., a rubbed PVC tube) so
they repel each other and then bring foil near something you want to test. You can observe whether
the Al foil is attracted or repelled.
2. Neon bulb: Pull the two wires from small neon bulb apart and hold one of the wires.
Bring the second wire near a charged object. Observe carefully what happens to the
neon bulb. Note that the gas near one of the electrodes in the bulb will glow. That is
the side where electrons are being emitted – or the negative side of the bulb. This
works best with conductors and has the advantage of determining positive or negative charges.
3. Electroscope: A simple device to detect the presence of static charge consists of an
aluminum can secured to a Styrofoam cup (which acts as an insulating stand). An
aluminum loop is hung from a straw and should barely touch the can when freely
suspended. Touching charged objects to the can should then make the foil loop be
repelled. If you bring a known charge near the can of the electroscope and the
aluminum bit moves farther away, the electroscope has the same charge as the object.
If it moves closer, then the charges are opposite.
These devices can be used to study the charges on various objects (here all conductors) that
have been charged by the process of induction.
Electrophorus
A Styrofoam cup secured to an aluminum pie pan makes a device to “hold” or “carry” static electricity.
While you will need to determine how this device works, the following procedure will “charge” the metal
plate of an electrophorus: Place a vinyl record on a table and rub it with a piece of
wool; holding the Styrofoam cup, place the electrophorus on the vinyl record;
touch the aluminum pie pan with your finger briefly; remove your finger and then
pick up the electrophorus using the Styrofoam cup. The electrophorus is now
charged. Bring your knuckle near the edge of the metal plate and you should see
a small spark jump from the plate to your knuckle. Repeat several times – without
re-rubbing the vinyl. Is the vinyl still charged? Does the electrophorus take
charge directly from the vinyl? Place a sheet of wax paper between the vinyl and
the electrophorus, and charge the electrophorus. With an insulator between them,
can any charge easily move from the vinyl to the electrophorus?
(1) Charge an Al foil bit with the electrophorus and then bring it near a charged PVC tube. What does this
tell you about the charge on the electrophorus? Bring the same Al foil near the vinyl record – what happens?
Can the electrophorus pick up bits of paper? (2) Use the neon bulb to test the charge on the electrophorus.
Honors Physics
2/16/16
(3) Use the electroscope to test the charge on the electrophorus.
Soda Cans
A uncharged metal soda can attached to a Styrofoam cup (which acts as an insulating stand) can also be
charged by induction. A negatively charged object (PVC rubbed with fur) is brought near the can. While the
PVC is held near, touch the can with your finger. When the PVC is pulled away, the can is charged. Test the
charge on the can (1) by using the Al foil bits – touch it to the PVC pipe and then see if the Al foil is
attracted or repelled by the can. Record your observations. How is the can charged? Draw a diagram
showing what happens during this process. (2) Test the charge using the neon bulb. (3) Charge the can on
the electroscope by induction. Determine the charge on the can.
QuickTime™ and a
decompressor
are needed to see this picture.
Neutralize the can by touching it with your finger. Repeat the process above but now using a positively
charged object (acetate or glass rubbed with silk). Record your observations and determine the charge on the
can now – including how you determined the sign of the charge.
QuickTime™ and a
decompressor
are needed to see this picture.
Neutralize the can again and obtain a second soda can on a Styrofoam cup. In the above induction charging
processes, there are two basic steps; a polarization step and a charging step . In the charging step, the hand
serves as a ground - an object which serves as a seemingly infinite source of or sink for electrons. During
the charging step, electrons move into or out of the ground (hand) in order to charge the soda can. Another
means of charging the can involves the use of another conducting object. For instance, another soda can
could be used. The diagrams below depict the induction charging process using a second can in place of the
hand.
QuickTime™ and a
decompressor
are needed to see this picture.
Test the charge on each can. Draw a diagram explaining how each can became charged. With the two cans
still charged and separated by an inch or two, place one of the Al foil bits in between them and observe what
happens. Explain what you see.
Lab Report
To complete this lab, write a conclusion that includes how the electrophorus became charged and the sign of
the charge (especially compared to the charge on the vinyl). Also describe how the cans became charged and
the charge on them (compared to the object used to charge them). Describe how the aluminum foil bit
behaved when placed between the two charged cans – including why it did this and why this behavior
eventually stops. In all cases, explain the role of your hand and identify the source of the charges deposited
on each conductor. Explain what happened to the charge deposited on the insulator used (PVC, vinyl,
acetate).