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Re: POSITIVE AND NEGATIVE

Three points:
1. I tried to be very careful to lead the students to distinguishing
between attraction by induction and attraction by creating charges of
opposite sign. That's why we don't get into "how many kinds of charge"
until Lab2.
2. Following Aarons, I tried to avoid the use of the terms "charge" and
charging until after introducing the concept of force acting over a
distance. We don't really get into "charge" until one or two labs later
when we charge dangling pith balls and deduce the amount of charge by
estimating the force (the hardest part of that experiment was getting a
reasonably accurate mass for a pith ball).
3. Ludwik's question #9 in Activity I is contradictory to the spirit of
my labs, because it uses the term "electical forces". What the student
observes are forces that are associated with friction (or, if you like,
tape separation). Making a connection with the student's concept of
"electrical" is a slow process built up over the next several labs. In
fact, the entire lab course is designed to build a unified concept of
electricity (and magnetism). The pacing of the labs, by the way, was
closely tied to the pacing of the "lecture" part of the course.
Regards,
Jack


On Wed, 23 Jan 2002, kowalskil wrote:

"More than 300 years old" was wrong. Make it 200. Also note
that 4 quotation marks were deformed (all is corrected below).

kowalskil wrote:

Inspired by Jack's posting (at http://www.hep.anl.gov/jlu/index.html ) I revised
the handout of my Sticky Tapes handout. Feel free to use it in any way you
wish. Critical comments and suggestions will be appreciated, as always.
Ludwik Kowalski

DISCOVERING TWO KINDS OF CHARGES

Based on "Electric and Magnetic Interactions" (by R. Chubay and B. Sherwood.)
To be used by Ludwik Kowalski, at Montclair State University in Spring of 2002.

ACTIVITY ONE
1) Take a 5" long piece of "Scotch" tape and fold over one-quarter inch to make a
non-sticky handle. Label it as B (base) tape. Stick it onto a smooth flat surface,
such as a desk. Smooth it with your finger.
2) Prepare another 5" piece with a handle, and label it U1 (upper). Stick it to the B
tape and smooth it with your finger.
3) With a rapid quick motion, pull the U1 tape up and off the B tape. The B tape
should remain stuck to the desk.
4) Let your partner hold the U1 tape (by the handle) and prepare U2 tape in the
same way as above.
5) Observe what happens when U1 and U2, suspended vertically, are brought
closer to each other. Describe what you see in your own words. The textbook
term charge should be part of your vocabulary.
6) How does the distance between the two tapes change the situation?
7) You can recharge tapes, if necessary, by repeating steps 2 and 3.
8) The amount of charge can be decreased by partially neutralizing a charged tape.
To do this run your finger along the slick side of the vertically suspended tape.
Make sure your finger touches only a portion of the width of a tape.
9) What happens to electric forces when tapes are partially neutralized?
10) Where charges on the U1 and U2 tapes like or unlike? Justify your answer.

ACTIVITY TWO
1) Take a 5" long piece of "Scotch" tape and fold over one-quarter inch to make a
non- sticky handle. Label it as B (base) tape. Stick it onto a smooth flat surface,
such as a desk. Smooth it with your finger.
2) Prepare another 5" piece with a handle, and label it U1 (upper). Stick it to the B
tape and smooth it with your finger.
3) Without separating the U1 tape from the B tape stick the U2 to U1. Smooth
the U2 tape with your finger.
4) Slowly lift the U1 tape, bringing the U2 tape along with it. The U1 and U2 tapes
are not yet separated. The B tape should remain stuck to the desk.
5) Run your finger along the slick side of the vertically suspended U2 tape. This, as
you know, will neutralize the setup. Tape which are not neutralized would be
attracted to a hand. Make sure your vertically suspended tapes are not attracted
by the hand.
6) Using the handles quickly separate U1 from U2. Is each of the two tapes
charged or neutral? How did you check this?
7) Do the tapes, U1 and U2, repel each other or do they attract each other?
8) What is the effect of the distance on the force between the tape?
9) Where charges on the U1 and U2 tapes like or unlike? Justify your answer.

ACTIVITY THREE
1) Prepare two sets of unlikely charged tapes, U1, U2 and V1, V2. To do this
follow steps 1 to 6 of the preceding activity.
2) Describe your observations (attraction or repulsion):
U1 and U2 ---> ............
U1 and V1 ---> ............
U1 and V2 ---> ............
U2 and V1 ---> ............
U2 and V2 ---> ............
V1 and V2 ---> ............
3) Summarize your result in the form of one short sentence. Hint: make sure
the adjectives "like" and "unlike" are used in this sentence.
4) Suppose that the kind of charge on U1 is DEFINED as positive. What
would you say about charges on U2, V1 and V2?
5) Suppose that the kind of charge on U1 is DEFINED as negative. What
would you say about charges on U2, V1 and V2?
6) Which definition of "positive" is consistent with what has already been
defined long time ago? According to the existing definition electrons are
negative and protons are positive. A tape is positive if it attracts electrons
and repels protons. Actually, the present convention about positivity and
negativity was made long before the discoveries of electrons and protons.
According to this (more than 200 years old) convention a charge on a
glass rod, after it is rubbed with silk, was arbitrarily named negative. Such
rod repels electrons and attracts protons; that is why we say that electrons
are negative and protons are positive.


--
"But as much as I love and respect you, I will beat you and I will kill
you, because that is what I must do. Tonight it is only you and me, fish.
It is your strength against my intelligence. It is a veritable potpourri
of metaphor, every nuance of which is fraught with meaning."
Greg Nagan from "The Old Man and the Sea" in
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