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electric charges (+ and -)



The activity described below was part of a teaching unit on Coulomb's
law composed at the modeling workshop for physics teachers two
years ago. I found it on an old diskette to be reformatted. Shows how
electric charges can be discovered by students in a simple activity.

Nothing special but perhaps worth sharing. It is based on Chapter 1 of:

R. Chabay and B. Sherwood, Electric and Magnetic Interactions,
John Wiley & Sons, Inc., New York, 1995.

Ludwik Kowalski
**********************************

Appendix 2 A qualitative investigation of electric forces

Students work in small groups and collect data which provide evidence
for the
existence of two kinds of electric charge. They are guided (5) to
discover that
charges either attract or repel each other. In the teaching sequence
which we
developed, this study is followed by a quantitative investigation of
Coulomb's law.

1. A strip of transparent tape, about 20 cm long, is firmly attached
(by its sticky side)
to the surface of a desk. It stays attached and serves as a base,
B, for all subsequent
manipulations.

2. Another strip of transparent tape is stuck to the base; it is bent at
one end to make a
short non-sticking handle. The handle is labeled as U, for upper,
and the strip is
firmly smoothed with a finger.

3. The U tape is pulled up rapidly from the B tape. It becomes
electrified. This is
verified by its tendency to be attracted to a hand.

4. Another U tape is prepared in the same way. The two U tapes repel
each other. The
repulsive force decreases when the distance between the U tapes
becomes longer.

5. Each U tape can be discharged by rubbing its slick side with a
finger. Discharged
tapes do not repel. Partial discharging can also be performed to
show that the force
depends on the amount of remaining charge. This is done by rubbing
the tapes
along the rims rather than over the entire surface.

6. A new strip of tape, labeled L, is stuck to B tape. Then another
tape, labeled U, is
placed on the L tape. The combined pair (U and L) is slowly removed
from B and
neutralized by rubbing the slick side of the U tape with a finger.
Finally, the U-L
pair is rapidly separated and its components are found to attract
each other.

7. Two U tapes and two L tapes are prepared, as above. They are used to
show that
like charges (L-L and U-U tapes) repel while the unlike charges
(U-L tapes) attract.
The forces depend on distances between the tapes and on the degree
of partial
discharging.

8. A plastic comb is electrified by rubbing it on hair and an agreement
is made to call
the charge on the comb negative. That negative charge acts
differently on U and L
tapes. The tape which is attracted must thus be positive while the
tape which is
repelled must be negative. The comb becomes an instrument for
recognizing
positive and negative charges. An absolute method to determine the
sign of a
charge on a comb is described in (7).

References used in Appendix 2:

5. R. Chabay and B. Sherwood, Electric and Magnetic Interactions,
John Wiley & Sons, Inc., New York, 1995. (See exercises in chapter 1)

7. E. Noll, "Are You Sure That Comb Has A Negative Charge," Science
Teach. 61,43, (1994).