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