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Critical Thinking



The outdoor temperature was below the freezing point and I did verify that
my copper wire (supporting two 4.5 kg weights, as described before) does
not penetrate ice at low temperatures (between -2 and -5 C). It has been
supported by the block of ice for more than 24 hours. The meaning is clear;
melting of ice under a loaded wire, often demonstrated at room temperatures,
is not caused by pressure; it is caused by heat traveling along the wire.
PLEASE DO NOT SAY "SO WHAT". A common textbook misconception is a great
teaching resource. Help your students discover that "this book statement is
wrong" and they will feel great about studying physics.

Do you remember the "capacitors in series" thread two years ago? It also
exposed a common misconception. Solve a textbook problem and try to verify
the answer in a laboratory. It is not hard to find good working capacitors
for which the "equal charges" assumption is violated by at least one order
of magnitude. In my case the 300 V from a source was distributed as about
3 V and 297 V on two capacitors in series (C1=C2=0.1 microFarads). The
textbook-predicted expectation is 150 V and 150 V.

PLEASE DO NOT SAY "IT'S A DEAD HORSE". The dead horse is in our physics
textbooks and nothing is done to eliminate it. It was there for more than
100 years. I did speak with authors of several widely used textbooks (over
a period of six years) but new editions of their books show no attempts
to correct the error. It is not a mathematical error; it is an error in
the initial assumption. (You may learn about the topic from two articles;
The Physics Teacher 26, 286, 1988 and 31, 156, 1993).

Are there other examples of textbooks misconceptions which high school
and college students can "discover" in physics labs or through assigned
"take home" projects? Critical thinking is worth cultivating.

Ludwik Kowalski kowalskiL@alpha.montclair.edu