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Chabay/Sherwood



SEE THE CHALLENGE AT THE END. BUT REPLY TO
IT UNDER THE "EQUIPOTENTIAL LINES" THREAD.

Joseph Bellina wrote:

On the contrary, there is a good literature on student
misconceptions, and any author should keep those
research results in mind when they write.

But there are many ways to do this. If I had to paraphrase
the line of reasoning presented by C&S in a textbook I
would be more explicit in labeling wrong ideas.

For example, something like this:

It would be wrong to think that .... (blah blah blah).
This kind of thinking is likely to result from .... (blah blah).
So why is it wrong with it? .... (blah)

This would not create situations in which a reader who does
not have a misconception feels like an idiot (not able to to
follow the arguments presented). In the one-to-one tutoring,
on the other hand, the Plato dialog approach (following the
idea of a student, showing where it leads, etc.) would be
highly desirable.

Ludwik Kowalski wrote:

I started reading "Electric & Magnetic Interactions" of Chabay
and Sherwood. I need some help.

1) ON PAGE 208 (Section 6.2.1) THEY SAY:

Now consider a circuit consisting of our mechanical "battery"
[Van de Graaff] and a nichrome wire that has some twists and
turns in it. You know from your own experiments that such
twists and turns do not seem to affect the amount of steady-
state current, which seems odd.

WHY SHOULD IT SEEM ODD? ....

*********************************************
Thanks to JohnM who answered the questions. My
next step will be to understand the equipotential lines
described yesterday. Can they be explained in terms
of charges accumulating at the strip bendings? But
first I hope somebody will either confirm or contradict
my findings. Meanwhile here is a challenge to those
who like to make predictions.

What kind of equipotential lines would you expect for the
silver strip painted on a carbon-impregnated paper if the
shape of the strip was an uncurling flat spiral. Suppose
the length is 100 cm, the DOP between the terminals is
1 V, the uniformly distributed resistance of the strip is
1 ohm.cm and the angle is 1800 degrees (5 round trips
from the grounded terminal at the center to the positive
terminal at outer end). Draw equipotential lines for
U=0.5 V and U=0.25 V. Assume that the resistance of
the carbon paper, (for example ohms/cm^2) is 1000
times larger than that of the silver layer.
Ludwik Kowalski