Re: CHARGED CAPACITOR TERM

[William Beaty <billb@eskimo.com>]

On Wed, 18 Feb 1998, Rauber, Joel              Phys wrote:

> My original point was only meant to say that I don't find this to be a 
> misconception commonly found in text books.  

If Ludwik's list is to be limited to physics texts at the undergrad level,
then you are probably right.  I don't recall seeing it in my small
collection of those texts.  But in textbooks as a whole, the assertion
that "capacitors are devices for storing charge" is very common.

As was recently mentioned part of the misconception involves an ignoring
of the second half of the circuit, coupled with a use of an incorrect
"source/sink"  model for charge.  In misconception research, I believe
this is even called "the source/sink fallacy", since K-6 students often
believe that batteries emit charge, and flashlight bulbs convert charges
into light. 

If all the components in a circuit have one side connected to ground, and
if we ignore that connection, then we can (misleadingly) state that a
battery sends charge to a capacitor.  The return current in the ground is
(wrongly)  ignored.  And if the capacitor's ungrounded lead is
disconnected from the battery and connected to a resistor, then the charge
flows from capacitor to resistor.  I would expect that students understand
this, except for some unanswered questions regarding why the ground wires
are necessary.  And a major inability to deal with ungrounded capacitors.


> They usually quite 
> transparently indicate what is actually going on with the standard two plate 
> capacitor.  Therefore it didn't belong on the list (there were others in 
> this category).  I did not mean to imply that this isn't a misconception 
> that many students may get from inadequate reading of their text and 
> inadequate instruction.  I might also add that when I introduce capacitors I 
> am very careful about this point; I hope this means many of my students go 
> away without this misconception.

I think you're to be congratulated for dwelling on this point in class. 
When we are surrounded by others who believe a physics misconception, and
who use language which has the misconception woven into it, then we don't
only need correct knowledge, we also need defenses.  I see these
"widespread misconceptions" as being group-think.  They continue to exist
because they take advantage of forces of "concensual reality" and of the
sort of subtle brainwashing which occurs in some religious cults.  If I go
and live among the cult members, I'd better have sturdy mental defenses,
otherwise their viewpoint will slowly become mine as well.  Physics
misconceptions are a communicable disease, they can infect people who
originally had the correct concepts.  But they also can be defeated by
innoculating people.  Even better, fight them with "engineered viruses": 
if upon hearing that "capacitors store charge", students were moved to
attempt conversion of the heathen infidels, then the meme for "capacitors
don't store charge" would slowly spread throughout the EE/technician
culture.

"Set a meme to catch a meme."  



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