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Re: Cause and effect.



I hear and understand Don's comments re: Cause and Effect. I guess that I
do think that one can equivalently think of the voltage difference as
being due to the charge separation and/or the charge separation as being
due to the voltage difference.


On Mon, 31 Mar 1997, Donald E. Simanek wrote:



On Mon, 31 Mar 1997, Prof. John P. Ertel (wizard) wrote:

This is a common problem when students forget the "true" meaning of the
elements of a formula. The defining equation for the linear capacitor is
Q = CV
This can be read (and should be read) as the magnitude of the charge, Q,
(the EFFECT) that resides on either plate of a capacitive device is
directly proportional to the potential difference, V*, (the CAUSE) between
the plates and the proportionality CONSTANT is the capacitance, C, of the
device.

I changed the header line, since I'm only responding to one small aspect
of John's excellent discussion of the student's conceptual problems with
the definition of capacitance.

Is "cause and effect" really helpful here? Can it sometimes be a trap?

It is "wrong" to say that the potential difference which arises when you
charge a capacitor is a *result* of the charge on the plates? After all,
wouldn't you say that the force on a charge shot between the plates is the
*result* of the field within the plates? The usual definition of potential
is the W/q in moving a charge through the field between two points (in
this case, the plates). Indeed, potential is usually defined *after* we
introduce the concept of the electric field, and the electric field is
defined after having introduced Coulomb's law of electric force. So is it
*wrong* to say that the charge is the cause of the force on another
charge, and the force is the cause of the field (defined through finding
the force on a test charge), and the field is the cause of the potential?

We've covered this ground before here, I know. Precisely how are *cause*
and *effect* defined? I challenge anyone to find a clear definition of
that in a textbook. Are we treading on dangerous ground here by using an
intuitive concept, failing to give it a precise technical definition, and
then expecting students to use it correctly? (I fear we do that sort of
think a *lot*.)

Personally, I can easily avoid ever using the words *cause* and *effect*
in the physics classroom, knowing its dangers. Now in a philosophy of
science course, this could get a thorough airing.

-- Donald

.....................................................................
Dr. Donald E. Simanek Office: 717-893-2079
Prof. of Physics Internet: dsimanek@eagle.lhup.edu
Lock Haven University, Lock Haven, PA. 17745 CIS: 73147,2166
Home page: http://www.lhup.edu/~dsimanek FAX: 717-893-2047
.....................................................................




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