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[Phys-L] Re: Definition of electric field



Hi all-
I have a problem with Bob's definitions.
1. Why does he need to go to the limit of zero charge?
2. He is, of course, free to make his own definition of
"operational definition" (provided he tells us what it is), but Bridgman
restricts "opserations" to "physical operations" (Bridgman 1948, p.5).
The "operation" in Bob's definition is the measurement of force, so he
will have given on operational definition of "force on a charge" when he
tells us how he will measure the force.
3. A definition that involves dividing by charge and taking a
limit is, therefore, not an operational definition.
Regards,
Jack

On Mon, 30 May 2005, Bob Sciamanda wrote:

1) The electric field E is operationally defined as the limiting value of
F/q as q (at rest) approaches zero, where F is the electric force on the
test charge q (E and F are vectors). This E is taken as the force on a unit
charge due to all the other charges in the universe, given their present
positions. These other charges are the sources of the measured field.

2) Having thus measured E at a point (or approximated it with a sufficiently
small test q), one can then predict the force on a finite charge Q placed at
this point to be F = EQ, provided that the source charges have not altered
their positions.

3) Indeed, one must acknowledge the possibility that the insertion of the
finite charge Q may have significantly altered the positions of the source
charges (all the other charges in the universe), thus altering the value of
E from the previously measured value. The field due to the universe of
sources IN THEIR NEW POSITIONS must be evaluated (experimentally or
theoretically) to get the force on the finite Q. Note that the field due to
Q itself is not included in this evaluation of E (ie., Q is not part of the
source of the E which acts upon Q).

I see only semantic differences between this and the Wikipedia definition
you quote.

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
http://www.winbeam.com/~trebor/
trebor@winbeam.com



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"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley
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