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[Phys-L] Re: Definition of electric field



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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