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[Phys-L] Re: Electric Field



Once I figured out what I had actually done years ago to program the
process, it is pretty much what John suggests--see below.

----- Original Message -----
From: "John Mallinckrodt" <ajm@CSUPOMONA.EDU>


David writes:

1. Is there a better way to introduce the notion of Electric Field
to beginning students? I understand that the inertia of the test
charge means that it will only start along a field line and not
stay on it but it sure is an appealing approach since they, at this
stage, understand forces fairly well.

Inertia should play no role in explaining the concept of the electric
field. Talking about the motion of charges in an electric field is
conceivably the worst way to introduce students to the concept. It
muddies the water by needlessly convolving two difficult ideas.

I see no fundamental problem with the standard approach involving an
operational definition of the electric field, which is, briefly:

Select a positive charge small enough so as not to disturb the
distribution of other nearby charges (or simply imagine for the sake
of this procedure that it DOESN'T disturb that distribution!), place
it at a position of interest, calculate all the electric forces on
it, and divide that force by the charge itself. Notice that the
result depends ONLY on the position of interest and NOT on any
property of the charge that was used to determine it. Use that fact
to appreciate the concept of a "vector field" (a vector quantity that
has a definite value at every position on space) and to see that it
is not so different from the concept of a "scalar field" like the
daily high temperatures on a weather map.

"Lines" are determined very simply by following the procedure above,
moving to a new point of interest a very short distance away in (or
opposite) the direction of the electric field, and repeating as
necessary.


Here is where my procedure was to find the net force components on the
charge, calculate acceleration components, then velocity components and then
new positions and move the charge to that point--having used a very small
time increment. What I had forgotten (had a serious 'senior moment' about)
was to accommodate the inertia problem I reset the velocity components to be
zero before calculating again. This then is an operational procedure for
picking that 'new point of interest' described by John above.

Rick
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