Re: GAUSS LAW

[Joel Rauber <Joel_Rauber@SDSTATE.EDU>]

> I am plannign on a flux ontro tomorrow. It is such an
> abstract idea! I try
> this analogy and seek your wisdom.

1) Electric Flux is horribly abstract the first time you hit it.  The I
introduce flux is to first draw a picture of  creek, (with hard clay bottom
and no tributaries.  Then I pick a point on the creek and draw a
cross-section (for simplicity I assume the creek has a rectangular
cross-section) and discuss the flux of water through the cross-section.

This is a very concrete idea (particularly if your creek is one of those
concrete water channels that cheap movies in the LA area like to have car
chase scenes in).

Students have little trouble thinking of how many gallons per second flow
past this point on the creek and realizing that how large a number this is
depends on the speed of the water (assumed constant and perpendicular to the
cross-section on the first pass with the concept) and the area of the cross
section.

I draw pictures with arrows representing the velocity of the water at
various points on the cross-section.

2) Next I make all the vectors be at  the same angle relative to the
rectangle.  and argue that its only the perpendicular component of the
velocity vectors that are important.

3) Next I make the velecity vectors have different angles and different
magnitudes; now to calculate flux we must divide the cross-section up into
itty-bitty areas and perform the same calculation (this is for a calculus
level intro course)

4) Next I show a picture of a uniform electric field (field line picture)
piercing a rectangular area perpendicular to the area; this is of course the
same picture as 1 above.

But now instead of gallons per second we are calculating "Electric Flux" or
N/C per meter^2; weird unfamiliar units and abstract ideas, like Electric
field; but the pictures are identical to the pictures of flux of water!

This isn't a royal road to getting the electric flux concept across, as
nothing beats lots of student CPU time pondering the concept; but it helps.

Joel Rauber