Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-L] charge distribution leading up to a capacitor



On 08/01/2016 12:43 PM, Jeff Spirko wrote:

Bruce Sherwood (or a
coauthor) has a visualization posted on the Matter & Interactions student
site that addresses exactly your situation:
http://www.glowscript.org/#/user/matterandinteractions/folder/matterandinteractions/program/18-SurfaceCharge

A few caveats:

1) The "snaky circuit" option there has almost the same geometry
as serpentine equipotential circuit at:
https://www.av8n.com/physics/img48/rwire_split_charge.png
but not exactly the same. For one thing, in the glowscript
circuit the two capacitor leads are connected together to
form a single wire.

2) Among other things, this implies that the wire is not
an equipotential, so the distribution of fields and charges
is quite different from the equipotential case. Presumably
the wire is highly resistive, so that it does not instantly
short out the capacitor.

3) Since this is supposed to be a steady-state situation, you
have to imagine that there is an invisible super-heavy-duty
battery somewhere, to maintain the voltage on the capacitor.

4) Be sure to rotate the diagram, so you can appreciate the
3D situation.

5) The capacitor plates appear to have zero thickness. The
surface-charge distribution appears to be the same, no matter
whether you view it from the front or the back. This is
unphysical. Real capacitor plates have nonzero thickness.
That means the front surface is distinct from the back surface.
This is important, because in reality the front charge distribution
is quite different from the back charge distribution.

6) The capacitor plates appear to have a uniform charge
distribution. This is unphysical. It would make more sense
to specify the plate potential rather than the charge density.

As a corollary, the electric field is unphysical in the region
where the wire contacts the back of the capacitor plate. There
is a tremendous amount of curl in the field. Professor Maxwell
says that's impossible in a steady-state situation.

You can learn more from comparing the two visualizations than from
either one separately.