Re: [Phys-L] interior charge

[John Denker <jsd@av8n.com>]

On 12/29/2013 01:41 PM, rjensen@ualberta.ca asked:

> Why are wires round? 

In many cases, they are not round.  Many wires are stranded.
Often the strands are large enough and few enough that the
overall cross-section is nowhere near round.

Also there is such a thing as "bus bar" which has a rectangular 
cross section.  This makes it convenient to bolt on connectors
for branch circuits.

Also the wires on printed circuit boards are nowhere near
round.

> Why are thicker wires used to carry more current?
> If the charge is on the surface, wouldn't it be better to create the
> wire as a ribbon?

Several different answers:  Let's be careful about the 
distinction between charge and current ... and between
AC and DC.

In the DC limit, the /current/ is carried throughout the
bulk of the wire.  So to a very good approximation, the
resistance per unit length is controlled by the cross-
sectional area, independent of the cross-sectional shape.

   The /steering charges/ are all in a layer just inside
   the surface, but that's the answer to a different question.

For high-frequency circuits, the current is carried in
a thin layer near the surface.
  http://en.wikipedia.org/wiki/Skin_effect

It is common for high-frequency circuits to use tubular
conductors.  One option is a silver-plated insulator
(or near-insulator).  Another option is an out-and-out 
hollow tube.