Doug Miller wrote:
In article .com
, wrote:

You, however, seem to be unclear on the concept of skin depth.

Skin depth is a measure of the depth to which the electric field
penetrates the material. It is not, as you seem to believe, the
depth at which the current flows.

Speaking of unclear on the concept... an electric field is simply a field in
which work is done on an electric charge -- IOW, where current flows.

In a force field the potential to do work is present, whether work
is actually being done or not. Surely you'll agree that the electric
field extends beyond the wire into the surrounding air where no
current flows.

You do not need current to have an electric field. Light is
an example. The skin depth for light is real small.


IOW, you're wrong.

You might want to grab yourself a high school physics text and [re]acquaint

Wikipedia has good articles on the subject.

yourself with a few concepts before you so glibly assure CW that he's
mistaken. First, it's *static* charge that resides on the surface of a
conductor, *not* electric current. Second, the cloud of free electrons in a
metal extends throughout it, rather than being confined to the surface.

OK, you got me there.

I wasn't familiar with the relationship between skin depth and
current density. The current density at the center of #12 wire
is almost 90% of that at the surface, right?

Last night I was thinking about this and remembered the
Hall effect. That would not be possible without current
passing through the conductor, rather than along the surface.

NOW, Mr CW can see me backpedaling.


Finally, you might want to ask yourself why the NEC-permitted ampacity of
conductors below about 4/0 is [roughly] proportional to their cross-sectional
area, not their diameter.

The NEC tables make assumptions about heat-dissipation
to the environment and 4/0 down to #8 conductors are
typically multi-standed, both of which complicate the issue.

If we look at the DC resistances vs cross sectional areas
for #10 and smaller in the table he

http://en.wikipedia.org/wiki/American_wire_gauge

We see that the resistance is inversely proportionate to the
the cross sectional area, confirming your point. In that
same range the NEC permitted ampacity goes up on a per/
cross-sectional area basis as the wire size goes down,
evidently because the larger surface to volume ratio
dissipates heat better.

How about closing an AC circuit through a capacitor?
Would you agree that the AC 'passes' through the capacitor
even though the electrons do not?

BTW, other than the description at the top of the page,
I don't see any difference between NEC 310-19 and
NEC 310-18 he
http://www.houwire.com/products/tech...cle310_18.html
http://www.houwire.com/products/tech...cle310_19.html

Are those pages correct?

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FF