Re: Current in a wire

[William Beaty <billb@ESKIMO.COM>]

On Wed, 22 Mar 2000, John P Lewis wrote:

> Hi Folks,
>  I'm in a bit of confusion now about the movement of charge (electrons)
> in a wire.  My belief is that if a wire were connected to some sort of
> emf current should flow throughout the entire wire.  I have a colleague
> in electronics who maintains that the charge travels only on the
> exterior surface of the conductor.

This might be part of a common misconception: the idea that because
net-charge only exists on the surface of the wire, then CURRENT can only
exist on the surface of the wire, since electric current is a flow of
charge, and there is no charge in an uncharged region of metal.  Not true!
Electric current in a metal is mostly a flow of non-net-charge or
cancelled charge, where for every electron there is a proton nearby.  Just
because we have equal and opposite quantities of charge in a metal does
not mean that those "cancelled" charges cannot flow along.  When electrons
flow among protons, the net charge is zero, yet there is charge, and it is
flowing.  Within wires, the charge flow is a flow of "cancelled charge."

This problem is reinforced by textbooks and educators who lable the
electron sea of metals with the term "current", as in:  "current flowing
inside of wires."  Well, no, charges flow inside of wires, but "current"
is not a "stuff" that can flow.  (In rivers, does water flow along, or is
there a mysterious substance called "current" which does the flowing?)

The origin of the problem is obvious: who really wants to tell students
about "charged charge" and "uncharged charge"?!!  The charged charge on
the outside of the wire forces the uncharged charge within the wire to
flow?!  It sounds crazy, but we must use language like this (or perhaps an
improved version) if we want students to be able to understand how wires
work.  The "hollow pipes" fallacy can be defeated.  Just speak of wires as
if they were always full of something.  That something is not "current",
it is cancelled-out charge.

An associated misconception: the surface charge on the wire is "static
electricity", therefore the surface charges cannot participate in the
electric current, since after all they are static.  Yet they DO flow.
"Static electricity" can and does flow.  Well, at least the elementary
charges which form the net-charge can physically move along.  The net
charge itself, when it moves, perhaps should be described as "propagating"
rather than "flowing," since the movement of net-charge is associated with
electromagnetic waves.

Net-charge can move across surfaces at nearly c, even though the charged
particles which create that net-charge might move at speeds on the order
of cm per hour, and can even move in the opposite direction compared to
the motion of the net-charge.

And if you dump 1000 gallons of dye into one end of a swimming pool, the
water level at the other end rises after a fairly short delay, but this
does not mean that the dyed water moved at such a high velocity across the
pool.  Suddenly remove 1000 gallons from one end of the pool, and a wave
of "anti-water"  spreads to the far end.  "Excess water" is not the same
thing as "water."  In a similar way, "net-charge" and "group of charges"
are two very different concepts. Regions of net-charge in an object can
appear and vanish, while no electrons or protons in that object are ever
created or destroyed.

To visually experiment with these concepts, see

  RED AND GREEN ELECTRICITY
  http://www.amasci.com/redgreen.html


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