Direction of current

[William Beaty <billb@eskimo.com>]

On Tue, 2 Jul 1996, David E G Sturm on tap-L wrote:

....
> This all reminds me of the electron-current/conventional-current flow 
> situation.  Does anyone have a really snazzy demonstration to show off 
> the difference among their collections?  
>
> I'm considering building a 6' high 3' wide board that would sit on the
> lecture desk with two side by side setups: bulbs, batteries, caps, etc;
> with graphics put on showing the effects are the same, but the notation's 
> (as we say in the south--'back-arsewards') different.

Have you ever seen the "Visual Electricity" devices sold by Arbor and
appearing on my page at http://www.eskimo.com/~billb/viselec/viselec.html? 
They display the proportional movement of charges within current-carrying
wires via digital circuitry and a "chaselight" effect in a row of LEDs. 

I put together an interesting version of this which displayed a 3-segment
"wire" having a piece of "positron" conductor in the center of a normal
electron-conductor wire:

                              Red LEDs(pos carriers)
      ---------------------- ++++++++++++++++ ------------------------
      Green LEDs (electrons)                     Green LEDs(e-)

When a hand-crank generator was clipped to the ends of this device and
cranked, the green LED "electrons" flowed one way and the red "positrons"
flowed the other, with regions of "creation" and "annihilation" at the
segment boundaries.  A good demo for starting discussions about the real
nature of electric current.

A couple of thoughts on the direction of current.  If we state that
"conventional" current is assumed positive, yet electric current is REALLY
negative, then we make a fundamental error.  Only in metals is electric
current a flow of negative particles.  Wet your fingers and touch them to
a 9V battery, and a current of a fraction of a mA appears within your
fingers and arms.  No electrons flow though, this current is entirely
composed of moving positive and negative ions.  The same goes for electric
currents flowing in the earth, in oceans, between the plates in batteries,
within the wet cardboard of electrolytic capacitors, etc.  In neon signs,
merc and sodium streetlights, sparks, and auroras, the electric current is
electrons flowing in one direction and positive gas ions flowing in the
other.  In some heated metal salts the current is entirely a flow of
protons.  I've heard it said that in water ice the conductivity is caused
by proton flow only.  (see Meiners' for info about visible electric
current in heated blocks of salt.) 

How can we measure electric current if it is sometimes positive charge
flow, sometimes negative, and sometimes interpenetrating, unequal flows of
both?  By the ingenious strategy of assuming a flow of positive charges! 
Are the heating, magnetism, and potential difference caused by electric
currents are identical for forward flows of positive charge or backwards
flows of negative?  Yes, and so we can arbitrarily create an abstract
"electric current" concept where there are no complicated charge flows,
but is only a transport of single-polarity charge.

But somehow over the decades this has been changed to read, "though
current is assumed to be a positive flow, is is really negative."  This is
wrong, and it's another of those pernicious physics myths that has found
its way into textbooks everywhere.  So much the worse for our
understanding that it covers up the true situation. 


......................uuuu / oo \ uuuu........,.............................
William Beaty  voice:206-781-3320   bbs:206-789-0775    cserv:71241,3623
EE/Programmer/Science exhibit designer        http://www.eskimo.com/~billb/
Seattle, WA 98117  billb@eskimo.com           SCIENCE HOBBYIST web page