Re: motor effect (fwd)

Date: Mon Mar 10 2003 - 14:45:31 PST

Hello Snakctalk

The model of electric current is just that, a model, so I choose the model that
works the easiest in the largest number of situations. In teaching college
physics and electronices I had to make the choice and long ago chose conventional
current, the flow of positive charges, over the flow of negative charges.

"Conventional electric current" is defined as the flow of positive charges.

In copper wires the electric current is carried by the flow of negative charges
(electrons) in the opposite direction.

In electrical circuits a positive charge flowing one direction creates the same
results as a negative charge flowing the other direction. In fact, no one knew
the sign of the charge carriers in wires until the Experiment of Tollman and
Stewart in 1916. And yet a great deal was done with electricity before that

Some high school textbooks use the flow of negative charge, however many others
and indeed almost all college texts in electricity use the flow of conventional
current. The books that use the flow of negative charges are inconsistent for
example when they treat charge flow in a salt water solution they do not say that
half the charges, the positives, moves one way and the other half, the negatives,
moves the opposite way and the effect is the sum of the absolute values of these
flows directed in the positive direction. They also don't treat the flow of
positive charges in water ice, or the flow of positive charges in Bismuth wires.
Whereas the conventional current is the same for all of these cases.

The real reason that I use the conventional current is that flowing positive
charges move from high voltages to low voltages. Thus the voltage around a
circuit can be modelled as height, with positive charges flowing down hill to
lower voltage. If the flow of negative charges is used they move uphill toward
higher voltages. They actually do, but this confuses students mightily.

Paul D

> Dear Paul,
> Can you reply to me so I can post this to snacktalk?
> Thanks,
> Deb
> ________________________
> Deb Hunt
> Snacktalk Moderator
> Exploratorium
> 3601 Lyon Street
> San Francisco, CA 94123-1099
> ---------- Forwarded message ----------
> Date: Sat, 8 Mar 2003 07:00:45 EST
> From:
> To:
> Subject: motor effect
> I enjoyed your link, but why do you say that electricity flows from the
> positive to the negative terminals when all American texbooks will teach that
> negative charge moves from the negative terminal to the positive terminal?
> Place your hand so that your thumb points along the wire in the direction
> that the electric current is flowing (current flows from the positive
> terminal of the battery to the negative terminal) and so that your fingers
> point from the north pole of the disk
> Thak you
> Dave Wille

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