# Rules for Electromagnetism

From: DonRath@aol.com
Date: Wed Oct 14 1998 - 15:27:57 PDT

TO: Louanne Marletta
FROM: Don Rathjen, Exploratorium Teacher Institute (donr@exploratorium.edu)

electromagnetism. It turns out that the Snackbook is correct -- but so are you
-- everybody is right -- it just depends on how you choose to define electric
current! As a longtime high school physics teacher, I'm actually on your side,
and if I had been the only one writing the Snackbook, I would indeed have
defined electric current as electron flow from the negative side of the
battery, through the wire, to the minus side, as you brought out. Some high
school physics books actually do define current this way, and I like this
definition because it allows high school physics students (who are often
encounteringthese ideas for the first time) to deal directly with what is
actually taking taking place in the wire -- that is, electron flow. So much
for the good news. The problem is that if you now take a look at most college
physics books you will find things like the following three quotes.

"The charge flowing through a surface...can be positive, negative, or both. It
is conventional to choosethe direction of the current to be in the direction
of flow of positive charge. In a metallic conductor such as copper, the
current is due to the motion of the negatively charged electrons. Therefore,
when we speak of current in such a conductor, the direction of the current
will be opposite to the flow of electrons." (Serway & Faughn, College Physics,
Third Edition, Saunders, 1992)

"Today, it is known that electrons flow in metal wires. Figure 20.5 shows the
negative electrons emerging from the negative terminal of the battery and
moving around the circuit toward the positive terminal. It is customary,
however, not to use the flow of electrons when discussing circuits. Instead,
a so-called conventional current is used, for reasons that date back to the
time when it was believed that positive charges moved through metal wires.
Conventional current is the hypothetical flow of positive charges that would
have the same effect in the circuit as the movement of negative charges that
actually does occur." (Cutnell and Johnson, Physics, Fourth Edition, Wiley,
1998)

"Since it is electrons that move in the wires of a circuit, the net flow of
charge is in the direction in which an electron experiences a force -- away
from the negative terminal of the battery and toward the positive
terminal...Historically, however, circuit analysis has generally been done in
terms of conventional current, which is the direction in which positive
charges would flow, or the direction opposite to the actual electron flow."
(Wilson and Buffa, College Physics, Third Edition, Prentice Hall, 1997)

When we wrote the Snackbook, we had a long-running and spirited debate about
whether to talk of conventional current, and right-hand rules -- or electron
flow, and left-hand rules. I was arguing from the teaching perspective, and my
co-author was arguing from the academic perspective. I eventually lost, but my
friendly protagonist has a PhD in physics from MIT, so I tried to take defeat
gracefully! So when the Circles of Magnetism and Motor Effects snacks in the
Snackbook talk of current flowing from the plus side of the battery through
the wire to the minus side, they aren't in error, and the right-hand rule
applies. If we were talking electron flow, then we would indeed use the left-
hand rule instead. And I still would argue that the latter would be more
appropriate for the Snackbook. So thanks for the support!

Finally, it is interesting to note that there are indeed places where positive
charge does flow -- in semiconductors, ionized gases, electrochemical cells,
etc. So in these areas the use of conventional current is just as easily
defensible as electron flow, and in some cases may even be more convenient.

Hope this helps. Let me know if you have any more questions or comments.

This archive was generated by hypermail 2.1.3 : Mon Apr 24 2006 - 11:34:46 PDT