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Re: [Phys-l] Learning barriers: was Physics first

----- Original Message ----- From: "Ludwik Kowalski" <>
To: "Forum for Physics Educators" <>
Sent: Sunday, December 03, 2006 1:49 PM
Subject: [Phys-l] Learning barriers: was Physics first

On 11/17/06 Rick Tarrara wrote:

Consider that there are three generations of scientists out there
using the terms oxidation and reduction. You, of course, could teach
your students using 'loss of electrons' or 'gain in electrons' and
then your students wouldn't be confused UNTIL they hit the outside
world and heard people saying oxidation and reduction.

In physics, positive and negative electrodes are usually referred to as
anode and cathode, respectively. I am thinking about diodes, X ray
tubes, fuel cells, etc. But in chemistry, as I wrote about two weeks
ago, the anode is defined as an electrode where oxidation takes place
while the cathode is defined as an electrode where reduction takes
place. The two ways of defining names of electrodes may be in conflict
with each other.

They are. The chemist is concerned with what is happening INSIDE the "box", and his description is of the process taking place there. The cathode is the place where electrons become available for use (reduction), while the anode is the place where electrons are generated (oxidation). For a physicist, the cathode is the electrical pole from which "cathode rays" appear, and in this sense there is no confusion; the place where electrons become available for "use" is the cathode. Otoh, what is happening externally is the reverse of what is happening internally; electrons "appearing" externally had to be "generated" internally at the same electrode. For a chemist, the terms anode and cathode refer to the internals of the "box", and the term electrode refers to what is happening externally. Thus the chemist's cathode is the physicist's anode.

Consider a 12 V car battery; it’s two electrodes are
always labeled as + and -. On that basis a physicist would say that
these two electrodes are anodes and cathodes, respectively.

A, however, chemist would point that the kind of reaction, taking place
at the interface between the electrode and the electrolyte (in each
cell), depends on the direction of the electric current. Writing down
the reactions:

Pb + SO4 --> PbSO4 at the lead plate (oxidaion of negative SO4 ions)
PbO2 + 4H + SO4 --> PbSO4 +2H2O at the PbO2 plate (reduction of
positive H ions)

s/he would point out that the PbO2 electrode, labeled as +, is an anode
when the battery is being charged, for example form a 20 V power
supply, but becomes a cathode when the battery is being discharged, for
example, into a 2 ohms resistor. Is there a way to consolidate two ways
of describing the words anode and cathode?

Several days ago, a young colleague pointed out that, in order to avoid
conflicts with chemists, physicists should stop refering to anode and
cathode as positive or negative electrodes. Here is a better
non-chemical nomenclature. An electrode should be called anode when a
conventional current flows from it into a connecting wire. Likewise, an
electrode should be called cathode when a conventional current flows
into it, from the connecting wire. Using these definitions a physicist
will never be in conflict with terminology used by chemists. I like
current-based definitions better than definitions that are useless,
unless one knows chemical reactions. It is easier to distinguish
cathodes from anodes by using a dc ampmeter than by analysing chemical
reactions at the interfaces between electrodes and the electrolyte.

Note that the suggested definition is expressed in terms of
conventional current. But that is not a big deal. In terms of
electrons one would say that an electrode is an anode when it sends
electrons into a connecting wire. Likewise, an electrode is a cathode
when it receives electrons from a connected wire. And it is neither
cathode nor anode when the current is zero. In my opinion the words
anode and cathode, like the word emf, are not needed. But I have no
doubt that they will be used by scientists for many more decades, if
not centuries. The same will be true with words like oxidation and

Certainly. Oxidation is the effect that a highly electronegative atom (oxygen, originally) has on a less highly electronegative atom (a metal, for example). Reduction is the effect on the more highly electronegative atom. Inherent in this exchange is that the oxidized atom gives up electrons to the reduced atom. The "reduction" is, of course, not of electrons, but of oxidation state. Therefore the term oxidation refers to the action of atoms that are oxygen-like in chemical activity on OTHER atoms, whereas reduction refers to the change in the oxidation state of that action on the oxidizing atom itself. Everything in the definitions, therefore, is predicated on, and refers to, the atom with the higher electronegativity.