[Phys-l] Learning barriers: was Physics first

[Ludwik Kowalski <kowalskil@mail.montclair.edu>]

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. They would be 
> confused for a while, then  they would realize these are just 
> different terms for what old LK called  'loss/gain of electrons". They 
> might curse you under their breadth for leaving them ignorant of this 
> common nomenclature and, if they then go off  to teach, will almost 
> certainly return to these old words. It would be wonderful if we 
> really had a clean slate and could rework all  the pedagogy without 
> regard to common practices, common nomenclature,  historical documents 
> and developments. . . . You could maybe start a movement towards more 
> descriptive nomenclature by  teaching 'loss/gain of electrons" but 
> also clearly identifying that others  call this oxidation and 
> reduction, but that you hold such words confuse  rather than clarify 
> the physics/chemistry. Maybe your students then will  spread your 
> teachings.

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. 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 
reduction.
_______________________________________________________
Ludwik Kowalski, a retired physicist
5 Horizon Road, apt. 2702, Fort Lee, NJ, 07024, USA
Also an amateur journalist at http://blake.montclair.edu/~kowalskil/cf/