Re: [Phys-l] battery redux and again (part deux)

[Brian Whatcott <betwys1@sbcglobal.net>]

At 02:40 PM 3/1/2007, Zeke, you wrote:

> As a high school  teacher, I often reach a point where the material is too
>  hard for the students or is mostly in another discipline. I try to sketch
>  what is going on anyway, but I often have to say, "You'll get more
> of this in chemistry (or biology)."
>
> Other times, like when trying to explain how a battery works or a
>  power supply, I preface the discussion with, "By a P2C2E..."
> which my students know means, "By a process too complicated to explain..."
>
> Marc "Zeke" Kossover


One is piqued by the thought of inventing a Just-So story: perhaps 
like this.....

There are liquids which are capable of conveying charge. They are called
electrolytes.  Water with a little acid, is such a liquid. It can 
carry positive
charge, and it can carry negative charge. Often the charge carriers
(called "ions" by Faraday,with a little classical help) weigh different
amounts - so as you might expect  they travel at different speeds.
  Really rather slowly, in fact. Slower - by  quite a bit
than the slow  progress of electron drift in conductive wire.

But ion transport can be visualized - because charges of a particular
 polarity may invoke color change in a transparent liquid.
 So in particular circumstances, you can measure ion transport speed
with an eyeball and a ruler and a stop watch.

Particular conductors planted in this electrolyte are found to collect
electrons on one  and donate electrons on the other conductor,
 each of which is called an electrode (by Faraday, with a little help).

If an isolated electrode  collects electrons, it becomes more
 negative until no more electron carriers (ions) can reach it.
And the same applies to the other electrode which becomes positive.

If an external conductive path is provided, the potential across
 the electrodes dips as current flows and more ions can again share
charge with the electrodes.
The 'clockwork' behind donating and accepting electrons involves
ions in chemical combination  and may cause irreversible changes
 to the electrodes - one may shrink away, another may grow.
Or the process can be driven backwards (reversibly) in the
 sort of battery which we are most   familiar with  - the lead
acid battery in our car.

Ah yes: just so!



Brian Whatcott    Altus OK    Eureka!