Re: Batteries

[Ludwik Kowalski <kowalskiL@MAIL.MONTCLAIR.EDU>]

A slightly expanded version of my tutorial handout
about the DOP has been posted on my website at:

http://blake.montclair.edu/~kowalskil

The document can also be accessed directly with

http://blake.montclair.edu/~kowalskil/elec/volts.html

Comments, as always will be appreciated. Sorry for the ASCII
pictures; I will eventually replace them with real illustrations.
What follows is the last paragraph from my handout.
Ludwik Kowalski
. . . . . . . . . .  . . . . . . . . .
The formula reflecting this fact is Q=C*U. Be aware that the
coefficient of proportionality C, linking Q and U, is called
capacity (of a capacitor). A set of plates storing more Q, for
a given U, than another set, is said to have a larger capacity C.
What is the unit of capacity? It is coulomb per volt, naturally.
That unit was named farad (F), after Faraday. Would you like
a unit of physical quantity be named after you? To make this
possible (but not certain) you must make a significant scientific
contribution. Keep in mind that possibilities for discovering or
explaining natural phenomena are unlimited. For example, we
know that positive and negative charges are separated in a
battery cell. This happens despite the fact that unlike charges
always attract each other. How can the separation of unlike
charges be explained in terms of concepts with which you are
already familiar? This question is not even asked in most
textbooks.
*********************************************
Hi William,
Your message was very instructive for me. But in order to
follow your advice I would have to violate a basic rule of
good teaching; I would have to explain something in terms
of something to be learned later. You wrote:

> I've explained WHAT happens, but haven't said WHY it
> happens.  Does  our understanding come from explaning
> "WHY"?  Not in my opinion.  I find that when I make a
> breakthrough in my own understanding, it's because I've
> connected some concepts together which had been separate.

True, but the concept of "work function" does not exist in
the minds of my students. Therefore they have nothing to
link battery cells with. The concept of the DOP produced
by a cell can not be linked with something which may
or may not be introduced to them later. But the link must
be made when the work function is introduced. Lean on
the past, do not lean on the future. That should be the
basic teaching principle.

William Beaty wrote:

> On Fri, 8 Feb 2002, Ludwik Kowalski wrote:
>
> > I am also puzzled by chemical cells. There was a discussion
> > of them last summer (August 2002). JohnD posted a message
> > which I saved. Part of it is reproduced below.
>
> "Unless you can explain something to your grandmother, you don't really
> understand it."  (I've seen this quote attributed both to Einstein and to
> Feynman.)
>
> Obviously there are plenty of situations where sheer complexity exposes
> the error in this quote. However, it still contains a large grain of
> truth: basic concepts in physics are extremely simple, and they knit
> together a varity of separate phenomena.
>
> In the case of batteries, there is a basic concept which rarely is
> mentioned:   electrification by contact.
>
> If you place a balloon against your hair, the surface of the balloon and
> the surface of the hair both become electrified.  The same thing happens
> when you place copper against iron, or place P-doped Germanium against
> N-doped Germanium, or hit an iron atom with an oxygen molecule.  The same
> thing happens when drop a lump of metal into an electrolyte: the metal
> becomes charged, while the electrolyte develops an opposite charge.  In
> other words, a single concept unites triboelectricity, batteries, LEDs,
> solar cells, diodes, thermocouples, and any other electrical device which
> depends on contact between dissimilar materials.
>
> Touch a piece of aluminum against a piece of iron.  One metal becomes
> charged positive, the other negative.  (If you had a very sensitive
> electrometer, you could detect this spontaneous charging.)  One metal
> "steals electrons" from the other one.  Yet only a small fraction of the
> metal's electrons are "stolen."  The two regions of opposite charge only
> grow until a certain level, then all growth stops.
>
> There seems to be an invisible electron-pump hidden between the two pieces
> of metal, but this is an electron pump which always creates the same
> "pressure" or potential difference (PD).  Try using other pairs of
> materials, and you'll get a variety of different PDs appearing between
> them, but the PD will always be only a few volts at most.  The "pump" only
> runs until a particular "pressure" is attained, then it stops.
>
> OK, here's a thought-experiment.  Take a bowl of salt water and place two
> lumps of copper into the liquid.  The salt water becomes charged, and the
> lumps of copper develop an opposite charge.  If you push the copper lumps
> together so that they touch, no charges flow.  This makes sense, because
> the "Contact Electrification" caused the two lumps of metal to become
> charged to the same polarity and the same voltage with respect to the salt
> water.
>
> Next, replace one of the lumps of copper with a lump of zinc.  The metal
> lumps still have a charge which is opposite the polarity of the charge on
> the salt water.  However, the lumps of metal have charged up to two
> DIFFERENT voltages.  If you touch them together, charges begin flowing.
> This charge-flow does not stop!  The "invisible pumps" which created the
> Contact Electrification are now running constantly.
>
> I've explained WHAT happens, but haven't said WHY it happens.  Does  our
> understanding come from explaning "WHY"?  Not in my opinion.  I find that
> when I make a breakthrough in my own understanding, it's because I've
> connected some concepts together which had been separate.  It's like
> finding that two partially-completed hunks of a picture puzzle can fit
> together to form one larger hunk.
>
> As far as batteries are concerned, the "contact electrification" concept
> provides a large amount of understanding.  It knits together the
> thermocouples with the balloons-rubbed-on-hair, the corroding metal with
> the solar cells.  All of these have a single explanation: the
> electron-stealing, the charge-pumping effect that arises between
> dissimilar materials.
>
> What if there is no explanation for this (perhaps it has the character of
> a physical "law.")  Does that mean we don't understand anything?  In
> physics, what does "understand" mean?
>
> Here's a better question:  why don't most K-12 textbooks teach about
> Contact Electrification and the important connection between "Static
> Electricity" and the rest of science?  Instead their goal seems to be to
> denigrate Electrostatics as obsolete "Ben Franklin" stuff which only
> applies to static cling, lightning, and Wimshurst Machines.