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Batteries (was Circuit Question)



Tina Fanetti wrote:

How does the battery "knows" that it is supposed to put
out more current for different lighbulbs? Mathematically
it makes sense but conceptually it doesn't to me.

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.

Flow of charges outside a battery seems to be natural; after all
like charged repel and unlike charges attract. But separation of
charges inside the battery seems to call for another "nonelectric
force." That force tends to keep the DOP constant between the
terminals. (The DOP in volts is often called EMF). We can not
explain the DOP without advanced physics. But this does not
prevent us from recognizing its existence and explaining things
in terms of it. What else can we do in the first physics course?
Ludwik Kowalski
------------------------------------------
Responding to this:
2) I am always puzzled by battery cells. Most introductory
physics texts describe their electrical properties. Chemical
reactions taking place in some cells are also described,
occasionally. But all this seems to be a description of what
happens rather than an explanation of why it happens.
I suspect that many on this list would appreciate if
somebody could post a short essay about batteries for an
elementary physics course.

John Denker wrote:
I thought somebody already had posted such an essay.
http://mailgate.nau.edu/cgi-bin/wa?A2=ind0007&L=phys-l&P=R27156

My impression is that electochemical cells can not be explained
at the elementary level;

How elementary? Elementary-school level? Obviously this subject is
not suitable for fourth grade. But it would be within the scope of an
ordinary modern-physics course.

they are what they are; period.

That seems like cop-out. It's just physics, after all.

[Using big words, like Gibbs
potential, QM or double layer, does not contribute to clarity
at the level of my teaching.]

Well, if you want to explain batteries in terms of 18th-century physics,
you're going to be disappointed. There is no ball-and-stick model that
explains why different metals have different work functions.

If you want to explain electrochemistry you're going to have to use the
methods (or at least the results) of modern physics in some form.
Battery voltages are on the order of a fraction of a Rydberg, and that's
not a coincidence. If you insist on no quantum mechanics, there will be
no atoms, no molecules, no metals, no work functions, no batteries,
and no humans to ask questions about the lack thereof.

Once you come to terms with the idea that nature does not require
materials to be electrically neutral in equilibrium, the rest of the
battery explanation requires little more than high-school physics.