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Re: Batteries



Several days ago, after reading the great tutorial JohnD
prepared for us, teachers,

http://www.monmouth.com/~jsd/physics/battery.htm

I wrote that a new sequence of teaching introductory
physics courses would have to be invented to make
modern explanations of electricity possible. Unfortunately,
I have nothing constructive to suggest. I am not sure this
can be done without sacrifying important principles.

In the existing situation I would not try to explain electric
batteries to students in the way John explained them to us.
To justify this reaction let me show how a hypothetical
student might react to the opening paragraphs of John's
essay. (Student’s objections are numbered to facilitate a
possible discussion. John's sentences begin with asterisks.)
Ludwik Kowalski

======================================

**** Start with a chunk of nickel. Make sure it is electrically
neutral; if in doubt you can just count positive and negative
charges and make sure they come out even.

1) It is easy for you to say this, teacher. How do we
count charges?

**** Now take a second chunk of nickel. It also starts out
electrically neutral. Now imagine that using some powerful
neutrino-ray or something, we cause an inverse-beta (electron
capture) reaction that turns the nickel nuclei into iron nuclei.
The crystal structure re-arranges itself accordingly. The chunk
of iron remains, by construction, electrically neutral.

2) I have no idea what the "inverse-beta, electron
capture" is, teacher. How can I imagine a "neutrino-ray"
without knowing what it is? As far as I know, a piece of
metal is not a crystal, like salt or calcite.

**** As we shall see, these two electrically-neutral pieces of
metal attract electrons differently. They have different work
functions.

3) What is the work function, teacher?

**** The work functions are different because of things
like the Pauli exclusion principle.

4) Who was Pauli and what principle are you talking about?

**** You have a different number of fermions in a
different-sized box, so the Fermi level will be different.

5) What are fermions teacher? And what are Fermi levels?
Why are you throwing at me words with which I am not
familiar? I did not skip classes and I read every assigned
textbook chapter. Nowhere did I see words you are using.
…………

**** The fact that different materials have different work
functions is perfectly understandable also. Different
materials have a different spacing between nuclei.

6) I did not know this, teacher.

**** So think of it as a particle-in-a-box problem: The smaller
the box, the higher the kinetic energy the electrons must have.

7) What kind of a box are you referring to? And why should
the size of a box have an effect on the kinetic energies of
electrons. I have no idea what the Heizenberg’s uncertainty
principles are, in case that is what you expect me to know.

**** You can even make a connection between the work
function (a purely electrical property) and the elastic properties
of the metal: when you squeeze the chunk of metal you squeeze
the electron wavefunctions, and that raises their kinetic energy.

8) What is a wavefunction, teacher? How can a change in the
0.5*m*v^2 result from squeezing a function?

LET ME STOP HERE.
===========================================

John, I hope this message will not discourage you from
posting tutorial messages for us. I am sure many physics
teachers appreciate and value them. We are lucky to have
on this list.

1) But do you agree that the existing sequence of topics,
in a typical introductory physics course, is not consistent
with your explanations?

2) Do you agree that EXPLAINING things to students
today we must not lean on what some of them may later
learn in more advanced courses? The alternative is to
describe experimental facts and relations and to say
that "they can be explained" in advanced courses.
Ludwik Kowalski

See two relevant messages below, they are important !

I WROTE :
A clever student may observe that "electrification by contact, as
far as we know, involves dielectric materials. What does a set
of metallic plates, for example, Cu and Zn, immersed in a dish
with salty water, have to do with rubbing a glass rod with silk?
I do not know why charges are separated through electrification
by friction; how can my ignorance help me to understand what
happens in the dish?"

And I do not remember any explanations of the electrification
by contact, only a description of it. Therefore I still feel that
I have nothing to lean on when trying to explain the nature
of something that takes electrons away from Cu and delivers
them to Zn via salty water. I can introduce proper vocabulary
and use it to describe what happens, I can measure how much
of it happens in different situations, I can describe chemical
reactions taking place, etc. All this is highly desirable and we
do it. But I can not explain batteries, or contact electrification,
in terms of something else. Compare this with the P*V=n*R*T,
for example. We can not only describe this relation, we can also
explain it in terms of molecular collisions.

May I suggest that somebody who has a good explanation of
electric batteries (for example, based on a textbook) posts it here,
or on a website. Then we can discuss its appropriateness for a
first physics course. The best I was able to do so far was to use
the water pump analogy, as described at the very end of my
handout:

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

It was an addendum to a comment made by William Betty last
summer. The analogy helps to figure out "how does a battery
know what to do?" The windmill stops turning when the
pressure of water matches the constant wind (and it turns
very rapidly when a lot of water is escaping lowering the
pressure. But the analogy has nothing to do with "forces
responsible for separation of unlike charges."

I am not criticizing what we do, I am only recognizing a
situation in which an explanation is not possible in terms
of what students already know in an introductory course.
There is nothing tragic in this; describing what happens is
a valuable preliminary step in learning something new. The
reason I was referring to "work function" has to do with
what JohnD wrote after my draft was posted in August.

JOHND RESPONDED:
Ludwik Kowalski wrote:

A clever student may observe that "electrification by contact, as
far as we know, involves dielectric materials.

Or the student may not observe any such thing.

1) Most electrical devices (contact electrification or
otherwise) involve dielectric materials. Try designing,
say, an electric motor or even a flashlight, using _only_
conductive metal parts. You can maybe sorta do it, but
the result would be highly impractical to say the least.

2) I can devise a contact-electrification machine (a very
powerful one) where all the critical contacts (the ones
that produce charge separation) are metal-to-metal.

What does a set
of metallic plates, for example, Cu and Zn, immersed in a dish
with salty water, have to do with rubbing a glass rod with silk?

The key idea in both cases is that different materials
sit at different voltages in equilibrium. In the same
way that people think "nature abhors a vacuum" they seem
to think that having nonzero electric fields running around
can't be the natural state ... but it is.

I do not know why charges are separated through electrification
by friction; how can my ignorance help me to understand what
happens in the dish?"

If you insist on a ball-and-stick model, you are
guaranteed to be disappointed. If you adhere to
19th-century physics, you will confidently predict
that atoms (if they even exist) will all behave the
same. Helium is just a heavy isotope of hydrogen.
And lithium the same, only heavier. And oxygen the
same, only heavier still. The alternative is to find a way
to accept that electrons stick to some atoms differently
from others. You can (a) accept this as an observed fact
(based on experiments), or you can (b) accept it as an
article of faith (revealed to you in a dream or
whatever), or you can (c) do the quantum mechanics.

And I do not remember any explanations of the electrification
by contact, only a description of it. Therefore I still feel that
I have nothing to lean on when trying to explain the nature
of something that takes electrons away from Cu and delivers
them to Zn via salty water. I can introduce proper vocabulary
and use it to describe what happens, I can measure how much
of it happens in different situations, I can describe chemical
reactions taking place, etc. All this is highly desirable and we
do it.

That's option (a) in the list above.

But I can not explain batteries, or contact electrification,
in terms of something else.

If "something else" is restricted to pre-20th-century
physics, neither Ludwik nor anybody else is going to
explain chemistry or atomic physics or materials science.

Compare this with the P*V=n*R*T,
for example. We can not only describe this relation, we
can also explain it in terms of molecular collisions.

OK, that's an example of something that can be
described in terms of 19th-century physics.
But ONE thing being possible doesn't prove
that ALL things are possible.

May I suggest that somebody who has a good explanation of
electric batteries (for example, based on a textbook) posts it
here, or on a website.

I've never seen a decent explanation in any textbook.
I had to figure it out for myself. Try this:
http://www.monmouth.com/~jsd/physics/battery.htm