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[Phys-L] Re: electricity



Alison Innes wrote:

I am an Education Coordinator at a museum in Canada. I didn't take
sciences in university (there's a reason I did a history degree) but I did
take physics in highschool and rather enjoyed it. I know find myself doing
science programmes (light, electricity, sound) here at the museum. I have
been following what was left for me, information wise. As I have been
trying to research electricity, I found this site
<http://amasci.com/miscon/elect.html> It seems to help me, but I just
want to check that it is somewhat reliable (I know that no source will
ever be 100% reliable). Anyone out there familiar with it?

I took a look at the site. I have an unfavorable opinion of
it, for several reasons.

As a point of pedagogy: One of the advantages of teaching
young kids is that you can teach them the right conceptions
before they've had a chance to form too many misconceptions.
Therefore a site featuring a massive list of misconceptions
is the wrong place to start.

For that matter, there is necessarily an infinite number
of possible misconceptions, so no matter whom you are
teaching, discussiong long lists of misconceptions is
generally a waste of time. Teach the right ideas and
move on. (There is an exception for misconceptions that
are particular common and/or particuarly deadly, but
that doesn't apply in this case.)

The choice of misconceptions is particularly poor. Too
many of them seem to be manufactured by abusing the
terminology in trivial ways, such as conflating
-- motion of electrons
-- electrical energy
-- electrical signals.

Again I say: don't focus on the misconceptions. Make
the correct distinctions and move on. Positive statements
are better than double negatives.

As a point of science and pedagogy both, I find the tone
of the site to be unscientific. In many cases it states
conclusions without offering adequate evidence ... in other
words, there's too much PbBA (Proof by Bold Assertion).

As a point of science, too many of the documents on
the page are only half right. They correctly identify
a misconception ... but then replace it with a different
misconception. This is not progress.

As an example of this, at one point it deprecates the
notion that sound consists of air particles that move
ballistically from the source to the receiver. So
far so good ... but then it goes on to boldly assert
that air molecules do not move at the speed of sound.
a) It doesn't say how fast the air molecules do
move, and
b) It turns out that the right answer is that at
any given instant, a typical air molecule *is*
moving at roughly the speed of sound.

(there's a reason I did a history degree)

That's a lame excuse. Historians don't accept PbBA in
place of historical evidence ... so why accept PbBA when
it comes to technical questions?

a lot of what I was taught in elementary school was WRONG.

Yes. Wrong history as well as wrong physics, chemistry
biology, et cetera.

Since I have just discovered that our 'weighing air using two
balloons and a meter stick' experiment isn't accurate
(apparently it only shows that air under pressure weighs more
than air that isn't), does anyone know of any simple
experiments/demonstrations that can show kids that air has
weight? Or is it something we have to take on faith?

Please, let's stop taking things on faith. That phrase
refers to believing things in the absence of evidence.
That's the opposite of what science is. For a discussion
of scientific methods in general, see
http://www.av8n.com/physics/scientific-methods.htm

As for the air: First of all, it would be more precise
to ask about the mass of the air. That's not quite the
same thing as its weight.

To prove the main point, why doesn't it suffice to show
that putting more air into a given volume increases the
mass? The balloons are a fairly direct demonstration
of this. What's the alleged problem?

If you want something even more direct, go the opposite
direction: pump out the air from a bell jar. Measure
the mass, then let the air back in and re-measure.

More importantly: Science in general is a huge lattice
of observed facts linked together by theory. If you
know anything about the theory of sound, the fact that
the speed of sound is less than infinity is powerful
evidence that the air has mass. Indeed, by measuring
the easily-observed compressibility of air, and the
speed of sound, you can determine the mass-density to
high accuracy.

Also, a wing develops force in proportion to the mass-
density of the air (and a few other factors). So by
observing the weight, size, and speed of a model airplane,
you can obtain a fair estimate of the mass-density of
the air.

Also, the fact that air pressure declines as you go up
in altitude is directly and precisely related to the
mass-density of the air. This is hard to demonstrate
on a table-top in a museum, but the effect is real
enough, and readily observable in the elevator of a
tall building.

etc. etc. etc.

The idea of "taking it on faith" is particularly
abhorrent when there is such a wealth of good
evidence available.