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Tidal bulge, Bohr atom & other myths



Concerning the spreading of physics misconceptions in class and in textbooks:
Regarding the oceanic tidal bulge Jim Green wrote:
...
But *later* some conscientious teacher is going to have to unteach that
stuff along with all the cartoon physics the kids learned from TV.

What is worse is that those kids are going to be the teachers soon enough
and will be saying -- as we hear frequently now -- hey this is what I learnt
in school and its good enough for me now.

I personally see no reason to talk about a tidal bulge. One could
productively talk about tidal FORCES which *do* make a daily march around
the Earth. Why pollute the student's heads with more cartoon physics?
...
But I urge that we teach no false doctrine.

I often disagree with Jim on things (e.g. mass/rest mass/rest energy termino-
logy, whether heat is work or not, use of "heat" as a noun, etc.), but in
this case I agree with Jim that the idea that the oceans possess a tidal bulge
is exaggerated to the point of being misleading and that we should not spread
"false doctrine". I think that it is appropriate to mention to students that
the equi-potential surface (geoid) DOES have a tidal bulge. It's just that
the instantaneous surface of the earth does not coincide with a geoid since
the whole earth is not a perfect uniform fluid with a zero equilibration time,
and the earth's surface is rotating too rapidly with respect to the bulge in
the equi-potential surface to respond and conform to it with all the under-
damped (Bay of Fundy) and overdamped parts (Eurasian continent) equilibrating
on the appropriate decay-time time scales.

Dave Dockstader brought up what seems to be a more egregious case of the
intentional propagation of misconceptions. He wrote:
...
Here's a quote from a geology text published in 1996 by DC Heath
"Electrons orbit at specific distances from the dense, positively charged
nucleus..." Jim has to like this! Anybody have any pull with the editors
at DC Heath?
...
This is one of my pet peeves. The Bohr Atom was dead by the mid-20s (after
having been proposed in 1913) and still today it is taught. Maybe we should
teach Ptolemaic cosmology, caloric, and phlogiston theories while we are at
it. (Teaching this stuff as part of a historical background and as a context
for the development the modern ideas of science is one thing; teaching it as
a viable explanation for how nature works is another.) The textbook in the
astronomy course that I teach here teaches the Bohr Atom when discussing
atomic spectra (at least the Rydberg spectrum which is so important in
astronomy) without a disclaimer. When we get to this part I tell the students
that the book is wrong and to ignore what it says about the mechanism for the
discrete atomic energy level spacings. I tell them if they want to really
know how and why atoms have the characteristic levels and spectra that they
do, they should become physics majors. This tactic hasn't resulted in a
noticable increase in the number of physics majors that we have here. I think
this may indicate that the students are not infinitely curious about how
atomic spectra work.

Some other errors and/or misconceptions propagated by textbooks that irritate
me are the usual explanations of rainbow formation and the use of the
constancy of the speed of *light* as the 2nd postulate of special relativity,
rather than the requirement of no instantaneous-action-at-a-distance (with the
corollary that there must be a speed limit of causation whose value we
conveniently label as c). The constancy of this speed limit c is itself a
theorem of the first two postulates. The reason that light happens to travel
at this speed limit is that the photon happens to be massless. If photons
happened to have a nonzero mass then the group velocity of light would be less
than c, but this would have no effect on the validity of special relativity
itself.

I better stop rambling here. I'm sure that we can all come up with a host of
commonly-propagated errors that bug us.

David Bowman
dbowman@gtc.georgetown.ky.us