Monday, January 19, 2009

Physics Textbook Writing: Medieval, Monastic Mimicry

I must admit that this was a rather enjoyable article, even if I don't completely agree with the whole thing.

The article is a guest editorial in the February 2009 issue of American Journal of Physics. It is written by Craig Bohren[1], Prof. Emeritus of Meteorology at Penn State.

In the article, he blasted many physics textbooks authors for doing nothing more than regurgitating what has been done by previous physics textbooks authors without even bothering to look at the source. This often resulted in the prorogation of errors and misleading/inaccurate statements, especially with regards to the historical accuracy. He cited an example of the former using the case of the "...velocity of light in a medium with refractive index n....". He shows why "c/n" is not THE "velocity of light", because it is one of many. It also leads to the misleading idea of light slowing down in denser media.

Like I said, a rather fun read.

He could have taken another example of a possible confusing "error" that has also been propagated from E&M textbooks. Seymore Margulies in the early 80's published a paper, also in AJP, about the confusing argument regarding the force acting on a dielectric slab that is partially inside a parallel plate capacitor[2]. He pointed several things that are utterly confusing, and such a thing is either never addressed, or simply glanced over in many E&M textbooks, leading to the idea that each subsequent authors simply used what was already written in previous textbooks.

Although the calculation is simple, the accompanying textbook discussions leave much to be desired. Almost invariably, the textbook presentations ignore the physical origin of the force and treat the problem formally. In addition, the approximations used do not clearly justify the result of the calculation. As a consequence, the result obtained is misleading, an opportunity to teach basic physics is lost, and many students are left confused. For example, how can the force act to pull the slab into the volume between the plates when the electric field there is perpendicular to this direction? If this is explained - the force is of course, due to the fringe field - an apparent paradox arises: How can the virtual-work calculation yield an answer when it is explicitly based on the assumption of a uniform electric field existing only in the region between the plates, and so does not include the fringe field at all?

Not one of the many texts and monographs examined contains a complete discussion of this familiar example. Indeed, only four even indicate, however minimally, the role of the fringe field in this type of phenomenon.

So this is certainly something that was addressed more than 20 years ago.


[1] C.F. Bohren, Am. J. Phys. v.77, p.101 (2009).
[2] S. Margulies, Am. J. Phys. v.52, p.515 (1984).

1 comment:

Anonymous said...

Is Bohren's article on the web somewhere? It sounds like an ad
for this internet physics text: