Re: Magnetic N and S poles

[Brian McInnes <bmcinnes@PNC.COM.AU>]

on 16/12/01 6:14 AM, John S. Denker at jsd@MONMOUTH.COM wrote:


> I would be surprised and disappointed if _any_ textbook,
> introductory or otherwise, pre-1945 or otherwise, defined
> B in terms of the Lorentz force law.

John, you may well be surprised and disappointed but the fact is, as quoted
in my e-mail earlier this week that Harnwell did in fac,t in his "Principles
of Electricity and Electromagnetism", write
" ..it is convenient to introduce a vector B, known as the magnetic
induction, which determines the force on a current element. The element of
induction is defined by the equation
 dF(sub1)  = i(sub1) cross dB(sub2) ".

The fact that you wrote, after you had my e-mail, that you "would be
surprised" indicates that you did not read it; if you had you would have
written, "I am surprised and disappointed"

Now, John, be prepared to be surprised and disappointed again.  The text,
Physics by Resnick and Halliday, used by so many instructors and read by so
many students over the last 40 years, devotes a whole Section (33-2, in the
1978 edition) to "The Definition of B".  That definition is "if we fire a
positive test charge q  with velocity v through a point P and if a force F
acts on the moving charge, a magnetic field B is present at point P, where B
is the vector that satisfies the relation F = q v cross B."  (The
definition, naturally employs bold notation for the vector quantities.)

Many, many "standard" texts go down a similar path.

It seems to me that you would be sympathetic to what Arnold Arons wrote, in
"Teaching Introductory Physics' on the subject of definitions:
"Very few students have ever encountered the idea of careful operational
definition: to most of them "defining" a term means seeking out a synonym
or memorising a short pat phrase.  They are initially very resistant to the
idea of telling the entire story, describing every action that goes into the
creation of a physical concept."

Brian McInnes