sci.physics.electromag

On Mon, 10 Mar 2008, FrediFizzx wrote:

> "Timo Nieminen" wrote:
>
>> That said, "changing E causes B" and vice versa is a common just-so story
>> in introductory textbooks. I wonder why?
>
> It probably comes from the purely experimental situations of demonstration.
> Quoting Halliday and Resnick in "Physics"; "A bar magnet, thrust through a
> closed loop of wire, will set up a current in the loop." And similar for a
> current in the wire sets up a magnetic field near the wire. But note that
> they say "current" and not E field. Clearly, thrusting the magnet through the
> loop *caused* the current in the wire. But in other places they use the term
> "induces" instead of causes.

Which goes to show that it isn't necessary to talk about "E causes B" and
vice versa. Some books talk about E causes B, B causes E in the
quasi-static context of induced fields, others only as window-dressing
when talking about radiation.

Stationary magnet, moving conductor vs stationary conductor, moving
magnetc - the very foundation of special relativity, offers a deep view of
E vs B, yet gets very little time in textbooks. The joys of tradition!

Similarly, the usual teaching of special relativity suffers from some
traditional flaws. A common approach is to emphasise how different
relativistic mechanics is from classical mechanics, but there's almost no
difference. I think the similarities should be emphasised instead. Teach
classical mechanics in a single reference frame, then SR compared to
Galilean relativity. Students don't like to work hard on a topic for a few
(or many) weeks and then be told that it's all wrong.

Not that I know the best way to teach about induction etc. In introductory
courses, typical of the ones that the "E causes B" books are aimed at, the
students often don't have the math background to grok vector algebra well
enough. Still, I think the symmetry of moving conductor vs moving magnet
has some real value - I used to set this as an advanced question for an
introductory course.

--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html