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Bar magnets, was magnetic circuits



I am often grateful to JohnD, and to others who know
physics better than I do, for sharing what they know.
Recent contributions on the subject of sparking switches,
and on the subject of magnetic circuits, are good examples.
What I am about to say is a not criticism, John; please
continue contributing and helping us to learn.

"John S. Denker" wrote:

Ludwik Kowalski [responding to Jack's question] wrote:

My high school textbook, I suppose. Thanks for a good hint.
Let me follow it? As one end of the bar magnet comes closer
to another it induces "poles of the opposite kind" at the surface.

It is similar to what happens in many dielectric materials. The
net pole becomes stronger at each bar magnet and the attractive
force between them changes more rapidly with the distance
than the 1/r^2 law [yes, for ideal monopoles] would suggest.
That is why magnets come toward each other suddenly.

That's not a good way to think about it.

Why did you say so? It is true that this kind of thinking does not
lead to a quantitative solution. But it is a good simple model for
qualitative reasoning. I suppose that an electrical engineer would
be able to calculate the force between the coaxial poles of two
bar magnets separated by a gap of any length. S/he would ask
for the value of B at the surface (when the other magnet is far
away), for example, 0.5 T, and for the permeability, for example
10,000 times mu_zero (assuming it remains constant).

But I do not know how to use such information in order to
predict, for example, that the attractive F is about 0.5 N when
the air gap is 3 mm. And even if I knew how to estimate the
force (using magnetic circuit theory) I would probably not
attempt to use that knowledge in an introductory physics
course.

Is it not true that the simple model I used is better than
nothing? I am often wrong and I appreciate being corrected.
But this time I do not thing I am wrong. The model of tiny
magnetic dipoles has been introduced more than a century
ago (by Weber ?) and it can still be used to make qualitative
predictions. What is wrong with this?

By the way, it is now clear to me that magnetic induction is
only one of two factors involved in the "rapid precipitation"
of two poles, as originally described. Even pure monopoles,
interacting with the 1/r^2 force would behave in that way.
Ludwik Kowalski