Re: magnetic north and south

[Hugh Logan <hlogan@ix.netcom.com>]

Zach Wolff wrote:
> I'll be the first to admit that my knowledge of em and is only at a
> high school level and I may be missing something far deeper.  Please
> correct anything in error here.  With that said, isn't this discussion
> of North and South somewhat ridiculous?

I thought the matter was settled in grade school when I was told that
the magnetic pole near the north geographic pole was really a south
magnetic pole, but I might not have thought of the possibility of a
reversal as determined with a conventionally labeled compass needle.

>  Aren't these names merely
> artifacts of geography?  It seems to me that with the failure of all
> efforts to identify a magnetic monopole switching the names on the
> poles would have no effect on the underlying laws.

I agree that the names of magnetic poles are a convention, just as the
the sign of electric charge is. I don't see what the discovery of a
magnetic monopole would have to do with it. It seems to me that it 
would just have to be given the appropriate polarity in accordance 
with your new convention. You would have to be careful about defining
the direction of the magnetic flux lines. For example, if you kept the
present conventions for charge and current direction, to map the
magnetic field about a straight conductor (Oersted's experiment),the
direction of the south pole of the revised compass needle used to map
the field would give the direction of the magnetic field, at least if
you wanted to keep the right hand rule. Magnetic lines would now leave
the south pole of a magnet and enter the north pole according to this
convention. Other conventions are possible. Years ago, most engineers
and physicists used conventional current (in the direction of the flow
of positive charge), but books for technicians used the opposite
convention (electron current). In the latter case the right hand rule
referred to above became a left hand rule. (I don't know if "electron
current" is still used in many books.)
> Like magnetic
> poles repel, opposite magnetic poles attract.  
O.K.
> Changing magnetic
> fields create current,

Changing magnetic fields are more directly related to the electric
field according to Faraday's law. The emf induced in a closed path
(circulation of the electric field) equals the negative time rate of
change of the flux of the magnetic field through the closed path. 
Of course, the emf can produce a current if the closed path is a
conductor. The minus sign in Faraday's law comes from Lenz's law. 
The emf must be such that the current it could produce is in such a
direction to oppose the change in the magnetic flux through the closed
path.  

> while changing current creates a magnetic
> field.  

Current produces a magnetic field whether it is changing or not.

Since Feynman's birthday is coming up, I might mention that the laws of
electromagnetism are summarized without too much mathematics in Sec. 1-4
of Volume 2 of _The Feynman Lectures on Physics_ by Feynman, Leighton,
and Sands. Faraday's law is stated more precisely on p. 17-2 (with the
minus sign).

Hugh Logan