 A current I creates a magnetic field that exerts forces on other
 currents in the field, but not on the current I itself.

 On the other hand, if the current is through a coil, then it has
 magnetic effect on itself, through selfinduction.

 Isn't this strange?
No.
Actually, you can't isolate "a current" so simply. In a wire with current
flowing through it, there are a very large number of parallel paths through
which current is flowing. Each of these paths creates own field that both
adds to the total field as well as exerts force on the other currents. The
end result with a sufficiently large current would be a wire that squishes
itself thinner. This force would also be acting in an arc to confine the
arc to a finite width. Consider a large 500pair telephone cable used as
a single conductor by connecting all 1000 wires together at each end. Now
consider a single conductor wire with the same total crosssectional area.
Both can conduct the current. Both will have a magnetic field around them.
Both will have the selfinduction force to try to squish the cable or wire
to a smaller crosssection. Try a 10MA of current through these and see
the effect (that's a big 'M', not a little 'm').
If you have 1 amp going through a 100 turn coil you'll get a certain level
of magnetic field from that. Now run 100 separately connected wires going
around the coil just once and feed them in parallel with 100 amps. You'll
get the same field strength. Put just one big thick wire around it and
run the 100 amps through. Again, same magnetic field strength. This is
assumping the
_changes_ in the current are not taking place. If that is
happening, there can be slight or great differences depending on the rate
of change (frequency of AC). To understand this, think of a transformer
with two 120V 60A (7.2kVA) windings. They can be wired in parallel or in
series and you get the same effect if you have 60A going through. You just
have to apply 240V in the series case to get the 60A, and have to have 120A
available in the parallel case to get the 60A in each winding.
Think of a measure called "amp*turns".
If course physical geomtry of coils and windings can be subtle effects on
the total shape of the magnetic field and just how uniformly it all adds
up.

/
 Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below 
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