| 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 self-induction. | | 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 500-pair 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 cross-sectional area. Both can conduct the current. Both will have a magnetic field around them. Both will have the self-induction force to try to squish the cable or wire to a smaller cross-section. 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.