loops of wire

In the two images below, there are two loops of wire shown. As far as my working out goes, the two coils are non-superimposable.

In the top coil, the direction of the current flow is in the same direction as the magnetic field outside the coil but opposite to the direction of the magnetic filed within the coil i.e. it's going from the north pole to the south pole.

However, in the bottom coil, the direction of the current is opposite to the magnetic field outside the coil and in the same direction as the magnetic field within the coil i.e. it's flowing from the south pole to the north pole.

In a book that I have, they use the simple right hand grip rule for solenoids and the current is always seen to point from the south to the north pole, as I have shown in the bottom coil. However, this doesn't apply to the top coil. Someone please tell me where I'm going wrong and whether I'm raising a legitimate question rather than just silly confusion.

[Extra note:

  1. I used the right hand grip rule for a wire in order to work out the directions of the poles.

  2. This is NOT an electromagnetic induction question.

  3. The current flows in the clockwise direction at the south pole and in the anticlockwise direction at the north pole, in both coils, so the 'S and N rule' is not broken.]


1 Answer 1


The current direction along the coil depends on the sense of the winding (or thread), so it can be either way. (I can't really make sense of the perspective in your drawings.)

The right-hand rule for coils and their magnetic field implicitly assumes that the current flows on circles around the solenoid axis. In reality, these are of course very slightly bent and form a helix, and the helix can be left-threaded or right-threaded. This does not apreciably change the way the current flows in the angular direction (tangential to the solenoid), and thus the magnetic field. However, the net current direction is reversed.

So if the solenoid is, say, vertical, and magnetic north is up, then the current (direction chosen as for the right-hand rule) is flowing up for a right-winding coil, and down for a left-winding one (if I didn't confuse myself here).


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