Polarity Change in AC Generator

These diagrams may not be the easiest for understanding the basic ac generator. It may be better not to show several turns in a helix spread out along a cylinder, but to show a single turn in one plane. There are several diagrams of this type on the web if you search for 'simple ac generator'.

1. The coil is rotating about an axis through the centre of the coil and at right angles to the screen of your computer. This is the axis of rotation. The axis of the coil is fixed relative to the coil, and so rotates as the coil rotates.

2. The emf is proportional to the rate of change of flux linked with the circuit. When the axis of the coil is lined up with the poles of the magnet the flux through the coil is a maximum in one direction. When the coil has turned through 90° from this position the flux linking the coil (going in one end and out the other] is zero. When the coil has turned another 90° the flux linkage is a maximum but in the opposite direction, and so on. Suggest you use these data to sketch a graph (smooth curve) of flux linkage against time. You will then see that the rate of change of flux linkage is zero when the flux linking the coil is greatest, and the rate of change is greatest when the flux linkage is zero.

In this diagram there is a rotating coil (like a solenoid) whose ends are connected to a slip ring commutator which allows the coil to rotate whilst maintaining electrical contact with the ends of the coil.
As the magnetic flux through the coil is changing and emf is induced (Faraday) and as there is a complete electrical circuit there is also an induced current passing through the coil.
The coil with the induced current passing through it acts as an electromagnet the poles s of which are labelled on the diagram. As shown the south pole of the electromagnet is approaching the south pole of the permanent magnet and the same is true of the north poles.
Like poles repel and so work has to be done to keep the coil rotating and that work manifests itself as electrical energy.
The like poles repelling each other is Lenz's law in action. as the induced current produces the south pole of the electromagnet to try and stop the coil rotating.
If a north pole had been produced then there would be a force of attraction between the north pole of the electromagnet and the south pole of the permanent magnet and no work would have to be done and yet electrical energy would still be produced which is not allowed according to the law of conservation of energy.

In this diagram the magnetic flux linked with the coil is not changing (no magnetic field lines are being cut) and so there is no induced emf and no induced current.

The magnet flux linked with the coil is changing so an emf is induced (Faraday) which produces an electric current and hence the coil becomes an electromagnet but this time there are forces of attraction between the poles of the electromagnet and the poles of the permanent magnet opposing the rotation of the coil.