Generating with a rotating coil

1. A generator uses electromagnetic induction to produce a potential difference from rotation.
2. In a simple generator a coil of wire is turned between the poles of a magnet, so its sides move across the magnetic field lines.
3. The magnetic field through the coil keeps changing as it turns, so a potential difference is induced across the ends of the coil.
4. The induced potential difference is largest when the plane of the coil is parallel to the field lines, because the sides are then moving straight across the lines and cutting them at the fastest rate.
5. The induced potential difference is zero when the plane of the coil is perpendicular to the field lines, because for that instant the sides are moving along the lines and not cutting them.
6. After every half turn, each side of the coil has moved to the other pole and is crossing the field in the opposite direction.
7. The potential difference induced in the coil therefore reverses every half turn, so the rotating coil always produces an alternating potential difference.
8. What reaches the external circuit depends on how the ends of the coil are connected to it, and this connection is the difference between an alternator and a dynamo.

The alternator

1. An alternator connects each end of the coil to its own slip ring, a complete metal ring that turns with the axle.
2. Fixed carbon brushes press against the rings, so each end of the coil stays connected to the same side of the external circuit throughout the rotation.
3. The reversal of potential difference in the coil every half turn is passed straight on, so the output is an alternating potential difference that drives an alternating current.
4. A graph of the output against time is a smooth wave that rises to a positive peak, falls through zero to a negative peak of the same size, and rises back to zero.
5. One complete cycle of this wave is produced by one complete rotation of the coil.
6. Many alternators keep the coil still and spin a magnet or electromagnet inside it instead, which produces the same changing field without the output passing through moving contacts.
7. Power stations use very large alternators of this kind to generate the mains supply, which in the UK alternates at $50Hz$.

The dynamo

1. A dynamo connects the ends of the coil to a split-ring commutator, a single ring cut into two halves that turns with the axle.
2. The brushes slip from one half of the ring to the other at the moment the coil is perpendicular to the field, when the induced potential difference is zero.
3. This is exactly the moment the potential difference in the coil reverses, so the connections to the external circuit are reversed at the same instant.
4. The two reversals cancel out, so the output always acts in the same direction and drives a direct current.
5. A graph of the output against time is a row of humps that all lie on the positive side of the time axis, with two humps for every rotation of the coil.
6. The current from a dynamo flows in one direction but is not steady, because its size still rises and falls as the coil turns.
7. A dynamo has the same construction as a simple d.c. motor, but it is turned by an outside force instead of being driven by a current.

Changing the output

1. Turning the coil faster makes the field through it change more quickly, so the peak potential difference increases.
2. Turning it faster also means each rotation takes less time, so the cycles are closer together on the graph and the frequency of an alternator's output increases.
3. A stronger magnet or a coil with more turns increases the peak potential difference without changing the frequency.
4. When the generator supplies a current, the current in the coil produces a magnetic field that opposes the rotation, so the coil becomes harder to turn.
5. This is why a hand-turned generator becomes noticeably stiffer when a lamp is connected and lit, because more work has to be done to keep it turning.