Heinrich LenzHeinrich Lenz formulated the following law:

An induced current has a magnetic field which opposes the external magnetic field that induced the current.

Basically, what this means is that if you move magnetic lines of force across a conductor, you induce an electrical current in that conductor. This is how the alternator works. The rotor provides the magnetic field (by means of an electromagnet), and this is then spun in close proximity to a conductor (the stator).

What Lenz's law tells us is that the induced current also has an associated magnetic field. This magnetic field opposses the magnetic field that induced the current in the first place.

You can observe this for yourself. Sit in a vehicle with the engine ticking over, but no major electrical loads switched on. Then switch on a big load like the headlamps. You will notice that the idle speed of the engine will drop slightly as the alternator comes under this load. The voltage regulator senses the voltage drop in the system caused by the load, and so energises the rotor, thus causing the alternator to generate electricity to power the headlamps. This induced electricity will also have its own associated magnetic field, which will oppose the magnetic field of the rotor. These two magnetic fields in opposition cause the idle speed of the engine to drop.

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