Question

How does the Lenz law work ?

Answer 1

Lenz's law describes how the Law of Conservation of Energy is upheld during the generation of an emf (voltage) and electric current using magnetic fields.

Generating electrical energy by electromagnetic induction requires work to be done so that energy can be transferred into electrical form - and this work has to be done against opposing magnetic forces to achieve this. It is no good doing the work against frictional forces or gravity - that doesn't transfer energy into electrical form. Likewise, gravitational energy is only 'created' by doing work against the opposition force of gravity and elastic potential energy is only 'created' by doing work against the opposing elastic forces between particles.
Lenz's law makes this point - that any generated emf ('voltage') and induced current will flow so that it's own magnetic effect will always provides an opposite force against the actions that are producing it .. like bringing repelling magnets together or moving attracting magnets apart.

In a generator the induced current in the windings creates it's own electromagnetic field - and these fields oppose any 'permanent' field within the generator (required to produce the induced emf in the first place).
Suppose the generated currents own field were to act in 'cooperation' with the generator's own field - his would increase the net field and improve the output from the generator .. from which a larger induced emf/current would result.. which would, in turn, increase the field even more .. giving an even greater induced emf/current ... and so on. This progressively increasing emf/current would give an ever increasing energy output that comes for no additional input energy .. it contravenes the Law of Conservation of Energy. In fact the generators mechanical energy input is ONLY transferred into electrical energy BECAUSE there is an electromagnetic opposition to do work against.

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