Lenz's Law

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Lenz's Law

This is a 'stroppy' law. Basically, it rebels against everything! Lenz's Law can be described as:

The emf is induced in a direction which opposes whatever causes the induction.

Example:

Lenz's Law

The thing causing the emf to be induced is movement of the bar to the left. So the emf induced will oppose this movement. Stop the bar and emf disappears.

How could you oppose this movement?

With a force to the right, of course!

What could produce the force?

A current flowing in a conductor in a magnetic field will produce a force. So, all we need is a current in the bar in the correct direction.

Here's where Fleming comes to the rescue. Fleming's Right Hand Rule allows you to work out the direction of the current induced. (Very similar to his Left Hand Rule, but this time we are generating a current not a force.)

As with left hand rule:

Thumb = direction of movement or Motion

First finger = Field direction

Second finger = induced conventional current

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In the case of his Right Hand Rule, the second finger is the important one. It shows the direction of the induced current (conventional current) if you are moving in the thumb direction through a magnetic field in the first finger direction.

You have to remember that:

The Left Hand Rule is for the motor effect (it tells you what force will be produced on a current carrying conductor in a field) - remember motorcars produce forces and drive on the left!

The Right Hand Rule is for the generator effect (it tells you in what direction a current is induced in a conductor moved through a magnetic field).

Apply the Right Hand Rule to the diagram...

Lenz's Law

Did you get the correct direction for current?

Now use the Left Hand Rule to check that this current would produce the force we expected (for instance, a force to the right that opposes the movement to the left). It does!

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