LENZ'S LAW

Heinrich Emil Lenz put a definite direction to induced currents when the field was changing ( strictly when "Flux" is changing ). Faraday did not do this - his writings are rather confused on the matter. ( Yah can't be perfect all the time - remember - meters in the modern sense had not been invented!)

In modernish terms:

"The Induced current is such as to OPPOSE the CHANGE in applied field."

This is most simply demonstrated in the animation.

• As the magnet approaches the loop, the applied B field in the centre increases. This is a change.

• An Induced Field is created which attempts to negate the applied field - ie to keep the total field at zero - its original value.

• This Induced field must be associated with a current - the INDUCED CURRENT in the loop whose direction is determined by the first version of the RH rules - the one for predicting B and current - NOT the force one. Of course, the Induced EMF direction is clearly predicted by thinking of conventional currents flowing from positive to negative.

Lenz's Law is all about conservation of energy. It guarantees that induced currents get their energy from the effect creating the change. The force acting against the conductor being moved earlier is actually an invocation of Lenz's Law. ( As the conductor moves down, the flux increases, so the induced field opposes this which leads to the direction of the Induced current - which in turn shows the direction of the force back on the current.)

Eddy Currents and other nasties.

If we have a changing B field cutting a conductor, then as soon as the B field changes, we will get an Induced current which has a direction decided by Lenz's Law.

This current may or may not be a bonus. When it isn't we do our damnedest to reduce the size by laminating the conductor or some such. When it is great we literally may cook with it. (Some types of stoves are induction stoves. They are cold to touch - unless you wear a conducting ring, but changing B Fields above induce currents in the base of metal pots placed on top which then resistively heat.)

MAGNETIC DAMPING

Of all the demonstrations, this one best demonstrates how Lenz's Law is merely a reinterpretation of F=qvB.

The metal plate advances into the B field as it swings. That means it is the same as a rod moving in a B field - the RHrule and F=qvB apply.

So - charge separation occurs to the top and bottom of the plate. An EMF is induced. But the rest of the plate is a conductor - so currents are set up and as we know, a force will be applied OPPOSITE to the motion slowing the plate.

Alternatively - By Lenz's Law,as the advancing plate notices an increase in B_applied, an induced B field opposes it- with an associated eddy current. As the plate departs, that part notices the decreasing B_applied, an induced field reinforces the applied field - with its induced current.

Energy is removed from the swing as a result of Ohmic heating of the plate.

METAL DETECTOR

This is almost a transformer. An AC current is sent through circular coil ( field coil) which is down near the ground. If a metal is underneath the coil, an eddy current will be set up in it. This will have a B field which in turn will affect the AC current in the field coil. The change is detected electronically - bingo - Instant riches. A $1 coin.

EDDY CURRENT TESTING FOR CRACKS IN STRUCTURES.

This is a standard technique of non destructive testing of aircraft structures and other structures for hairline cracks. Induce eddy currents in the metal being tested. If the eddy current value suddenly drops, a crack is probably present stopping the current.