Energy Considerations - Where does all the Ek go?

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Energy Considerations - Where does all the Ek go?

If you roll a metal bar into a magnetic field it will slow and stop due to the force opposing its motion (the force that Lenz says will be set up).

Where does all the kinetic energy of the bar go?

Well in fact, the Ek is transformed into electrical energy.

So this is the source of the emf, transferred from other energy into electrical energy.

Energy Considerations

Pull this magnet out of the coil. This produces a changing flux in the coil and therefore induces an emf and current in the coil (so long as the coil forms part of a circuit). The emf will oppose the direction of the moving magnet.


What direction is the induced current?


It opposes the movement of magnet. How? It sets up a magnetic field in coil with a North pole at the left hand end to attract the South pole of the magnet back into coil.

If the magnet moved back into the coil then the current would have to flow the other way, so that the coil would be repelling the South pole of the magnet.

Look at this - another common example...


If you spin this wheel, it slows and stops quite quickly.

Movement of the wheel through the magnetic field induces an emf and therefore a current in the metal wheel.

Lenz's Law says that the induced current opposes the motion of the wheel (because it's this motion which is setting up the current).

Which direction is the current?

To understand this example, think of one 'spoke' of the wheel as it passes through the magnetic field. The 'spoke' moves through the field just as if it were a piece of wire moving through a field. The 'spoke' moves into the screen, the field direction is from left to right, so using Fleming's Right Hand Rule, line up your thumb (motion) going into the screen, your first finger (field) going from left to right and your second finger (direction of conventional current) should be pointing downwards.

We've just used Fleming's Right Hand Rule to predict that a current flows from the axle to the rim of the wheel.