Motors in Reverse


The opposite effect to the motor effect is that if a wire is moved in a magnetic field, a voltage (EMF) is produced across its ends, and if there is a complete loop, a current will flow in the loop.


 

In other words – Field + Force = Current

The direction of the current is given by Fleming’s RIGHT hand rule:

 

Moving a wire in a magnetic field: cutting magnetic field (or flux) lines

  • If the wire or magnet is moved faster, more flux lines are cut per second and a larger induced current will flow.
  • If the wire and magnet is stationary, no current will flow.
  • It doesn’t matter if the magnet or the wire are moved, it’s relative motion that matters.

Moving a magnet into and out of a coil of wire: changing the flux linking the coil.

This is a bicycle dynamo. The harder you pedalled the faster the magnet rotated inside the coil and the brighter the lights glowed on your bike.

To increase the current generated:

Spin the coil faster.

Wind more turns or loops on the coil

Use a stronger magnetic field.

Use a coil with a larger area.

Relative Motion

  • Pushing a magnet into a coil induces a current in one direction; pulling it out induces a current in the opposite direction.
  • The direction of  induced poles opposes the change which caused them – in other words if we push the N pole of the magnet into the coil the induced current sets up a N pole to repel the moving N pole being pushed into the coil and vice versa. (Lenz’s law)
  • If the magnetic poles are swapped around, the current will flow the other way.
  • If the magnet is stationary, or both are moving at the same speed with respect to each other, no current will flow.

So which way will the current flow in the doughnut-shaped bundle of wires below when we

push the N pole upwards?


An alternating current is generated by spinning a coil of wire in a magnetic field – the generator principle.

 

 

To increase the current generated:

  • Spin the coil faster.

  • Wind more turns or loops on the coil.

  • Use a stronger magnetic field.

  • Use a coil with a larger area.

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