Electromagnetic induction

All these effects and uses are due to something called electromagnetic induction. This may sound rather complicated but all it means is a way of generating electricity by using moving wires, moving magnets or changing the voltages in one coil to make electrical energy in another.

The way of making electricity using magnets was discovered by Faraday in 1831 and this method is the basis of all dynamos used for making electricity today.

We can show this by two simple experiments.

In the first experiment we use a wire connected to a sensitive meter and passing between the poles of a strong magnet (Figure 1). The wire can be moved between the poles of the magnet.



The next experiment uses a coil of wire instead of just a straight piece (Figure 2). If the north end of the magnet is pushed into the 600 turn coil a current is produced, pushing in the south end makes a current in the opposite direction, as does pulling a north end out.











These results show that:


Numbers 3, 4 and 5 form the basis of Faraday's Law

If you work out the polarity of the coil as the magnet is pushed towards it you will always find that the induced current in the coil tries to prevent the motion.
For example, if you move a north pole towards a coil then that end of the coil becomes north, trying to push the magnet away.

This is called Lenz's Law.



Another way of inducing a current in a coil is shown in Figure 3. If the wire is connected to the battery, a current will flow in coil A. This will be like bringing up a magnet to coil B and so a current will flow in B. If the switch is held fixed to the battery the current will fall to zero, but if the wire is disconnected a current will flow in B in the opposite direction but will stop after the switch has been opened. Connecting quickly will give a larger current than if the wire is slowly pressed against the battery terminals.

NB – strictly speaking in all these experiments it is a voltage that is generated and this then gives a current in the meter if the circuit is complete.