Electromagnetic induction
The ignition and sparking system in many cars uses an induction coil
Transformers are used in thousands of different appliances
Electricity is generated in power stations using generators
Electricity is transmitted round the country at very high voltage
Electric guitars are ways of making music
Clockwork radios can pick up signals in remote places
Video tape recorders can store information
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.
A current is only produced in the wire if it is moved up or down in the field cutting through the lines of the magnetic field.
Side-to-side or end-to-end motion produces nothing and when the wire is held still in the field there is no reading.
If the wire is moved upwards the flow of current is in the opposite direction to that when the wire is moved downwards.
If the magnet is moved faster, a bigger current is produced.
Leaving the magnet still in the coil gives no current.
Using a coil with more turns, say 1200 gives a bigger current.
schoolphysics: Electromagnetic induction animation
To see an animation of the movement of a magnet in and out of a coil click on the animation link.
These results show that:
1. A current is produced when either the wire or magnet move (the wire must cut the magnetic field lines).
2. The faster the movement, the bigger the current.
3. Changing the direction of the movement changes the direction of the current.
4. The stronger the magnet, the bigger the current.
5. The more coils of wire, the bigger the current.
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.
Lenz's law: The induced voltage is always trying to prevent the change.
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.
