If a magnet is moved into a coil of wire which is part of a complete circuit a voltage is induced across the ends of the wire - a current is produced (induced) in the wire. If the magnet is then moved out of the coil, or the other pole of the magnet is moved into the coil, the direction of the induced voltage (current) is reversed.

See here for an interactive Java application

• If the magnet is stationary with respect to the magnetic field no voltage is induced and therefore no current flows. If the wire 'cuts through' the lines of magnetic flux (crosses through field lines) a current is registered on a sensitive galvanometer (either a voltmeter or ammeter)
• The faster the magnet 'cuts the magnetic flux lines' the bigger the voltage and the bigger the current flow. As if you move the magnet faster you cut through more lines of magnetic magnetic flux in a given time and you therefore get a bigger induced current and voltage.
• The more turns of the wire that 'cut the magnetic flux lines' (possible if you wind the wire into a coil!) the bigger the voltage and current induced.
• If you use a stronger magnet the magnetic flux lines are closer together - therefore as you move the magnet it cuts through more lines in a given time and you get a bigger induced current and voltage.
• If the coil face has a bigger area the total flux intercepted by it will be bigger

The direction that the induced voltage (and therefore the current) is produced in ALWAYS opposes the field that produces it (so that you have to do work to change kinetic energy into electrical energy). This is called Lenz's Law - the induced voltage always opposes the change producing it.

Click here for an animation to illustrate this.

here is a vidclip that illustrates Lenz's law in action. A copper pipe is NOT magnetic but when a magnet is dropped through it it travels slower than a non-magnetic piece of metal. Why? Because an electric current is induced in the copper pipe that produces a field that opposes the field of the magnet - the repulsion therefore acts to oppose its movement and it falls slowly...

Electric Generator

Electricity can be generated by rotating a coil of wire in a magnetic field or by rotating a magnet inside a coil of wire. This is how a generator works.

If a wire, or coil of wire, cuts through a magnetic field, or vice-versa, a voltage (potential difference) is produced between the ends of the wire. This induced voltage causes a current to flow if the wire is part of a complete circuit. This is called the generator effect,

The size of the induced voltage increases when:

A changing magnetic field will also produce an induced voltage in a coil.

The direction of the induced current is reversed if either the direction of the movement or the direction of the magnetic field is reversed. It can be found using Fleming's Right Hand Dynamo Rule. The right hand rule predicts the direction of an induced current and RIGHT has an I in it - the symbol for current!

You should be able, when provided with a diagram, to explain how an a.c. generator works, including the purposes of the slip rings and brushes

Here are the links for interactive demonstrations of generators (AC generator DC generator)

Consider the example below:

Hold up your right hand with the fingers mutually at right angles.

The Field is going from N to S (make this your First finger)
The wire is being Moved upwards (make this your thuMb)
This results in the current flowing into the page (away from you) - indicated by the cross

Try these examples (mouseover for the solution!):

Try the electromagnetic induction wordsearch: click here

A metal detector works using electromagnetic induction. Click here to see how it works.

Transformers are used to change the voltage of an a.c. supply. At power stations, transformers are used to produce very high voltages before the electricity is transmitted to where it is needed through power lines (National Grid). Local transformers reduce the voltage to safer levels before the electricity is supplied to consumers. You should understand how a transformer works by electromagnetic induction, and know why they are used for power transmission (that the higher the voltage, the smaller the current needed to transmit energy at the same rate therefore less energy is wasted by heat loss to the atmosphere).

A transformer consists of two separate coils wound around an iron core. When an alternating voltage is applied across one coil (the primary) an alternating voltage is induced across the other coil by electromagnetic induction (secondary).

The voltages across the primary and secondary coils of a transformer are related as shown: