Learn Electronics/Alternating Current

A (coil of) wire and a magnet, generating AC

Magnets attract iron and a few other metals, but here we are concerned with generating electricity. A simple experiment with iron filings and a magnet shows that there is such a thing as a magnetic field; that field is strongest (the force of attraction is highest) close to the poles of the magnet, and gets weaker further away from the poles.

A wire is usually made of copper, and copper is neither attracted nor repelled by a magnet, however if a wire, usually of copper, is inside a CHANGING magnetic field, or if the wire itself is moved to a place where the magnetic field is of DIFFERENT strength, then a voltage is generated between the two ends of that wire. If there are two wires that are connected in series, then that voltage will be twice as much, and if it is a coil, in that CHANGING magnetic field, with n number of turns, then the voltage will be n times as large. If a voltmeter is connected to the terminals of that coil, and the coil is moved, or the magnet is moved, then the voltage will be shown on the voltmeter. That voltage's direction will keep changing, it will ALTERNATE, as the changes are made, therefore that voltage is an Alternating Voltage, and if there is a circuit completed between the two terminals of the coil, then an Alternating Current (AC) will flow in that circuit.

• Usually one or more coils of wire are fixed on the outside of a circle, and an electromagnet is rotated inside, causing an alternating voltage and/or alternating current (AC) to be generated.

1 Think of a simple magnet that rotates; what happens? Starting with the magnet in a position so that the coil is half-way between the poles of the magnet, the magnetic field strength is zero at the coil, and therefore the voltage across the terminals of the coil is zero.

2. As the magnet rotates a little bit further, one of the poles gets closer to the coil, the fieldstrength changes, and therefore a voltage appears across the coil's terminals; the polarity of that voltage depends on how the voltmeter is connected; reversing the leads will reverse the indicated polarity of the voltage. Also the polarity depends on whether the North-pole or the South-pole is closest to the coil. Because the magnet rotates, its poles ALTERNATE getting closer to the coil, and therefore the indicated voltage ALTERNATES as well.

• Normally the resulting AC voltage is connected to a transformer in order to increase the voltage to a suitable level for transmission over great distances, but in addition there are sensors and switches designed to switch off if the current is too high, and also to ensure the rotation of the magnet or equivalent is then stopped - or else the speed of rotation would increase, causing considerable destruction, even, maybe, loss of life.