GENERATING ELECTRIC CURRENTS

We saw that a change in the magnetic flux passing through a transformer's secondary coil causes current to flow in that coil. Since the magnetic flux through the secondary coil changes whenever the current through the primary coil changes, an alternating current in

the transformer's primary coil induces an alternating current in its secondary coil.

But there's another way to change the magnetic flux passing through a coil of wire: move the magnetic flux. That's how a generator works. Whenever a magnet moves past a coil of wire or a coil of wire moves past a magnet, the flux through the coil changes and current flows in the coil and its circuit.

Most generators use rotary motion to produce electricity. The generator has a permanent magnet that spins between two fixed coil of wire. As the magnet spins, its magnetic flux lines sweep through the two coils and drive a current through them. This current experiences a volt age rise as it passes through the coils and a volt age drop as it passes through the light bulb, so it transfers power from the generator to the bulb.

The iron core inside each coil extends the magnet's flux lines so that they are sure to sweep through the coil each time a pole of the magnet passes by. These cores are temporarily magne­tized by the nearby magnet and effectively in­crease its length. Without the iron cores, most of the rotating magnet's flux lines would bend around before passing through the entire coil and the generator would be less effective at producing electricity.

In the above diagram, you can see two magnets in the motor: The armature (or rotor) is an electromagnet, while the field magnet is a permanent magnet (the field magnet could be an electromagnet as well, but in most small motors it isn't in order to savepower). The principle design of the motor is identical to the generator. A generator produces an alternating current in the circuit it powers. This current flows in one direction as the magnet's north pole approaches a coil and in the opposite direction as the south pole approaches it. To generate the 60 Hz alternating current used in the United States, the generator must turn 60 times each second so that the current completes one full cycle of reversals every l/60th of a second. In Europe, the generator must turn 50 times each second to supply 50 Hz alternating current. The generators throughout the continent-wide power distribution networks all turn together in perfect synchronization. That way, power can be redirected within each network so that any generator can provide the power consumed by any user.

Some devices require direct current electric power. A car is a good example. It generates DC electric power to charge its battery and to run its headlights, ignition system, and other electric components. While this power is actually produced by an AC generator or alternator, the car uses special electronic switches to send current from alternator one way through its electric system. While the current in the alternator's reverses, the current through the car's electric system always travels in one direction.

Because large permanent magnets are extremely expensive, most industrial generators actually use iron-core electromagnets instead. These rotating electromagnets drive currents through generator coils just as effectively as permanent magnets would. Al- though these electromagnets consume some electric power, they are much more соя effective than real permanent magnets.

An alternating current (AC)is an electrical current whose magnitude and direction vary cyclically, as opposed to direct current, whose direction remains constant. The usual waveform of an AC power circuit is a sine wave, as this result in the most efficient

transmission of energy. However in certain applications different waveforms are used, such as triangular or square waves.

Used generically, AC refers to the form in which electricity is delivered to busi­nesses and residences. However, audio and radio signals carried on electrical wire are also examples of alternating current. In these applications, an important goal is often the recovery of information encoded (or modulated) onto the AC signal.