Electric generators and motors
Electricity produces magnetism
Electricity and magnetism are so closely linked that one can produce the other. An electric current flowing in a cable produces a magnetic field around the cable. A magnetic field moving near a wire causes electricity to flow along the wire. Electromagnetism is the basis of how both generators and electric motors work.
Ørsted demonstrates his discovery
In 1820, the Danish scientist Hans Christian Ørsted (1777–1851) observed that when he passed an electric current through a wire, a magnetic compass needle nearby was slightly deflected. He realised that the electric current had produced magnetism: he had discovered electromagnetism.
How an electric bell works
If the wire is wound around a core of iron to form a long coil, or solenoid, and an electric current passed through it, the strength of the magnetism increases. When current is flowing, the core of this electromagnet is magnetized and both the core and wire produce a magnetic field. Switch off the electricity and the field disappears, because the core loses its magnetization and the wire no longer carries a current. Electromagnets have many uses, including magnetic cranes to lift heavy pieces of metal or, on a smaller scale, inside electric doorbells.
Using movement to produce electricity
An illustration of Faraday's experiment
After Ørsted discovered that electricity could produce magnetism, the English scientist Michael Faraday (1791–1867) investigated whether the reverse effect—that magnetism could produce electricity—was possible. In an experiment he carried out in 1831, he found that an electric current was produced in a coil of wire when a magnet was moved in and out of it. The faster the magnet moved, the greater the current produced.
Generators
A simple a.c. generator or alternator
This principle established a way of generating electricity: a means of converting motion, or kinetic energy, into electrical energy. The same effect that Faraday had discovered could be achieved by moving a wire while keeping the magnet stationary. Inside a simple generator, a coil of wire is made to spin between the poles of a magnet, which induces (creates) an electric current in the wire.
The first half of each turn of the coil produces a current flowing in one direction; the second half produces one flowing in the opposite direction. So the current varies, or alternates, from zero to a maximum and back again. This is known as an alternating current (a.c.). The number of times this cycle is completed in one second is called the frequency.
The first half of each turn of the coil produces a current flowing in one direction; the second half produces one flowing in the opposite direction. So the current varies, or alternates, from zero to a maximum and back again. This is known as an alternating current (a.c.). The number of times this cycle is completed in one second is called the frequency.
Power station generator
Cross-section through a generator in a power station
In a generator at a modern power station, the coil that produces the electric current, sometimes called the armature, is kept stationary: it is known as the stator. The changing magnetic field is produced by a rotating electromagnet: the rotor. The rotor is connected to a turbine, driven by a power source such as the steam from water heated by burning fuel or nuclear reactions, or simply the movement of water itself. A current flows when the generator is connected to a circuit.
A simple d.c. generator or dynamo
To produce a current flowing continuously in the same direction only—a direct current (d.c.)—the generator, known as a dynamo, is connected to the circuit through a device called a commutator. This consists of a contact ring, split so that the connection (via carbon brushes) to the circuit is such that the electric current generated flows in one direction only.
Using electricity to produce movement
Faraday's experiment
In an experiment of 1821, Faraday showed that when an electric current was passed through a wire it rotated around a stationary magnet. So, acting in the opposite way to a generator, in which movement produced electricity, electricity could produce movement. This is the basic principle of an electric motor.
Electric motor
How an electric motor worksAn electric motor is a device that turns electric current into a rotary (spinning) movement. When electricity flows through a coil of wire situated between the opposite poles of a magnet, the coil produces its own magnetic field. The two magnetic fields interact, and the attraction and repulsion that occurs causes the coil to spin. If the electrical supply is d.c., it must first pass through a commutator which makes the electric current in the coil regularly reverse and consequently keeps the motor turning in one direction only.
Inside an electric drill
Electric motors are clean and quiet, and can be small enough to run a wristwatch, or large enough to power a vehicle. Washing machines, food processors, vacuum cleaners, drills, hair dryers and many other appliances and devices are driven by electric motors.
Transformers
Transformers at an electricity sub-station
Most generators produce a.c., and are called alternators. Alternating current is more convenient than direct current, because it is easier to transmit than d.c. and can be changed to a higher or lower voltage by a device called a transformer, whereas d.c. cannot. High-voltage electricity travels across country from power stations (which house the generators) along cables suspended above ground by pylons.
A network of cables delivering electricity from power stations to users across a country is called the national grid. The high voltage (at 400,000 V) electricity is transmitted at needs to be reduced or "stepped down" for use in the home (230 V). A series of transformers, contained inside electricity sub-stations, do this job.
A network of cables delivering electricity from power stations to users across a country is called the national grid. The high voltage (at 400,000 V) electricity is transmitted at needs to be reduced or "stepped down" for use in the home (230 V). A series of transformers, contained inside electricity sub-stations, do this job.
How a transformer works
How a step-down transformer works
A simple transformer consists of an iron core with a coil wound around each "arm". An alternating electric current first passes through one coil, known as the primary coil. This produces an alternating magnetic field in the core, which in turn induces (creates) an alternating electric current in the other coil, called the secondary coil.
In a transformer designed to "step up" the voltage, the secondary coil has more turns than the primary. A “step-down” transformer, which decreases voltage, has fewer turns in the secondary coil than in the primary coil.
In a transformer designed to "step up" the voltage, the secondary coil has more turns than the primary. A “step-down” transformer, which decreases voltage, has fewer turns in the secondary coil than in the primary coil.
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