Generator

A generator is a motor working in reverse: a motor changes electrical energy into mechanical energy, but a generator produces electrical energy from mechanical energy. Superficially the diagram of a generator appears identical to that of a motor. Each consists of a loop that can rotate in a magnetic field. In a motor, electric current is fed into the loop, resulting in rotation of the loop. In the generator, the loop is rotated, resulting in the production of electric current in the loop. For 180 degrees of the rotation, electron deflection produces an electric current in the loop that moves in one direction; for the next 180 degrees, the electron deflection is reversed. As the current leaves the loop to an external circuit, the current will be observed to move in one direction and then the other. This is called alternating current. For a generator to generate direct current it is necessary to use a split-ring commutator at the point where the generator feeds current to the external circuit. The current in the loop is still alternating, but it is direct in the external circuit.

Michael Faraday, the English scientist, and Joseph Henry of the United States independently showed in 1831 that moving a magnet through coils of wire would generate a current in the wire. If the magnet was plunged into the coil, current flowed one way. When the magnet was removed, the current direction was reversed. This phenomenon is called electromagnetic induction, and it is the principle underlying the operation of the generator. As long as the magnet and the coil move relative to each other, a potential difference is produced across the coil and current flows in the coil. A potential difference is also produced if the magnetic field through the coil grows stronger or weaker. The greater the rate at which the magnetic flux through the coil changes, the greater the potential difference produced. The key is that the magnetic field through the coil must be changing. In 1864 James Clerk Maxwell suggested: (1) If an electric field changes with time, a magnetic field is induced at right angles to the changing electric field. The greater the rate at which the electric field changes, the stronger the induced magnetic field. (2) If a magnetic field changes with time, an electric field is induced at right angles to the changing magnetic field. The greater the rate at which the magnetic field changes, the stronger the induced electric field. Maxwell calculated that these electric and magnetic fields would propagate each other and travel through space as time-varying fields. The speed of these electromagnetic waves is 3.0 x 108 (300,000,000) meters per second. That happens to be the same as the speed of light. In fact, visible light is merely a narrow range of frequencies in what is known as the electromagnetic spectrum. As people read a printed page, electromagnetic waves reflected from the page pass into their eyes. As the electric field of that wave reaches the eye's retina, electrons in molecules of the retina interact with the field, change position, and start the message to the brain that eventually allows a person to understand what has been read.

Lenz's law.

Whenever a changing magnetic field generates a current in a coil of wires, the current produced will generate a magnetic field. That induced magnetic field will always tend to oppose the original change in the magnetic flux through the coil. This rule was first suggested by Heinrich F.E. Lenz of Germany in 1834. The effects of the induced field can be observed during the operation of a hand-cranked generator. When the generator is cranked slowly, little current is produced and weak electromagnetic forces oppose the rotation. But as the cranking rate is increased and more current is produced, the forces on the rotating loop become stronger, and the loop is correspondingly more difficult to turn. Lenz's law also applies to motors, where a current-carrying wire moves in a magnetic field. That movement, in turn, produces a current in the wire that opposes the original direction of current in the wire. Because electric current cannot occur without a potential difference, this opposition effect is sometimes called a back-emf. When a motor is started, a large current flows at first, and, as the motor begins to turn rapidly, a large back-emf is induced and the net current in the motor drops. If a large load is suddenly added to the motor, slowing it drastically, the back-emf will drop, and the sudden rise in current may cause overheating and burn out the motor. Even a simple coil of wire in a DC circuit exhibits the effects of back-emf. As the current in the coil increases, the changing magnetic field produced around the coil will tend to produce a back-emf. This is called self-inductance. Normally the current in a circuit rises rapidly after the switch is closed. But in this circuit, the current rises relatively slowly. On the other hand, when the switch is opened, the current in the circuit normally falls to zero almost instantly. But as the magnetic field around the coil decreases, an emf is generated that tends to keep the current from decaying as rapidly. A coil like this is used in devices designed to prevent damage to electronic equipment caused by voltage spikes sudden increases in potential difference that would tend to produce rapidly changing currents.

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