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CIRCUITS

An electrical circuit provides pathways along which current can flow to do work. Current is driven by a power source, such as a BATTERY. This produces an electrical pressure, known as voltage, which pushes electrons along the wires. Engineers classify circuits into two types. In a series circuit, the same current flows through all the components (such as light bulbs) in the circuit. In a parallel circuit, the same voltage is applied to all the components.

ELECTRICITY IN THE HOME

Modern homes depend on electrical circuits. They carry power to the electric motors in toasters, refrigerators, DVD players, and many other machines. Electricity also supplies heat and light. Forcing current through something with RESISTANCE turns electrical energy into heat. The result may be a red glow that makes toast, or the brilliant white of a light bulb.

PROTECTING CIRCUITS

Fuses protect wires from too much current, which could make them hot enough to start a fire. A fuse is a thin wire in a fireproof casing. Too much current makes it melt, breaking the circuit safely. An earth wire protects people from electric shock if the metal casing of an electrical machine accidentally gets connected to the electricity supply. Instead of current flowing to earth through a person when they touch the machine (and possibly killing them), it flows harmlessly through the earth wire.

The simplest mains supply uses only two wires, so it needs only two-pin plugs and sockets.

Most mains supplies use three-pin plugs and sockets. They have an earth wire to make sure the metal parts do not cause electric shocks.

BATTERIES

A battery turns chemical energy into electrical energy. It consists of one or more cells. Each cell contains two electrodes (pieces of metal or another substance), and a chemical called the electrolyte that transports electrons between them. The electrodes are made of different materials, so one gets more electrons than the other. The excess electrons can flow around a circuit connected to a battery as an electric current. Different kinds of battery are used for different purposes. Some, such as torch batteries, can be used only once. Others, including nickel-cadmium (NiCad) batteries and car batteries, can be recharged and used again.

INSIDE A CAR BATTERY

A car battery can be used and recharged by the car’s alternator for years. It can also deliver the huge current needed to start the car. Its electrodes of lead and lead oxide are immersed in dilute sulphuric acid.

TORCH BATTERIES

Modern batteries have a steel case around a layer of manganese dioxide and a core of zinc powder. Both are coated in a strong alkali (the opposite of an acid) electrolyte. The manganese dioxide gives up electrons to the zinc. The electrons travel to the battery’s negative end through a collector. The current stops when the chemicals are used up.

BIOGRAPHY: ALESSANDRO VOLTA Italian, 1745-1827

In 1800, the scientist Volta made the first battery, a pile of silver and zinc discs separated by salt-soaked card. His friend Luigi Galvani had noticed that a frog’s leg twitched when in contact with two different metals. Galvani thought it was the frog that produced this electrical effect. Volta showed it was the metals.

RESISTANCE

Electrons moving along a wire bump into lots of atoms, which slow the electrons down and make them lose energy. This effect is called resistance. It limits the current that can flow when a particular voltage is applied. The energy lost by the electrons makes the wire hotter – hot enough, perhaps, to light a room.

CIRCUIT TERMS AND SYMBOLS

Voltage

The electrical pressure that drives current through a circuit. It is measured between two points, one of which is often the surface of the Earth. Unit: volt. Symbol: V. A single alkaline battery gives a voltage of 1.4 V.

Current

The flow of electrical charge through a circuit. It is measured as the charge per second passing one point. Unit: ampère. Symbol: A. Starting a car can draw a current of 200 A.

Resistance

The property of a circuit that opposes the flow of current. It is measured as voltage divided by current. Unit: ohm. Symbol: Ω. An ordinary torch bulb has a resistance of about 8Ω.

Power

The rate at which energy is consumed or released by a circuit. It is measured as voltage times current. Unit: watt. Symbol: W. An electric train uses about 3,000,000 W or 3 MW (megawatts).

Ohm’s law

(First stated by German physicist Georg Ohm.) The current through a circuit is given by the voltage across it divided by its resistance. A triangle helps to show the three relationships: V = I x R, I = V ÷ R, and R = V ÷ I, where V = voltage, I = current, and R = resistance.

LIGHT WORK

Inside a light bulb is a filament – a length of very thin wire coiled up twice so it looks shorter and thicker than it really is. The filament is made from tungsten, a metal that withstands high temperatures. A 60W bulb filament has a resistance of 88Ω, allowing a current of 0.26A to flow when connected to a 230V supply.

RESISTANCE AT WORK

A current is only forced to do some work when it encounters resistance. In the end, it just generates heat, but on the way it may do something more interesting, like produce music. The 33Ω resistor here would draw a current similar to that of a personal radio.

TORCH BATTERIES

Modern batteries have a steel case around a layer of manganese dioxide and a core of zinc powder. Both are coated in a strong alkali (the opposite of an acid) electrolyte. The manganese dioxide gives up electrons to the zinc. The electrons travel to the battery’s negative end through a collector. The current stops when the chemicals are used up.

Copyright © 2007 Dorling Kindersley

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