How Electricity is Produced

8/4/2019 How Electricity is Produced

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How Electricity Is Produced

Static Electricity

The most familiar way of producing static electricity is by rubbing, or friction. Rubbing together

two different kinds materials that are insulators can transfer electrons from one substance to

another. The substance that gains electrons acquires a negative charge, and the one that loses

them acquires a positive charge. For example, rubbing a balloon against dry hair produces anopposite electric charge in the balloon and the hair (which will be drawn to the balloon). Similarly,

shuffling over a carpet in dry weather will produce a sufficient electrostatic charge on a person's

body to give a slight shock when the person touches a conductor.

Objects can also acquire an electric charge through a process called electrostatic induction. In

the illustration Electrostatic Induction, a charged object (the negatively charged rod) is brought

near an electrically insulated metal sphere, but not into contact with it. The excess electrons in

the rod will repel the electrons from the part of the sphere nearest the rod to the part farthest from

the rod. If electrons are allowed to escape from the sphere through an electrical connection to the

ground, the sphere will be left with a net positive charge.

Current Electricityis produced by creating a difference in electric potential between two points connected by a

conductor. A potential difference exists between two points when one has more electrons than

the other. The point with excess electrons is called the negative terminal; the other, the positive

terminal. The potential difference between the two terminals creates an electrical pressure called

electromotive force (emf), or voltage.

The two most common ways of creating a voltage to produce current are chemically (using

batteries) and by electromagnetic induction (using generators). A voltage can also be created by

heat, light, or mechanical pressure.

In the chemical method, explained in the article Battery, Electric, complementary chemical

reactions cause one terminal to gain electrons, making it negative, and cause the other to lose

electrons, making it positive.

The electromagnetic induction method for producing an electric current involves the use of a

permanent magnet or an electromagnet. When a wire is moved through a magnetic field, the

electrons in the wire are displaced and move toward one end of the wire. This action makes one

end negative, the other positive.

Under certain conditions, heat will cause electrons to flow between two different materials. One

device for producing this effect is the thermocouple, which is used as a measuring instrument and

as a control device.

Light falling on certain metals and semiconductors will release electrons to produce an electriccurrent. This effect is used in certain kinds of batteries.

Some crystals, including quartz crystals cut into certain geometric shapes, will produce a voltage

when squeezed and will vibrate when subjected to a voltage. This phenomenon, called the

piezoelectric effect is used in a variety of electrical devices, including microphones, radio

transmitters, and electronic watches.


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How Electricity Works For Us

One of the most useful properties of electricity is its ability to produce heat. Electricity produces

heat in a conductor as it overcomes the conductor's resistance to the flow of electrons through it,

just as mechanical energy produces heat in overcoming friction. The heat-producing effect of

electricity is used in electric ranges, toasters, soldering irons, and many other devices. In

incandescent lightbulbs, the effect is used to make a filament glow brightly.

Another very useful property of electricity is that it can be a source of magnetism; electrons

flowing through a wire create a magnetic field around the wire. This effect is the basis for the

operation of electromagnets, which make possible electric motors, telephones, loudspeakers, and

many other devices.

Electricity has several other useful properties. It can be made to jump a narrow gap separating

two conductors, creating a spark. An important application of such sparks is in igniting the fuel in

the cylinders of gasoline engines. Electricity can also produce chemical changes.

In a gas or vacuum, electrons can be accelerated to high speed and directed in specific

directions. Electrons used in this way make possible many kinds of devices, including televisionpicture tubes, fluorescent lights, and X-ray tubes. In certain semiconductor devices, such as

transistors, the movement of electrons can be readily controlled by electrical means. Such

devices are used in computers, radios, tape recorders, and many other products.

Units of Electricity

Five common units used in working with electricity and electric circuits are the volt, ampere, watt,

ohm, and hertz.

The Volt (V)

is a unit for measuring both electric potential difference and electromotive force. The voltage

supplied by most automobile storage batteries is 12 volts. In many countries, including most of

those of Europe, electricity is supplied to homes at 220 or 240 volts. In the United States and

Canada, homes are typically supplied with electricity at around 120 volts for ordinary use and 240

volts for such appliances as electric ranges and electric water heaters. High-tension power lines

have voltages of hundreds of thousands of volts. These lines are used for the transmission of

electric energy over long distances.

The Ampere (A)

is a unit for measuring electric currentthe flow of electric charges. The ampere (or amp) is a

base unit of the SI (metric system) and is defined in terms of the magnetic force produced

between two parallel wires carrying an electric current. Houses are typically wired to provide a

total of 60 or more amperes. The amount of electric charge transferred by a current of oneampere in one second is one coulomb.

The Watt (W)

Electric powerthat is, the rate at which electric energy is used to perform workis measured in

watts. Lightbulbs and appliances are usually marked with their wattage, indicating the rate at

which they consume energy. The normal 120-volt, 15-ampere household circuit can safely handle

electrical devices drawing a total of 1,800 watts (1.8 kilowatts). The mechanical power an electric

motor can provide is usually given in watts or horsepower. (One horsepower equals 746 watts.)


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Consumption of electricity is measured in terms of the kilowatt-hourthe work done by 1,000

watts in one hour.

The relationship of voltage (in volts) to current (in amperes) and resistance (in ohms) is

expressed by Ohm's Law:

Voltage = current X resistance

The Ohm (W)

is a unit for measuring the resistance a material has to the flow of an electric current.

The relationship between power (in watts), voltage (in volts), and current (in amperes) is:

Power = voltage X current

The Hertz (Hz)

is a measure of the rate at which an alternating current reverses direction. Each two consecutive

reversals in an alternating current are called a cycle. Commercially generated alternating current

in the United States has a frequency of 60 hertz (cycles per second). In many countries, including

European countries, alternating current has a frequency of 50 hertz.

Electric Circuits

An electric circuit consists of the various conductors that lead from the negative to the positive

terminal of a source of electricity. The various parts of a typical house circuit include a fuse or

circuit breaker, wires, switches, wall outlets, and light sockets and bulbs.

A circuit through which electricity is flowing is said to be closed. The circuit can be opened, or

broken, by turning off a switch or by removing a fuse, pulling out a plug, or disconnecting the

wires. A circuit generally contains a load, a device such as a lightbulb or appliance that provides

resistance in the circuit. If a current is allowed to flow from one terminal to another with very little

resistance, a short circuit exists. Unless such a current is quickly stopped by a fuse or circuit

breaker, the wires may heat up enough to start a fire.

There are two basic methods of wiring a circuitin series and in parallel. In the series circuit the

current flows through one device (such as a lightbulb) to reach the next. In the parallel circuit the

current enters and leaves each device separately. Devices connected in series each carry the

same amount of current; devices connected in parallel are each subjected to the same voltage.

Many electrical applications use a combination of these two types of circuits.

History

Static electricity was observed by Thales, a Greek philosopher who lived in the sixth century B. C.He thought static electricity was a form of magnetism, and it was not until 1600 that electricity was

recognized as something different from magnetism. This discovery was made by William Gilbert,

an English physician. He called the force involved electric,; taken from elektron, the Greek word

for amber. (Static electricity was first observed in amber.)

In the mid-1700's, E. Georg von Kleist of Prussia and Pieter van Musschenbroek of Holland,

working independently, invented a device for storing electricity. In 1780 Luigi Galvani, an Italian


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physiologist, found that a severed frog's leg hung by a copper hook would twitch when touched

with iron. Galvani had discovered current electricity (which for many years was called galvanism).

Another Italian, Alessandro Volta, used Galvani's discovery when he invented the first electric

battery in 1800. His studies led to the discovery of electrolysis, the decomposition of a substance

with an electric current.

In his kite experiment of 1752 Benjamin Franklin proved that lightning is an electrical discharge.

He suggested, incorrectly, that electricity is a kind of fluid and that it flows from a point where

there is a surplus, or positive amount, to a point where there is a deficiency, or negative amount.

This theory, called the conventional (or fluid) theory of current, persisted even after the

development of the electron theory early in the 20th century. Today, the direction of a current is

still considered to be from the positive terminal to the negative, even though electrons flow in the

opposite direction.

In 1820, Hans Christian Oersted, a Danish physicist, discovered that an electric current can

produce magnetic effects. In 1831 Michael Faraday, an English physicist and chemist, discovered

the complementary phenomenon: that a magnet can produce electrical effects. (Joseph Henry, a

United States physicist, observed the phenomenon in 1830 but did not publish his findings untilafter Faraday.) Faraday used this discoveryelectromagnetic inductionto invent an

experimental electric generator. He also formulated the laws of electrolysis and developed the

first transformer.

Between 1870 and 1880, Sir William Crookes, an English scientist, experimenting with vacuum

tubes, concluded that the rays produced in these tubes were composed of particles smaller than

atoms. J. J. Thomson, an English physicist, demonstrated the existence of these particles (later

called electrons) in 1897. Thomson's work led to the theory of electric current being the flow of

electrons.

As the basic principles of electricity were discovered and electrical theories were developed,

inventors began putting them to use. Among the electrical inventions of the late 19th century are

Samuel F. B. Morse's telegraph; Alexander Graham Bell's telephone; Thomas A. Edison's electric

light; Guglielmo Marconi's wireless telegraph; and electric generators and motors.

By the 1920's, scientists studying the electrical structure of the atom had developed a complex

mathematical description of its nature. On a more practical level, the development during the

early 20th century of such electrical appliances as electric refrigerators and washing machines

helped raise the standard of living in many countries.

The understanding of the behavior of electrons in a vacuum permitted the development of

electronics in the first half of the 20th century. Following the invention of the transistor in 1948,

the use of electronic devices based on semiconductors came to dominate the field of electronics.

The miniaturization of electronic devices led to the development of a large number of new

electrical devices, such as the personal computer, and to the widespread use of electronic

controls.


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Transformer

Transformer, a device that transfers electric energy from one circuit to another, usually with a

change in voltage. Transformers work only with a varying electric current, such as alternating

current (AC). Transformers are important in the distribution of electric power. They raise thevoltage of the electricity generated at a power plant to the high levels needed to transmit the

electricity efficiently. Other transformers reduce the voltage at the locations where the electricity is

used. Many household devices contain transformers to raise or lower house-current voltage as

needed. Television sets and stereo equipment, for example, require high voltages; doorbells and

thermostats, low voltages.

How A Transformer Works

A simple transformer consists essentially of two coils of insulated wire. In most transformers, the

wires are wound around an iron-containing structure called the core. One coil, called the primary,

is connected to a source of alternating current that produces a constantly varying magnetic field

around the coil. The varying magnetic field, in turn, produces an alternating current in the other

coil. This coil, called the secondary, is connected to a separate electric circuit.

The ratio of the number of turns in the primary coil to the number of turns in the secondary coil

the turns ratiodetermines the ratio of the voltages in the two coils. For example, if there is one

turn in the primary and ten turns in the secondary coil, the voltage in the secondary coil will be 10

times that in the primary. Such a transformer is called a step-up transformer. If there are ten turns

in the primary coil and one turn in the secondary the voltage in the secondary will be one-tenth

that in the primary. This kind of transformer is called a step-down transformer. The ratio of the

electric current strength, or amperage, in the two coils is in inverse proportion to the ratio of the

voltages; thus the electrical power (voltage multiplied by amperage) is the same in both coils.

The impedance (resistance to the flow of an alternating current) of the primary coil depends onthe impedance of the secondary circuit and the turns ratio. With the proper turns ratio, the

transformer can, in effect, match the impedances of the two circuits. Matched impedances are

important in stereo systems and other electronic systems because they permit the maximum

amount of electric power to be delivered from one component to another.

In an autotransformer, there is only one coil and both circuits are connected to it. They are

connected at different points, so that one circuit contains a larger portion of the coil (that is, has

more turns) than the other.

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How Electricity is Produced