Electricity
| I | INTRODUCTION |
Electricity, one of the basic forms of energy.
Electricity is associated with electric charge, a property of certain elementary
particles such as electrons and protons, two of the basic particles that make up
the atoms of all ordinary matter. Electric charges can be stationary, as in
static electricity, or moving, as in an electric current.
Electrical activity takes place constantly
everywhere in the universe. Electrical forces hold molecules together. The
nervous systems of animals work by means of weak electric signals transmitted
between neurons (nerve cells). Electricity is generated, transmitted, and
converted into heat, light, motion, and other forms of energy through natural
processes, as well as by devices built by people.
Electricity is an extremely versatile form of
energy. It can be generated in many ways and from many different sources. It can
be sent almost instantaneously over long distances. Electricity can also be
converted efficiently into other forms of energy, and it can be stored. Because
of this versatility, electricity plays a part in nearly every aspect of modern
technology. Electricity provides light, heat, and mechanical power. It makes
telephones, computers, televisions, and countless other necessities and luxuries
possible.
| II | ELECTRIC CHARGE |
Electricity consists of charges carried by
electrons, protons, and other particles. Electric charge comes in two forms:
positive and negative. Electrons and protons both carry exactly the same amount
of electric charge, but the positive charge of the proton is exactly opposite
the negative charge of the electron. If an object has more protons than
electrons, it is said to be positively charged; if it has more electrons than
protons, it is said to be negatively charged. If an object contains as many
protons as electrons, the charges will cancel each other and the object is said
to be uncharged, or electrically neutral.
Electricity occurs in two forms: static
electricity and electric current. Static electricity consists of electric
charges that stay in one place. An electric current is a flow of electric
charges between objects or locations.
| III | STATIC ELECTRICITY |
Static electricity can be produced by rubbing
together two objects made of different materials. Electrons move from the
surface of one object to the surface of the other if the second material holds
onto its electrons more strongly than the first does. The object that gains
electrons becomes negatively charged, since it now has more electrons than
protons. The object that gives up electrons becomes positively charged. For
example, if a nylon comb is run through clean, dry hair, some of the electrons
on the hair are transferred to the comb. The comb becomes negatively charged and
the hair becomes positively charged. The following materials are named in
decreasing order of their ability to hold electrons: rubber, silk, glass,
flannel, and fur (or hair). If any two of these materials are rubbed together,
the material earlier in the list becomes negative, and the material later in the
list becomes positive. The materials should be clean and dry.
| A | Charging by Contact |
Objects become electrically charged in
either of two ways: by contact or by induction.
A charged object transfers electric charge
to an object with lesser charge if the two touch. When this happens, a charge
flows from the first to the second object for a brief time. Charges in motion
form an electric current. When charge flows between objects in contact, the
amount of charge that an object receives depends on its ability to store charge.
The ability to store charge is called capacitance and is measured in units
called farads.
Charging by contact can be demonstrated
by touching an uncharged electroscope with a charged comb. An electroscope is a
device that contains two strips of metal foil, called leaves, that hang from one
end of a metal rod. A metal ball is at the other end of the rod. When the
charged comb touches the ball, some of the charges on the comb flow to the
leaves, which separate because they now hold like charges and repel each other.
If the comb is removed, the leaves remain apart because they retain their
charges. The electroscope has thus been charged by contact with the comb.
This flow of charge between objects with
different amounts of charge will occur whenever possible. However, it requires a
pathway for the electric charge to move along. Some materials, called
conductors, allow an electric current to flow through them easily. Other
materials, called insulators, strongly resist the passage of an electric
current.
Under normal conditions, air is an
insulator. However, if an object gains a large enough charge of static
electricity, part of the charge may jump, or discharge, through the air to
another object without touching it directly. When the charge is large enough,
the air becomes a conductor. Lightning is an example of a discharge.
| B | Coulomb’s Law |
Objects with opposite charges attract
each other, and objects with similar charges repel each other. Coulomb’s law,
formulated by French physicist Charles Augustin de Coulomb during the late 18th
century, quantifies the strength of the attraction or repulsion. This law states
that the force between two charged objects is directly proportional to the
product of their charges and inversely proportional to the square of the
distance between them. The greater the charges on the objects, the larger the
force between them; the greater the distance between the objects, the lesser the
force between them. The unit of electric charge, also named after Coulomb, is
equal to the combined charges of 6.24 × 1018 protons (or
electrons).
If two charged objects in contact have
the same capacitance, they divide the charge evenly. Suppose, for example, that
one object has a charge of +4 coulombs and the other a charge of +8 coulombs.
When they touch, charge will flow from the 8-coulomb object to the 4-coulomb
object until each has a charge of +6 coulombs. If each object originally had a
charge of +6 coulombs, no charge would flow between them.
If two objects have different
capacitances, they divide the charge in proportion to their capacitances. If an
object with a capacitance of 10 farads touches an object with a capacitance of 5
farads, the 10-farad object will end up with twice the amount of charge of the
5-farad object. Suppose that the objects are oppositely charged and that one has
a charge of +20 coulombs and the other a charge of -8 coulombs. Their total
charge is therefore +12 coulombs. After they touch, the 10-farad object will
have a charge of +8 coulombs and the 5-farad object will have +4 coulombs.
| C | Charging by Induction |
A charged object may induce a charge in a
nearby neutral object without touching it. For example, if a positively charged
object is brought near a neutral object, the electrons in the neutral object are
attracted to the positive object. Some of these electrons flow to the side of
the neutral object that is nearest to the positive object. This side of the
neutral object accumulates electrons and becomes negatively charged. Because
electrons leave the far side of the neutral object while its protons remain
stationary, that side becomes positively charged.
Since the negatively charged side of the
neutral object is closest to the positive object, the attraction between this
side and the positive object is greater than the repulsion between the
positively charged side and the positive object. The net effect is an attraction
between the objects. Similarly, when a negatively charged object is brought near
a neutral object, the negative object induces a positive charge on the near side
of the neutral object and a negative charge on the far side. As before, the net
effect is an attraction between the objects.
The induced charges described above are
not permanent. As soon as the charged object is taken away, the electrons on the
other object redistribute themselves evenly over it, so that it again becomes
neutral.
An object can also be charged permanently
by induction. If a negatively charged object, A, is brought near a neutral
object, B, the electrons on B are repelled as far as possible from A and flow to
the other side of B. If that side of B is then connected to the ground by a good
conductor, such as a metal wire, the electrons flow out through the wire into
the ground. The ground can receive almost any amount of charge because Earth,
being neutral, has an enormous capacitance. Object B is said to be grounded by
the wire connecting it to Earth.
If this wire is then removed, B has a
positive charge, since it has lost electrons to Earth. Thus B has been
permanently charged by induction. Even if A is subsequently removed, B still
remains positive because the wire has been disconnected and B cannot regain
electrons from Earth to neutralize its positive charge.
| IV | ELECTRIC CURRENT |
An electric current is a movement of charge.
When two objects with different charges touch and redistribute their charges, an
electric current flows from one object to the other until the charge is
distributed according to the capacitances of the objects. If two objects are
connected by a material that lets charge flow easily, such as a copper wire,
then an electric current flows from one object to the other through the wire.
Electric current can be demonstrated by connecting a small light bulb to an
electric battery by two copper wires. When the connections are properly made,
current flows through the wires and the bulb, causing the bulb to glow.
Current that flows in one direction only,
such as the current in a battery-powered flashlight, is called direct current.
Current that flows back and forth, reversing direction again and again, is
called alternating current. Direct current, which is used in most
battery-powered devices, is easier to understand than alternating current. Most
of the following discussion focuses on direct current. Alternating current,
which is used in most devices that are “plugged in” to electrical outlets in
buildings, will be discussed in the Alternating Current section of this
article.
Other properties that are used to quantify
and compare electric currents are the voltage (also called electromotive force)
driving the current and the resistance of the conductor to the passage of the
current. The amount of current, voltage, and resistance in any circuit are all
related through an equation called Ohm’s law.
| A | Conductors and Insulators |
Conductors are materials that allow an
electric current to flow through them easily. Most metals are good conductors.
Substances that do not allow electric
current to flow through them are called insulators, nonconductors, or
dielectrics. Rubber, glass, and air are common insulators. Electricians wear
rubber gloves so that electric current will not pass from electrical equipment
to their bodies. However, if an object contains a sufficient amount of charge,
the charge can arc, or jump, through an insulator to another object. For
example, if you shuffle across a wool rug and then hold your finger very close
to, but not in contact with, a metal doorknob or radiator, current will arc
through the air from your finger to the doorknob or radiator, even though air is
an insulator. In the dark, the passage of the current through the air is visible
as a tiny spark.
| B | Measuring Electric Current |
Electric current is measured in units
called amperes (amp). If 1 coulomb of charge flows past each point of a wire
every second, the wire is carrying a current of 1 amp. If 2 coulombs flow past
each point in a second, the current is 2 amp. See also Electric
Meters.
| C | Voltage |
When the two terminals of a battery are
connected by a conductor, an electric current flows through the conductor. One
terminal continuously sends electrons into the conductor, while the other
continuously receives electrons from it. The current flow is caused by the
voltage, or potential difference, between the terminals. The more willing the
terminals are to give up and receive electrons, the higher the voltage. Voltage
is measured in units called volts. Another name for a voltage produced by a
source of electric current is electromotive force.
| D | Resistance |
A conductor allows an electric current to
flow through it, but it does not permit the current to flow with perfect
freedom. Collisions between the electrons and the atoms of the conductor
interfere with the flow of electrons. This phenomenon is known as resistance.
Resistance is measured in units called ohms. The symbol for ohms is the Greek
letter omega, Ω.
A good conductor is one that has low
resistance. A good insulator has a very high resistance. At commonly encountered
temperatures, silver is the best conductor and copper is the second best.
Electric wires are usually made of copper, which is less expensive than
silver.
The resistance of a piece of wire depends
on its length, and its cross-sectional area, or thickness. The longer the wire
is, the greater its resistance. If one wire is twice as long as a wire of
identical diameter and material, the longer wire offers twice as much resistance
as the shorter one. A thicker wire, however, has less resistance, because a
thick wire offers more room for an electric current to pass through than a thin
wire does. A wire whose cross-sectional area is twice that of another wire of
equal length and similar material has only half the resistance of the thinner
wire. Scientists describe this relationship between resistance, length, and area
by saying that resistance is proportional to length and inversely proportional
to cross-sectional area.
Usually, the higher the temperature of a
wire, the greater its resistance. The resistance of some materials drops to zero
at very low temperatures. This phenomenon is known as superconductivity.
| E | Ohm’s Law |
The relationship between current, voltage,
and resistance is given by Ohm’s law. This law states that the amount of current
passing through a conductor is directly proportional to the voltage across the
conductor and inversely proportional to the resistance of the conductor. Ohm’s
law can be expressed as an equation, V = IR, where V is the
difference in volts between two locations (called the potential difference),
I is the amount of current in amperes that is flowing between these two
points, and R is the resistance in ohms of the conductor between the two
locations of interest. V = IR can also be written R =
V/I and I = V/R. If any two of the quantities are known,
the third can be calculated. For example, if a potential difference of 110 volts
sends a 10-amp current through a conductor, then the resistance of the conductor
is R = V/I = 110/10 = 11 ohms. If V = 110 and R =
11, then I = V/R = 110/11 = 10 amp.
Under normal conditions, resistance is
constant in conductors made of metal. If the voltage is raised to 220 in the
example above, then R is still 11. The current I will be doubled,
however, since I = V/R = 220/11 = 20 amp.
| F | Heat and Power |
A conductor’s resistance to electric
current produces heat. The greater the current passing through the conductor,
the greater the heat. Also, the greater the resistance, the greater the heat. A
current of I amp passing through a resistance of R ohms for
t seconds generates an amount of heat equal to
I2Rt joules (a joule is a unit of energy equal
to 0.239 calorie).
Energy is required to drive an electric
current through a resistance. This energy is supplied by the source of the
current, such as a battery or an electric generator. The rate at which energy is
supplied to a device is called power, and it is often measured in units called
watts. The power P supplied by a current of I amp passing through
a resistance of R ohms is given by P =
I2R.
| V | HOW ELECTRIC CURRENT IS CONDUCTED |
All electric currents consist of charges in
motion. However, electric current is conducted differently in solids, gases, and
liquids. When an electric current flows in a solid conductor, the flow is in one
direction only, because the current is carried entirely by electrons. In liquids
and gases, however, a two-directional flow is made possible by the process of
ionization (see Electrochemistry).
| A | Conduction in Solids |
The conduction of electric currents in
solid substances is made possible by the presence of free electrons
(electrons that are free to move about). Most of the electrons in a bar of
copper, for example, are tightly bound to individual copper atoms. However, some
are free to move from atom to atom, enabling current to flow.
Ordinarily the motion of the free electrons
is random; that is, as many of them are moving in one direction as in another.
However, if a voltage is applied to the two ends of a copper bar by means of a
battery, the free electrons tend to drift toward one end. This end is said to be
at a higher potential and is called the positive end. The other end is said to
be at a lower potential and is called the negative end. The function of a
battery or other source of electric current is to maintain potential difference.
A battery does this by supplying electrons to the negative end of the bar to
replace those that drift to the positive end and also by absorbing electrons at
the positive end.
Insulators cannot conduct electric currents
because all their electrons are tightly bound to their atoms. A perfect
insulator would allow no charge to be forced through it, but no such substance
is known at room temperature. The best insulators offer high but not infinite
resistance at room temperature.
Some substances that ordinarily have no
free electrons, such as silicon and germanium, can conduct electric currents
when small amounts of certain impurities are added to them. Such substances are
called semiconductors. Semiconductors generally have a higher resistance to the
flow of current than does a conductor, such as copper, but a lower resistance
than an insulator, such as glass.
| B | Conduction in Gases |
Gases normally contain few free electrons
and are generally insulators. When a strong potential difference is applied
between two points inside a container filled with a gas, the few free electrons
are accelerated by the potential difference and collide with the atoms of the
gas, knocking free more electrons. The gas atoms become positively charged ions
and the gas is said to be ionized. The electrons move toward the high-potential
(more positive) point, while the ions move toward the low-potential (more
negative) point. An electric current in a gas is composed of these opposite
flows of charges.
| C | Conduction in Liquid Solutions |
Many substances become ionized when they
dissolve in water or in some other liquid. An example is ordinary table salt,
sodium chloride (NaCl). When sodium chloride dissolves in water, it separates
into positive sodium ions, Na+, and negative chlorine ions,
Cl-. If two points in the solution are at different potentials, the
negative ions drift toward the positive point, while the positive ions drift
toward the negative point. As in gases, the electric current is composed of
these flows of opposite charges. Thus, while water that is absolutely pure is an
insulator, water that contains even a slight impurity of an ionized substance is
a conductor.
Since the positive and negative ions of a
dissolved substance migrate to different points when an electric current flows,
the substance is gradually separated into two parts. This separation is called
electrolysis.
| VI | SOURCES OF ELECTRIC CURRENT |
There are several different devices that can
supply the voltage necessary to generate an electric current. The two most
common sources are generators and electrolytic cells.
| A | Generators |
Generators use mechanical energy, such as
water pouring through a dam or the motion of a turbine driven by steam, to
produce electricity. The electric outlets on the walls of homes and other
buildings, from which electricity to operate lights and appliances is drawn, are
connected to giant generators located in electric power stations. Each outlet
contains two terminals. The voltage between the terminals drives an electric
current through the appliance that is plugged into the outlet. See
Electric Power Systems.
| B | Electrolytic Cells |
Electrolytic cells use chemical energy to
produce electricity. Chemical reactions within an electrolytic cell produce a
potential difference between the cell’s terminals. An electric battery consists
of a cell or group of cells connected together.
| C | Other Sources |
There are many sources of electric current
other than generators and electrolytic cells. Fuel cells, for example, produce
electricity through chemical reactions. Unlike electrolytic cells, however, fuel
cells do not store chemicals and therefore must be constantly refilled.
Certain sources of electric current
operate on the principle that some metals hold onto their electrons more
strongly than other metals do. Platinum, for example, holds its electrons less
strongly than aluminum does. If a strip of platinum and a strip of aluminum are
pressed together under the proper conditions, some electrons will flow from the
platinum to the aluminum. As the aluminum gains electrons and becomes negative,
the platinum loses electrons and becomes positive.
The strength with which a metal holds its
electrons varies with temperature. If two strips of different metals are joined
and the joint heated, electrons will pass from one strip to the other.
Electricity produced directly by heating is called thermoelectricity.
Some substances emit electrons when they
are struck by light. Electricity produced in this way is called
photoelectricity. When pressure is applied to certain crystals, a potential
difference develops across them. Electricity thus produced is called
piezoelectricity. Some microphones work on this principle.
| VII | ELECTRIC CIRCUITS |
An electric circuit is an arrangement of
electric current sources and conducting paths through which a current can
continuously flow. In a simple circuit consisting of a small light bulb, a
battery, and two pieces of wire, the electric current flows from the negative
terminal of the battery, through one piece of connecting wire, through the bulb
filament (also a type of wire), through the other piece of connecting wire, and
back to the positive terminal of the battery. When the electric current flows
through the filament, the filament heats up and the bulb lights.
A switch can be placed in one of the
connecting wires. A flashlight is an example of such a circuit. When the switch
is open, the connection is broken, electric current cannot flow through the
circuit, and the bulb does not light. When the switch is closed, current flows
and the bulb lights.
The bulb filament may burn out if too much
electric current flows through it. To prevent this from happening, a fuse
(circuit breaker) may be placed in the circuit. When too much current flows
through the fuse, a wire in the fuse heats up and melts, thereby breaking the
circuit and stopping the flow of current. The wire in the fuse is designed to
melt before the filament would melt.
The part of an electric circuit other than
the source of electric current is called the load. The load includes all
appliances placed in the circuit, such as lights, radios, fans, buzzers, and
toasters. It also includes the connecting wires, as well as switches, fuses, and
other devices. The load forms a continuous conducting path between the terminals
of the current source.
There are two basic ways in which the parts
of a circuit are arranged. One arrangement is called a series circuit, and the
other is called a parallel circuit.
| A | Series Circuits |
If various objects are arranged to form a
single conducting path between the terminals of a source of electric current,
the objects are said to be connected in series. The electron current first
passes from the negative terminal of the source into the first object, then
flows through the other objects one after another, and finally returns to the
positive terminal of the source. The current is the same throughout the circuit.
In the example of the light bulb, the wires, bulb, switch, and fuse are
connected in series.
When objects are connected in series, the
electric current flows through them against the resistance of the first object,
then against the resistance of the next object, and so on. Therefore the total
resistance to the current is equal to the sum of the individual resistances. If
three objects with resistances R1,
R2, and R3 are
connected in series, their total resistance is R1 +
R2 + R3. For example,
if a motor with a resistance of 48 ohms is connected to the terminals of a
current source by two wires, each with a resistance of 1 ohm, the total
resistance of the motor and wires is 48 + 1 + 1 = 50 ohms. If the voltage is 100
volts, a current of 100/50 = 2 amp will flow through the circuit.
Voltage can be thought of as being used
up by the objects in a circuit. The voltage that each object uses up is called
the voltage drop across that object. Voltage drop can be calculated from the
equation V = IR, where V is the voltage drop across the
object, I is the amount of current, and R is the resistance of the
object.
In the example of the motor, the voltage
drop in each wire is V = IR = 2 × 1 = 2 volts, and the voltage
drop in the motor is 2 × 48 = 96 volts. Adding up the voltage drops (2 + 2 + 96)
gives a total drop of 100 volts. In a series circuit the sum of the voltage
drops across the objects always equals the total voltage supplied by the
source.
| B | Parallel Circuits |
If various objects are connected to form
separate paths between the terminals of a source of electric current, they are
said to be connected in parallel. Each separate path is called a branch of the
circuit. Current from the source splits up and enters the various branches.
After flowing through the separate branches, the current merges again before
reentering the current source.
The total resistance of objects connected
in parallel is less than that of any of the individual resistances. This is
because a parallel circuit offers more than one branch (path) for the electric
current, whereas a series circuit has only one path for all the current.
The electric current through a parallel
circuit is distributed among the branches according to the resistances of the
branches. If each branch has the same resistance, then the current in each will
be equal. If the branches have different resistances, the current in each branch
can be determined from the equation I = V/R, where I is the
amount of current in the branch, V is the voltage, and R is the
resistance of the branch.
The total resistance of a parallel
circuit can be calculated from the equation
 |
 |
The greater the resistance of a given
branch, the smaller the portion of the electric current flowing through that
branch. If a parallel circuit of three branches, with resistances of 10, 15, and
30 ohms, is connected to a 150-volt source, the branch with a resistance of 10
ohms would receive a current of V/R = 150/10 = 15 amp. Similarly, the
15-ohm branch receives 10 amp, and the 30-ohm branch receives 5 amp. These
branch currents add up to a total current of 30 amp, which is the value obtained
by dividing the voltage by the total resistance.
| C | Series-Parallel Circuits |
Many circuits combine series and parallel
arrangements. One branch of a parallel circuit, for example, may have within it
several objects in a series. The resistances of these objects must be combined
according to the rules for a series circuit. On the other hand, a series circuit
may at one point divide into two or more branches and then rejoin. The branches
are parallel and must be treated by the rules for parallel circuits.
Complicated series-parallel circuits may
be analyzed by means of two rules called Kirchhoff’s laws. These rules make it
possible to find the amount of electric current flowing through each part of any
circuit, as well as the voltage across it. The first of Kirchhoff’s laws states
that at any junction in a circuit through which a steady current is flowing, the
sum of the currents flowing to the junction is equal to the sum of the currents
flowing away from that point. The second law states that, starting at any point
in a circuit and following any closed path back to the starting point, the net
sum of the voltage encountered will be equal to the net sum of the products of
the resistances encountered and the currents flowing through them. In other
words, Ohm’s law applies not only to a circuit as a whole, but also to any given
section of a circuit.
| D | Series and Parallel Sources |
Sources of electric current can also be
connected in various ways. Sources can be arranged in series by connecting a
terminal of one source to the opposite terminal of the next source. For example,
if the positive terminal of battery A is connected to the negative terminal of
battery B, and the positive terminal of battery B to the negative terminal of
battery C, then batteries A, B, and C are in series. The load is then placed
between the positive terminal of battery C and the negative terminal of battery
A.
When sources of electric current are
connected in series, their total voltage is equal to the sum of their individual
voltages. For example, three 1.5-volt batteries connected in series furnish a
total of 4.5 volts. If the load is 9 ohms, the batteries send a current of 4.5/9
= 0.5 amp through the load.
Current sources may be arranged in
parallel by connecting all the positive terminals together and all the negative
terminals together. The load is then placed between the group of positive
terminals and the group of negative terminals.
Arranging sources in parallel does not
increase the voltage. If three 1.5-volt batteries are connected in parallel, the
total voltage is still 1.5 volts. Batteries should not be connected in parallel
unless they have approximately the same voltage. If a high voltage battery is
connected in parallel with a low voltage battery, the high voltage battery will
force an electric current through the low voltage battery and damage it.
| VIII | ELECTRIC FIELDS |
A single electric charge can attract or
repel, and it will demonstrate this ability as soon as another charge is brought
near it. The ability to attract or repel can be thought of as being stored in
the region around the charge. This region is called the electric field of force
of the charge. All charged objects have electric fields around them.
| A | Lines of Force |
An electric field can be visualized as
consisting of imaginary lines called lines of force. Each line corresponds to
the path that a positive charge would take if placed in the field on that line.
The lines in the field around a positively charged object radiate in all
directions away from the object, since the object repels positive charges.
Conversely, the lines in the field around a negatively charged object are
directed toward the object. If a positive and a negative object are placed near
each other, their lines of force connect. If two objects with similar charges
are placed near each other, the lines do not connect. Lines of force never cross
each other.
Lines of force are only imaginary.
Nevertheless, the idea of lines of force helps in visualizing an electric
field.
| B | Field Direction |
When a charge is placed at any given
point in an electric field, it is acted on by a force that tends to push it in a
certain direction. This direction is called the direction of the field at that
point. The field direction can be represented graphically by the lines of force
near an electric charge.
| C | Field Strength |
The strength, or intensity, of a field
at any point is defined as the force exerted on a charge of 1 coulomb placed at
that point. For example, if a point charge of 1 coulomb is subjected to a force
of 10 newtons, the electric field is 10 newtons per coulomb at that point. An
object with a charge of 5 coulombs would be subjected to a force of 50 newtons
at the same point.
Field strength is represented
graphically by the closeness (density) of the lines of force. Where the lines
are close together, the field is strong. Where they are far apart, the field is
weak. Near a charge, the field is strong and the lines are close together. At
greater distances from the charge, the field weakens and the lines are not as
close together. The field strength values that the lines represent are relative,
since a field can be drawn with as many lines as desired.
| IX | ELECTRICITY AND MAGNETISM |
Many similarities exist between electric and
magnetic phenomena. A magnet has two opposite poles, referred to as north and
south. Opposite magnetic poles attract each other, and similar magnetic poles
repel each other, exactly as happens with electric charges.
The force with which magnetic poles attract
or repel each other depends on the strength of the poles and the distance
between them. This relationship is similar to the Coulomb’s inverse square law
for electric charges. See also Magnetism.
The similarities between electric and
magnetic phenomena indicate that electricity and magnetism are related.
Electricity produces magnetic effects and magnetism produces electric effects.
The relationship between electricity and magnetism is called electromagnetism.
See also Quantum Electrodynamics.
| A | Magnetic Effects of Electricity |
It has been noted that an electric field
exists around any electric charge. If electric charges are moving, they
constitute an electric current. The magnetic effect of electricity is
demonstrated by the fact that a magnetic field exists around any electric
current. The field can be detected when a magnet is brought close to the
current-carrying conductor.
The magnetic field around an electric
current can be thought of as lines of magnetic force that form closed circular
loops around the wire that carries the current. The direction of the magnetic
field can be determined by a convenient rule called the right-hand rule. To
apply this rule, the thumb of the right hand is pointed in the direction in
which the current is flowing and the fingers are curled around the wire. The
direction of the fingers then indicates the direction of the lines of magnetic
force. (The right-hand rule assumes that current flows from positive to
negative.)
| B | Motor Effect |
As already stated, a magnetic field exists
around a wire carrying an electric current, and a magnetic field exists between
the two poles of a magnet. If the wire is placed between the poles, the magnetic
fields interact to produce a force that tends to push the wire out of the field.
This phenomenon, known as the motor effect, is used in electric motors. See
also Electric Motors and Generators.
| C | Solenoids |
If a wire is bent into many continuous
loops to form a long spiral coil, then the magnetic lines of force tend to go
through the center of the coil from one end to the other rather than around the
individual loops of wire. Such a coil, called a solenoid, behaves in the same
way as a magnet and is the basis for all electromagnets. The end from which the
lines exit is the north pole and the end into which the lines reenter is the
south pole. The polarity of the coil can be determined by applying the left-hand
coil rule. If the left hand grasps the coil in such a way that the fingers curl
around in the direction of the electron current, then the thumb points in the
direction of the north pole.
| D | Electric Effects of Magnetism |
If a wire is moved through a magnetic
field in such a way that it cuts the magnetic lines of force, a voltage is
created across the wire. An electric current will flow through the wire if the
two ends of the wire are connected by a conductor to form a circuit. This
current is called an induced current, and the induction of a current in this
manner is called electromagnetic induction.
It does not matter whether the wire moves
or the magnetic field moves, provided that the wire cuts through lines of force.
If a magnet is moved near a stationary wire, the lines of magnetic force are cut
by the wire and an electric current is induced in the wire.
Like any electric current, an induced
current generates a magnetic field around it. Lenz’s law expresses an important
fact concerning this magnetic field: The motion of an induced current is always
in such a direction that its magnetic field opposes the magnetic field that is
causing the current.
| X | ALTERNATING CURRENT |
An alternating current is an electric current
that changes direction at regular intervals. When a conductor is moved back and
forth in a magnetic field, the flow of current in the conductor will reverse
direction as often as the physical motion of the conductor reverses direction.
Most electric power stations supply electricity in the form of alternating
currents. The current flows first in one direction, builds up to a maximum in
that direction, and dies down to zero. It then immediately starts flowing in the
opposite direction, builds up to a maximum in that direction, and again dies
down to zero. Then it immediately starts in the first direction again. This
surging back and forth can occur at a very rapid rate.
Two consecutive surges, one in each
direction, are called a cycle. The number of cycles completed by an electric
current in one second is called the frequency of the current. In the United
States and Canada, most currents have a frequency of 60 cycles per second.
Although direct and alternating currents
share some characteristics, some properties of alternating current are somewhat
different from those of direct current. Alternating currents also produce
phenomena that direct currents do not. Some of the unique traits of alternating
current make it ideal for power generation, transmission, and use.
| A | Amperage and Voltage |
The strength, or amperage, of an
alternating current varies continuously between zero and a maximum. Since it is
inconvenient to take into account a whole range of amperage values, scientists
simply deal with the effective amperage. Like a direct current, an alternating
current produces heat as it passes through a conductor. The effective amperage
of an alternating current is equal to the amperage of a direct current that
produces heat at the same rate. In other words, 1 effective amp of alternating
current through a conductor produces heat at the same rate as 1 amp of direct
current flowing through the same conductor. Similarly, the voltage of an
alternating current is considered in terms of the effective voltage.
| B | Impedance |
Like direct current, alternating current is
hindered by the resistance of the conductor through which it passes. In
addition, however, various effects produced by the alternating current itself
hinder the alternating current. These effects depend on the frequency of the
current and on the design of the circuit, and together they are called
reactance. The total hindering effect on an alternating current is called
impedance. It is equal to the resistance plus the reactance.
The relationship of effective current,
effective voltage, and impedance is expressed by V = IZ, where
V is the effective voltage in volts, I is the effective current in
amperes (amp), and Z is the impedance in ohms.
| C | Advantages of Alternating Current |
Alternating current has several
characteristics that make it more attractive than direct current as a source of
electric power, both for industrial installations and in the home. The most
important of these characteristics is that the voltage or the current may be
changed to almost any value desired by means of a simple electromagnetic device
called a transformer. When an alternating current surges back and forth through
a coil of wire, the magnetic field about the coil expands and collapses and then
expands in a field of opposite polarity and again collapses. In a transformer, a
coil of wire is placed in the magnetic field of the first coil, but not in
direct electric connection with it. The movement of the magnetic field induces
an alternating current in the second coil. If the second coil has more turns
than the first, the voltage induced in the second coil will be larger than the
voltage in the first, because the field is acting on a greater number of
individual conductors. Conversely, if there are fewer turns in the second coil,
the secondary, or induced, voltage will be smaller than the primary
voltage.
The action of a transformer makes possible
the economical transmission of electric power over long distances. If 200,000
watts of power is supplied to a power line, it may be equally well supplied by a
potential of 200,000 volts and a current of 1 amp or by a potential of 2,000
volts and a current of 100 amp, because power is equal to the product of voltage
and current. The power lost in the line through heating, however, is equal to
the square of the current times the resistance. Thus, if the resistance of the
line is 10 ohms, the loss on the 200,000-volt line will be 10 watts, whereas the
loss on the 2,000-volt line will be 100,000 watts, or half the available power.
Accordingly, power companies tend to favor high voltage lines for long distance
transmission.
| XI | HISTORY |
Humans have known about the existence of
static electricity for thousands of years, but scientists did not make great
progress in understanding electricity until the 1700s.
| A | Early Theories |
The ancient Greeks observed that amber,
when rubbed, attracted small, light objects. About 600 bc Greek philosopher Thales of Miletus
held that amber had a soul, since it could make other objects move. In a
treatise written about three centuries later, another Greek philosopher,
Theophrastus, stated that other substances also have this power.
For almost 2,000 years after Theophrastus,
little progress was made in the study of electricity. In 1600 English physician
William Gilbert published a book in which he noted that many substances besides
amber could be charged by rubbing. He gave these substances the Latin name
electrica, which is derived from the Greek word elektron (which
means “amber”). The word electricity was first used by English writer and
physician Sir Thomas Browne in 1646.
The fact that electricity can flow through
a substance was discovered by 17th-century German physicist Otto von Guericke,
who observed conduction in a linen thread. Von Guericke also described the first
machine for producing an electric charge in 1672. The machine consisted of a
sulfur sphere turned by a crank. When a hand was held against the sphere, a
charge was induced on the sphere. Conduction was rediscovered independently by
Englishman Stephen Gray during the early 1700s. Gray also noted that some
substances are good conductors while others are insulators.
Also during the early 1700s, Frenchman
Charles Dufay observed that electric charges are of two kinds. He found that
opposite kinds attract each other while similar kinds repel. Dufay called one
kind vitreous and the other kind resinous.
American scientist Benjamin Franklin
theorized that electricity is a kind of fluid. According to Franklin’s theory,
when two objects are rubbed together, electric fluid flows from one object to
the other. The object that gains electric fluid acquires a vitreous charge,
which Franklin called positive charge. The object that loses electric fluid
acquires a resinous charge, which Franklin called negative charge.
Franklin demonstrated that lightning is a
form of electricity. In 1752 he constructed a kite and flew it during a storm.
When the string became wet enough to conduct, Franklin, who stood under a shed
and held the string by a dry silk cord, put his hand near a metal key attached
to the string. A spark jumped. Electric charge gathered by the kite had flowed
down the wet string to the key and then jumped across an air gap to flow to the
ground through Franklin’s body. Franklin also showed that a Leyden jar, a device
able to store electric charge, could be charged by touching it to the key when
electric current was flowing down the string.
Around 1766 British chemist Joseph
Priestley proved experimentally that the force between electric charges varies
inversely with the square of the distance between the charges. Priestley also
demonstrated that an electric charge distributes itself uniformly over the
surface of a hollow metal sphere and that no charge and no electric field of
force exists within such a sphere. French physicist Charles Augustin de Coulomb
reinvented a torsion balance to measure accurately the force exerted by electric
charges. With this apparatus he confirmed Priestley’s observations and also
showed that the force between two charges is proportional to the product of the
individual charges.
In 1791 Italian biologist Luigi Galvani
published the results of experiments that he had performed on the muscles of
dead frogs. Galvani had found earlier that the muscles in a frog’s leg would
contract if he applied an electric current to them.
| B | 19th and 20th Centuries |
In 1800 another Italian scientist,
Alessandro Volta, announced that he had created the voltaic pile, a form of
electric battery. The voltaic pile made the study of electric current much
easier by providing a reliable, steady source of current. Danish physicist Hans
Christian Oersted demonstrated that electric currents are surrounded by magnetic
fields in 1819. Shortly afterward, André Marie Ampère discovered the
relationship known as Ampere’s law, which gives the direction of the magnetic
field. Ampère also demonstrated the magnetic properties of solenoids. Georg
Simon Ohm, a German high school teacher, investigated the conducting abilities
of various metals. In 1827 Ohm published his results, including the relationship
now known as Ohm’s law.
In 1830 American physicist Joseph Henry
discovered that a moving magnetic field induces an electric current. The same
effect was discovered a year later by English scientist Michael Faraday. Faraday
introduced the concept of lines of force, a concept that proved extremely useful
in the study of electricity.
About 1840 British physicist James
Prescott Joule and German scientist Hermann Ludwig Ferdinand von Helmholtz
demonstrated that electricity is a form of energy and that electric circuits
obey the law of the conservation of energy.
Also during the 19th century, British
physicist James Clerk Maxwell investigated the properties of electromagnetic
waves and light and developed the theory that the two are identical. Maxwell
summed up almost all the laws of electricity and magnetism in four mathematical
equations. His work paved the way for German physicist Heinrich Rudolf Hertz,
who produced and detected electric waves in the atmosphere in 1886, and for
Italian engineer Guglielmo Marconi, who harnessed these waves in 1895 to produce
the first practical radio signaling system.
The electron theory, which is the basis
of modern electrical theory, was first advanced by Dutch physicist Hendrik
Antoon Lorentz in 1892. American physicist Robert Andrews Millikan accurately
measured the charge on the electron in 1909. The widespread use of electricity
as a source of power is largely due to the work of pioneering American engineers
and inventors such as Thomas Alva Edison, Nikola Tesla, and Charles Proteus
Steinmetz during the late 19th and early 20th centuries.
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