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Heat/Overview
Heat is a form of energy.
According to the Kinetic Theory of Matter, heat is the result of
the continuous motion and vibration of the atoms and molecules
that constitute all matter. The transfer of heat between objects
of different temperatures by thermal flow processes involves a
reduction in the average motion of the particles of the hotter
object and an increase in the average motion of the particles of
the cooler object. The branch of physics comprising the
comprehensive study of the transfer of heat, and of the
conversion of heat into WORK and work into heat in physical and
chemical processes, is known as Thermodynamics.
Cold is the absence of heat. The
coldest possible temperature is Absolute Zero, - 273.15 degrees
C. At this temperature, all molecular motion would cease. Even
though temperatures within a few millionths of a degree of
absolute zero have been achieved, it is impossible in any real
process to attain absolute zero. This impossibility is known as
the third law of thermodynamics.
Development of the Concept of Heat
Until the growth of classical
physics in the 18th century, scientists did not comprehend the
true nature of heat. Even though earlier investigators such as
English scientist Robert Boyle had considered heat to be some
form of manifestation of the movement of small particles in
objects, throughout most of the 18th century heat was still
mainly thought of in terms dating back to the days of ancient
Greek philosophy. That is, heat was considered to be a kind of
basic "element," a form of actual substance. This
substance, while all-pervasive, was conceived of as an invisible,
weightless material--a fluid called caloric by French chemist
Antoine Lavoisier. If a hot object were to be placed in contact
with a cooler object, for example, this invisible fluid would
enter the cooler object to make it hotter. The notion was
sometimes expanded to a two-fluid form--the other, cold fluid
being called frigoric.
In the mid-18th century the
Scottish chemist Joseph Black, while adhering to the caloric
idea, was able to advance scientific understanding of heat's true
nature by developing the concepts of heat capacity and latent
heat. He also made clear the difference between heat and
temperature.
By the end of the 18th century the
notion of heat as a fluid, while still prevalent among
scientists, was on the way out. It was helped along in particular
by the work of Count Rumford and English chemist Humphry Davy.
Experiments that they conducted gave strong support to Boyle's
concept of heat as a result of motions. After that, in the course
of the 19th century, the basic concepts of thermodynamics were
worked out by a number of noted physicists, including British
scientists James Prescott Joule and Lord Kelvin. From their work
arose the modern understanding of heat as a form of energy in
transit.
Thermodynamics/Overview
Thermodynamics is the branch of
the physical sciences that studies the transfer of HEAT and the
interconversion of heat and work in various physical and chemical
processes. The term is derived from the Greek words thermos
(heat) and dynamics (power). The study of Thermodynamics is
central to both chemistry and physics and is becoming
increasingly important in understanding biological and geological
processes.
There are several subdisciplines
within this blend of chemistry and physics. Classical
thermodynamics considers the transfer of energy and work in
macroscopic systems--that is, without any consideration of the
nature of the forces and interactions between microscopic
individual particles. Statistical thermodynamics, on the other
hand, links the atomic nature of matter on a microscopic level
with the observed behavior of materials on the macroscopic level.
(In a further subdivision, statistical thermodynamics proper is
concerned with macroscopic processes that are independent of
time, while statistical mechanics is concerned with
time-dependent processes.) Statistical thermodynamics describes
energy relationships based on the statistical behavior of large
groups of individual atoms or molecules, and it relies heavily on
the mathematical implications of Quantum Mechanics. Chemical
thermodynamics focuses on energy transfer during chemical
reactions, and on the work done by chemical systems.
Thermodynamics is limited in its
scope. It emphasizes the initial and the final state of a
system--a given system being all of the components that interact
in the process under study, and the path, or manner, by which the
interaction takes place. It provides no information concerning
either the speed of the change or what occurs at the atomic and
molecular levels during the course of the change..
Development
of Thermodynamics
The early studies of
thermodynamics were motivated by the desire to derive useful work
from heat energy. The first reaction turbine was described by
Hero of Alexandria (AD c.120). It consisted of a pivoted copper
sphere fitted with two bent nozzles and partially filled with
water. When the sphere was heated over a fire, steam would escape
from the nozzles and the sphere would rotate. The device was not
designed to do useful work. It was instead a curiosity, and the
nature of heat and heat transfer at that time remained mere
speculation. The changes that occur when substances burn were
initially accounted for, in the late 17th century, by proposing
the existence of an invisible material substance called
phlogiston, which was supposedly lost when combustion took place.
In 1789, Antoine Lavoisier
prepared oxygen from mercuric oxide. In doing so, he demonstrated
the law of conservation of mass and thus overthrew the phlogiston
theory. Lavoisier proposed that heat, which he called caloric,
was an element, probably a weightless fluid surrounding the atoms
of substances, and that this fluid could be removed during the
course of a reaction. The observation that heat flowed from
warmer to colder bodies when such bodies were placed in thermal
contact was explained by proposing that particles of caloric
repelled one another.
Roughly simultaneous to these
advances, the actual conversion of heat to useful work was
progressing as well. At the end of the 17th century Thomas Savery
invented a machine to pump water from a well, using steam and a
system of tanks and hand-operated valves. Savery's pump is
generally hailed as the first practical application of steam
power. Thomas Newcomen developed Savery's invention into the
first piston engine in 1712. The design of the steam-powered
piston engine was further refined by James Watt later in the 18th
century.
Section Two
Active
Systems
Active solar heating systems
commonly consist of several hundred square meters of solar
collector panels, plus a storage medium to hold the
heat collected during the day, and a set of automatic controls
that monitor and regulate both heat collection and delivery
between the storage medium and the living space. Active systems
use either a liquid (of which the most popular is a mixture of
water and an antifreeze, such as propylene glycol) or air as the
heat-transfer medium. Insulated pipes or ducts carry the
heat-transfer medium, called the working fluid, to the collector
panels--where the fluid absorbs heat--and then back to the
storage, which in liquid-based (hydronic) systems is an insulated
tank or in air systems is an insulated bin of fist-sized rocks.
(Alternatively, phase-change materials may be used to store
heat.) The absorbed heat is transferred to the storage medium,
and the cooled working fluid is then returned to the collectors
to pick up more heat. Heat is removed from storage and delivered
to the living space as needed. Most types of active systems
require an auxiliary heating system to provide extra heat during
extended periods of cloudiness or extreme cold. A typical active
heating system might cost between $2,000 and $5,000 per thousand
square feet of living space in the northeastern United
States--depending on the type of system, its efficiency, and so
forth. A large portion of a building's annual domestic hot water
(DHW) needs can be supplied by a relatively inexpensive (between
$2,000 and $2,500) active hydronic system using about 9 sq m (100
sq ft) of collectors for a typical residence. A heat exchanger,
usually in the hot-water tank, keeps the working fluid separate
from the potable water supply. Such systems, although they
require a backup energy source, may pay for themselves in energy
savings in less than 10 years.
High-temperature solar collector
panels may be used to power absorption-chiller air
conditioning. Such systems are relatively expensive but may
be cost-effective in climates where plentiful sunshine and a
substantial need for air conditioning exist. Also, heat pumps
may be used in conjunction with solar panels; solar heat boosts
the heat pump's source during the winter, and during the summer
the heat pump can discharge heat to the outdoors at night through
the collectors.
American Generating Plant Fuel Usage
The central-station generating
plants built throughout the United States were generally designed
to use the most accessible and economical fuels. Hydroelectric
plants were built at locations where dams could be built to
impound the water needed to supply the hydraulic energy for the
turbogenerators. Power plants near coalfields were likely to have
coal-fired furnaces, whereas others were more likely to utilize
oil or natural gas as the primary fuel. In time the price of fuel
became an important factor in the generating process as fuel
transportation systems developed. Many coal-burning plants were
converted to use either oil or gas as competition between the
fuels and fuel suppliers increased.
Air pollution from coal-fired
plants became a major issue in some parts of the country in the
1960s and '70s, leading more utilities to switch to gas or oil.
Later, however, shortages of oil and gas required some plants to
be converted back to coal, either because high costs or uncertain
supplies of the desired fuel meant that it was not available or
because of governmental regulatory action. To reduce pollutants
to within new statutory limits, some utilities shifted to
new--and frequently distant--sources of coal, and some installed
sophisticated and expensive devices to cleanse pollutants from
plant emissions.
The locations for power plants run
by nuclear energy are usually determined not by the source of
their fuel but by land availability, access to suitable sources
of cooling water, and other physical considerations. At one time
the breeder reactor, a nuclear reactor that makes more fuel than
it uses, was looked upon as a major potential source of energy
for electric power production in the United States, but recent
concern about the spread of plutonium sufficient supplies of
uranium and has resulted in a decreased interest in
breeder-reactor power plants. Sharp differences of opinion exist
concerning the safety of nuclear plants, and safety has become a
growing public concern, particularly since the reactor accident
(1979) at Three Mile Island in Pennsylvania and the nuclear
disaster (1986) at Chernoble in the Soviet Union. As a result,
the future of nuclear power in the United States appears
uncertain. Increasing concern about the contribution of the
burning of fossil fuels to the greenhouse effect has led to a
reevaluation of nuclear power, however. Nuclear power plants do
not emit "greenhouse gases" such as carbon dioxide.
In 1987, production of electric
energy by utilities in the United States totaled 2,570 billion
kilowatt-hours (kW h). Of this, 56.9% was produced by
coal-burning plants, 4.6% by oil, 10.6% by gas, 9.7% by
hydroelectric plants, and 17.7% by nuclear plants. The remainder
came from geothermal, wood, waste, and solar plants. This
distribution of sources represents a significant decrease in oil
use by utilities and an increase in coal and uranium use.
The highly industrialized nature
of the United States, together with its population, economic
development, and overall size, have made it the world's largest
user of electric energy. In 1988, about 35% of the total
electricity sold in the United States was used for residential
purposes, 35% for industrial activities, and 27% for commercial
purposes. The remainder was used for farm purposes and
miscellaneous uses, and some was lost in the generation,
transmission, and distribution system.
Battery
In experimenting with what he
called atmospheric electricity, Galvani found that a frog muscle
would twitch when hung by a brass hook on an iron lattice.
Another Italian, Alessandro Volta, a professor at the University
of Pavia, affirmed that the brass and iron, separated by the
moist tissue of the frog, were generating electricity, and that
the frog's leg was simply a detector. In 1800, Volta succeeded in
amplifying the effect by stacking plates made of copper, zinc,
and moistened pasteboard respectively and in so doing he invented
the battery.
A battery separates electrical
charge by chemical means. If the charge is removed in some way,
the battery separates more charge, thus transforming chemical
energy into electrical energy. A battery can affect charges, for
instance, by forcing them through the filament of a light bulb.
Its ability to do work by electrical means is measured by the
VOLT, named for Volta. A volt is equal to 1 joule of work or
energy (1 Joule = 2.78/10,000,000 kilowatt-hours) for each
coulomb of charge. The electrical ability of a battery to do work
is called the electromotive force, or emf.
Btu
The British thermal unit (Btu) is
a quantity of energy usually associated with the production or
transfer of heat. Before 1929 it was defined as the amount of
heat required to raise the temperature of 1 pound of water 1
degree Fahrenheit (from 59.5 deg F to 60.5 deg F). In 1929 it was
redefined in terms of electrical units and is equivalent to
251.996 calories, 778.26 ft-lb, or approximately one-third
watt-hours.
Calorie
A calorie is a unit of heat
energy, originally defined as the amount of energy, as heat
(calor in Latin means heat), required to raise the temperature of
1 g of liquid water from 14.5 deg to 15.5 deg C. Today a calorie
is defined in mechanical rather than thermal terms. In this
system, 1 calorie (cal) equals 4.184000 watt-seconds (W-s), or
joules (J).
The energy required to melt 1 g of
ice is 80 cal; to boil 1 g of water, 540 cal must be expended.
The Earth receives from the Sun approximately 2 cal/min/sq cm of
surface area.
The combustion of 1 g of carbon
liberates 7,830 cal, or 7.830 kcal (kilocalorie--nutritionists
write Cal for kcal). Metabolism of carbohydrates liberates about
4 kcal/g. Fats yield about 9 kcal/g. The caloric requirement of
an average adult is about 2,000 kcal/d (day), or about 1.4
kcal/min. Vigorous running expends about 15 kcal/min.
A 1-ft free-fall of a 1-lb mass a
sea level (defined as 1 foot-pound of force, or ft lbf) produces
0.324 cal. One horsepower (hp) is equal to 550 ft lbs/s (second),
which is equal to 10.7 kcal/min.
Capacitor
Another device capable of
electrical work is the capacitor, a descendant of the Leyden jar,
which is used to store charge. If a charge Q is placed on the
metal plates the voltage rises to amount V. The measure of a
capacitor's ability to store charge is the capacitance C, where C
= Q/V. Charge flows from a capacitor just as it flows from a
battery, but with one significant difference. When the charge
leaves a capacitor's plates, no more can be obtained without
recharging. This happens because the electrical force is
conservative. The energy released cannot exceed the energy
stored. This ability to do work is called electric potential.
This type of conservation of
energy is also associated with emf. The electrical energy
obtainable from a battery is limited by the energy stored in
chemical molecular bonds. Both emf and electric potential are
measured in volts, and, unfortunately, the terms voltage,
potential, and emf are used rather loosely. For example, the term
battery potential is often used instead of emf.
Heat Capacity /Specific Heat
Heat can be measured
quantitatively. The units of measurement are typically either the
calorie or the British thermal unit (Btu). Both of these units
were at one time defined in terms of the amount of heat energy
required to raise a given amount of liquid water by a given
thermometric degree (within a certain degree range). Now the
units are defined in mechanical terms--by the amount of work they
can do, which can be expressed in electrical-unit equivalents, as
well. temperature is a measurement of the degree of heat of an
object, not the total quantity of heat energy that the object
possesses. Thus an object that is at a higher temperature than
another does not necessarily have a greater total heat content
than the object at a lower temperature. The sizes and types of
material of the objects, as well as their temperatures, determine
the total quantities of heat energy the contain.
In thermodynamics, the heat
capacity of an object or substance is the amount of heat energy
required to raise the temperature of the object or substance by
one degree Celsius. Specific heat is a closely related concept.
It is the amount of heat necessary to raise a unit mass of matter
by one degree. Common units of heat, such as the calorie and
British thermal unit, have been defined in terms of the specific
heat of water at a standard temperature. The specific heat of
copper is 0.093 cal/(g) at room temperature, and the heat
capacity of a 100-g copper bar is 9.3 cal/C degree. The specific
heats of most materials remain essentially constant over the
common range of temperatures. At extremely low temperatures,
however, specific heats become considerably smaller.
A volume of gas will accept more
heat energy per degree of temperature rise if it is allowed to
expand freely than if it is confined. Thus a gas has two distinct
values of specific heat: one value at constant pressure, and
another, smaller value at constant volume. The ratio of these
values is
different for different gases and
is of importance in describing the behavior of a gas undergoing a
thermodynamic process.
Combustion
Turbines
More recently, combustion turbine
generators have become popular as peaking units, not only because
of their quick-start and intermittent-operation capabilities, but
often because of the short time they require for installation. In
recent years many U.S. utilities have found themselves deficient
in generating capacity when the installation of new facilities
has been delayed by problems in procurement, licensing, or
construction. Lengthy delays have occurred in many planned
nuclear, fossil-fueled, and hydroelectric facilities. Procurement
and construction of a large steam-electric station takes from 5
to 10 years even after advance procedural requirements have been
met. Thus a combustion turbine unit of 30,000 kW or more, which
can be installed and operated within a year or two after
procurement, is an attractive alternative. Such units can also
provide emergency service during power outages are often valuable
as sources of start-up power for conventional generating plants
following a blackout. Capital costs of combustion turbines are
lower than those of conventional steam units, but fuel efficiency
is usually not as high and maintenance is more expensive.
Conduction
Conduction heat transfer is the
flow of thermal energy in matter as a result of molecular
collisions. For example, if one end of a metal bar is held in a
flame, heat is conducted along the bar. This conduction is
initiated by the excitation, or increased vibration, of metal
molecules at the hot end of the bar. The excited molecules then
collide with other molecules, exciting them also. This process
passes thermal energy along the length of the bar. It continues
as long as a temperature difference is maintained between the two
ends.
Convection
While conduction involves energy
transfer on a microscopic, or atomic, scale, convective heat
transfer results from the motion of large-scale quantities of
matter. Convection is important in gases and liquids, which are
able to expand significantly when they accept thermal energy and
can develop currents of material flow. For example, convective
heat transfer occurs in a pan of water being heated on a stove.
The water at the bottom of the pan accepts heat energy from the
pan by conduction. The water in this region then undergoes
thermal expansion and is buoyed upward by the surrounding, denser
water. The lighter water carries thermal energy throughout the
pan by this convection process. That is, the convection current
that has been established travels throughout the body of the
water, transferring heat and causing a temperature
redistribution. Convection currents permit buildings to be heated
without the use of circulatory devices. The heated air moves
solely by gravity.
Electric
Current
An electric charge in motion is
called electric current. The strength of a current is the amount
of charge passing a given point (as in a wire) per second, or I =
Q/t, where Q coulombs of charge pass in t seconds. The unit for
measuring current is the ampere or amp, which equals 1
coulomb/sec. Because it is the source of magnetism as well,
current is the link between electricity and magnetism. In 1819
the Danish physicist Hans Christian Oersted found that a compass
needle was affected by a current-carrying wire. Almost
immediately, Andre Ampere in France discovered the magnetic force
law. Michael Faraday in England and Joseph Henry in the United
States added the idea of magnetic induction, whereby a changing
magnetic field produces an electric field. The stage was then set
for the encompassing electromagnetic theory of James Clerk
Maxwell .
The variation of actual currents
is enormous. A modern electrometer can detect currents as low as
1/100,000,000,000,000,000 amp, which is a mere 63 electrons per
second. The current in a nerve impulse is approximately 1/100,000
amp; a 100-watt light bulb carries 1 amp; a lightning bolt peaks
at about 20,000 amps; and a 1,200-megawatt nuclear power plant
can deliver 10,000,000 amps at 115 V.
Most materials are insulators. In
them, all electrons are bound in individual atoms and do not
permit a flow of charge unless the electric field acting on the
material is so high that breakdown occurs. Then, in a process
called ionization, the most loosely bound electrons are torn from
the atoms, allowing current flow. This condition exists during a
lightning storm. The separation of charge between the clouds and
the ground creates a large electric field that ionizes the air
atoms, thereby forming a conducting path from cloud to ground.
Electricity
Electricity is a form of energy, a
phenomenon that is a result of the existence of electrical
charge. The theory of electricity and its inseparable effect,
magnetism, is probably the most accurate and complete of all
scientific theories. The understanding of electricity has led to
the invention of motors, generators, telephones, radio and
television, X-ray devices, computers, and nuclear energy systems.
Electricity is a necessity to modern civilization.
Electrical
History
Amber is a yellowish, translucent
mineral. As early as 600 BC the Greeks were aware of its peculiar
property: when rubbed with a piece of fur, amber develops the
ability to attract small pieces of material such as feathers. For
centuries this strange, inexplicable property was thought to be
unique to amber.
Two thousand years later, in the
16th century, William Gilbert proved that many other substances
are electric (from the Greek word for amber, elektron) and that
they have two electrical effects. When rubbed with fur, amber
acquires resinous electricity; glass, however, when rubbed with
silk, acquires vitreous electricity. Electricity repels the same
kind and attracts the opposite kind of electricity. Scientists
thought that the friction actually created the electricity (their
word for charge). They did not realize that an equal amount of
opposite electricity remained on the fur or silk. In 1747,
Benjamin Franklin in America and William Watson (1715-87) in
England independently reached the same conclusion: all materials
possess a single kind of electrical "fluid" that can
penetrate matter freely but that can be neither created nor
destroyed. The action of rubbing merely transfers the fluid from
one body to another, electrifying both. Franklin and Watson
originated the principle of conservation of charge: the total
quantity of electricity in an insulated system is constant.
Franklin defined the fluid, which
corresponded to vitreous electricity, as positive and the lack of
fluid as negative. Therefore, according to Franklin, the
direction of flow was from positive to negative--the opposite of
what is now known to be true. A subsequent two-fluid theory was
developed, according to which samples of the same type attract,
whereas those of opposite types repel.
Franklin was acquainted with the
Leydon Jar, a glass jar coated inside and outside with tinfoil.
It was the first capacitor, a device used to store charge. The
Leyden jar could be discharged by touching the inner and outer
foil layers simultaneously, causing an electrical shock to a
person. If a metal conductor was used, a spark could be seen and
heard. Franklin wondered whether lightning and thunder were also
a result of electrical discharge. During a thunderstorm in 1752,
Franklin flew a kite that had a metal tip. At the end of the wet,
conducting hemp line on which the kite flew he attached a metal
key, to which he tied a nonconducting silk string that he held in
his hand. The experiment was extremely hazardous, but the results
were unmistakable: when he held his knuckles near the key, he
could draw sparks from it. The next two who tried this extremely
dangerous experiment were killed.
It was known as early as 1600 that
the attractive or repulsive force diminishes as the charges are
separated. This relationship was first placed on a numerically
accurate, or quantitative, foundation by Joseph Priestley, a
friend of Benjamin Franklin. In 1767, Priestley indirectly
deduced that when the distance between two small, charged bodies
is increased by some factor, the forces between the bodies is
reduced by the square of the factor. For example, if the distance
between charges is tripled, the force decreases to one-ninth its
former value. Although rigorous, Priestley's proof was so simple
that he did not strongly advocate it. The matter was not
considered settled until 18 years later, when John Robinson of
Scotland made more direct measurements of the electrical force
involved.
The French physicist Charles A. de
Coulomb, whose name is used as the unit of electrical charge,
later performed a series of experiments that added important
details, as well as precision, to Priestley's proof. He also
promoted the two-fluid theory of electrical charges, rejecting
both the idea of the creation of electricity by friction and
Franklin's single-fluid model.
Today the electrostatic force law,
also known as Coulomb’s Law, is expressed as follows: if two
small objects, a distance r apart, have charges p and q and are
at rest, the magnitude of the force (F) on either is given by F = kpq/rr, where K is a constant. According to the International
System of Units, the force is measured in newtons (1 newton =
0.225 lb), the distance in meters, and the charges in Coulombs.
The constant k then becomes 8.988 billion. Charges of opposite
sign attract, whereas those of the same sign repel. A coulomb (C)
is a large amount of charge. To hold a positive coulomb (+ C) 1
meter away from a negative coulomb (- C) would require a force of
9 billion newtons (2 billion pounds). A typical charged cloud
about to give rise to a lightning bolt has a charge of about 30
coulombs.
Because of an accident the
18th-century Italian scientist Luigi GALVANI started a chain of
events that culminated in the development of the concept of
voltage and the invention of the battery. In 1780 one of
Galvani's assistants noticed that a dissected frog leg twitched
when he touched its nerve with a scalpel. Another assistant
thought that he had seen a spark from a nearby charged electric
generator at the same time. Galvani reasoned that the electricity
was the cause of the muscle contractions. He mistakenly thought,
however, that the effect was due to the transfer of a special
fluid, or "animal electricity," rather than to
conventional electricity.
Electricity/Overview
Electric power has become an
indispensable form of energy throughout much of the world. Even
systems that use forms of energy other than electricity are
likely to contain controls or equipment that run on electric
power. For example, modern home heating systems may burn natural
gas, oil, or coal, but most systems have combustion and
temperature controls that require electricity in order to
operate. Similarly, most industrial and manufacturing processes
require electric power, and the computers and business machines
of many offices and commercial establishments are paralyzed if
electric service is interrupted.
During the first part of the 20th
century, only about 10% of the total energy generated in the
United States was converted to electricity. By 1990 electric
power accounted for about 40% of the total. Developing countries
are usually not as dependent on electricity as are the more
industrialized nations, but the growth rate of electricity use in
some of those countries is comparable to the rate of growth in
the early years of electricity availability in the United States.
Electrical
Puzzles
In spite of many spectacular
successes, important unanswered questions remain within the field
of electricity. One basic question remains unanswered: how does
the force get from here to there? Perhaps it is by the exchange
between charged particles of quanta of electromagnetic radiation.
These hypothetical quanta are small, chargeless, massless
particles in a so-called virtual state. This idea is part of the
theory of quantum electrodynamics, developed by Richard Feynman
of the California Institute of Technology and Julian Schwinger of
Harvard. This theory is puzzling, however. The complete answer
might never be known.
Another unsolved problem involves
the electrical theory of matter. The electron is considered a
small body packed with negative electrical charge. According to
some scientists, it is a ball of charge having a radius of
approximately 1/10,000,000,000,000,000 meters. What holds it
together? Unless some other force, an attractive one, is
involved, the negative charge on one side repelling the negative
charge on the other side would tear the particle apart. Another
force may exist, although no such force has been found.
Speed
of Electricity
As electrons bounce along through
the wire, the general charge drift constitutes the current. The
average, or drift, speed is defined as the speed the electrons
would have if all were moving with constant velocity parallel to
the field. The drift speed is actually small even in good
conductors. In a 1.0-mm-diameter copper wire carrying a current
of 10 amps at room temperature, the drift speed of the electrons
is 0.2 mm per second. In copper, the electrons rarely drift
faster than one hundred-billionth the speed of light.
On the other hand, the speed of
the electric signal is the speed of light. This means that, at
the speed of light, the removal of one electron from one end of a
long wire would affect electrons elsewhere. For example, consider
a long, motionless freight train, with the cars representing
electrons in a wire. Because the couplings between cars have play
in them, the caboose is affected a short while after the engine
begins moving.
During this time the engine moves
forward a short distance. The signal telling the caboose to start
moves backward quickly, traveling the length of the train in the
same time it takes the engine to go forward a meter or so.
Similarly, the electron drift speed in a conductor is low, but
the signal moves at the speed of light in the opposite direction.
Evaporation
Evaporation is the conversion of a
liquid substance into the gaseous state. If the liquid is in an
open container, eventually it will evaporate completely. If a
liquid is placed in a closed container of larger volume, some
molecules leave the liquid and go into the excess space. This
process continues until an equilibrium is reached, in which the
molecules of vapor return to the liquid at the same rate as they
evaporate. The pressure exerted by the vapor in equilibrium with
its liquid is called the vapor pressure; it is a characteristic
property of each substance at a given temperature, and it
increases as temperature increases.
Evaporation
causes a decrease in the temperature of the liquid; to maintain a constant temperature, heat
must be supplied. The secretion and evaporation of sweat is the
principal mechanism by which the human body gets rid of excess
heat. High humidity hinders evaporation; in conjunction with high
temperature, it causes a person to feel uncomfortable.
The amount of water evaporated
from the Earth's surface each year is, on the average, equivalent
to a layer 100 cm (39 in) thick over the entire surface of the
planet; the process absorbs about one-fourth of the solar
radiation that reaches the surface. The water vapor remains in
the atmosphere for about 10 days before being returned to the
surface as rain or snow. This hydrologic cycle of evaporation and
condensation is essential to life on land and is largely
responsible for weather and climate.
The
Future of Power
Researchers in both government and
industry are seeking new technology, methods, and equipment for
the years ahead. Among the issues under investigation are the
more efficient use of power during peak periods, and the
development and greater utilization of power-saving devices, such
as high-efficiency light bulbs. The search for economic methods
of synthetic fuel creation continues, as well as experimentation
in such promising areas as the use of hydrogen for fuel. In
addition, recent discoveries of higher-temperature
superconducting materials that present lower resistance to
current flow may open up important new approaches.
As awareness grows of the need to
conserve energy resources, increasing interest also is being
shown in the development of small-scale hydroelectric power
plants. In the United States, legislation now favors the
development of such plants. The Public Utilities Regulatory
Policies Act (1978) states that utilities must buy electric power
fed into their lines from small, privately owned generators. Such
small-scale facilities can make more efficient user of power
resources.
In the 1990s, faced with the
possibility of government deregulation and increasing energy
costs, the electric power industry in the United States was
forced to seek more economical ways of generating electricity.
Underground Transmission Cables
Many transmission circuits utilize
underground cables, although these installations have been
limited largely to locations where rights-of-way for overhead
lines could not be obtained or where overhead lines were not
feasible because they would have interfered with other
activities. In general the costs of underground circuits are
several times those of comparable overhead circuits.
Insulation problems are very
different with underground cables from those with overhead lines,
in which air serves as a major insulating medium. A number of
different types of cable designs and insulation have been used in
the United States. Solid synthetic insulating materials have
given satisfactory results in the lower voltage ranges, but for
high-voltage applications the principal insulator is gas, or an,
oil-paper combination. Some extruded synthetic insulation have
recently been developed that use materials such as polyethylene.
One common kind of gas- and
oil-insulated cable, known as self-contained cable, uses a
conductor formed around a hollow core that is later filled with
oil under low pressure. The conductor is insulated with an
oil-impregnated paper, and the entire assembly is covered with a
metal sheath. Three such cables are required, one for each phase
of a three-phase power circuits normally used for alternating
current transmission throughout the world. Another cable system,
known as pipe-type, utilizes conductors insulated with
oil-impregnated paper and covered with metallic and synthetic
sheathing tapes. Three of these cables are pulled into a single
pipe that is then filled with either gas or oil under high
pressure. In the United States, the pipe-type system has been
used most.
One significant problem with
underground AC circuits is the continuous flow of charging
current between the energized conductors and the metallic cable
sheaths. Unless expensive compensation devices are used, this
charging current can utilize the entire current-carrying capacity
of the cable within a few miles of circuit and introduce other
operating problems as well. Although these problems do not occur
with DC cable systems, DC transmission involves the additional
cost of converters.
Aesthetic concerns and
difficulties in obtaining rights-of-way have increased the
pressures to place power circuits underground, and the future
will probably see a significant expansion in the use of
underground systems. The most extensive extra-high-voltage (EHV)
underground cable system at present is the 345 kV network that
supplies the New York City area.
Growth of The Electric Power Industry
The first commercial
electric-power installations in the United States were
constructed in the latter part of the 19th century. The
Rochester, N.Y., Electric Light Co. was established in 1880. In
1882, Thomas A. Edison's Pearl Street steam-electric station
began operation in New York City and within a year was reported
to have had 500 customers for the lighting services it supplied.
A short time later a central station powered by a small
waterwheel began operation in Appleton, Wis.
In 1886 the feasibility of sending
electric power greater distances from the point of generation by
using alternating current (AC) was demonstrated at Great
Barrington, Mass. The plant there utilized transformers to raise
the voltage from the generators for a high-voltage transmission
line.
The electric power industry of the
United States grew from small beginnings such as these to become,
in less than 10 years, the most heavily capitalized industry in
the country. It now comprises about 3,100 different corporate
entities, including systems of private investors, federal and
other government bodies, and cooperative-user groups. Less than
one-third of the corporate groups have their own generating
facilities; the others are directly involved only in the
transmission and distribution of electric power.
For several decades electric power
use in the United States grew at an average annual rate of about
7%, a rate that results in a doubling every 10 years. The rate of
growth remained constant, with only minor year-to-year
variations, until the early 1970s, when fuel shortages and rising
concern over possible environmental damage, together with reduced
expansion of the U.S. economy, slowed the growth rate. In the
period from 1974 to 1985 the annual increase in electricity use
varied between 1.7% and 6.2%. Although total energy use in the
United States has either declined or remained unchanged since
1973, electricity use has continued to grow.
Heat
Engines
The steam engine developed by
James Watt in 1769 was a type of heat engine. A heat engine is
any device that withdraws heat from a heat source, converts some
of this heat into useful work, and transfers the remainder of the
heat to a cooler reservoir. A major advance in the understanding
of the heat engine was provided in 1824 by N. L. Sadi Carnot, a
French engineer, in his discussion of the cyclic nature of the
heat engine. This theoretical approach is known as the Carnot
Cycle.
Heat
Exchanger
A heat exchanger is a device in
which heat is transferred from one fluid, across a tube or other
solid surface, to another fluid. When two fluids at different
temperatures enter the heat exchanger, the temperature of the
cold fluid increases. In this process, none of the transferred
heat is lost. Any device in which a temperature difference exists
may be classified as a heat exchanger. However, a heat exchanger
generally is considered to be a device for the transfer,
elimination, or recovery of heat without a change of the fluids'
state. If a fluid condenses, the heat exchanger is a condenser.
If a fluid evaporates, the heat exchanger is an evaporator.
The simplest heat exchanger is a
tube through which a hot liquid flows. Cool air flows around the
outside of the tube to carry away the heat, thereby heating the
air and cooling the liquid inside the tube. The automobile
radiator s an example. Usually a liquid-cooled heat exchanger is
of the shell-and-tube type. In it a smaller tube runs inside a
larger tube, tank, or shell. Cold liquid flows through one tube
to cool the hot liquid flowing in the other tube. This type of
heat exchanger is used in automobiles with automatic
transmissions to cool the hot automatic-transmission fluid. The
hot fluid is circulated through a tube located in the lower tank
of the radiator. There the cooler liquid in the engine cooling
system surrounds the tube to pick up and carry away the excess
heat.
Heat exchangers may be fired or
unfired. Examples of fired heat exchangers are boilers, furnaces,
and engines. The typical forced-air furnace widely used to heat
homes has a large heating chamber, or heat exchanger. Cool air is
circulated in close contact with the hot iron firebox. This heats
the air for distribution through ducts. Unfired heat exchangers
are condensers, coolers, and evaporators. These are used in
heating and refrigeration systems, power-plant cooling, and
chemical and food-processing plants.
History
of Plastics
The first synthetic plastic was
celluloid, a mixture of cellulose nitrate and camphor. Invented
in 1856 by Alexander Parkes, it was used initially as a
substitute for ivory in billiard balls, combs, and piano keys.
The high flammability of celluloid has restricted its use to
products that are small in size. For years celluloid was widely
used in photographic and motion picture film stock, until it was
superseded by the less dangerous polymer cellulose acetate.
In 1909 the second synthetic
plastic, phenol-formaldehyde (also called Bakelite), was invented
by Leo Baekeland when he simply heated a mixture of phenol and
formaldehyde. Shortly before World War II a number of synthetic
polymers were developed, including Casein, Nylon, Polyesters,
polyvinyl chloride, polystyrene, and polyethylene. Since then the
number as well as the types and qualities of plastics have
greatly increased, producing superior materials such as epoxies,
polycarbonate, Teflon, silicones, and polysulfones.
Two modern trends found in the
development of plastic materials are of interest. One is the
increased number of foamed plastics --plastics that are imbedded
with gas--and the other is the specific designing of plastics to
satisfy particular service requirements. The ability of chemists
to tailor the properties of plastics has become powerful and
dramatic. This may be illustrated by polyethylene, which is soft
and waxy when used as a film, but hard and abrasion-resistant
when used as a socket for an artificial hip joint.
Heat Pump
A heat pump efficiently heats and
cools air. It works on a direct expansion-refrigeration cycle for
cooling and a reverse-refrigeration cycle for heating. During
cooling, the refrigerant is compressed and discharged through a
four-way reversing valve that sends the hot gas to a condenser,
where it is liquefied. The high-pressure liquid flows through the
expansion valve, where it is expanded to a low-pressure gas in
the evaporator, a heat exchanger that transfers the heat of the
air to be cooled to the refrigerant to be vaporized. The gas is
returned to the compressor to repeat the cycle.
As it heats, the four-way
reversing valve sends the hot gas from the compressor into the
evaporator, where it heats air passing over its coils. The
high-pressure, high-temperature gas becomes a liquid that is
forced through the expansion valve into the outdoor coil
(condenser), which functions as an evaporator. Heat from outside
air vaporizes the liquid refrigerant, which becomes a
low-pressure, low-temperature gas and returns to the compressor.
Heat pumps that use air as the
cooling and heating medium work best in relatively mild climates.
At very high or very low temperatures the heat pump's efficiency
decreases rapidly. This is especially true in the heating cycle.
Most heat pumps are small units with heating and cooling
capacities between 6,000 and 300,000 British thermal units per
hour. Larger systems using well water instead of air deliver
unmatched efficiency in the heating cycle. A ground system heat
pump extracts the heat in the ground below the frost line through
pipes buried in the soil. Such a system can also extract heat
from groundwater.
It is possible to miniaturize all
the components in a heat pump except for the compressor.
Scientists theorize that eventually miniature heat pumps,
positioned on room walls, could heat and cool houses far more
efficiently and inexpensively than present heating and cooling
systems.
Heat
Transfer
Heat transfer concerns the flow of
heat energy in matter as a result of differences in temperature.
The energy, whether in the form of molecular motion or
electromagnetic radiation, obeys certain natural laws of heat
transfer in flowing from one body to another. Heat energy flows
naturally in only one direction, that is, from hotter objects to
cooler ones, and specialized devices are needed to reverse this
natural direction of transfer. Heat transfer takes place through
conduction, convection, or radiation. The science of
thermodynamics relates the rates of heat flow to temperature
differences and material properties. The efficient operation of
any device that uses energy is likely to depend on reducing
certain rates of heat transfer and increasing others. For
example, a home heating system operates most efficiently when the
heat loss through the building walls is minimized and the
heat-transfer rate from the burning fuel to the room air is
maximized.
Infrared
radiation
Infrared radiation is the region
of the electromagnetic spectrum between visible light and
microwaves, containing radiation with wavelengths ranging from
about 0.75 microns (1 micron equals 1 one-millionth of a meter)
to about 1,000 microns (1 mm). These limits are arbitrary,
because the characteristics of the radiation are unchanged on
either side of the limits. The discovery of infrared radiation is
attributed to Sir William Herschel who, in 1800, dispersed
sunlight into its component colors with a prism and showed that
most of the heat in the beam fell in the spectral region beyond
the red, where no visible light existed. In 1847, Armand Fizeau
and Jean Foucault of France showed that infrared radiation,
although invisible, behaved similarly to light in its ability to
produce interference effects.
Infrared radiation is generally
associated with heat because heat is its most easily detected
effect . Most materials, in fact, readily absorb infrared
radiation in a wide range of wavelengths, which causes an
increase in the temperatures of the materials. All objects with a
temperature greater than absolute zero emit infrared energy, and
even incandescent objects usually emit far more infrared energy
than visible radiation; about 60% of the Sun's rays are infrared.
Sources of infrared radiation other than hot, solid bodies
include the emissions of electrical discharges in gases and the
laser, which can emit highly monochromatic (single-wavelength)
infrared radiation.
Infrared radiation can be used to
detect the temperature of a distant object and therefore has many
temperature-sensing applications, such as in astronomy or in
heat-seeking military missiles. Photographs taken by infrared
radiation reveal information not detectable by visible light. In
the laboratory, infrared spectroscopy is an important method for
identifying unknown chemicals.
Joule
The joule is the unit of energy or
work in the mks (meter-kilogram-second) system of units. It is
the work done when a force of 1 newton acts through a distance of
1 meter, and is thus synonymous with a newton-meter of work. One
joule is equivalent to 1 watt-second, 10 million ergs, 0.7376
foot-pounds, and 9.48 X .0001 Btu.
Natural
Gas
Natural gas, a flammable gas
within the Earth's crust, is a form of petroleum and is second
only to crude oil in importance as a fuel. Natural gas consists
mostly (88 to 95 percent) of the hydrocarbon Methane (CH4), but
proportions of hydrocarbons (see HYDROCARBON) higher in the
methane series are usually present, including Ethane (C2H6), 3 to
8 percent; Propane (C3H8), 0.7 to 2 percent; Butane (C4H10), 0.2
to 0.7 percent; and Pentane (C5H12), 0.03 to 0.5 percent. Other
gases present include carbon dioxide (CO2), 0.6 to 2.0 percent;
nitrogen (N2), 0.3 to 3.0 percent; and helium (He), 0.01 to 0.5
percent. Carbon dioxide, nitrogen, and helium detract slightly
from the heating value of natural gas. Helium and carbon dioxide,
however, are valuable in their own right; in certain natural
gases where their concentrations are relatively high, they may be
extracted commercially.
The hydrocarbons that make up
natural gas area component of in -ground petroleum. In the past
the gas was considered a useless by-product of oil production and
was burned off in the oil fields as waste. Coal beds also contain
appreciable quantities of methane, the principal component of
natural gas.
Natural gas is produced on all
continents except Antarctica. The world's largest producer is
Russia. The United States, Canada, and the Netherlands are also
important producers.
The most efficient, least costly
means of transporting natural gas is via pipeline. The United
States has nearly 3.2 million km (2 million mi) of natural-gas
pipeline, much of it built during World War II. The
Siberian-Western Europe gas pipeline, completed in 1983, was
built to exploit the huge natural gas reserves of the former
USSR, primarily in present-day Russia.
The gas may also be transported in
pressurized tanks. Liquefied natural gas (LNG) must be kept under
very high pressures and at very low temperatures during
transport, but it requires far less space than the substance in
its gaseous state.
Natural gas is used primarily as a
fuel and as a raw material in manufacturing. It fuels home
furnaces and water heaters, clothes dryers, and cooking stoves.
It is used in brick, cement, and ceramic-tile kilns; in glass
making; for generating steam in water boilers; and as a clean
heat source for sterilizing instruments and processing foods.
As a raw material in petrochemical
manufacturing, its uses are widespread. They include the
production of sulfur, carbon black, and ammonia. Ammonia is used
as a source of nitrogen in
a range of fertilizers and as a
secondary feedstock for manufacturing other chemicals including
nitric acid and urea. Ethylene, perhaps the most important basic
petrochemical produced from natural gas, is used in manufacturing
plastics and many other products.
Widespread concern about the
environmental damage caused by the burning of coal and petroleum,
and the realization that reserves of natural gas may be much
greater than was once estimated, have spurred new technologies
that have already increased its use significantly. Small
gas-turbine generators add capacity to power-generation plants,
and utilities anticipate lowered pollution as more natural gas
replaces coal and oil. Because it is a clean-burning fuel that
emits less carbon monoxide and carbon dioxide than gasoline,
natural gas is already being used instead of gasoline in some
U.S. truck, bus, and auto fleets.
By the end of the 20th century,
the greatest growth in the use of natural gas will occur in the
Pacific region. Projections for a 70 percent increase in energy
use in that region by the year 2000 have spurred plans for the
construction of international pipelines running east and south
from Russia and Kazakhstan, north from Australia and Indonesia,
and west from Alaska.
Overhead Transmission Lines
Many of the first high-voltage
transmission lines in the United States were built principally to
transmit electrical energy from hydroelectric plants to distant
industrial locations and population centers. High-voltage
transmission lines were originally designed to permit the
construction of large generating units and central stations on
attractive, remote sites close to fuel sources and supplies of
cooling water. Today, however, they connect different power
networks in order to achieve greater economy by exchanges of
low-cost power, to achieve savings in reserve generating
capacity, to improve the reliability of the system, and to take
advantage of diversity in the peak loads of different systems and
thereby reduce operating costs.
At one time power lines in the
33-kV or 44-kV class were classified as high-voltage lines. As
loads increased and transmission distances became greater,
transmission voltages were increased. Electrical losses increase
proportionately to the square of the current--the higher the
voltage of the line, the lower the current needed to carry an
equivalent amount of power. Moreover, one high-voltage line can
usually carry as much power as several lower-voltage ones, so the
use of higher voltages reduces the number of lines required and
conserves the space required for rights-of-way. Voltage levels
increased to 69, 115, 138, and 161 kV in various sections of the
United States. Before World War II the highest-voltage lines in
the United States were 230 kV, with the exception of one 287-kV
line from Boulder Dam to Los Angeles. In the early 1950s several
345-kV lines were constructed. By 1964 the first 500-kV lines in
the United States were being completed, and in 1969 the first
765-kV line was put into service. All of these involved AC
systems.
In 1970 a 1,380-km (856-mi),
800-kV direct-current (DC) line was placed in commercial service
to connect northwestern U.S. hydroelectric sources with the Los
Angeles area. Such systems offer an economical means of
transferring large quantities of power over long distances. They
also avoid stability problems sometimes encountered by AC
systems; DC systems are sometimes used to connect AC systems even
over short transmission distances.
Peak Load
All electric-utility systems
experience cyclic load patterns involving higher demands for
electric power at some hours of the day and some seasons of the
year than at others. Such considerations affect the design of a
utility's generating capacity plant because some types of
generating equipment are better suited to supplying base, or
continuous, loads and may not operate satisfactorily or
economically over a varying load cycle; others are better
designed for the variable loading, intermittent use, and frequent
start-up and shutdown required by such patterns of operation.
Hydroelectric plants are often well adapted to intermittent
operation and may be useful for supplying peaking power. They can
be constructed only in special locations, however, and they must
often rely on fuel plants to supply peaking needs. Steam plants
especially designed for peaking service have been installed in a
few systems, and internal combustion units have sometimes been
used for such service.
Properties
of Plastics
The bonding properties and
chemical versatility of carbon account for the great number of
plastics. Although carbon is the backbone of polymer chains,
other elements are included, to varying degrees, in the chemical
structures of plastics. These include hydrogen, oxygen, nitrogen,
chlorine, fluorine, and occasionally other elements, such as
sulfur and silicon.
While progress in polymer
technology makes it increasingly difficult to make general
statements about these materials, the following properties are
characteristic of most plastics:
low strength--for the
familiar plastics, about one-sixth the strength of structural
steel
low stiffness
(technically, modulus of elasticity)--less then one-tenth
that of metals, except for reinforced plastics
a tendency to creep,
that is, to increase in length under a tensile stress
low hardness (except
formaldehyde plastics)
low density, usually an
advantage, the density of most plastics being close to that
of water
brittleness at low
temperatures and loss of strength and hardness at moderately
elevated temperatures (thermal expansion of plastics is about
ten times that of metals)
flammability, although
many plastics do not burn
outstanding electrical
characteristics, such as electrical resistance
degradation of some
plastics by environmental agencies such as ultraviolet
radiation, although most plastics are highly resistant to
chemical attack
Almost all of the characteristics
mentioned above can be modified to some degree by the addition to
a given plastic of suitable fillers or reinforcing fibers. For
example, a number of plastics have been developed that can
sustain elevated temperatures, including Teflon and the
silicones. Addition of other materials to plastics generally
reduces their property of electrical resistance. On the other
hand, a number of plastics have more recently been developed for
the specific purpose of making them electrically conductive. The
aim of such research is to produce cheap and lightweight
components for use in the electronics industry.
Plastics
The bonding properties and
chemical versatility of carbon account for the great number of
plastics. Although carbon is the backbone of polymer chains,
other elements are included, to varying degrees, in the chemical
structures of plastics. These include hydrogen, oxygen, nitrogen,
chlorine, fluorine, and occasionally other elements, such as
sulfur and silicon. @bWhile progress in polymer technology makes
it increasingly difficult to make general statements about these
materials, the following properties are characteristic of most
plastics: 1. low strength--for the familiar plastics, about
one-sixth the strength of structural steel 2. low stiffness
(technically, modulus of elasticity)--less then one-tenth that of
metals, except for reinforced plastics 3. a tendency to creep,
that is, to increase in length under a tensile stress 4. low
hardness (except formaldehyde plastics) 5. low density, usually
an advantage, the density of most plastics being close to that of
water 6. brittleness at low temperatures and loss of strength and
hardness at moderately elevated temperatures (thermal expansion
of plastics is about ten times that of metals) 7. flammability,
although many plastics do not burn 8. outstanding electrical
characteristics, such as electrical resistance 9. degradation of
some plastics by environmental agencies such as ultraviolet
radiation, although most plastics are highly resistant to
chemical attack Almost all of the characteristics mentioned above
can be modified to some degree by the addition to a given plastic
of suitable fillers or reinforcing fibers. For example, a number
of plastics have been developed that can sustain elevated
temperatures, including Teflon and the silicones. Addition of
other materials to plastics generally reduces their property of
electrical resistance. On the other hand, a number of plastics
have more recently been developed for the specific purpose of
making them electrically conductive. The aim of such research is
to produce cheap and lightweight components for use in the
electronics industry.
Ethylene-Based
Plastics
The simplest structure among the
many thermoplastics is that of polyethylene. Addition
polymerization is the name given to the process in which each
ethylene monomer opens up at a double bond and joins to the end
of the lengthening chain. The earliest thermoplastics to be
developed had the basic structure of polyethylene and were made
by addition polymerization. These polymers could be created
simply by substituting other atoms or groups of atoms for one or
more of the four hydrogen atoms in the ethylene monomer. Polyvinyl
chloride is made from an ethylene monomer in which one
chlorine atom has replaced one hydrogen atom. The result is a
polymer that is nonflammable. Polyvinyl fluoride is made from an
ethylene monomer in which a fluorine atom has replaced a hydrogen
atom. The result is another polymer with improved heat
resistance. Polyvinyl alcohol involves the substitution of an OH
group, which causes the polymer to be water soluble.
Polytetrafluoroethylene (Teflon) contains fluorine atoms in place
of all hydrogen atoms. The well-known properties of this plastic
include remarkable heat resistance as well as the inability to be
softened by heat. In polypropylene a methyl group (CH3) replaces
one hydrogen atom. In the monomer of polystyrene a phenyl ring of
six carbon atoms is attached to the ethylene unit in place of one
hydrogen atom. This bulky side group results in a brittle
plastic.
Except for the fluorinated
polymers and the acrylic polymers, thermoplastics must be
protected from destruction caused by ultraviolet radiation. Carbon
black provides such protection in polyethylene pipe, but
other additives must be used if the product must be white or
pigmented.
The consumption of polyethylene
exceeds that of any other plastic. This soft, flexible, waxy
material is produced in five grades: low density, medium density,
high density, ultrahigh molecular weight (UHMW), and irradiated
(cross-linked by radiation). It is also made into a flexible
foam. The differences in density result from differences in the
degree of crystallinity. When the long polymer chains are ordered
in a parallel arrangement like the atoms in a metal crystal, the
result is a higher density than would be possible in a random or
disordered distribution. The branching of polymer chains also
leads to lower densities. Although low-density polyethylene has
the highest vapor transmission rate, it is the least expensive of
the five grades and is used as a vapor barrier in buildings.
High-density polyethylene is used in blown bottles and pipes. The
UHMW grade is a harder, stronger material.
Polypropylene is hard and
strong, and has a higher useful temperature range than
polyethylene, polyvinyl chloride, and polystyrene. It is highly
crystalline. At low temperatures it becomes brittle, but this is
overcome by copolymerization with ethylene or other monomers.
Polymethyl methacrylate (PMMA),
also called acrylic, is known by its trade names Lucite and
Plexiglass. Its monomer contains a complex side group, which
prevents crystallization. PMMA has outstanding resistance to
outdoor environments, including ultraviolet radiation. It has
excellent optical properties and unlimited coloring
possibilities. It is also harder and stronger than the plastics
previously mentioned, although it is brittle. PMMA is familiar in
lighting fixtures, outdoor signs, aircraft windows, and
automobile taillights.
The fluorocarbon group consists of
several polymers, all containing fluorine. The presence of
fluorine makes these polymers nonflammable. The carbon-fluorine
bond is extremely stable and provides chemical and heat stability
and low surface tension, thus leading to low friction and
nonwetting, nonstaining, nonsticking properties. New resins
called Teflon AFs are also amorphous, enhancing their physical
properties and making them of potential great usefulness in
optical and electronic circuits for computers and instruments.
Polyvinylidene dichloride (PVDC) is a tough, protective plastic
that can be processed to exhibit Piezioelectricity, making it
valuable for many applications in electronics.
Polyvinyl chloride (PVC) is
a stiff plastic made soft and flexible by adding plasticizers. It
is used as shower curtains, hoses, and electrical insulation.
Polystyrene is a clear, hard, brittle plastic that is attacked by
many solvents.
Power
Power, in physics, is the rate at
which work is done. A given amount of work done over a long
period of time represents less power than that work done over a
short period of time. The average power required to accomplish a
certain amount of work is found by dividing the work by the time
period during which it is done. The instantaneous power
requirement at any moment during the job may be found from the
time derivative of the work function, a concept of calculus. For
example, the familiar 1/4-horsepower electric motor in many
household appliances may deliver several horsepower for a short
period just after it is turned on; its average power output over
a long running period is likely to be somewhat less than 1/4
horsepower. Units of power are properly expressed in terms of
work per unit time. In the international metric (SI) system,
power is expressed in joules per second or watts (W). A machine
capable of delivering 746 W of continuous power is rated at one
Horsepower in the English system of physical units. Other units
of power include the Btu/h and the foot-pound/sec.
Propane
Propane is a Hydrocarbon of
molecular weight 44.09 and boiling point -42.1 deg C (-43.8 deg
F) at atmospheric pressure. Like ethane and methane, propane is a
member of the Alkane series of hydrocarbons. It is an important
fuel gas and chemical feedstock and is a major constituent of
Liquefied Petroleum Gas. The net calorific value of propane is
about 12,000 cal/g (21,600 Btu/lb). When used as a fuel, 23.8 cu
m of air are required for the combustion of 1 cu m of propane
gas, with the products of combustion being carbon dioxide, water
and nitrogen. The ignition temperature is 466 deg C, and the
flame temperature is 1,970 deg C.
Electric Generating Plants
Virtually all commercial electric
energy is now produced by generators driven by steam from the
burning of fossil fuels or from nuclear sources or by hydropower.
Developed nations depend mainly on fossil fuels, but some
countries now depend more heavily on nuclear energy produced by
materials such as uranium. France, for example, generates about
70% of its electricity from nuclear power plants; power costs in
that nation are the lowest in Europe.
A basic steam-power plant includes
a furnace or reactor for raising the temperature of the water in
a boiler, or steam generator, until it changes into steam, and a
turbine, which drives the generator to produce electric power.
Throughout the history of the electric power industry,
improvements in design, metallurgy, fabrication techniques, and
control systems have permitted continual increases in the size,
operating temperatures, pressures, and efficiencies of electric
generating units. These improvements and increasing demands for
electric power have led generating facilities to develop from the
early steam-engine-driven generator, which could produce a few
kilowatts (kW), today's giants, with outputs as high as 1,300,000
kW. Hydroelectric, or waterpower, generators have grown from the
12-kW machines of 1882 to the 600,000-kW units at the Grand
Coulee station in Washington state.
Electrical Power Transmission
Electric power transmission
systems consist of step-up transformer stations to connect the
lower-voltage power-generating equipment to the higher-voltage
transmission facilities; high-voltage transmission lines and
cables for transferring power from one point to another and
pooling generation resources; switching stations, which serve as
junction points for different transmission circuits; and
step-down transformer stations that connect the transmission
circuits to lower-voltage distribution systems or other user
facilities. In addition to the transformers, these transmission
substations contain circuit breakers and associated connection
devices to switch equipment into and out of service, lightning
arresters to protect the equipment, and other appurtenances for
particular applications of electricity. Highly developed control
systems, including sensitive devices for rapid detection of
abnormalities and quick disconnection of faulty equipment, are an
essential part of every installation in order to provide
protection and safety for both the electrical equipment and the
public.
Radiation
Radiative heat transfer involves
the flow of energy in the form of electromagnetic waves.
Radiation thus differs fundamentally from conduction and
convection, in that it does not depend on the presence of matter.
The energy of electromagnetic radiation is not the same thing as
heat, but when the radiation strikes an absorbing material it is
converted into heat. Heat may even be transmitted across a vacuum
in this way, through conversion processes. To be transmitted,
however, the energy must originate in matter at a higher
temperature than the matter receiving the energy.
The term radiation refers both to
the transmission of energy in the form of waves, and to the
transmission of streams of atomic particles through space. Any
energy that is transmitted in the form of waves is some kind of
electromagnetic radiation. Each kind is distinguished by its
wavelength, or frequency. All kinds of electromagnetic radiation
obey the same physical laws, they all travel at the speed of
light, and when they fall on a surface they exert a pressure
proportional to the net flux of energy divided by the speed of
light. Roughly in the order of decreasing wavelength, the kinds
of electromagnetic radiation are radio waves, radiant heat energy
and microwaves, infrared radiation, light, ultraviolet radiation,
X rays, and gamma rays. Many forms of particulate radiation are
possible. In the phenomenon of radioactivity, alpha radiation and
beta radiation are observed, along with gamma rays. Very
energetic particles from outer space are called cosmic rays. Any
particulate or electromagnetic radiation that can dissociate
atoms into ions is called ionizing radiation. Such radiation can
produce harmful effects in organisms, and it is of concern in
matters dealing with nuclear energy. It is also widely used in
medicine, however, for both diagnosis and therapy as well as
being widely employed in scientific research.
Resistance
Although a conductor permits the
flow of charge, it is not without a cost in energy. The electrons
are accelerated by the electric field. Before they move far,
however, they collide with one of the atoms of the conductor,
slowing them down or even reversing their direction. As a result,
they lose energy to the atoms. This energy appears as heat, and
the scattering is a resistance to the current.
In 1827 a German teacher named
George OHM demonstrated that the current in a wire increases in
direct proportion to the voltage V and the cross-sectional areas
of the wire A, and in inverse proportion to the length I. Because
the current also depends on the particular material, Ohm's law is
written in two steps, I = V/R, and R = pI/A X the resistivity.
The quantity R is called the Resistance. The Resistivity depends
only on the type of material. The unit of resistance is the Ohm,
where 1 ohm is equal to 1 volt/mp.
In lead, a fair conductor, the
resistivity is 22/100,000,000 ohm-meters; in copper, an excellent
conductor, it is only 1.7/100,000,000 ohm-meters. Where high
resistances between 1 and 1 million ohms are needed, Resistors
are made of materials such as carbon, which has a resistivity of
1,400/100,000,000 ohm-meters.
Certain materials, such as lead,
lose their resistance almost entirely when cooled to within a few
degrees of absolute zero. Such materials are called
superconductors. Substances have recently been found that become
superconductive at much higher temperatures.
The resistive heating caused by
electron scattering is a significant effect and is used in
electric stoves and heaters as well as in incandescent light
bulbs. In a resistor the power P, or energy per second, is given
by P = (I squared) R.
Solar
Radiation
Radiation given off by the Sun,
consisting mainly of visible light, *ultraviolet radiation, and
*infrared radiation, although the whole spectrum of
electromagnetic waves is present, from radio waves to X-rays.
High-energy charged particles such as electrons are also emitted,
especially from solar flares. When these reach the Earth, they
cause magnetic storms (disruptions of the Earth's magnetic
field), which interfere with radio communications.
Thermostat
A thermostat is an
electro-mechanical on/off switch that is activated by temperature
changes. It is typically used to control a heating or cooling
system. The sensing element is usually a spiral bimetallic strip
that coils and uncoils in response to temperature changes because
of differential expansion of the two bonded metals. The switch
element is either a set of electrical contacts or a
glass-encapsulated mercury switch that controls a low-voltage
relay. The relay can actuate a motor starter and igniter for an
oil burner, a heavy-duty switch for electrical units, or a
solenoid-operated valve on a gas furnace. The thermostat may also
control a house-type air conditioner or heat pump. To reduce
temperature swings, a small electrical heater unit is energized
during the warming period, causing the switch to break
prematurely in anticipation of room-heater override.
Thermometer
The thermometer, a device for
measuring temperature, is used in many forms, basically divided
into mechanical and electrical types. The best-known mechanical
type is the liquid-in-glass thermometer, and an important
electrical type is the resistance thermometer. To cover the full
range of temperature measurement, from near Absolute Zero to
thousands of degrees, other instruments are also used, such as
the Bolometer, Pyrometer, Thermocouple, and thermopile. The
temperature scales most commonly used on thermometers are the
Celsius scale, the Fahrenheit scale, and the Kelvin scale. The
first two are based on the freezing and boiling points of water,
although the Celsius (C) is numerically more useful than the
Fahrenheit (F) scale because those two points are assigned the
numbers 0 and 100, respectively. The Kelvin (K) scale has the
widest scientific applications because 0 degrees on the scale
corresponds to absolute zero.
The liquid-in-glass thermometer
consists of a small bulb reservoir and a calibrated fine-bore
capillary tube. The liquid in the bulb rises or falls in the tube
as it expands or contracts in response to temperature changes.
The height of the column is measured against the markings on the
tube. Mercury is the preferred liquid in quality thermometers. It
freezes at - 38.9 degrees C ( - 38 degrees F) and boils at 357
degrees C (675 degrees F). The accuracy of industrial mercury
thermometers is 1 percent of the column. Other liquids used are
dye-colored alcohol, toluene, and pentane, the last with a
freezing point of - 200 degrees C ( - 328 degrees F).
A second type of liquid-expansion
thermometer consists of a liquid-filled metal bulb and capillary
tube attached to either a spiral tube or a bellows. As the
temperature of the bulb changes, the pressure or the volume of
the liquid changes, moving an indicator across a scale.
A typical gas or vapor thermometer
similarly consists of a bulb and a capillary tube connected to a
pressure-measuring device. The gas thermometer is simple, rugged,
and accurate and has a wide response. Vapor-pressure thermometers
respond to the pressure exerted by saturated vapor in equilibrium
with a volatile liquid. It is similar to the gas thermometer in
construction. The principal advantage of the vapor-pressure type
is the large change in pressure obtained for small temperature
changes, resulting in high sensitivity.
Electrical resistance thermometers
operate on the principle that the resistivity of most metals
increases with increased temperature. This principle was
discovered in 1821 by Sir Humphry Davy, but this phenomenon was
not used until the construction of a platinum resistance
thermometer in 1861 by the German engineer Ernst W. von Siemens.
In 1886 the British physicist Hugh L. Callendar proposed this
thermometer as a new standard of accuracy in temperature
measurement. Today the U.S. National Institute of Standards and
Technology uses high-precision platinum resistance thermometers,
accurate to 0.001 degrees C, to define the key points on the
International Practical Temperature Scale, established in 1968.
Both copper-wire and nickel-wire resistance thermometers are much
lower in cost than platinum thermometers and have a precision of
0.05 degrees C. In the range of 10 degrees to 2 degrees above
absolute zero, impurity-doped germanium resistance thermometers
are used, calibrated against the temperature of liquid helium.
Ultraviolet
Electromagnetic radiation having
wavelengths shorter than visible light but longer than X rays is
called ultraviolet light, or ultraviolet radiation. This light is
invisible to human eyes and is also known as black light. The
ultraviolet region of the spectrum was discovered in 1801 by
German physicist Johann Ritter in the course of photochemical
experiments.
Ultraviolet light is generally
divided into the near, far, and extreme ultraviolet regions . The
extreme wavelengths, which are particularly harmful to life, are
strongly absorbed by the Earth's atmosphere and particularly by
the Ozone Layer.
Ultraviolet light is created by
the same processes that generate visible light--transitions in
atoms in which an electron in a high-energy state returns to a
less energetic state. Fluorescent and mercury-vapor lamps produce
large amounts of ultraviolet light, which is filtered out when
the lamps are intended for optical use. Visible light may instead
be filtered out to achieve black-light effects through the
induced Luminescence of objects by ultraviolet light.
Biological effects of ultraviolet
light include sunburn and tanning. Excessive exposure has been
linked to the development of skin cancers and of cataracts in the
eye. Far ultraviolet light, which has the ability to destroy
certain kinds of bacteria, is used for sterilizing foodstuffs and
medical equipment.
Voltage
Whether as an emf or an electric
potential, voltage is a measure of the ability of a system to do
work on a unit amount of charge by electrical means. Voltage is a
better-known quantity than electric field. For instance, voltages
measured in an electrocardiogram peak at 5 millivolts; many are
familiar with the 115-volt potential of a house. The potential
between a cloud and the ground just before a typical lightning
bolt is a minimum of 10,000 volts.
Devices for developing or changing
potential or emf include batteries, generators, transformers, and
Van De Graaff Generators. Sometimes high voltages are needed. For
instance, the electron beams in television tubes require more
than 30,000 volts. Electrons "falling" through such a
potential reach velocities as high as one-third the speed of
light and have sufficient energy to cause a spot of light on the
screen. Such high potentials may be developed from low
alternating potentials by using a transformer.
By scuffing shoes on a carpet on a
dry day, an electric potential of more than 20,000 volts can be
developed, resulting in a spark.
Watt
The watt is the unit of power
ordinarily employed in mechanics and electricity. One watt equals
1 joule per second, or 10 million ergs, and 746 watts equal 1
horsepower (h.p.). The power in watts developed in an electrical
circuit is equal to the potential (volts) times the current
(amperes). In heat measurement, which customarily uses calories
and Btu's as energy units, 1 watt equals 0.239 calories per
second or 3.4192 Btu/h. Multiplying a unit of power by a time
unit, such as is done to obtain kilowatt hours (kW h), gives
units of energy.
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Thursday May 15, 2008 - 07:22 AM
