TEXT
Atom and Atomic Theory, the study of the nature
of atoms and the forces which hold them together. In ancient Greek philosophy
the word atom was used to describe the smallest bit of matter that could
be conceived. This “fundamental particle,” to use the present-day term
for this concept, was thought of as indestructible; in fact, the Greek
word for atom means “not divisible.” Knowledge about the size and nature
of the atom grew slowly throughout the centuries when people were content
merely to speculate about it.
With the advent of experimental science in
the 16th and 17th centuries (see CHEMISTRY; SCIENCE), progress in atomic
theory quickened. Chemists soon recognized that all liquids, gases, and
solids can be analyzed into their ultimate components, or elements (see
ELEMENTS, CHEMICAL). For example, salt was found to be composed of two
distinct and different elements, sodium and chlorine, which are joined
together in an intimate form known as a chemical compound. Air was discovered
to consist of a mixture of the gases nitrogen and oxygen. Water was symbolized
as HOH, meaning that it consists of two atoms of hydrogen for every atom
of oxygen.
Dalton's Theory
John Dalton, a British schoolmaster and chemist,
was fascinated by the patchwork puzzle of the elements. Early in the 19th
century he made studies of the way in which the various elements combine
with one another to form chemical compounds. Other scientists, among them
the English physicist Sir Isaac Newton, had already speculated that the
smallest units of a substance are atoms. Dalton was regarded as the founder
of atomic theory because he made the theory quantitative. He showed how
these atoms link together in definite proportions. Subsequent investigations
proved that the smallest unit of a chemical substance such as water is
a molecule. Each molecule of water consists of a single atom of oxygen
and two atoms of hydrogen joined by an electrical force called a “chemical
bond.” See CHEMICAL REACTION.
All atoms of any given element behave in
the same way chemically. Thus, from a chemical viewpoint, the atom is the
smallest entity to be considered. The chemical properties of the various
elements are quite different; their atoms combine in many different ways
to form a multitude of different chemical compounds. Some elements, such
as the gases helium and argon, are inert, that is, they fail to react with
other elements. Unlike oxygen, which has a diatomic molecule (two atoms
combined in a single molecule), helium and other inert gases are monatomic
elements, with a single atom per molecule. See NOBLE GASES.
Avogadro's Law
The study of gases attracted the attention
of the Italian physicist Amedeo Avogadro, who in 1811 formulated an important
law bearing his name (see AVOGADRO'S LAW). This law states that equal volumes
of different gases contain the same number of molecules when compared under
the same conditions of temperature and pressure. Given these conditions,
two identical bottles, one filled with oxygen and the other with helium,
will contain exactly the same number of molecules. Twice as many atoms
of oxygen will be present, however, because oxygen is diatomic.
Atomic Weight
Measurement of the weights of standard volumes
(that is, the densities) of different gases permits direct comparison of
the weights of individual gas molecules. When oxygen is taken as a standard
and the oxygen atom is assigned a value of 16.0000 atomic mass units (amu),
helium is found to have an atomic weight of 4.003 amu, fluorine 19.000,
and sodium 22.997. (Note that it is customary to speak of “atomic weights,”
although “atomic masses” would perhaps be more accurate. Mass is a measure
of the quantity of matter in a body, whereas weight is the force exerted
on the body by the influence of gravity. Thus, “atomic weight” is measured
in amu. In processes that occur within the nuclei of atoms, such as nuclear
fission, mass is converted into energy.)
The observation that many atomic weights
are close to whole numbers led the British chemist William Prout to suggest
in 1816 that all elements might be composed of hydrogen atoms. Subsequent
measurements of atomic weights revealed that chlorine, for example, has
an atomic weight of 35.455. The discovery of such fractional atomic weights
appeared to invalidate Prout's hypothesis until a century later, when it
was discovered that the atoms of most elements do not all have the same
weight. Atoms of the same element that differ in weight are known as isotopes
(see ISOTOPE). In the case of chlorine two isotopes occur in nature. Experiments
show that chlorine is a mixture of three parts of chlorine-35 for every
one part of the heavier chlorine-37 isotope. This proportion accounts for
the observed atomic weight of chlorine. Atomic scientists can measure isotopes
with great precision. For example, the light isotope of chlorine is measured
at 34.97867 amu.
The standard used for the calculation of
atomic weights has recently been changed. During the first part of the
20th century it was customary to use natural oxygen as the standard against
which atomic weights or masses were computed; oxygen was assigned an integral
atomic weight of 16. This standard was used by chemists even after the
rare isotopes of oxygen (oxygen-17 and oxygen-18) were discovered in 1929,
because the small amounts of these isotopes in natural oxygen are relatively,
although not absolutely, in constant proportion to the abundant isotope,
oxygen-16. Physicists found it easier, however, to compute atomic masses
against only the oxygen-16 isotope. This method resulted in two slightly
different tables of atomic weights or masses. The situation was resolved
in the early 1960s, when the international unions of chemistry and physics
agreed on a single new standard, the abundant isotope of carbon, carbon-12.
The new standard completely replaced the two earlier standards for all
scientists. The new standard is particularly appropriate because carbon-12
is often used as a reference standard in computations of atomic masses
using the mass spectrometer. Moreover, the table of atomic weights based
on carbon-12 is in close agreement with the old table based on natural
oxygen.
Periodic Table
By the middle of the 19th century several
chemists recognized that similarities in the chemical properties of various
elements implied a regularity that might be illustrated by arranging the
elements in a tabular or periodic form. The Russian chemist Dmitry Mendeleyev
proposed a chart of elements called the periodic table (see PERIODIC LAW),
in which the elements are arranged in rows and columns so that elements
with similar chemical properties are grouped together. According to this
arrangement, each element was assigned a number (atomic number) ranging
from 1 for hydrogen to 92 for uranium. Because not all the elements were
known at the time of Mendeleyev, blank spaces were left in the periodic
table, each of which corresponded to a missing element. Further research,
aided by the arrangement of the known elements in the chart, led to the
discovery of missing elements. Elements of higher atomic number have correspondingly
heavier atomic weights; this fact could have been predicted from Prout's
hypothesis.
Size of the Atom
Curiosity about the size of the atom and
its weight tantalized hundreds of scientists for a long period during which
lack of adequate instruments and proper techniques prevented them from
obtaining satisfactory answers. Subsequently, a variety of ingenious experiments
was devised to determine the size and weight of the various atoms. The
lightest of all atoms, hydrogen, has a diameter of 1 × 10-8 cm (0.00000001
cm) and weighs 1.7 × 10-24 (the fraction of a gram represented by
17 preceded by 23 zeros and a decimal point). An atom is so small that
a single drop of water contains more than a million million billion atoms.
Radioactivity
That the atom is not a solid bit of matter,
incapable of further subdivision, became evident with the discovery of
radioactivity. In 1896 the French physicist Antoine Henri Becquerel found
that certain substances, such as uranium salts, give off penetrating rays
of mysterious origin. Only a year earlier the German scientist Wilhelm
Conrad Roentgen had announced the discovery of X rays, which can penetrate
sheets of lead. The French scientists Marie Curie and her husband Pierre
Curie contributed further to an understanding of radioactive substances
(see RADIUM). As a result of the research of the British physicist Ernest
Rutherford and his contemporaries, it was shown that uranium and some other
heavy elements, such as thorium and radium, emit three different kinds
of radiation, initially called alpha (a), beta (b), and gamma (g) rays.
The first two, which were found to consist of electrically charged bits
of matter, are now called alpha and beta particles. Gamma rays eventually
were identified as electromagnetic waves, similar to X rays but of shorter
wavelengths (see ELECTROMAGNETIC RADIATION).
Rutherford Nuclear Atom
Recognition of the nature of radioactive
emissions enabled physicists to penetrate into the mystery of the atom.
Far from being a solid bit of matter, the atom was found to consist mostly
of space. At the center of this space is an infinitesimally small core
called the nucleus. Rutherford established that the mass of the atom is
concentrated in its nucleus. He also proposed that satellites called electrons
travel in orbits around the nucleus (see ELECTRON). The nucleus has a positive
charge of electricity; the electrons each have a negative charge. The charges
carried by the electrons add up to the same amount of electricity as resides
in the nucleus, and thus the normal electrical state of the atom is neutral.
Bohr Atom
To explain the structure of the atom, the
Danish physicist Niels Bohr developed in 1913 a hypothesis known as the
Bohr theory of the atom (see QUANTUM THEORY). He assumed that electrons
are arranged in definite shells, or quantum levels, at a considerable distance
from the nucleus. The arrangement of these electrons is called the electron
configuration. The number of such electrons equals the atomic number of
the atom; hydrogen has a single orbital electron, helium has 2, and uranium
has 92. The electron shells are built up in a regular fashion from a first
shell to a total of seven shells, each of which has an upper limit to the
number of electrons that it can accommodate. The first shell is complete
with two electrons, the second can hold up to eight electrons, and successive
shells hold still larger numbers. The “last” electrons, those which are
outermost or added last to the atom's structure, determine the chemical
behavior of the atom.
The inert, or noble, gases (helium, neon,
argon, krypton, xenon, and radon) all have completely filled outer shells.
They do not enter into chemical combinations in nature, although the three
heaviest inert gases (krypton, xenon, and radon) have formed chemical compounds
in the laboratory. On the other hand, the outermost shells of such elements
as lithium, sodium, and potassium contain only one electron. These elements
combine readily with other elements (transferring their outermost electrons
to them) to form a great many chemical compounds.
Atomic shells do not necessarily fill up
with electrons in consecutive order. The electrons of the first 18 elements
in the periodic table are added in a regular manner, each shell being filled
to a designated limit before a new shell is started. Beginning with the
19th element, the outermost electron starts a new shell before the previous
shell is completely filled. A regularity is still maintained, however,
as electrons fill successive shells in a repetitious back-and-forth pattern.
The result is the regular repetition of chemical properties for atoms of
increasing atomic weight that corresponds to the arrangement of the elements
in the periodic table.
It is convenient to visualize the electrons
moving about the nucleus of an atom much as if they were planets moving
about the sun. This view is much more precise than that held by contemporary
physicists, however. It is now known that it is impossible to pinpoint
the precise position of an electron in the atom's space without disturbing
its predicted location at some future time. This uncertainty is resolved
by attributing to the atom a cloudlike form, in which the electron's position
is defined in terms of the probability of finding it at some distance from
the nucleus. This rather fuzzy schematic conception of the atom may be
reconciled with the solar-system model by noting that in the tiny space
of the atom the electron, which makes many billions of orbits around the
nucleus in a single second, is everywhere at once. The cloud view thus
gives a form to the atom that is not supplied by a solar-system model.
Line Spectra
One of the great successes of theoretical
physics was the explanation of the characteristic line spectra of various
elements (see SPECTROSCOPY: SPECTRUM LINES). Atoms excited by a supply
of energy from an external source emit light of well-defined frequencies.
If hydrogen gas, for example, is held at low pressure in a glass tube and
an electrical current is passed through it, visible light of a reddish
color is given off. Careful examination of this light with a prism spectroscope
shows a line spectrum, a series of regularly spaced lines of light, each
of which has a definite wavelength and associated energy. The Bohr theory
permits the physicist to calculate these wavelengths in a straightforward
fashion. It is assumed that in the hydrogen atom the outer electron can
move in stable orbits. While the electron remains in an orbit at a fixed
distance from the nucleus, the atom does not radiate energy. When the atom
is excited, the electron jumps to a higher-energy orbit farther from the
nucleus, and as it falls back to its normal orbit, it emits a discrete
amount of energy corresponding to a certain wavelength of light. Each line
of light observed represents an electronic transition between a higher
and lower energy orbit.
In many heavier elements, if an atom is sufficiently
excited so that inner electrons close to the nucleus are affected, then
penetrating radiation, or X rays, will be emitted. These electronic transitions
involve large amounts of energy.
Atomic Nucleus
In 1905 Albert Einstein developed his mass-energy
equation, E = mc2, as part of his special theory of relativity. This equation
states that with a given mass (m) is associated an amount of energy (E)
equal to this mass multiplied by the square of the velocity of light (c).
A very small amount of mass is equivalent to a vast amount of energy. Because
more than 99 percent of the atom's mass is in the nucleus, any release
of the atom's energy would have to come from the nucleus.
In 1919 Rutherford exposed nitrogen gas to
a radioactive source that emitted alpha particles. Some of the alpha particles
collided with the nuclei of the nitrogen atoms. As a result of these collisions,
the nitrogen atoms were transmuted into oxygen atoms. A positively charged
particle was emitted from the nucleus of each of the atoms undergoing transmutation.
These particles were recognized as being identical to the nuclei of hydrogen
atoms. They are called protons (see PROTON). Although further research
proved that protons are constituents of the nuclei of all elements, no
more clues to the structure of the nucleus were found until 1932, when
the British physicist Sir James Chadwick discovered in the nucleus another
particle, known as the neutron, having the same weight as the proton but
without an electrical charge. It was then realized that the nucleus is
made up of protons and neutrons. In any given atom, the number of protons
is equal to the number of electrons and hence to the atomic number of the
atom. Isotopes are then explained as atoms of the same element (that is,
containing the same number of protons) that have different numbers of neutrons.
In the case of chlorine, one isotope is identified by the symbol of 35Cl
and its heavy relative by 37Cl. The superscripts identify the mass number
of the isotope and are numerically equal to the total number of neutrons
and protons in the nucleus of the atom. Sometimes the atomic number is
given as a subscript, as in }Cl.
The least stable arrangement of nuclei is
one in which an odd number of neutrons and an odd number of protons are
present; all but four isotopes containing nuclei of this kind are radioactive.
The presence of a large excess of neutrons over protons detracts from the
stability of a nucleus; nuclei in all isotopes of elements above bismuth
in the periodic table contain this type of arrangement, and they are all
radioactive. Most known stable nuclei contain an even number of protons
and an even number of neutrons.
Artificial Radioactivity
Experiments by the French physicists Frédérick
and Irène Joliot-Curie in the early 1930s showed that stable atoms
of an element may be made artificially radioactive by suitable bombardment
with nuclear particles or rays. Such radioactive isotopes (radioisotopes)
are produced as a result of a nuclear reaction, or transformation. In such
reactions the 270-odd isotopes found in nature serve as targets for nuclear
projectiles. The development of atom smashers, or accelerators, for hurling
these projectile-particles to high energy has made it possible to observe
thousands of nuclear reactions.
Nuclear Reactions
In 1932 two British scientists, Sir John
D. Cockcroft and Ernest T. S. Walton, were the first to use artificially
accelerated particles to successfully disintegrate the nucleus. They produced
a beam of protons, which were boosted to high speed by means of a high-voltage
device called a voltage multiplier. These particles were then used to bombard
a lithium target. In this nuclear reaction, lithium-7 (7Li) splits into
two fragments, which are nuclei of helium atoms. The reaction is expressed
by the equation
7Li + 1H = 4He + 4He
Physicists have measured the weights of these atoms precisely—7Li has a weight of 7.018242 amu; 1H, 1.008137 amu; and 4He, 4.003910 amu. The weights on the left side of the equation add up to 8.026379 amu, whereas those on the right side total 8.007820 amu; a “loss” of 0.018559 amu has occurred. Using Einstein's E = mc2 relation, 1 amu is found to be the equivalent of 931.3 million electron volts (MeV) of energy. On this basis the nuclear reaction with lithium releases 17.28 MeV of energy. The “lost” mass appears as energy in the form of the violent motion of the helium nuclei. See NUCLEAR CHEMISTRY.
Particle Accelerator
The American physicist Ernest O. Lawrence
developed about 1930 a particle accelerator called a cyclotron. This machine
generates electrical attractive and repulsive forces that accelerate atomic
particles while they are confined to a circular orbit by the electromagnetic
force of a large magnet. The particles spiral outward under the influence
of these electric and magnetic forces, reaching extremely high speeds.
The acceleration takes place in a vacuum so that the particles do not collide
with molecules of air. Because the equipment necessary for producing intense
magnetic forces is massive, high-energy machines are huge and expensive
installations. See PARTICLE ACCELERATORS.
Nuclear Forces
Modern nuclear theory is based on the notion
that nuclei consist of neutrons and protons that are held together by extremely
powerful “nuclear” forces. The elucidation of these nuclear forces requires
physicists to disrupt neutrons and protons by bombarding nuclei with extremely
energetic particles. Such bombardments have revealed more than 200 so-called
elementary particles, or tiny bits of matter, most of which exist for much
less than one hundred-millionth of a second.
This subnuclear world was first revealed
in cosmic rays (see COSMIC RAYS). These rays consist of highly energetic
particles that constantly bombard the earth from outer space, penetrating
down through the atmosphere and even into the earth's crust. Cosmic radiation
includes many types of particles, some having energies far exceeding anything
achieved in particle accelerators. When these energetic particles strike
nuclei, new particles are created. Among the first such particles to be
observed were the muons (detected in 1937) and pions (1947). The existence
of the pion had been predicted in 1935 by the Japanese physicist Yukawa
Hideki.
According to the most widely accepted theory,
nuclear particles are held together by “exchange forces,” in which pions
common to both neutrons and protons are continuously exchanged between
them. The binding of protons and neutrons by pions is similar to the binding
of two atoms in a molecule through sharing or exchanging a common pair
of electrons. These particles are about 200 times as heavy as electrons.
The muon is essentially a heavy electron and can be either positively or
negatively charged. The pion, slightly heavier than the muon, can carry
a positive or negative charge, or no charge.
Elementary Particles
Accelerator studies eventually established
that each kind of particle also has an antiparticle of the same mass but
opposite in charge or other electromagnetic property. Physicists have long
sought a theory that would put this bewildering array of particles in order.
Particles are now grouped according to the force that usually controls
their interactions. Hadrons (strong nuclear force) include hyperons, mesons,
and the neutron and proton. Leptons (electromagnetic and weak forces) include
the tau, muon, electron, and neutrinos. Bosons (particlelike objects associated
with interactions) include the photon and the hypothetical carriers of
the weak force and of gravitation. The weak nuclear force is evident in
such radioactive or particle-decay reactions as alpha decay (the release
of a helium nucleus from an unstable atomic nucleus). See ANTIMATTER.
In 1963 the U.S. physicists Murray Gell-Mann
and George Zweig proposed that hadrons are actually combinations of more
fundamental particles called quarks, the interactions of which are carried
by particlelike gluons. This theory underlies current investigations and
has served to predict the existence of further particles. See ELEMENTARY
PARTICLES; GLUON; QUARK.
Release of Atomic Energy
Two nuclear processes of great practical
significance because they provide vast amounts of energy are fission, the
splitting of a heavy nucleus into lighter ones, and thermonuclear fusion,
the fusion of two light nuclei (at extremely high temperatures) to form
a heavier one. The Italian-born American physicist Enrico Fermi achieved
fission in 1934, but the reaction was not recognized as such until 1939,
when the German scientists Otto Hahn and Fritz Strassmann announced that
they had split uranium nuclei by bombarding them with neutrons. Neutrons
are also released by the reaction and can cause a chain reaction with other
nuclei. An uncontrolled chain reaction is seen in the explosion of an atomic
bomb. Heat from controlled reactions, however, as in nuclear reactors,
can be used to produce electric power.
Thermonuclear fusion occurs in stars, including
the sun, and is the source of their heat and light. Uncontrolled fusion
is seen in the explosion of a hydrogen bomb, but physicists are currently
trying to develop a practical controlled-fusion device. See NUCLEAR ENERGY;
NUCLEAR WEAPONS.
Heat, in physics, transfer of energy from
one part of a substance to another, or from one body to another by virtue
of a difference in temperature. Heat is energy in transit; it always flows
from a substance at a higher temperature to the substance at a lower temperature,
raising the temperature of the latter and lowering that of the former substance,
provided the volume of the bodies remains constant. Heat does not flow
from a lower to a higher temperature unless another form of energy transfer,
work, is also present. See also POWER.
Until the beginning of the 19th century,
the effect of heat on the temperature of a body was explained by postulating
the existence of an invisible substance or form of matter termed caloric.
According to the caloric theory of heat, a body at a high temperature contains
more caloric than one at a low temperature; the former body loses some
caloric to the latter body on contact, increasing that body's temperature
while lowering its own. Although the caloric theory successfully explained
some phenomena of heat transfer, experimental evidence was presented by
the American-born British physicist Benjamin Thompson (later known as Count
von Rumford) in 1798 and by the British chemist Sir Humphry Davy in 1799
suggesting that heat, like work, is a form of energy in transit. Between
1840 and 1849 the British physicist James Prescott Joule, in a series of
highly accurate experiments, provided conclusive evidence that heat is
a form of energy in transit and that it can cause the same changes in a
body as work.
Temperature
The sensation of warmth or coldness of a
substance on contact is determined by the property known as temperature.
Although it is easy to compare the relative temperatures of two substances
by the sense of touch, it is impossible to evaluate the absolute magnitude
of the temperatures by subjective reactions. Adding heat to a substance,
however, not only raises its temperature, causing it to impart a more acute
sensation of warmth, but also produces alterations in several physical
properties, which may be measured with precision. As the temperature varies,
a substance expands or contracts, its electrical resistivity (see RESISTANCE)
changes, and in the gaseous form, it exerts varying pressure. The variation
in a standard property usually serves as a basis for an accurate numerical
temperature scale (see below).
Temperature depends on the average kinetic
energy of the molecules of a substance, and according to kinetic theory
(see GASES; THERMODYNAMICS), energy may exist in rotational, vibrational,
and translational motions of the particles of a substance. Temperature,
however, depends only on the translational molecular motion. Theoretically,
the molecules of a substance would exhibit no activity at the temperature
termed absolute zero. See MOLECULE.
Temperature Scales
Five different temperature scales are in
use today: the Celsius scale, known also as the centigrade scale, the Fahrenheit
scale, the Kelvin scale, the Rankine scale, and the international thermodynamic
temperature scale (see THERMOMETER). The centigrade scale, with a freezing
point of 0° C and a boiling point of 100° C, is widely used throughout
the world, particularly for scientific work, although it was superseded
officially in 1950 by the international temperature scale. In the Fahrenheit
scale, used in English-speaking countries for purposes other than scientific
work and based on the mercury thermometer, the freezing point of water
is defined as 32° F and the boiling point as 212° F (see MERCURY).
In the Kelvin scale, the most commonly used thermodynamic temperature scale,
zero is defined as the absolute zero of temperature, that is, -273.15°
C, or -459.67° F. Another scale employing absolute zero as its lowest
point is the Rankine scale, in which each degree of temperature is equivalent
to one degree on the Fahrenheit scale. The freezing point of water on the
Rankine scale is 492° R, and the boiling point is 672° R.
In 1933 scientists of 31 nations adopted
a new international temperature scale with additional fixed temperature
points, based on the Kelvin scale and thermodynamic principles. The international
scale is based on the property of electrical resistivity, with platinum
wire as the standard for temperature between -190° and 660° C.
Above 660° C, to the melting point of gold, 1063° C, a standard
thermocouple, which is a device that measures temperature by the amount
of voltage produced between two wires of different metals, is used; beyond
this point temperatures are measured by the so-called optical pyrometer,
which uses the intensity of light of a wavelength emitted by a hot body
for the purpose.
In 1954 the triple point of water—that is,
the point at which the three phases of water (vapor, liquid, and ice) are
in equilibrium—was adopted by international agreement as 273.16 K. The
triple point can be determined with greater precision than the freezing
point and thus provides a more satisfactory fixed point for the absolute
thermodynamic scale. In cryogenics, or low-temperature research, temperatures
as low as 0.003 K have been produced by the demagnetization of paramagnetic
materials. Momentary high temperatures estimated to be greater than 100,000,000
K have been achieved by nuclear explosions (see NUCLEAR WEAPONS).
Heat Units
Heat is measured in terms of the calorie,
defined as the amount of heat necessary to raise the temperature of 1 g
of water at a pressure of 1 atm from 15° to 16° C. This unit is
sometimes called the small or gram calorie to distinguish it from the large
calorie, or kilocalorie, equal to 1000 cal, which is used in nutrition
studies. In mechanical engineering practice in the United States and Great
Britain, heat is measured in British thermal units, or Btu (see BRITISH
THERMAL UNIT). One Btu is the quantity of heat required to raise the temperature
of 1 lb of water 1° F and is equal to 252 cal. Mechanical energy can
be converted into heat by friction, and the mechanical work necessary to
produce 1 cal is known as the mechanical equivalent of heat. It is equal
to 4.1855 × 107 ergs/cal or 778 ft-lb Btu. According to the law of
conservation of energy, all the mechanical energy expended to produce heat
by friction appears as energy in the objects on which the work is performed.
This fact was first conclusively proven in a classic experiment performed
by Joule, who heated water in a closed vessel by means of rotating paddle
wheels and found that the rise in water temperature was proportional to
the work expended in turning the wheels.
If heat is converted into mechanical energy,
as in an internal-combustion engine, the law of conservation of energy
also applies. In any engine, however, some energy is always lost or dissipated
in the form of heat because no engine is perfectly efficient. See HORSEPOWER.
Latent Heat
A number of physical changes are associated
with the change of temperature of a substance. Almost all substances expand
in volume when heated and contract when cooled. The behavior of water between
0° and 4° C (32° and 39° F) constitutes an important exception
to this rule. The phase of a substance refers to its occurrence as either
a solid, liquid, or gas, and phase changes in pure substances occur at
definite temperatures and pressures (see PHASE RULE). The process of changing
from solid to gas is referred to as sublimation, from solid to liquid as
melting, and from liquid to vapor as vaporization. If the pressure is constant,
these processes occur at constant temperature. The amount of heat required
to produce a change of phase is called latent heat, and hence, latent heats
of sublimation, melting, and vaporization exist (see DISTILLATION; EVAPORATION).
If water is boiled in an open vessel at a pressure of 1 atm, the temperature
does not rise above 100° C (212° F), no matter how much heat is
added. The heat that is absorbed without changing the temperature of the
water is the latent heat; it is not lost but is expended in changing the
water to steam and is then stored as energy in the steam; it is again released
when the steam is condensed to form water (see CONDENSATION). Similarly,
if a mixture of water and ice in a glass is heated, its temperature will
not change until all the ice is melted. The latent heat absorbed is used
up in overcoming the forces holding the particles of ice together and is
stored as energy in the water. To melt 1 g of ice, 79.7 cal are needed,
and to convert 1 g of water to steam at 100° C, 541 cal are needed.
Specific Heat
The heat capacity, or the measure of the
amount of heat required to raise the temperature of a unit mass of a substance
one degree is known as specific heat. If the heating process occurs while
the substance is maintained at a constant volume or is subjected to a constant
pressure the measure is referred to as a specific heat at constant volume
or at constant pressure. The latter is always larger than, or at least
equal to, the former for each substance. Because 1 cal causes a rise of
1° C in 1 g of water, the specific heat of water is 1 cal/g/° C.
In the case of water and other approximately incompressible substances,
it is not necessary to distinguish between the constant-volume and constant-pressure
specific heats, as they are approximately equal. Generally, the two specific
heats of a substance depend on the temperature.
Transfer of Heat
The physical methods by which energy in the
form of heat can be transferred between bodies are conduction and radiation.
A third method, which also involves the motion of matter, is called convection.
Conduction requires physical contact between the bodies or portions of
bodies exchanging heat, but radiation does not require contact or the presence
of any matter between the bodies. Convection occurs when a liquid or gas
is in contact with a solid body at a different temperature and is always
accompanied by the motion of the liquid or gas. The science dealing with
the transfer of heat between bodies is called heat transfer.
Gases, collective term for one of the three visibly different states of ordinary matter, liquid and solid being the other two. Solids have well-defined shapes and are difficult to compress. Liquids are free-flowing and bounded by self-formed surfaces. Gases expand freely to fill their containers and are much lower in density than liquids and solids.
The Ideal Gas Law
The atomic theory of matter defines states,
or phases, in terms of order. Molecules have a certain freedom of motion
in space. These microscopic degrees of freedom are associated with the
concept of macroscopic order. Molecules in a solid are arranged in a regular
lattice, their freedom restricted to small vibrations about lattice sites.
In contrast, there is no macroscopic spatial order in a gas. Molecules
move at random, bounded only by the walls of their container.
Empirical laws have been developed that correlate
macroscopic variables. For common gases, the macroscopic variables include
pressure (P), volume (V), and temperature (T). Boyle's law states that
in a gas held at a constant temperature the volume is inversely proportional
to the pressure. Charles's law, or Gay-Lussac's law, states that if a gas
is held at a constant pressure the volume is directly proportional to the
absolute temperature. Combining these laws gives the ideal gas law: PV/T
= R (per mole), also known as the equation of state of an ideal gas. The
constant R on the right-hand side of the equation is a universal constant,
the discovery of which is a cornerstone of modern science.
The Kinetic Theory of Gases
With the advent of the atomic theory of matter,
the above-mentioned empirical laws acquired a microscopic basis. The volume
of a gas reflects simply the position distribution of its constituent molecules.
More exactly, the macroscopic variable V represents the available amount
of space in which a molecule can move. The pressure of a gas, which can
be measured with gauges placed on the container walls, registers the average
change of momentum experienced by molecules as they collide with, and subsequently
rebound from, the walls. The temperature of a gas is proportional to the
average kinetic energy of the molecules, or to the square of the average
velocity of the molecules. The reduction of these macroscopic measures
to such mechanical variables as position, velocity, momentum, and kinetic
energy of the molecules, which can be correlated through Newton's laws
of mechanics, should yield all the empirical gas laws. This turns out to
be generally true.
The physics that relates the properties of
gases to classical mechanics is called the kinetic theory of gases. Besides
providing a basis for the ideal gas equation of state, the kinetic theory
can also be used to predict many other properties of gases, including the
statistical distribution of molecular velocities and transport properties
such as thermal conductivity, the coefficient of diffusion, and viscosity.
Van der Waals equation
The ideal gas equation of state is only approximately
correct. Real gases do not behave exactly as predicted. In some cases the
deviation can be extremely large. For example, ideal gases could never
become liquids or solids, no matter how much they were cooled or compressed.
Thus, modifications of the ideal gas law, PV = RT, were proposed. Particularly
useful and well known is the van der Waals equation of state: (P + a/V2)
(V - b) = RT, where a and b are adjustable parameters determined from experimental
measurements carried out on actual gases. They are material parameters
rather than universal constants, in the sense that their values vary from
gas to gas.
The van der Waals equation also has a microscopic
interpretation. Molecules interact with one another. The interaction is
strongly repulsive in close proximity, becomes mildly attractive at intermediate
range, and vanishes at long distance. The ideal gas law must be corrected
when attractive and repulsive forces are considered. For example, the mutual
repulsion between molecules has the effect of excluding certain territory
around each molecule from intrusion by its neighbors. Thus, a fraction
of space becomes unavailable to each molecule as it executes random motion.
In the equation of state, a volume of exclusion (b) should be subtracted
from the volume of the container (V); thus, (V - b).
Phase Transitions
At low temperatures (reduced molecular motion)
and at high pressures or reduced volumes (reduced intermolecular spacing),
the molecules in a gas come under the influence of one another's attractive
force. Under certain critical conditions, the entire system enters a high-density
bound state and acquires a bounding surface. This signifies the onset of
the liquid state. The process is known as a phase transition. The van der
Waals equation permits such a phase transition. It also describes a two-phase
coexistence region that terminates on a critical point, above which no
physical distinction can be found between the gas and the liquid phases.
These phenomena are consistent with experimental observations. For actual
use one has to go to equations that are more sophisticated than the van
der Waals equation.
Improved understanding of the properties
of gases over the past century has led to large-scale exploitation of the
principles of physics, chemistry, and engineering for industrial and consumer
applications.
See ATOM AND ATOMIC THEORY; MATTER, STATES
OF; THERMODYNAMICS.
Pressure, in mechanics, the force per unit area exerted by a liquid or gas on a body or surface, with the force acting at right angles to the surface uniformly in all directions. In the British system, pressure is usually measured in pounds per square inch (PSI); in international usage, in kilograms per square centimeters, or in atmospheres; and in the international metric system (SI), in newtons per square meter (see INTERNATIONAL SYSTEM OF UNITS). The unit atmosphere (atm) is defined as a pressure of 1.03323 kg/sq cm (14.696 lb/sq in), which, in terms of the conventional mercury barometer, corresponds to 760 mm (29.921 in) of mercury. The unit kilopascal (kPa) is defined as a pressure of 0.0102 kg/sq cm (0.145 lb/sq in).
Pressure Gauges
Most gauges record the difference between
the fluid pressure and local atmospheric pressure. For small pressure differences,
a U-tube manometer is used. It consists of a U-shaped tube with one end
connected to the container and the other open to the atmosphere. Filled
with a liquid, such as water, oil, or mercury, the difference in the liquid
surface levels in the two manometer legs indicates the pressure difference
from local atmospheric conditions. For higher pressure differences, a Bourdon
gauge, named after the French inventor Eugène Bourdon, is used.
This consists of a hollow metal tube with an oval cross section, bent in
the shape of a hook. One end of the tube is closed, the other open and
connected to the measurement region. If pressure (above local atmospheric
pressure) is applied, the oval cross section will become circular, and
at the same time the tube will straighten out slightly. The resulting motion
of the closed end, proportional to the pressure, can then be measured via
a pointer or needle connected to the end through a suitable linkage. Gauges
used for recording rapidly fluctuating pressures commonly employ piezoelectric
or electrostatic sensing elements that can provide an instantaneous response.
As most pressure gauges measure the difference
between the fluid and the local atmospheric pressure, the atmospheric pressure
must be added to the gauge pressure to arrive at the true absolute pressure.
A negative gauge-pressure reading corresponds to a partial vacuum.
Low gas pressure (down to about 10-6 mm mercury
absolute) can be measured by the so-called McLeod gauge, in which a measured
volume of gas at the unknown low pressure is compressed at constant temperature
to a much smaller volume, and then the pressure is measured directly with
a manometer. The unknown pressure is then calculated from Boyle's law (see
GASES). For still lower pressures, various gauges depending on radiation,
ionization, or molecular effects are used (see VACUUM TECHNOLOGY).
Range
Depending on the use, pressures may range
from 10-8 to 10-2 mm of mercury (absolute) for high-vacuum work to thousands
of kilograms per square centimeter for hydraulic presses and controls.
Pressures in the range of millions of kilograms per square centimeter have
been obtained for experimental purposes and for the manufacture of artificial
diamonds, where pressures of about 70,000 kg/sq cm (about 1 million lb/sq
in), together with temperatures in excess of 2770° C (5000° F),
are required.
In the atmosphere the decreasing weight of
the air column with altitude leads to a reduction in local atmospheric
pressure. Thus the pressure decreases from its sea-level value to 0.85
kg/sq cm (12.1 lb/sq in) at 1.6 km (1 mi), the elevation of Denver, Colorado;
and to about 0.24 kg/sq cm (3.4 lb/sq in) at 10,700 m (35,000 ft) elevation,
a normal jet flight altitude.
Partial pressure is the term applied to the
effective pressure a single constituent exerts in a mixture of gases. In
the atmosphere the total pressure (atmospheric pressure) is equal to the
sum of the partial pressures of its constituents (oxygen, nitrogen, carbon
dioxide, and rare gases).
Temperature, in physics, property of systems that determines whether
they are in thermal equilibrium (see THERMODYNAMICS). The concept of temperature
stems from the idea of measuring relative hotness and coldness and from
the observation that the addition of heat to a body leads to an increase
in temperature as long as no melting or boiling occurs. In the case of
two bodies at different temperatures, heat will flow from the hotter to
the colder until their temperatures are identical and thermal equilibrium
is reached (see HEAT TRANSFER). Thus, temperatures and heat, although interrelated,
refer to different concepts, temperature being a property of a body and
heat being an energy flow to or from a body by virtue of a temperature
difference. See ENERGY.
Temperature changes have to be measured in terms of other property
changes of a substance. Thus, the conventional mercury thermometer measures
the expansion of a mercury column in a glass capillary, the change in length
of the column being related to the temperature change. If heat is added
to an ideal gas contained in a constant-volume vessel, the pressure increases,
and the temperature change can be determined from the pressure change by
Gay-Lussac's law (see GASES), provided the temperature is expressed on
the absolute scale.
Temperature Scales
One of the earliest temperature scales was that devised by the German
physicist Gabriel Daniel Fahrenheit. According to this scale, at standard
atmospheric pressure, the freezing point (and melting point of ice) is
32° F, and the boiling point is 212° F. The centigrade, or Celsius
scale, invented by the Swedish astronomer Anders Celsius, and used throughout
most of the world, assigns a value of 0° C to the freezing point and
100° C to the boiling point. In scientific work, the absolute or Kelvin
scale, invented by the British mathematician and physicist William Thomson,
1st Baron Kelvin, is most widely used. In this scale, absolute zero is
at -273.16° C, which is zero K, and the degree intervals are identical
to those measured on the centigrade scale (see ABSOLUTE ZERO). The corresponding
“absolute Fahrenheit” or Rankine scale, devised by the British engineer
and physicist William J. M. Rankine, places absolute zero at -459.69°
F, which is 0° R, and the freezing point at 491.69° R. A more consistent
scientific temperature scale, based on the Kelvin scale, was adopted in
1933.
Effects of Temperature
Temperature plays an important part in determining the conditions in
which living matter can exist. Thus, birds and mammals demand a very narrow
range of body temperatures for survival and must be protected against extreme
heat or cold (see BODY TEMPERATURE). Aquatic species can exist only within
a narrow temperature range of the water, which differs for various species.
Thus, for example, the increase in temperature of river water by only a
few degrees as a result of heat discharged from power plants may kill most
of the native fish. See WATER POLLUTION.
The properties of all materials are also markedly affected by temperature
changes. At arctic temperatures, for example, steel becomes very brittle
and breaks easily, and liquids either solidify or become very viscous,
offering high frictional resistance to flow (see VISCOSITY). At temperatures
near absolute zero, many materials exhibit strikingly different characteristics
(see CRYOGENICS). At high temperatures, solid materials liquefy or become
gaseous; chemical compounds may break up into their constituents.
The temperature of the atmosphere is greatly influenced by both the
land and the sea areas. In January, for example, the great landmasses of
the northern hemisphere are much colder than the oceans at the same latitude,
and in July the situation is reversed. At low elevations the air temperature
is also determined largely by the surface temperature of the earth. The
periodic temperature changes are due mainly to the sun's radiant heating
of the land areas of the earth, which in turn convect heat to the overlying
air. As a result of this phenomenon, the temperature decreases with altitude,
from a standard reference value of 15.5° C (60° F) at sea level
(in temperate latitudes), to about -55° C (about -67° F) at about
11,000 m (about 36,000 ft). Above this altitude, the temperature remains
nearly constant up to about 33,500 m (about 110,000 ft). For the temperature-humidity
index, see HUMIDITY.
A study of flow, is a study of movement.In the mechanical world this flow may be of a fluid (liquid or gas) or the movement of heat.The flow of heat is the nature of work in the most basic form.
HEAT:Heat will always flow from a region of high temperature to low temperature.The rate of flow is a function of temperature differential and the nature of the conductor. (See HEAT TRANSFER)It is interesting to note that for a heat engine to function, heat energy must pass from the inlet to the exhaust. To maintain flow, some energy must be sacrificed to the universe (HEAT SINK).In other words, to have heat flow through a mechanical system, some heat must be lost at the exhaust.
Fluid flow most flow through a containing structure (Piping system.)The rate of fluid flow is a function of pressure differential, the nature of the fluid (temperature, viscosity) and the condition of the system ( fluid frictional losses.) and the size of the system.
Flow is a kinetic energy, pressure which will determine the intensity of flow is a potential energy.The size and condition of the piping system offers resistance to flow and will determine the actual volume of delivery.
Because of internal friction in the system, there is always a loss associated with a fluid system.Loss is a function of system design, condition and operation.
Given any two points in a fluid system, a pressure drop will occur between these two points, this drop is due to friction and fluid turbulence.As flow increases across these two points, pressure drop across the points alsoincreases and this increase is proportional to flow.
The pressure drop across two fixed points in a system can be used to measure the volume of flow in the system.
Steam, water in vapor state, used in the generation
of power and on a large scale in many industrial processes. The techniques
of generating and using steam, therefore, are important components of engineering
technology. The generation of electricity is largely accomplished by first
generating steam, whether the heat is produced by burning coal or gas or
by the nuclear fission of uranium (see NUCLEAR ENERGY; STEAM ENGINE; TURBINE).
Steam also is still much in use for space heating purposes (see HEATING,
VENTILATING, AND AIR CONDITIONING), and it propels most of the world's
naval vessels and commercial ships (see SHIPS AND SHIPBUILDING).
The boiling point of water at sea-level atmospheric
pressure (1.03 kg/sq cm or 14.7 lb/sq in) is about 100° C (212°
F). At this critical temperature, the addition of 970.3 Btu of heat will
convert 0.454 kg (1 lb) of water to 0.454 kg of steam at the same temperature.
For water under pressure, the boiling point rises with the increase of
pressure up to a pressure of 225.8 kg/sq cm (3208.2 lb/sq in) according
to Boyle's law (see GASES). At this pressure, water boils at a temperature
of 374.15° C (705.47° F), its critical point. Beyond critical pressure
and temperature there is no distinction between liquid water and steam.
Pure steam is a dry and invisible vapor.
In many cases, however, when water is boiling, a quantity of small droplets,
or particles, of water are taken up with the steam, and the resulting mixture
is visible as a white vapor. A similar effect occurs when dry steam is
exhausted into the comparatively cool atmosphere. Some of the steam cools
and condenses, forming the familiar white vapor seen when a kettle boils
on a stove. Such steam is said to be wet.
Steam that is heated to the exact boiling
point corresponding to the existing pressure is called saturated steam.
Heating steam beyond this temperature produces so-called superheated steam.
Superheating also occurs if saturated steam is compressed or if saturated
steam is throttled by passing the steam through a valve from a high-pressure
vessel to a low-pressure vessel. Throttling causes the temperature of the
steam to drop somewhat, but the temperature of the throttled steam is still
higher than that of saturated steam at the corresponding pressure. Steam
in its superheated state is generally used in modern power generation systems.
Heat Transfer, in physics, process by which energy in the form of heat is exchanged between bodies or parts of the same body at different temperatures. Heat is generally transferred by convection, radiation, or conduction. Although these three processes can occur simultaneously, it is not unusual for one mechanism to overshadow the other two. Heat, for example, is transferred by conduction through the brick wall of a house, the surfaces of high-speed aircraft are heated by convection, and the earth receives heat from the sun by radiation. See also ENERGY; HEAT; TEMPERATURE.
Conduction
This is the only method of heat transfer in opaque solids. If the temperature
at one end of a metal rod is raised by heating, heat is conducted to the
colder end, but the exact mechanism of heat conduction in solids is not
entirely understood. It is believed, however, to be partially due to the
motion of free electrons in the solid matter, which transport energy if
a temperature difference is applied. This theory helps to explain why good
electrical conductors also tend to be good heat conductors (see CONDUCTOR,
ELECTRICAL). Although the phenomenon of heat conduction had been observed
for centuries, it was not until 1882 that the French mathematician Jean
Baptiste Joseph Fourier gave it precise mathematical expression in what
is now regarded as Fourier's law of heat conduction. This physical law
states that the rate at which heat is conducted through a body per unit
cross-sectional area is proportional to the negative of the temperature
gradient existing in the body.
The proportionality factor is called the thermal conductivity of the
material. Materials such as gold, silver, and copper have high thermal
conductivities and conduct heat readily, but materials such as glass and
asbestos have values of thermal conductivity hundreds and thousands of
times smaller, conduct heat poorly, and are referred to as insulators (see
INSULATION). In engineering applications it is frequently necessary to
establish the rate at which heat will be conducted through a solid if a
known temperature difference exists across the solid. Sophisticated mathematical
techniques are required to establish this, especially if the process varies
with time, the phenomenon being known as transient-heat conduction. With
the aid of analog and digital computers, these problems are now being solved
for bodies of complex geometry. See COMPUTER.
Convection
Conduction occurs not only within a body but also between two bodies
if they are brought into contact, and if one of the substances is a liquid
or a gas, then fluid motion will almost certainly occur. This process of
conduction between a solid surface and a moving liquid or gas is called
convection. The motion of the fluid may be natural or forced. If a liquid
or gas is heated, its mass per unit volume generally decreases. If the
liquid or gas is in a gravitational field, the hotter, lighter fluid rises
while the colder, heavier fluid sinks. This kind of motion, due solely
to nonuniformity of fluid temperature in the presence of a gravitational
field, is called natural convection (see GRAVITATION). Forced convection
is achieved by subjecting the fluid to a pressure gradient and thereby
forcing motion to occur according to the law of fluid mechanics.
If, for example, water in a pan is heated from below, the liquid closest
to the bottom expands and its density decreases; the hot water as a result
rises to the top and some of the cooler fluid descends toward the bottom,
thus setting up a circulatory motion. Similarly, in a vertical gas-filled
chamber, such as the air space between two window panes in a double-glazed,
or Thermopane, window, the air near the cold outer pane will move down
and the air near the inner, warmer pane will rise, leading to a circulatory
motion.
The heating of a room by a radiator depends less on radiation than
on natural convection currents, the hot air rising upward along the wall
and cooler air coming back to the radiator from the side of the bottom.
Because of the tendencies of hot air to rise and of cool air to sink, radiators
should be placed near the floor and air-conditioning outlets near the ceiling
for maximum efficiency. Natural convection is also responsible for the
rising of the hot water and steam in natural-convection boilers (see BOILER)
and for the draft in a chimney. Convection also determines the movement
of large air masses above the earth, the action of the winds, rainfall,
ocean currents, and the transfer of heat from the interior of the sun to
its surface.
Radiation
This process is fundamentally different from both conduction and convection
in that the substances exchanging heat need not be in contact with each
other. They can, in fact, be separated by a vacuum. Radiation is a term
generally applied to all kinds of electromagnetic-wave phenomena (see ELECTROMAGNETIC
RADIATION). Some radiation phenomena can be described in terms of wave
theory (see WAVE MOTION), and others can be explained in terms of quantum
theory. Neither theory, however, completely explains all experimental observations.
The German-born American physicist Albert Einstein conclusively demonstrated
(1905) the quantized behavior of radiant energy in his classical photoelectric
experiments. Before Einstein's experiments the quantized nature of radiant
energy had been postulated, and the German physicist Max Planck used quantum
theory and the mathematical formalism of statistical mechanics to derive
(1900) a fundamental law of radiation (see STATISTICS). The mathematical
expression of this law, called Planck's distribution, relates the intensity
or strength of radiant energy emitted by a body to the temperature of the
body and the wavelength of radiation. This is the maximum amount of radiant
energy that can be emitted by a body at a particular temperature. Only
an ideal body (blackbody,) emits such radiation according to Planck's law.
Real bodies emit at a somewhat reduced intensity. The contribution of all
frequencies to the radiant energy emitted by a body is called the emissive
power of the body, the amount of energy emitted by a unit surface area
of a body per unit of time. As can be shown from Planck's law, the emissive
power of a surface is proportional to the fourth power of the absolute
temperature. The proportionality factor is called the Stefan-Boltzmann
constant after two Austrian physicists, Joseph Stefan and Ludwig Boltzmann,
who, in 1879 and 1884, respectively, discovered the fourth power relationship
for the emissive power. According to Planck's law, all substances emit
radiant energy merely by virtue of having a positive absolute temperature.
The higher the temperature, the greater the amount of energy emitted. In
addition to emitting, all substances are capable of absorbing radiation.
Thus, although an ice cube is continuously emitting radiant energy, it
will melt if an incandescent lamp is focused on it because it will be absorbing
a greater amount of heat than it is emitting.
Opaque surfaces can absorb or reflect incident radiation. Generally,
dull, rough surfaces absorb more heat than bright, polished surfaces, and
bright surfaces reflect more radiant energy than dull surfaces. In addition,
good absorbers are also good emitters; good reflectors, or poor absorbers,
are poor emitters. Thus, cooking utensils generally have dull bottoms for
good absorption and polished sides for minimum emission to maximize the
net heat transfer into the contents of the pot. Some substances, such as
gases and glass, are capable of transmitting large amounts of radiation.
It is experimentally observed that the absorbing, reflecting, and transmitting
properties of a substance depend upon the wavelength of the incident radiation.
Glass, for example, transmits large amounts of short wavelength (ultraviolet)
radiation, but is a poor transmitter of long wavelength (infrared) radiation
(see INFRARED RADIATION; ULTRAVIOLET RADIATION). A consequence of Planck's
distribution is that the wavelength at which the maximum amount of radiant
energy is emitted by a body decreases as the temperature increases. Wien's
displacement law, named after the German physicist Wilhelm Wien, is a mathematical
expression of this observation and states that the wavelength of maximum
energy, expressed in microns (millionths of a meter), multiplied by the
Kelvin temperature of the body is equal to a constant, 2878. Most of the
energy radiated by the sun, therefore, is characterized by small wavelengths.
This fact, together with the transmitting properties of glass mentioned
above, explains the greenhouse effect. Radiant energy from the sun is transmitted
through the glass and enters the greenhouse. The energy emitted by the
contents of the greenhouse, however, which emit primarily at infrared wavelengths,
is not transmitted out through the glass. Thus, although the air temperature
outside the greenhouse may be low, the temperature inside the greenhouse
will be much higher because there is a sizable net heat transfer into it.
In addition to heat transfer processes that result in raising or lowering
temperatures of the participating bodies, heat transfer can also produce
phase changes such as the melting of ice or the boiling of water. In engineering,
heat transfer processes are usually designed to take advantage of these
phenomena. In the case of space capsules reentering the atmosphere of the
earth at very high speed, a heat shield that melts in a prescribed manner
by the process called ablation is provided to prevent overheating of the
interior of the capsule. Essentially, the frictional heating produced by
the atmosphere is used to melt the heat shield and not to raise the temperature
of the capsule (see FRICTION).
Thermodynamics, field of physics that describes and correlates the physical
properties of macroscopic systems of matter and energy. The principles
of thermodynamics are of fundamental importance to all branches of science
and engineering.
A central concept of thermodynamics is that of the macroscopic system,
defined as a geometrically isolable piece of matter in coexistence with
an infinite, unperturbable environment. The state of a macroscopic system
in equilibrium can be described in terms of such measurable properties
as temperature, pressure, and volume, which are known as thermodynamic
variables. Many other variables (such as density, specific heat, compressibility,
and the coefficient of thermal expansion) can be identified and correlated,
to produce a more complete description of an object and its relationship
to its environment.
When a macroscopic system moves from one state of equilibrium to another,
a thermodynamic process is said to take place. Some processes are reversible
and others are irreversible. The laws of thermodynamics, discovered in
the 19th century through painstaking experimentation, govern the nature
of all thermodynamic processes and place limits on them.
Zeroth Law of Thermodynamics
The vocabulary of empirical sciences is often borrowed from daily language.
Thus, although the term temperature appeals to common sense, its meaning
suffers from the imprecision of nonmathematical language. A precise, though
empirical, definition of temperature is provided by the so-called zeroth
law of thermodynamics as explained below.
When two systems are in equilibrium, they share a certain property.
This property can be measured and a definite numerical value ascribed to
it. A consequence of this fact is the zeroth law of thermodynamics, which
states that when each of two systems is in equilibrium with a third, the
first two systems must be in equilibrium with each other. This shared property
of equilibrium is the temperature.
If any such system is placed in contact with an infinite environment
that exists at some certain temperature, the system will eventually come
into equilibrium with the environment—that is, reach the same temperature.
(The so-called infinite environment is a mathematical abstraction called
a thermal reservoir; in reality the environment need only be large relative
to the system being studied.)
Temperatures are measured with devices called thermometers (see THERMOMETER).
A thermometer contains a substance with conveniently identifiable and reproducible
states, such as the normal boiling and freezing points of pure water. If
a graduated scale is marked between two such states, the temperature of
any system can be determined by having that system brought into thermal
contact with the thermometer, provided that the system is large relative
to the thermometer.
First Law of Thermodynamics
The first law of thermodynamics gives a precise definition of heat,
another commonly used concept.
When an object is brought into contact with a relatively colder object,
a process takes place that brings about an equalization of temperatures
of the two objects. To explain this phenomenon, 18th-century scientists
hypothesized that a substance more abundant at higher temperature flowed
toward the region at a lower temperature. This hypothetical substance,
called “caloric,” was thought to be a fluid capable of moving through material
media. The first law of thermodynamics instead identifies caloric, or heat,
as a form of energy. It can be converted into mechanical work, and it can
be stored, but is not a material substance. Heat, measured originally in
terms of a unit called the calorie, and work and energy, measured in ergs,
were shown by experiment to be totally equivalent. One calorie is equivalent
to 4.186 × 107 ergs, or 4.186 joules.
The first law, then, is a law of energy conservation. It states that,
because energy cannot be created or destroyed—setting aside the later ramifications
of the equivalence of mass and energy (see NUCLEAR ENERGY)—the amount of
heat transferred into a system plus the amount of work done on the system
must result in a corresponding increase of internal energy in the system.
Heat and work are mechanisms by which systems exchange energy with one
another.
In any machine some amount of energy is converted into work; therefore,
no machine can exist in which no energy is converted into work. Such a
hypothetical machine (in which no energy is required for performing work)
is termed a “perpetual-motion machine of the first kind.” Since the input
energy must now take heat into account (and in a broader sense chemical,
electrical, nuclear, and other forms of energy as well), the law of energy
conservation rules out the possibility of such a machine ever being invented.
The first law is sometimes given in a contorted form as a statement that
precludes the existence of perpetual-motion machines of the first kind.
Second Law of Thermodynamics
The second law of thermodynamics gives a precise definition of a property
called entropy. Entropy can be thought of as a measure of how close a system
is to equilibrium; it can also be thought of as a measure of the disorder
in the system. The law states that the entropy—that is, the disorder—of
an isolated system can never decrease. Thus, when an isolated system achieves
a configuration of maximum entropy, it can no longer undergo change: It
has reached equilibrium. Nature, then, seems to “prefer” disorder or chaos.
It can be shown that the second law stipulates that, in the absence of
work, heat cannot be transferred from a region at a lower temperature to
one at a higher temperature.
The second law poses an additional condition on thermodynamic processes.
It is not enough to conserve energy and thus obey the first law. A machine
that would deliver work while violating the second law is called a “perpetual-motion
machine of the second kind,” since, for example, energy could then be continually
drawn from a cold environment to do work in a hot environment at no cost.
The second law of thermodynamics is sometimes given as a statement that
precludes perpetual-motion machines of the second kind.
Thermodynamic Cycles
All important thermodynamic relations used in engineering are derived
from the first and second laws of thermodynamics. One useful way of discussing
thermodynamic processes is in terms of cycles—processes that return a system
to its original state after a number of stages, thus restoring the original
values for all the relevant thermodynamic variables. In a complete cycle
the internal energy of a system depends solely on these variables and cannot
change. Thus, the total net heat transferred to the system must equal the
total net work delivered from the system.
An ideal cycle would be performed by a perfectly efficient heat engine—that
is, all the heat would be converted to mechanical work. The 19th-century
French scientist Nicolas Léonard Sadi Carnot, who conceived a thermodynamic
cycle that is the basic cycle of all heat engines, showed that such an
ideal engine cannot exist. Any heat engine must expend some fraction of
its heat input as exhaust. The second law of thermodynamics places an upper
limit on the efficiency of engines; that upper limit is less than 100 percent.
The limiting case is now known as a Carnot cycle.
Third Law of Thermodynamics
The second law suggests the existence of an absolute temperature scale
that includes an absolute zero of temperature. The third law of thermodynamics
states that absolute zero cannot be attained by any procedure in a finite
number of steps. Absolute zero can be approached arbitrarily closely, but
it can never be reached.
Microscopic Basis of Thermodynamics
The recognition that all matter is made up of molecules provided a
microscopic foundation for thermodynamics. A thermodynamic system consisting
of a pure substance can be described as a collection of like molecules,
each with its individual motion describable in terms of such mechanical
variables as velocity and momentum. At least in principle, it should therefore
be possible to derive the collective properties of the system by solving
equations of motion for the molecules. In this sense, thermodynamics could
be regarded as a mere application of the laws of mechanics to the microscopic
system.
Objects of ordinary size—that is, ordinary on the human scale—contain
immense numbers (on the order of 1024) of molecules. Assuming the molecules
to be spherical, each would need three variables to describe its position
and three more to describe its velocity. Describing a macroscopic system
in this way would be a task that even the largest modern computer could
not manage. A complete solution of these equations, furthermore, would
tell us where each molecule is and what it is doing at every moment. Such
a vast quantity of information would be too detailed to be useful and too
transient to be important.
Statistical methods were devised therefore to obtain averages of the
mechanical variables of the molecules in a system and to provide the gross
features of the system. These gross features turn out to be, precisely,
the macroscopic thermodynamic variables. The statistical treatment of molecular
mechanics is called statistical mechanics, and it anchors thermodynamics
to mechanics.
Viewed from the statistical perspective, temperature represents a measure
of the average kinetic energy of the molecules of a system. Increases in
temperature reflect increases in the vigor of molecular motion. When two
systems are in contact, energy is transferred between molecules as a result
of collisions. The transfer will continue until uniformity is achieved,
in a statistical sense, which corresponds to thermal equilibrium. The kinetic
energy of the molecules also corresponds to heat and—together with the
potential energy arising from interaction between molecules—makes up the
internal energy of a system.
The conservation of energy, a well-known law of mechanics, translates
readily to the first law of thermodynamics, and the concept of entropy
translates into the extent of disorder on the molecular scale. By assuming
that all combinations of molecular motion are equally likely, thermodynamics
shows that the more disordered the state of an isolated system, the more
combinations can be found that could give rise to that state, and hence
the more frequently it will occur. The probability of the more disordered
state occurring overwhelms the probability of the occurrence of all other
states. This probability provides a statistical basis for definitions of
both equilibrium state and entropy.
Finally, temperature can be reduced by taking energy out of a system,
that is, by reducing the vigor of molecular motion. Absolute zero corresponds
to the state of a system in which all its constituents are at rest. This
is, however, a notion from classical physics. In terms of quantum mechanics,
residual molecular motion will exist even at absolute zero. An analysis
of the statistical basis of the third law goes beyond the scope of the
present discussion.
See GASES; QUANTUM THEORY; UNCERTAINTY PRINCIPLE.
ASSIGNMENT SHEET
PROPERTIES OF WATER AND STEAM
Assignment Sheet Number 62B-201
INTRODUCTION
The process by which we convert water into steam and use the steam to turn a propulsion shaft encompasses the generation and expansion phases of the steam cycle. A study of the properties of water and steam at these critical phases is necessary to understand the steam cycle. This lesson defines terms associated with these properties and processes, and explains the use of steam tables to calculate the work and efficiency created by steam.
LESSON TOPIC LEARNING OBJECTIVES
Terminal Objective:
EXPLAIN the fundamentals and principles of thermodynamics and fluid mechanics in relation to the design, construction and operation of engineering plant equipment.
Enabling Objectives:
DEFINE the following terms:
.Enthalpy
.Entropy
.Working Fluid
.Density
.Specific Volume
.Specific Weight
.Reversible Process
.Irreversible Process
.Adiabatic Process
.Isothermal Process
.Isobaric Process
CALCULATE properties of water and/or steam at a given condition to 100%
accuracy using steam tables.
STUDY ASSIGNMENT
STUDY QUESTIONS
1. What is the difference between enthalpy and entropy? Explain their significance in the description of a steam/water mixture.
2. Determine the saturation temperature or pressure for the following:
a. 5 psia
b. 700°F
c. 450°F
d. 160 psia
e. 175 psig
3. In a naval boiler, discuss all places where sensible heat and latent heat are added to the working fluid.
STUDY SCENARIOS
We boil water to make steam so that this medium will acquire higher levels of enthalpy (useful energy) to turn a turbine. Warm water also contains enthalpy, but in significantly lower amounts. Superheated steam contains high levels of enthalpy.
1. What do we call water that is at a temperature below its saturation temperature?
2. How do we calculate it's enthalpy?
The steam entering an LP turbine is 14.7 psia and the exhaust is 28 inches Hg Vac. The enthalpy of the steam is 1150 BTU/lbm and the moisture content is 5%.
3. How much work does the turbine extract?
INTRODUCTION
The process by which we convert water into steam and use the steam to turn a propulsion shaft encompasses the generation and expansion phases of the steam cycle. A study of the properties of water and steam at these critical phases is necessary to understand the steam cycle. This lesson defines terms associated with these properties and processes, and explains the use of steam tables to calculate the work and efficiency created by steam.
REFERENCES
(a) Elements of Applied Thermodynamics, Robert M. Johnson, et
al.
(b) Principles of Naval Engineering NAVPERS 10788 series
(c) Introduction to Naval Engineering, Edward F. Gritzen.
INFORMATION
Basic Thermodynamic Terms
Enthalpy (h), measured in British thermal units per pound (mass), or BTU/lbm, represents the total energy content of steam. It expresses the internal energy and flow work, or the total potential energy and kinetic energy contained within a substance. The advantage of enthalpy is that we can express in one term all of the energy in a substance which is due to its pressure and temperature. Enthalpy values are used to represent the energy level of steam entering a turbine, a value useful for determining turbine efficiency. By superheating steam, we can add enthalpy to steam without raising the pressure of the steam. For example, steam at 620 psig and 850°F can do more work in a turbine than steam that is 620 psig and 650°F.
Entropy (s), measured in BTU/lbm-°R, represents the unavailability of energy (°R=Rankine temperature scale where 0°R = absolute zero and 460°R = 0°F). The second law of thermodynamics states that when heat is transferred from high temperature to low temperature regions, some of the heat will be rejected and not converted into mechanical work. Entropy is a measure of how much heat must be rejected to a lower temperature receiver at a given pressure and temperature.
A complex explanation of the mathematical significance of the definition of entropy is unnecessary. It is a term which attempts to describe the universe's tendency to evenly distribute all mass and energy throughout space. Processes which produce entropy are possible and those which destroy entropy are impossible.
Bodies with a high temperature will, when brought in contact with a body of a lower temperature, always cause heat to transfer from the hot body to the cold body. This will lower the internal energy of the hot body and raise the internal energy of the cold body. This is the principle that guides the design and operation of all naval heat exchangers. For example, a main engine lube oil cooler directs hot lube oil over cool seawater piping, so that the hot lube oil will transfer some of its heat to the cooler seawater. If left together indefinitely, the property of entropy would cause the heat from the lube oil to be equally distributed between the oil and the water, so that both would have the same temperature.
Entropy would not be important except for the fact that the purpose of any engine is to collect, transfer, and use energy. Thus, in a steam plant for example, it is not possible to add energy to water, boil it and transmit the resulting high energy steam across the relatively cooler engineroom without some of that energy being lost. Some of this energy will always be lost through system conditions such as ineffective pipe lagging, piping leaks, and dirty or fouled tubes which retard heat transfer. Operators must constantly attempt to minimize the effects of these conditions to maximize plant efficiency and reduce fuel and water costs.
A working fluid is a substance which receives, transfers and transmits energy in a thermodynamic system. In most systems, the working substance is a fluid (liquid, vapor or gas). In a steam system, water is the working fluid.
Density (r), measured in lbm/ft, represents the mass of a substance per unit volume, or how tightly packed the molecules are. The more molecules packed in a given space, the more dense the material. The density of water in a given location of the boiler is critical to the steam generation process because relatively dense feedwater will naturally push a less dense steam/water mixture through the boiler generating tubes.
Specific volume (vSP), measured in ft3/lbm, represents the space occupied per unit mass of a substance. It is the mathematical inverse of density. Most engineering equipment is designed for size and strength taking into consideration the specific volume of the intended working fluid.
Specific weight (g), measured in lbf/ft3, represents the weight of a
substance per unit volume. This is the density of a substance acted
upon by gravity. The pressure of a fluid at the bottom of a storage
tank is a direct function of the height of the fluid in the tank and the
specific weight of the feedwater. This resultant pressure is an important
shipboard consideration with respect to providing a minimum suction pressure
for a pump below the tank to move the fluid through a system.
The state of a working fluid refers to the physical properties it possesses
at a particular pressure, temperature and volume. If each of these
are known with respect to a substance, the state of the substance is known.
The substance can be a subcooled, saturated, or superheated solid, liquid,
or gas. Many systems operate the working fluid with very specific
temperature/pressure relationships. Water is subcooled in the condensate
and feed phases of the steam cycle to allow it to be pumped, saturated
in portions of the generation and feed phases for natural flow or for maintaining
proper chemistry, and superheated in the expansion phase to extract maximum
work from the steam to turn a propulsion turbine.
A thermodynamic process is any process which changes the state of the working fluid. These processes can be classified by the nature of the state change that takes place. Common types of thermodynamic processes include the following:
A reversible process is an ideal process where the working fluid returns to its original state by conducting the original process in the reverse direction. For a process to be reversible, it must be able to occur in precisely the reverse order. All energy that was transformed or distributed during the original process must be capable of being returned to its exact original form, amount and location. Reversible processes do not occur in real life.
An irreversible process is any process which is not reversible. All real life processes, such as the basic steam cycle, are irreversible.
An adiabatic process is a state change where there is no transfer of heat to or from the system during the process. Because heat transfer is relatively slow, any rapidly performed process can approach being adiabatic. Compression and expansion of working fluids are frequently achieved adiabatically with pumps and turbines.
An isothermal process is a state change in which no temperature change occurs. Note that heat transfer can occur without causing a change in temperature of the working fluid. In the DFT, auxiliary exhaust heats incoming condensate, then condenses to liquid and falls to the bottom of the tank. Throughout this process, the temperature of the auxiliary exhaust remains constant at 246-249°F.
An isobaric process is a state change in which the pressure of the working fluid is constant throughout the change. An isobaric state change occurs in the boiler superheater, as the heat of the exiting steam is increased without increasing its associated pressure.
A thermodynamic cycle is a recurring series of thermodynamic processes through which an effect is produced by the transformation or redistribution of energy. In other words, it is a series of processes repeated over and over again in the same order. Thermodynamic cycles contain five basic elements: (1) a working fluid, (2) an engine, (3) a heat source, (4) a heat receiver, and (5) a pump. All thermodynamic cycles may be classified as being open cycles or closed cycles.
A closed cycle is one in which the working fluid is reused. Steam plants and refrigeration cycles are closed cycles. In a steam plant, the water undergoes a series of processes that change the state of the water. Eventually the water returns to its original state and is ready to begin the cycle again.
An open cycle is one in which the working fluid is not reused. Open cycles typically use the atmosphere as a working fluid. An internal combustion engine represents a typical open cycle. Air is drawn into the engine, combusted in the cylinders, and exhausted back to the atmosphere. Fresh air is drawn into the engine to begin the cycle again.
Heat Addition and Temperature
When heat is added to a material, one of two things will occur: the material will change temperature or the material will change state. When a substance is below the temperature at a given pressure required to change state, the addition of sensible heat will raise the temperature of the substance. Sensible heat applied to a pot of water will raise its temperature until it boils. Once the substance reaches the necessary temperature at a given pressure to change state, the addition of latent heat causes the substance to change state. Adding latent heat to the boiling water does not get the water any hotter, but changes the liquid (water) into a gas (steam).
One can state that a certain amount of heat is required to raise the temperature of a substance 1°F. This energy is called the specific heat capacity. The specific heat capacity of a substance depends upon the volume and pressure of the material. For water, the specific heat capacity is 1 BTU/lbm-°F and remains constant. This means that if we add 1 BTU of heat to 1 lbm of water, the temperature will rise 1°F.
Introduction to Steam Tables
When a teapot of water is placed on a hot burner, sensible heat begins to heat the water. The energy added to the water raises its internal energy and its temperature. When the water reaches 212°F, the temperature no longer rises as latent heat begins to change the water from a liquid to a vapor. The mass inside the teapot is slowly changing from a 100% water / 0% steam mixture into a 0% water / 100% steam mixture. If we add only half the necessary latent heat, then only half the water will boil into steam. The result would be a 50% water / 50% steam mixture at 212°F. If we add all the latent heat necessary, then the water at 212°F changes completely into steam at 212°F. Continuing to add heat to the 212°F steam results in a temperature increase (superheating), and we are again raising the temperature by adding sensible heat. Refer to figure 3.2-1 (sensible/latent heat and enthalpy).
While the properties of water at atmospheric pressure are commonly known, water under different pressures will exhibit different properties. When water is boiled at pressures higher than atmospheric, the same events occur as described above with two exceptions. First, the boiling temperature will be higher than 212°F. Second, less latent heat is required to be added to change the water completely into steam. If water were to be boiled at a pressure lower than atmospheric pressure, then we would find that the boiling temperature would be less than 212°F and a larger amount of latent heat would be required to change the water completely into steam. Refer to figure 3.2-2 (temperature vs. latent heat).
When water is below the boiling point, the addition of heat is seen as sensible heat. This water is said to be a subcooled liquid. When enough sensible heat is added so that the temperature of the water approaches saturation temperature but no steam has yet been formed, the water is said to be a saturated liquid.
As the water is transformed from a saturated liquid to saturated steam, boiling is occurring. As latent heat is added, the temperature of the water remains the same but the saturated liquid is being changed into a saturated vapor. During this period the water is referred to as a liquid/vapor mixture. When enough latent heat is added so that all of the liquid is converted into vapor, the water becomes a saturated vapor. Note that the saturated vapor is 100% vapor and exists at the same temperature as the saturated liquid. Above the saturated steam point, vapor exists at a temperature higher than saturation temperature. This is the superheated vapor region.
Steam tables are a useful tool for determining the properties of steam and water at various temperatures and pressures. The steam tables are broken into three tables.
Steam table 1 is used for looking up values of specific volume (v), enthalpy (h), and entropy (s) for water when it exists as a saturated liquid, a liquid-vapor mixture or as a saturated vapor. Table 1 is used when saturation temperature is known.
Steam table 2 is also used for looking up values of specific volume (v), enthalpy (h), and entropy (s) for water when it exists as a saturated liquid, a liquid-vapor mixture or as a saturated vapor. The only difference is that table 2 is used when saturation pressure is known. Notice that table 1 and 2 both list the exact same information. Both are used for water when it exists between a saturated liquid and a saturated vapor.
Steam table 3 is used for superheated steam. It is arranged in a different format because when steam is superheated, the relationship between pressure and temperature changes. To use table 3, you must know both the temperature and the pressure of the superheated steam. Pressure is listed vertically down the right side of the page and temperature is listed horizontally across the top of each page. Specific volume, enthalpy and entropy for the superheated steam are calculated at the intersection of the two columns.
For subcooled liquids, notice that none of the tables are appropriate. For this situation, the enthalpy of a subcooled liquid is approximately the same as the temperature of the subcooled liquid minus 32°F (T - 32), or you can approximate the enthalpy of a subcooled liquid at a given temperature by using the value of enthalpy of a saturated liquid at the same temperature.
If we know that non-superheated water is at 200.0 psia, then we can use table 2 to find out the following data:
Example Steam Table Columns (Table 2)
Specific Volume Enthalpy Entropy Abs.
Sat. Sat. Sat. Sat. Sat. Sat. Abs.
Press. Temp. liquid Evap vapor liquid Evap. vapor liquid Evap. vapor
Press.
(psia) °F vf vfg vg hf hfg hg sg sfg sg (psia)
200.0 381.79 .01839 2.2689 2.2873 355.5 842.8 1198.3 .5438 1.0016 1.5454 200.0
The saturation temperature corresponding to its saturation pressure is 381.79°F. The vf, hf and sf terms are the parameters associated with water as a saturated liquid (the subscript f means "fluid"). Recall that a saturated liquid is 100% liquid/ 0% vapor at Tsat. The vg, hg and sg terms are the parameters associated with a saturated vapor (the subscript g means "gas"). Recall that saturated vapor is 0% liquid/100% vapor at Tsat.
The vfg, hfg and sfg terms represent the difference between the saturated liquid value (subscript f) and the saturated vapor value (subscript g). Thus the following relationship exists:
vfg = vg - vf hfg = hg - hf sfg = sg - sf
Whenever it is desired to know the v, h, or s of a water/steam mixture, the following equations are used:
vmix = vf + x vfg hmix = hf + x hfg
smix = sf + xhfg
vmix = vg - y vfg hmix = hg - y hfg
smix = sg - yhfg
Note: The mixture value is greater than the saturated liquid value but not as great as the saturated vapor value.
In the equation above, x = quality = % of steam in the steam/water mixture by mass, and y = moisture = % of water in a steam/water mixture by mass. Note that x + y = 1 ( or 100%)
The following is an example problem using the steam table 2 excerpt from the previous page. We have a liquid-vapor mixture at 200 psia. Assuming that the moisture content of the steam is 30%, we want to determine the amount of enthalpy, or useable energy, contained in the steam.
To determine the specific enthalpy of this steam, we choose the equation that solves for enthalpy (h) of a mixture of liquid and vapor:
hmix = hg - yhfg
Imputing the values for steam at 200 psia from the steam tables, and given that the moisture content is 30% gives us the following:
hmix = 1198.3 - (.3)(842.8)
hmix = 945.46 BTU/lbm
If this steam was of greater quality, we would assume that it would possess greater internal energy and thus greater enthalpy. Let's see how enthalpy of this steam is affected by a moisture content of only 10%:
hmix = 1198.3 - (.1)(842.8)
hmix = 1114.02 BTU/lbm
Table 3 is used in conjunction with superheated steam. There are no subscripts associated with the h, s, and v because saturated conditions no longer exist. In order to add additional sensible heat through superheating, the moisture must be removed from the liquid/vapor mixture. Superheating the steam causes the remaining liquid to change into vapor. Once the steam is no longer in contact with liquid, it is no longer said to be saturated. The symbol "Sh" means "degrees superheat," and represents the amount of degrees that the superheated steam is above saturation temperature at that particular pressure.
Mollier Diagram
The Mollier diagram is a small portion of data from the steam tables graphed onto enthalpy-entropy coordinates. It presents the region that is commonly found in propulsion plant steam systems. Examine the last section of the steam tables for a representation of a Mollier diagram.
Locating information off the Mollier diagram is done as follows: The horizontal axis is entropy (s) in BTU/lbm-°R. The vertical axis is enthalpy (h) in BTU/1bm. The dark line across the middle of the chart is called a "steam dome" because of its shape. Above this line, the data is for superheated steam. Below this line, the data is for a steam-water mixture. The data directly on the line is for saturated steam.
To find data in the steam-water mixture region of the chart, enter the chart using the absolute pressure and %-moisture (y). Once you find the intersection of these two parameters, read off the number directly across from the intersection point for enthalpy. Read off the number directly below the intersection point for entropy.
To find data in the superheated region of the chart, enter the chart
using the measured temperature and pressure of the steam. Again,
find the intersection point of these two parameters and read off the values
for entropy and enthalpy. Notice that moisture does not plot in the
superheat region. This is because moisture is a parameter which only
exists in saturated conditions.