What temperature does indicate is the tion of internal energy flow between bodies, andthe average molecular kinetic energy in transitbetween those bodies.. Heat and Temperature Thermal e
Trang 1it becomes an ion.
sub-stance made up of atoms of more than onechemical element These atoms are usuallyjoined in molecules
CHEMICAL ELEMENT: A substancemade up of only one kind of atom
law of physics which holds that within asystem isolated from all other outside fac-tors, the total amount of energy remainsthe same, though transformations of ener-
gy from one form to another take place
CONSERVATION OF MASS: A ical principle which states that total mass isconstant, and is unaffected by factors such
phys-as position, velocity, or temperature, in anysystem that does not exchange any matterwith its environment Unlike the otherconservation laws, however, conservation
of mass is not universally applicable, butapplies only at speeds significant lowerthan that of light—186,000 mi (297,600km) per second Close to the speed of light,mass begins converting to energy
CONSERVE: In physics, “to conserve”
something means “to result in no net loss
of ” that particular component It is ble that within a given system, the compo-nent may change form or position, but aslong as the net value of the componentremains the same, it has been conserved
possi-ELECTRON: Negatively charged cles in an atom Electrons, which spinaround the nucleus of protons and neu-trons, constitute a very small portion of theatom’s mass In most atoms, the number ofelectrons and protons is the same, thuscanceling out one another When an atomloses one or more electrons, however—
parti-thus becoming an ion—it acquires a netelectrical charge
FRICTION: The force that resistsmotion when the surface of one objectcomes into contact with the surface ofanother
FLUID: Any substance, whether gas orliquid, that tends to flow, and that con-forms to the shape of its container Unlikesolids, fluids are typically uniform inmolecular structure for instance, one mol-ecule of water is the same as another watermolecule
GAS: A phase of matter in which cules exert little or no attraction towardone another, and therefore move at highspeeds
mole-ION: An atom that has lost or gainedone or more electrons, and thus has a netelectrical charge
LIQUID: A phase of matter in whichmolecules exert moderate attractionstoward one another, and therefore move atmoderate speeds
MATTER: Physical substance that hasmass; occupies space; is composed ofatoms; and is ultimately (at speedsapproaching that of light) convertible toenergy There are several phases of matter,including solids, liquids, and gases
K E Y T E R M S
Trang 2of Matter
The cholesteric class of liquid crystals is sonamed because the spiral patterns of lightthrough the crystal are similar to those whichappear in cholesterols Depending on the physi-cal properties of a cholesteric liquid crystal, onlycertain colors may be reflected The response ofliquid crystals to light makes them useful in liq-uid crystal displays (LCDs) found on laptopcomputer screens, camcorder views, and in otherapplications
In some cholesteric liquid crystals, high peratures lead to a reflection of shorter visiblelight waves, and lower temperatures to a display
tem-of longer visible waves Liquid crystal ters thus show red when cool, and blue as theyare warmed This may seem a bit unusual tosomeone who does not understand why the ther-
thermome-mometer displays those colors, since people ically associate red with heat and blue with cold
typ-T H E typ-T R I P L E P O I N typ-T A liquid tal exhibits aspects of both liquid and solid, andthus, at certain temperatures may be classifiedwithin the crystalline quasi-state of matter Onthe other hand, the phenomenon known as thetriple point shows how an ordinary substance,such as water or carbon dioxide, can actually be aliquid, solid, and vapor—all at once
crys-Again, water—the basis of all life on Earth—
is an unusual substance in many regards Forinstance, most people associate water as a gas orvapor (that is, steam) with very high tempera-tures Yet, at a level far below normal atmospher-
ic pressure, water can be a vapor at temperatures
as low as -4°F (-20 °C) (All of the pressure values
MOLE: A unit equal to 6.022137 1023(more than 600 billion trillion) molecules
Their size makes it impossible to weighmolecules in relatively small quantities;
hence the mole facilitates comparisons ofmass between substances
MOLECULE: A group of atoms,
usual-ly of more than one chemical element,joined in a structure
NEUTRON: A subatomic particle thathas no electrical charge Neutrons arefound at the nucleus of an atom, alongsideprotons
PHASES OF MATTER: The variousforms of material substance (matter),which are defined primarily in terms of thebehavior exhibited by their atomic ormolecular structures On Earth, three prin-cipal phases of matter exist, namely solid,liquid, and gas Other forms of matterinclude plasma
PLASMA: One of the phases of matter,closely related to gas Plasma apparently
does not exist on Earth, but is found, forinstance, in stars and comets’ tails Con-taining neither atoms nor molecules, plas-
ma is made up of electrons and positiveions
PROTON: A positively charged particle
in an atom Protons and neutrons, whichtogether form the nucleus around whichelectrons orbit, have approximately thesame mass—a mass that is many timesgreater than that of an electron
SOLID: A phase of matter in whichmolecules exert strong attractions towardone another, and therefore move slowly
SYSTEM: In physics, the term “system”usually refers to any set of physical interac-tions isolated from the rest of the universe.Anything outside of the system, includingall factors and forces irrelevant to a discus-sion of that system, is known as the envi-ronment
K E Y T E R M S C O N T I N U E D
Trang 3of Matter
in the discussion of water at or near the triple
point are far below atmospheric norms: the
pressure at which water would turn into a vapor at
-4°F, for instance, is about 1/1000 normal
atmos-pheric pressure.)
As everyone knows, at relatively low atures, water is a solid—ice But if the pressure of
temper-ice falls below a very low threshold, it will turn
straight into a gas (a process known as
sublima-tion) without passing through the liquid stage
On the other hand, by applying enough pressure,
it is possible to melt ice, and thereby transform it
from a solid to a liquid, at temperatures below its
normal freezing point
The phases and changes of phase for a givensubstance at specific temperatures and pressure
levels can be plotted on a graph called a phase
diagram, which typically shows temperature on
the x-axis and pressure on the y-axis The phase
diagram of water shows a line between the solid
and liquid states that is almost, but not quite,
exactly perpendicular to the x-axis: it slopes
slightly upward to the left, reflecting the fact that
solid ice turns into water with an increase of
pressure
Whereas the line between solid and liquidwater is more or less straight, the division
between these two states and water vapor is
curved And where the solid-liquid line intersects
the vaporization curve, there is a place called the
triple point Just below freezing, in conditions
equivalent to about 0.7% of normal atmosphericpressure, water is a solid, liquid, and vapor all atonce
W H E R E T O L E A R N M O R E
Biel, Timothy L Atom: Building Blocks of Matter San
Diego, CA: Lucent Books, 1990.
Feynman, Richard Six Easy Pieces: Essentials of Physics Explained by Its Most Brilliant Teacher New intro-
duction by Paul Davies Cambridge, MA: Perseus Books, 1995.
Hewitt, Sally Solid, Liquid, or Gas? New York: Children’s
Press, 1998.
“High School Chemistry Table of Contents—Solids and Liquids” Homeworkhelp.com (Web site) <http://www.
homeworkhelp.com/homeworkhelp/freemember/text /chem/hig h/topic09.html> (April 10, 2001).
“Matter: Solids, Liquids, Gases.” Studyweb (Web site).
<http://www.studyweb.com/links/4880.html> (April
10, 2001).
“The Molecular Circus” (Web site) <http://www.cpo.
com/Weblabs/circus.html> (April 10, 2001).
Paul, Richard A Handbook to the Universe: Explorations
of Matter, Energy, Space, and Time for Beginning entific Thinkers Chicago: Chicago Review Press,
Trang 4T H E R M O D Y N A M I C SThermodynamics
C O N C E P TThermodynamics is the study of the relation-ships between heat, work, and energy Thoughrooted in physics, it has a clear application tochemistry, biology, and other sciences: in a sense,physical life itself can be described as a con-tinual thermodynamic cycle of transformationsbetween heat and energy But these transforma-tions are never perfectly efficient, as the secondlaw of thermodynamics shows Nor is it possible
to get “something for nothing,” as the first law ofthermodynamics demonstrates: the work output
of a system can never be greater than the netenergy input These laws disappointed hopefulindustrialists of the early nineteenth century,many of whom believed it might be possible tocreate a perpetual motion machine Yet the laws
of thermodynamics did make possible such
high-ly useful creations as the internal combustionengine and the refrigerator
H O W I T W O R K S
Historical Context
Machines were, by definition, the focal point ofthe Industrial Revolution, which began in Eng-land during the late eighteenth and early nine-teenth centuries One of the central preoccupa-tions of both scientists and industrialists thusbecame the efficiency of those machines: theratio of output to input The more output thatcould be produced with a given input, the greaterthe production, and the greater the economicadvantage to the industrialists and (presumably)society as a whole
At that time, scientists and captains ofindustry still believed in the possibility of a per-petual motion machine: a device that, uponreceiving an initial input of energy, would con-tinue to operate indefinitely without furtherinput As it emerged that work could be convert-
ed into heat, a form of energy, it began to seempossible that heat could be converted directlyback into work, thus making possible the opera-tion of a perfectly reversible perpetual motionmachine Unfortunately, the laws of thermody-namics dashed all those dreams
S N O W ’ S E X P L A N A T I O N Sometexts identify two laws of thermodynamics, whileothers add a third For these laws, which will bediscussed in detail below, British writer and sci-entist C P Snow (1905-1980) offered a witty,nontechnical explanation In a 1959 lecture pub-
lished as The Two Cultures and the Scientific
Rev-olution, Snow compared the effort to transform
heat into energy, and energy back into heat again,
as a sort of game
The first law of thermodynamics, in Snow’sversion, teaches that the game is impossible towin Because energy is conserved, and thus, itsquantities throughout the universe are always thesame, one cannot get “something for nothing” byextracting more energy than one put into amachine
The second law, as Snow explained it, offers
an even more gloomy prognosis: not only is itimpossible to win in the game of energy-workexchanges, one cannot so much as break even.Though energy is conserved, that does not meanthe energy is conserved within the machinewhere it is used: mechanical systems tend towardincreasing disorder, and therefore, it is impossi-
Trang 5Thermo-ble for the machine even to return to the original
level of energy
The third law, discovered in 1905, seems tooffer a possibility of escape from the conditions
imposed in the second law: at the temperature of
absolute zero, this tendency toward breakdown
drops to a level of zero as well But the third law
only proves that absolute zero cannot be
attained: hence, Snow’s third observation, that it
is impossible to step outside the boundaries of
this unwinnable heat-energy transformation
game
Work and Energy
Work and energy, discussed at length elsewhere
in this volume, are closely related Work is the
exertion of force over a given distance to displace
or move an object It is thus the product of force
and distance exerted in the same direction
Ener-gy is the ability to accomplish work
There are many manifestations of energy,including one of principal concern in the present
context: thermal or heat energy Other
manifes-tations include electromagnetic (sometimes
divided into electrical and magnetic), sound,
chemical, and nuclear energy All these, however,
can be described in terms of mechanical energy,
which is the sum of potential energy—the
ener-gy that an object has due to its position—andkinetic energy, or the energy an object possesses
by virtue of its motion
M E C H A N I C A L E N E R G Y Kineticenergy relates to heat more clearly than doespotential energy, discussed below; however, it ishard to discuss the one without the other To use
a simple example—one involving mechanicalenergy in a gravitational field—when a stone isheld over the edge of a cliff, it has potential ener-
gy Its potential energy is equal to its weight(mass times the acceleration due to gravity) mul-tiplied by its height above the bottom of thecanyon below Once it is dropped, it acquireskinetic energy, which is the same as one-half itsmass multiplied by the square of its velocity
Just before it hits bottom, the stone’s kineticenergy will be at a maximum, and its potentialenergy will be at a minimum At no point can thevalue of its kinetic energy exceed the value of thepotential energy it possessed before it fell: themechanical energy, or the sum of kinetic andpotential energy, will always be the same, thoughthe relative values of kinetic and potential energymay change
A WOMAN WITH A SUNBURNED NOSE S UNBURNS ARE CAUSED BY THE S UN ’ S ULTRAVIOLET RAYS (Photograph by Lester
V Bergman/Corbis Reproduced by permission.)
Trang 6Thermo-dynamics
system Rather than being “energy-in-residence,”heat is “energy-in-transit.”
This may be a little hard to comprehend, but
it can be explained in terms of the stone-and-cliffkinetic energy illustration used above Just as asystem can have no kinetic energy unless some-thing is moving within it, heat exists only whenenergy is being transferred In the above illustra-tion of mechanical energy, when the stone wassitting on the ground at the top of the cliff, it wasanalogous to a particle of internal energy in body
A When, at the end, it was again on the
ground—only this time at the bottom of thecanyon—it was the same as a particle of internal
energy that has transferred to body B In
between, however, as it was falling from one tothe other, it was equivalent to a unit of heat
T E M P E R A T U R E In everyday life, ple think they know what temperature is: a meas-ure of heat and cold This is wrong for two rea-sons: first, as discussed below, there is no suchthing as “cold”—only an absence of heat So,then, is temperature a measure of heat? Wrongagain
peo-Imagine two objects, one of mass M and the other with a mass twice as great, or 2M Both
have a certain temperature, and the question is,how much heat will be required to raise theirtemperature by equal amounts? The answer is
that the object of mass 2M requires twice as
much heat to raise its temperature the sameamount Therefore, temperature cannot possibly
be a measure of heat
What temperature does indicate is the tion of internal energy flow between bodies, andthe average molecular kinetic energy in transitbetween those bodies More simply, though a bitless precisely, it can be defined as a measure ofheat differences (As for the means by which athermometer indicates temperature, that isbeyond the parameters of the subject at hand; it
direc-is ddirec-iscussed elsewhere in thdirec-is volume, in the text of thermal expansion.)
con-M E A S U R I N G T E con-M P E R A T U R E
A N D H E A T Temperature, of course, can bemeasured either by the Fahrenheit or Centigradescales familiar in everyday life Another tempera-ture scale of relevance to the present discussion isthe Kelvin scale, established by William Thom-son, Lord Kelvin (1824-1907)
Drawing on the discovery made by Frenchphysicist and chemist J A C Charles (1746-
from the ground
If the stone’s mechanical energy—at least inrelation to the system of height between the cliffand the bottom—has dropped to zero, where did
it go? A number of places When it hit, the stonetransferred energy to the ground, manifested asheat It also made a sound when it landed, andthis also used up some of its energy The stoneitself lost energy, but the total energy in the uni-verse was unaffected: the energy simply left thestone and went to other places This is an exam-ple of the conservation of energy, which is close-
ly tied to the first law of thermodynamics
But does the stone possess any energy at thebottom of the canyon? Absolutely For one thing,its mass gives it an energy, known as mass or restenergy, that dwarfs the mechanical energy in thesystem of the stone dropping off the cliff (Massenergy is the other major form of energy, asidefrom kinetic and potential, but at speeds wellbelow that of light, it is released in quantities thatare virtually negligible.) The stone may have elec-tromagnetic potential energy as well; and ofcourse, if someone picks it up again, it will havegravitational potential energy Most important tothe present discussion, however, is its internalkinetic energy, the result of vibration among themolecules inside the stone
Heat and Temperature
Thermal energy, or the energy of heat, is really aform of kinetic energy between particles at theatomic or molecular level: the greater the move-ment of these particles, the greater the thermalenergy Heat itself is internal thermal energy thatflows from one body of matter to another It isnot the same as the energy contained in a sys-tem—that is, the internal thermal energy of the
Trang 7Thermo-1823), that gas at 0°C (32°F) regularly contracts
by about 1/273 of its volume for every Celsius
degree drop in temperature, Thomson derived
the value of absolute zero (discussed below) as
-273.15°C (-459.67°F) The Kelvin and Celsius
scales are thus directly related: Celsius
tempera-tures can be converted to Kelvins (for which
nei-ther the word nor the symbol for “degree” are
used) by adding 273.15
M E A S U R I N G H E A T A N D H E A T
C A P A C I T Y Heat, on the other hand, is
meas-ured not by degrees (discussed along with the
thermometer in the context of thermal
expan-sion), but by the same units as work Since
ener-gy is the ability to perform work, heat or work
units are also units of energy The principal unit
of energy in the SI or metric system is the joule
(J), equal to 1 newton-meter (N • m), and the
primary unit in the British or English system is
the foot-pound (ft • lb) One foot-pound is equal
to 1.356 J, and 1 joule is equal to 0.7376 ft • lb
Two other units are frequently used for heat
as well In the British system, there is the Btu, or
British thermal unit, equal to 778 ft • lb or 1,054
J Btus are often used in reference, for instance, to
the capacity of an air conditioner An SI unit that
is also used in the United States—where British
measures typically still prevail—is the
kilocalo-rie This is equal to the heat that must be added
to or removed from 1 kilogram of water to
change its temperature by 1°C As its name
sug-gests, a kilocalorie is 1,000 calories A calorie is
the heat required to change the temperature in 1
gram of water by 1°C—but the dietary Calorie
(capital C), with which most people are familiar
is the same as the kilocalorie
A kilocalorie is identical to the heat capacityfor one kilogram of water Heat capacity (some-
times called specific heat capacity or specific
heat) is the amount of heat that must be added
to, or removed from, a unit of mass for a given
substance to change its temperature by 1°C this
is measured in units of J/kg • °C (joules per
kilo-gram-degree Centigrade), though for the sake of
convenience it is typically rendered in terms of
kilojoules (1,000 joules): kJ/kg • °c Expressed
thus, the specific heat of water 4.185—which is
fitting, since a kilocalorie is equal to 4.185 kJ
Water is unique in many aspects, with regard to
specific heat, in that it requires far more heat to
raise the temperature of water than that of
mer-cury or iron
R E A L - L I F E
A P P L I C A T I O N S
Hot and “Cold”
Earlier, it was stated that there is no such thing as
“cold”—a statement hard to believe for someonewho happens to be in Buffalo, New York, orInternational Falls, Minnesota, during a Febru-ary blizzard Certainly, cold is real as a sensoryexperience, but in physical terms, cold is not a
“thing”—it is simply the absence of heat
People will say, for instance, that they put anice cube in a cup of coffee to cool it, but in terms
of physics, this description is backward: whatactually happens is that heat flows from the cof-fee to the ice, thus raising its temperature Theresulting temperature is somewhere between that
of the ice cube and the coffee, but one cannotobtain the value simply by averaging the twotemperatures at the beginning of the transfer
For one thing, the volume of the water in theice cube is presumably less than that of the water
in the coffee, not to mention the fact that theirdiffering chemical properties may have someminor effect on the interaction Most important,however, is the fact that the coffee did not simplymerge with the ice: in transferring heat to the icecube, the molecules in the coffee expended some
of their internal kinetic energy, losing furtherheat in the process
C O O L I N G M A C H I N E S Even ing machines, such as refrigerators and air condi-tioners, actually use heat, simply reversing theusual process by which particles are heated Therefrigerator pulls heat from its inner compart-ment—the area where food and other perish-ables are stored—and transfers it to the regionoutside This is why the back of a refrigerator iswarm
cool-Inside the refrigerator is an evaporator, intowhich heat from the refrigerated compartmentflows The evaporator contains a refrigerant—agas, such as ammonia or Freon 12, that readilyliquifies This gas is released into a pipe from theevaporator at a low pressure, and as a result, itevaporates, a process that cools it The pipe takesthe refrigerant to the compressor, which pumps
it into the condenser at a high pressure Located
at the back of the refrigerator, the condenser is along series of pipes in which pressure turns thegas into liquid As it moves through the condens-
Trang 8man-Thus, cooling machines do not defy theprinciples of heat discussed above; nor do theydefy the laws of thermodynamics that will be dis-cussed at the conclusion of this essay In accor-dance with the second law, in order to move heat
in the reverse of its usual direction, externalenergy is required Thus, a refrigerator takes inenergy from a electric power supply (that is, theoutlet it is plugged into), and extracts heat
Nonetheless, it manages to do so efficiently,removing two or three times as much heat fromits inner compartment as the amount of energyrequired to run the refrigerator
Transfers of Heat
It is appropriate now to discuss how heat is ferred One must remember, again, that in orderfor heat to be transferred from one point toanother, there must be a difference of tempera-ture between those two points If an object orsystem has a uniform level of internal thermalenergy—no matter how “hot” it may be in ordi-nary terms—no heat transfer is taking place
trans-Heat is transferred by one of three methods:
conduction, which involves successive molecularcollisions; convection, which requires the motion
of hot fluid from one place to another; or tion, which involves electromagnetic waves andrequires no physical medium for the transfer
radia-C O N D U radia-C T I O N Conduction takesplace best in solids and particularly in metals,whose molecules are packed in relatively closeproximity Thus, when one end of an iron rod isheated, eventually the other end will acquire heatdue to conduction Molecules of liquid or non-metallic solids vary in their ability to conductheat, but gas—due to the loose attractionsbetween its molecules—is a poor conductor
When conduction takes place, it is as though
a long line of people are standing shoulder to
shoulder, passing a secret down the line In thiscase, however, the “secret” is kinetic thermalenergy And just as the original phrasing of thesecret will almost inevitably become garbled bythe time it gets to the tenth or hundredth person,some energy is lost in the transfer from molecule
to molecule Thus, if one end of the iron rod issitting in a fire and one end is surrounded by air
at room temperature, it is unlikely that the end inthe air will ever get as hot as the end in the fire.Incidentally, the qualities that make metallicsolids good conductors of heat also make themgood conductors of electricity In the firstinstance, kinetic energy is being passed frommolecule to molecule, whereas in an electricalfield, electrons—freed from the atoms of whichthey are normally a part—are able to move alongthe line of molecules Because plastic is much lessconductive than metal, an electrician will use
a screwdriver with a plastic handle Similarly,
a metal pan typically has a handle of wood orplastic
C O N V E C T I O N There is a term, vection oven,” that is actually a redundancy: allovens heat through convection, the principalmeans of transferring heat through a fluid Inphysics, “fluid” refers both to liquids and gases—anything that tends to flow Instead of simplymoving heat, as in conduction, convectioninvolves the movement of heated material—that
“con-is, fluid When air is heated, it displaces cold (that
is, unheated) air in its path, setting up a tion current
convec-Convection takes place naturally, as forinstance when hot air rises from the land on awarm day This heated air has a lower densitythan that of the less heated air in the atmosphereabove it, and, therefore, is buoyant As it rises,however, it loses energy and cools This cooledair, now more dense than the air around it, sinksagain, creating a repeating cycle
The preceding example illustrates naturalconvection; the heat of an oven, on the otherhand, is an example of forced convection—a sit-uation in which some sort of pump or mecha-nism moves heated fluid So, too, is the coolingwork of a refrigerator, though the refrigeratormoves heat in the opposite direction
Forced convection can also take place within
a natural system The human heart is a pump,and blood carries excess heat generated by thebody to the skin The heat passes through the
Trang 9Thermo-skin by means of conduction, and at the surface
of the skin, it is removed from the body in a
number of ways, primarily by the cooling
evapo-ration of moisture—that is, perspievapo-ration
R A D I A T I O N If the Sun is hot—hotenough to severely burn the skin of a person who
spends too much time exposed to its rays—then
why is it cold in the upper atmosphere? After all,
the upper atmosphere is closer to the Sun And
why is it colder still in the empty space above the
atmosphere, which is still closer to the Sun? The
reason is that in outer space there is no medium
for convection, and in the upper atmosphere,
where the air molecules are very far apart, there
is hardly any medium How, then, does heat
come to the Earth from the Sun? By radiation,
which is radically different from conduction or
convection The other two involve ordinary
ther-mal energy, but radiation involves
electromag-netic energy
A great deal of “stuff ” travels through theelectromagnetic spectrum, discussed in another
essay in this book: radio waves, microwaves for
television and radar, infrared light, visible light, x
rays, gamma rays Though the relatively narrow
band of visible-light wavelengths is the only part
of the spectrum of which people are aware in
everyday life, other parts—particularly the
infrared and ultraviolet bands—are involved in
the heat one feels from the Sun (Ultraviolet rays,
in fact, cause sunburns.)
Heat by means of radiation is not as worldly” as it might seem: in fact, one does not
“other-have to point to the Sun for examples of it Any
time an object glows as a result of heat—as for
example, in the case of firelight—that is an
example of radiation Some radiation is emitted
in the form of visible light, but the heat
compo-nent is in infrared rays This also occurs in an
incandescent light bulb In an incandescent bulb,
incidentally, much of the energy is lost to the heat
of infrared rays, and the efficiency of a
fluores-cent bulb lies in the fact that it converts what
would otherwise be heat into usable light
The Laws of Thermodynamics
Having explored the behavior of heat, both at the
molecular level and at levels more easily
per-ceived by the senses, it is possible to discuss the
laws of thermodynamics alluded to throughout
this essay These laws illustrate the relationships
between heat and energy examined earlier, and
show, for instance, why a refrigerator or air ditioner must have an external source of energy
con-to move heat in a direction opposite con-to its normalflow
The story of how these laws came to be covered is a saga unto itself, involving the contri-butions of numerous men in various places over
dis-a period of more thdis-an dis-a century In 1791, Swissphysicist Pierre Prevost (1751-1839) put forth histheory of exchanges, stating correctly that allbodies radiate heat Hence, as noted earlier, there
is no such thing as “cold”: when one holds snow
in one’s hand, cold does not flow from the snowinto the hand; rather, heat flows from the hand tothe snow
Seven years later, an American-British cist named Benjamin Thompson, Count Rum-ford (1753) was boring a cannon with a bluntdrill when he noticed that this action generated agreat deal of heat This led him to question theprevailing wisdom, which maintained that heatwas a fluid form of matter; instead, Thompsonbegan to suspect that heat must arise from someform of motion
physi-C A R N O T ’ S E N G I N E The nextmajor contribution came from the French physi-cist and engineer Sadi Carnot (1796-1832)
B ENJAMIN T HOMPSON , C OUNT R UMFORD (Illustration by
H Humphrey UPI/Corbis-Bettmann Reproduced by permission.)
Trang 10Thermo-dynamics
Though he published only one scientific work,
Reflections on the Motive Power of Fire (1824), this
treatise caused a great stir in the European tific community In it, Carnot made the firstattempt at a scientific definition of work,describing it as “weight lifted through a height.”
scien-Even more important was his proposal for ahighly efficient steam engine
A steam engine, like a modern-day internalcombustion engine, is an example of a largerclass of machine called heat engine A heatengine absorbs heat at a high temperature, per-forms mechanical work, and, as a result, gives offheat a lower temperature (The reason why thattemperature must be lower is established in thesecond law of thermodynamics.)
For its era, the steam engine was what thecomputer is today: representing the cutting edge
in technology, it was the central preoccupation ofthose interested in finding new ways to accom-plish old tasks Carnot, too, was fascinated by thesteam engine, and was determined to help over-come its disgraceful inefficiency: in operation, asteam engine typically lost as much as 95% of itsheat energy
In his Reflections, Carnot proposed that the
maximum efficiency of any heat engine wasequal to (TH-TL)/TH, where TH is the highestoperating temperature of the machine, and TL
the lowest In order to maximize this value, TLhas to be absolute zero, which is impossible toreach, as was later illustrated by the third law ofthermodynamics
In attempting to devise a law for a perfectlyefficient machine, Carnot inadvertently provedthat such a machine is impossible Yet his workinfluenced improvements in steam enginedesign, leading to levels of up to 80% efficiency
In addition, Carnot’s studies influenced Kelvin—
who actually coined the term ics”—and others
“thermodynamT H E F I R S “thermodynamT L A W O F “thermodynamT H E R M O
-D Y N A M I C S During the 1840s, JuliusRobert Mayer (1814-1878), a German physicist,published several papers in which he expoundedthe principles known today as the conservation
of energy and the first law of thermodynamics
As discussed earlier, the conservation of energyshows that within a system isolated from all out-side factors, the total amount of energy remainsthe same, though transformations of energyfrom one form to another take place
The first law of thermodynamics states thisfact in a somewhat different manner As with theother laws, there is no definitive phrasing;instead, there are various versions, all of whichsay the same thing One way to express the law is
as follows: Because the amount of energy in asystem remains constant, it is impossible to per-form work that results in an energy outputgreater than the energy input For a heat engine,this means that the work output of the engine,combined with its change in internal energy, isequal to its heat input Most heat engines, how-ever, operate in a cycle, so there is no net change
in internal energy
Earlier, it was stated that a refrigeratorextracts two or three times as much heat from itsinner compartment as the amount of energyrequired to run it On the surface, this seems tocontradict the first law: isn’t the refrigerator put-ting out more energy than it received? But theheat it extracts is only part of the picture, and notthe most important part from the perspective ofthe first law
A regular heat engine, such as a steam orinternal-combustion engine, pulls heat from ahigh-temperature reservoir to a low-temperaturereservoir, and, in the process, work is accom-plished Thus, the hot steam from the high-tem-perature reservoir makes possible the accom-plishment of work, and when the energy isextracted from the steam, it condenses in thelow-temperature reservoir as relatively coolwater
A refrigerator, on the other hand, reversesthis process, taking heat from a low-temperaturereservoir (the evaporator inside the cooling com-partment) and pumping it to a high-temperaturereservoir outside the refrigerator Instead of pro-ducing a work output, as a steam engine does, itrequires a work input—the energy supplied viathe wall outlet Of course, a refrigerator does pro-duce an “output,” by cooling the food inside, butthe work it performs in doing so is equal to theenergy supplied for that purpose
T H E S E C O N D L A W O F T H E R
-M O D Y N A -M I C S Just a few years afterMayer’s exposition of the first law, another Ger-man physicist, Rudolph Julius Emanuel Clausius(1822-1888) published an early version of thesecond law of thermodynamics In an 1850paper, Clausius stated that “Heat cannot, of itself,pass from a colder to a hotter body.” He refined
Trang 11Thermo-this 15 years later, introducing the concept of
entropy—the tendency of natural systems
toward breakdown, and specifically, the tendency
for the energy in a system to be dissipated
The second law of thermodynamics beginsfrom the fact that the natural flow of heat is
always from a high-temperature reservoir to a
low-temperature reservoir As a result, no engine
can be constructed that simply takes heat from a
source and performs an equivalent amount of
work: some of the heat will always be lost In
other words, it is impossible to build a perfectly
efficient engine
Though its relation to the first law is ous, inasmuch as it further defines the limita-
obvi-tions of machine output, the second law of
ther-modynamics is not derived from the first
Else-where in this volume, the first law of
thermody-namics—stated as the conservation of energy
law—is discussed in depth, and, in that context,
it is in fact necessary to explain how the behavior
of machines in the real world does not contradict
the conservation law
Even though they mean the same thing, thefirst law of thermodynamics and the conserva-
tion of energy law are expressed in different ways
The first law of thermodynamics states that “the
glass is half empty,” whereas the conservation of
energy law shows that “the glass is half full.” The
thermodynamics law emphasizes the bad news:
that one can never get more energy out of a
machine than the energy put into it Thus, all
hopes of a perpetual motion machine were
dashed The conservation of energy, on the other
hand, stresses the good news: that energy is never
lost
In this context, the second law of namics delivers another dose of bad news:
thermody-though it is true that energy is never lost, the
energy available for work output will never be as
great as the energy put into a system A car
engine, for instance, cannot transform all of its
energy input into usable horsepower; some of the
energy will be used up in the form of heat and
sound Though energy is conserved, usable
ener-gy is not
Indeed, the concept of entropy goes farbeyond machines as people normally understand
them Entropy explains why it is easier to break
something than to build it—and why, for each
person, the machine called the human body will
inevitably break down and die, or cease to tion, someday
funcT H E funcT H I R D L A W O F funcT H E R M O
-D Y N A M I C S The subject of entropy leadsdirectly to the third law of thermodynamics, for-mulated by German chemist Hermann WalterNernst (1864-1941) in 1905 The third law statesthat at the temperature of absolute zero, entropyalso approaches zero From this statement,Nernst deduced that absolute zero is thereforeimpossible to reach
All matter is in motion at the molecularlevel, which helps define the three major phases
of matter found on Earth At one extreme is agas, whose molecules exert little attractiontoward one another, and are therefore in constantmotion at a high rate of speed At the other end
of the phase continuum (with liquids somewhere
in the middle) are solids Because they are closetogether, solid particles move very little, andinstead of moving in relation to one another,they merely vibrate in place But they do move
Absolute zero, or 0K on the Kelvin scale oftemperature, is the point at which all molecularmotion stops entirely—or at least, it virtuallystops (In fact, absolute zero is defined as thetemperature at which the motion of the averageatom or molecule is zero.) As stated earlier,Carnot’s engine achieves perfect efficiency if itslowest temperature is the same as absolute zero;
but the second law of thermodynamics showsthat a perfectly efficient machine is impossible
This means that absolute zero is an unreachableextreme, rather like matter exceeding the speed
of light, also an impossibility
This does not mean that scientists do notattempt to come as close as possible to absolutezero, and indeed they have come very close In
1993, physicists at the Helsinki University ofTechnology Low Temperature Laboratory in Fin-land used a nuclear demagnetization device toachieve a temperature of 2.8 • 10-10 K, or0.00000000028K This means that a fragmentequal to only 28 parts in 100 billion separatedthis temperature from absolute zero—but it wasstill above 0K Such extreme low-temperatureresearch has a number of applications, mostnotably with superconductors, materials thatexhibit virtually no resistance to electrical cur-rent at very low temperatures
Trang 12Thermo-dynamics
ABSOLUTE ZERO: The temperature,defined as 0K on the Kelvin scale, at whichthe motion of molecules in a solid virtual-
ly ceases The third law of thermodynamicsestablishes the impossibility of actuallyreaching absolute zero
BTU (BRITISH THERMAL UNIT): Ameasure of energy or heat in the Britishsystem, often used in reference to thecapacity of an air conditioner A Btu isequal to 778 foot-pounds, or 1,054 joules
CALORIE: A measure of heat or energy
in the SI or metric system, equal to the heatthat must be added to or removed from 1gram of water to change its temperature by33.8°F (1°C) The dietary Calorie (capitalC) with which most people are familiar isthe same as the kilocalorie
CONDUCTION: The transfer of heat
by successive molecular collisions duction is the principal means of heattransfer in solids, particularly metals
law of physics which holds that within asystem isolated from all other outside fac-tors, the total amount of energy remainsthe same, though transformations of ener-
gy from one form to another take place
The first law of thermodynamics is thesame as the conservation of energy
CONSERVE: In physics, “to conserve”
something means “to result in no net loss
of ” that particular component It is ble that within a given system, the compo-nent may change form or position, but aslong as the net value of the componentremains the same, it has been conserved
possi-CONVECTION: The transfer of heatthrough the motion of hot fluid from oneplace to another In physics, a “fluid” can be
either a gas or a liquid, and convection isthe principal means of heat transfer, forinstance, in air and water
ENERGY: The ability to accomplishwork
ENTROPY: The tendency of naturalsystems toward breakdown, and specifical-
ly, the tendency for the energy in a system
to be dissipated Entropy is closely related
to the second law of thermodynamics
FIRST LAW OF THERMODYNAMICS:
A law which states the amount of energy in
a system remains constant, and therefore it
is impossible to perform work that results
in an energy output greater than the
ener-gy input This is the same as the tion of energy
conserva-FOOT-POUND: The principal unit ofenergy—and thus of heat—in the British
or English system The metric or SI unit isthe joule A foot-pound (ft • lb) is equal to1.356 J
HEAT: Internal thermal energy thatflows from one body of matter to another.Heat is transferred by three methods con-duction, convection, and radiation
HEAT CAPACITY: The amount of heatthat must be added to, or removed from, aunit of mass of a given substance to changeits temperature by 33.8°F (1°C) Heatcapacity is sometimes called specific heatcapacity or specific heat A kilocalorie isthe heat capacity of 1 gram of water
absorbs heat at a high temperature, forms mechanical work, and as a resultgives off heat at a lower temperature
per-KINETIC ENERGY: The energy that
an object possesses by virtue of its motion
K E Y T E R M S
Trang 13Thermo-JOULE: The principal unit of energy—
and thus of heat—in the SI or metric tem, corresponding to 1 newton-meter (N
sys-• m) A joule (J) is equal to 0.7376 pounds
foot-KELVIN SCALE: Established byWilliam Thomson, Lord Kelvin (1824-1907), the Kelvin scale measures tempera-ture in relation to absolute zero, or 0K
(Units in the Kelvin system, known asKelvins, do not include the word or symbolfor degree.) The Kelvin and Celsius scalesare directly related; hence Celsius tempera-tures can be converted to Kelvins by adding273.15
KILOCALORIE: A measure of heat orenergy in the SI or metric system, equal tothe heat that must be added to or removedfrom 1 kilogram of water to change itstemperature by 33.8°F (1°C) As its namesuggests, a kilocalorie is 1,000 calories Thedietary Calorie (capital C) with whichmost people are familiar is the same as thekilocalorie
MECHANICAL ENERGY: The sum ofpotential energy and kinetic energy in agiven system
POTENTIAL ENERGY: The energythat an object possesses due to its position
RADIATION: The transfer of heat bymeans of electromagnetic waves, whichrequire no physical medium (e.g., water orair) for the transfer Earth receives the Sun’sheat by means of radiation
SECOND LAW OF
THERMODYNAM-ICS: A law of thermodynamics whichstates that no engine can be constructedthat simply takes heat from a source andperforms an equivalent amount of work
Some of the heat will always be lost, and
therefore it is impossible to build a
perfect-ly efficient engine This is a result of thefact that the natural flow of heat is alwaysfrom a high-temperature reservoir to alow-temperature reservoir—a factexpressed in the concept of entropy Thesecond law is sometimes referred to as “thelaw of entropy.”
SYSTEM: In physics, the term “system”
usually refers to any set of physical tions isolated from the rest of the universe
interac-Anything outside of the system, includingall factors and forces irrelevant to a discus-sion of that system, is known as the envi-ronment
TEMPERATURE: The direction ofinternal energy flow between bodies whenheat is being transferred Temperaturemeasures the average molecular kineticenergy in transit between those bodies
THERMAL ENERGY: Heat energy, aform of kinetic energy produced by themovement of atomic or molecular parti-cles The greater the movement of theseparticles, the greater the thermal energy
THERMODYNAMICS: The study ofthe relationships between heat, work, andenergy
THIRD LAW OF THERMODYNAMICS:
A law of thermodynamics which states that
at the temperature of absolute zero,entropy also approaches zero Zero entropywould contradict the second law of ther-modynamics, meaning that absolute zero istherefore impossible to reach
WORK: The exertion of force over agiven distance to displace or move anobject Work is thus the product of forceand distance exerted in the same direction
K E Y T E R M S C O N T I N U E D
Trang 14Brown, Warren Alternative Sources of Energy
Introduc-tion by Russell E Train New York: Chelsea House, 1994.
Encyclopedia of Thermodynamics (Web site).
<http://therion.minpet.unibas.ch/minpet/groups/
thermodict/> (April 12, 2001).
Entropy and the Second Law of Thermodynamics
(Web site) <http://www.2ndlaw.com> (April 12, 2001).
Fleisher, Paul Matter and Energy: Principles of Matter and Thermodynamics Minneapolis, MN: Lerner Pub-
Thermo-ham Mahwah, N.J.: Troll Associates, 1985.
Suplee, Curt Everyday Science Explained Washington,
D.C.: National Geographic Society, 1996.
“Temperature and Thermodynamics” PhysLINK.com
(Web site) <http://www.physlink.com/ae_thermo cfm> (April 12, 2001).
Trang 15H E A THeat
C O N C E P T
Heat is a form of energy—specifically, the energy
that flows between two bodies because of
differ-ences in temperature Therefore, the scientific
definition of heat is different from, and more
precise than, the everyday meaning Physicists
working in the area of thermodynamics study
heat from a number of perspectives, including
specific heat, or the amount of energy required to
change the temperature of a substance, and
calorimetry, the measurement of changes in heat
as a result of physical or chemical changes
Ther-modynamics helps us to understand such
phe-nomena as the operation of engines and the
gradual breakdown of complexity in physical
sys-tems—a phenomenon known as entropy
H O W I T W O R K S
Heat, Work, and Energy
Thermodynamics is the study of the
relation-ships between heat, work, and energy Work is the
exertion of force over a given distance to displace
or move an object, and is, thus, the product of
force and distance exerted in the same direction
Energy, the ability to accomplish work, appears
in numerous manifestations—including thermal
energy, or the energy associated with heat
Thermal and other types of energy, ing electromagnetic, sound, chemical, and
includ-nuclear energy, can be described in terms of two
extremes: kinetic energy, or the energy associated
with movement, and potential energy, or the
energy associated with position If a spring is
pulled back to its maximum point of tension, its
potential energy is also at a maximum; once it is
released and begins springing through the air to
return to its original position, it begins gainingkinetic energy and losing potential energy
All manifestations of energy appear in bothkinetic and potential forms, somewhat like theway football teams are organized to play bothoffense or defense Just as a football team takes anoffensive role when it has the ball, and a defensiverole when the other team has it, a physical systemtypically undergoes regular transformationsbetween kinetic and potential energy, and mayhave more of one or the other, depending onwhat is taking place in the system
What Heat Is and Is Not
Thermal energy is actually a form of kineticenergy generated by the movement of particles atthe atomic or molecular level: the greater themovement of these particles, the greater the ther-mal energy Heat is internal thermal energy thatflows from one body of matter to another—or,more specifically, from a system at a higher tem-perature to one at a lower temperature Thus,temperature, like heat, requires a scientific defi-nition quite different from its common meaning:
temperature measures the average molecularkinetic energy of a system, and governs the direc-tion of internal energy flow between them
Two systems at the same temperature aresaid to be in a state of thermal equilibrium
When this occurs, there is no exchange of heat
Though in common usage, “heat” is an sion of relative warmth or coldness, in physicalterms, heat exists only in transfer between twosystems What people really mean by “heat” is theinternal energy of a system—energy that is aproperty of that system rather than a property oftransferred internal energy