Science of everyday things vol 2 physics

Whereas the parti-cles in a liquid or gas move fast enough to be inrelative motion with regard to one another, how-ever, solid particles merely vibrate from a fixedposition.. The melting

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SCIENCE EVERYDAY

THINGS

OF

SCIENCE

EVERYDAY

THINGS

OF

volume 2: REAL-LIFE PHYSICS

edited by NEIL SCHLAGER written by JUDSON KNIGHT

of Matter

geometric arrangement that is repeated in alldirections Metals, for instance, are crystallinesolids Other solids are said to be amorphous,meaning that they possess no definite shape

Amorphous solids—clay, for example—eitherpossess very tiny crystals, or consist of severalvarieties of crystal mixed randomly Still othersolids, among them glass, do not contain crystals

V I B R A T I O N S A N D F R E E Z I N G

Because of their strong attractions to one

anoth-er, solid particles move slowly, but like all cles of matter, they do move Whereas the parti-cles in a liquid or gas move fast enough to be inrelative motion with regard to one another, how-ever, solid particles merely vibrate from a fixedposition

parti-This can be shown by the example of asinger hitting a certain note and shattering aglass Contrary to popular belief, the note doesnot have to be particularly high: rather, the noteshould be on the same wavelength as the vibra-tion of the glass When this occurs, sound energy

is transferred directly to the glass, which shattersbecause of the sudden net intake of energy

As noted earlier, the attraction and motion

of particles in matter has a direct effect on heatand temperature The cooler the solid, the slowerand weaker the vibrations, and the closer the par-ticles are to one another Thus, most types ofmatter contract when freezing, and their densityincreases Absolute zero, or 0K on the Kelvin scale

of temperature—equal to -459.67°F (-273°C)—

is the point at which vibration virtually stops

Note that the vibration virtually stops, but doesnot stop entirely In any event, the lowest temperature actually achieved, at a Finnishnuclear laboratory in 1993, is 2.8 • 10-10K, or0.00000000028K—still above absolute zero

Below that point, it starts to decrease again

Not only does the density of ice begindecreasing just before freezing, but its volumeincreases This is the reason ice floats: its weight

is less than that of the water it has displaced, and

therefore, it is buoyant Additionally, the buoyantqualities of ice atop very cold water explain whythe top of a lake may freeze, but lakes rarelyfreeze solid—even in the coldest of inhabitedregions

Instead of freezing from the bottom up, as itwould if ice were less buoyant than the water, thelake freezes from the top down Furthermore, ice

is a poorer conductor of heat than water, and,thus, little of the heat from the water belowescapes Therefore, the lake does not freeze com-pletely—only a layer at the top—and this helpspreserve animal and plant life in the body ofwater On the other hand, the increased volume

of frozen water is not always good for humans:when water in pipes freezes, it may increase involume to the point where the pipe bursts

M E LT I N G When heated, particles begin

to vibrate more and more, and, therefore, movefurther apart If a solid is heated enough, it losesits rigid structure and becomes a liquid The tem-perature at which a solid turns into a liquid iscalled the melting point, and melting points aredifferent for different substances For the mostpart, however, solids composed of heavier parti-cles require more energy—and, hence, highertemperatures—to induce the vibrations neces-sary for freezing Nitrogen melts at -346°F (-210°C), ice at 32°F (0°C), and copper at 1,985°F(1,085°C) The melting point of a substance,incidentally, is the same as its freezing point: thedifference is a matter of orientation—that is,whether the process is one of a solid melting tobecome a liquid, or of a liquid freezing to become

a solid

The energy required to change a solid to aliquid is called the heat of fusion In melting, allthe heat energy in a solid (energy that exists due

to the motion of its particles) is used in breaking

up the arrangement of crystals, called a lattice.This is why the water resulting from melted icedoes not feel any warmer than when it wasfrozen: the thermal energy has been expended,with none left over for heating the water Once allthe ice is melted, however, the absorbed energyfrom the particles—now moving at much greaterspeeds than when the ice was in a solid state—causes the temperature to rise

From Liquid to Gas

The particles of a liquid, as compared to those of

a solid, have more energy, more motion, and less

of Matter

attraction to one another The attraction,

how-ever, is still fairly strong: thus, liquid particles are

in close enough proximity that the liquid resists

compression

On the other hand, their arrangement isloose enough that the particles tend to move

around one another rather than merely vibrating

in place, as solid particles do A liquid is therefore

not definite in shape Both liquids and gases tend

to flow, and to conform to the shape of their

con-tainer; for this reason, they are together classified

as fluids

Owing to the fact that the particles in a uid are not as close in proximity as those of a

liq-solid, liquids tend to be less dense than solids

The liquid phase of substance is thus inclined to

be larger in volume than its equivalent in solid

form Again, however, water is exceptional in this

regard: liquid water actually takes up less space

than an equal mass of frozen water

B O I L I N G When a liquid experiences anincrease in temperature, its particles take on

energy and begin to move faster and faster They

collide with one another, and at some point the

particles nearest the surface of the liquid acquire

enough energy to break away from their

neigh-bors It is at this point that the liquid becomes a

gas or vapor

As heating continues, particles throughoutthe liquid begin to gain energy and move faster,

but they do not immediately transform into gas

The reason is that the pressure of the liquid,

combined with the pressure of the atmosphere

above the liquid, tends to keep particles in place

Those particles below the surface, therefore,

remain where they are until they acquire enough

energy to rise to the surface

The heated particle moves upward, leavingbehind it a hollow space—a bubble A bubble is

not an empty space: it contains smaller trapped

particles, but its small weight relative to that of

the liquid it disperses makes it buoyant

There-fore, a bubble floats to the top, releasing its

trapped particles as gas or vapor At that point,

the liquid is said to be boiling

T H E E F F E C T O F A T M O S P H E R

-I C P R E S S U R E As they rise, the particles

thus have to overcome atmospheric pressure, and

this means that the boiling point for any liquid

depends in part on the pressure of the

surround-ing air This is why cooksurround-ing instructions often

vary with altitude: the greater the distance from

sea level, the less the air pressure, and the shorterthe required cooking time

Atop Mt Everest, Earth’s highest peak atabout 29,000 ft (8,839 m) above sea level, thepressure is approximately one-third normalatmospheric pressure This means the air is one-third as dense as it is as sea level, which explainswhy mountain-climbers on Everest and other tallpeaks must wear oxygen masks to stay alive Italso means that water boils at a much lower tem-perature on Everest than it does elsewhere At sealevel, the boiling point of water is 212°F (100°C),but at 29,000 ft it is reduced by one-quarter, to158°F (70°C)

Of course, no one lives on the top of Mt

Everest—but people do live in Denver, Colorado,where the altitude is 5,577 ft (1,700 m) and theboiling point of water is 203°F (95°C) Given thelower boiling point, one might assume that foodwould cook faster in Denver than in New York,Los Angeles, or some other city close to sea level

In fact, the opposite is true: because heated cles escape the water so much faster at high alti-tudes, they do not have time to acquire the ener-

parti-gy needed to raise the temperature of the water

It is for this reason that a recipe may contain astatement such as “at altitudes above XX feet, add

XX minutes to cooking time.”

If lowered atmospheric pressure means alowered boiling point, what happens in outerspace, where there is no atmospheric pressure?

Liquids boil at very, very low temperatures This

is one of the reasons why astronauts have to wearpressurized suits: if they did not, their bloodwould boil—even though space itself is incredi-bly cold

The behavior of water in boiling and densation makes possible distillation, one of theprincipal methods for purifying seawater in vari-ous parts of the world First, the water is boiled,

of Matter

then, it is allowed to cool and condense, thusforming water again In the process, the waterseparates from the salt, leaving it behind in theform of brine A similar separation takes placewhen salt water freezes: because salt, like mostsolids, has a much lower freezing point thanwater, very little of it remains joined to the water

in ice Instead, the salt takes the form of a brinyslush

G A S A N D I T S L A W S Havingreached the gaseous state, a substance takes oncharacteristics quite different from those of asolid, and somewhat different from those of a liq-uid Whereas liquid particles exert a moderateattraction to one another, particles in a gas exertlittle to no attraction They are thus free to move,and to move quickly The shape and arrangement

of gas is therefore random and indefinite—and,more importantly, the motion of gas particlesgive it much greater kinetic energy than the otherforms of matter found on Earth

The constant, fast, and random motion ofgas particles means that they are always collidingand thereby transferring kinetic energy back andforth without any net loss These collisions alsohave the overall effect of producing uniformpressure in a gas At the same time, the charac-teristics and behavior of gas particles indicatethat they will tend not to remain in an open con-tainer Therefore, in order to maintain any pres-sure on a gas—other than the normal atmos-pheric pressure exerted on the surface of the gas

by the atmosphere (which, of course, is also agas)—it is necessary to keep it in a closed con-tainer

There are a number of gas laws (examined inanother essay in this book) describing theresponse of gases to changes in pressure, temper-ature, and volume Among these is Boyle’s law,which holds that when the temperature of a gas

is constant, there is an inverse relationshipbetween volume and pressure: in other words,the greater the pressure, the less the volume, andvice versa According to a second gas law,Charles’s law, for gases in conditions of constantpressure, the ratio between volume and tempera-ture is constant—that is, the greater the temper-ature, the greater the volume, and vice versa

In addition, Gay-Lussac’s law shows that thepressure of a gas is directly related to its absolutetemperature on the Kelvin scale: the higher thetemperature, the higher the pressure, and vice

versa Gay-Lussac’s law is combined, along withBoyle’s and Charles’s and other gas laws, in theideal gas law, which makes it possible to find thevalue of any one variable—pressure, volume,number of moles, or temperature—for a gas, aslong as one knows the value of the other three

Other States of Matter

P L A S M A Principal among states ofmatter other than solid, liquid, and gas is plasma,which is similar to gas (The term “plasma,” whenreferring to the state of matter, has nothing to dowith the word as it is often used, in reference toblood plasma.) As with gas, plasma particles col-lide at high speeds—but in plasma, the speeds areeven greater, and the kinetic energy levels evenhigher

The speed and energy of these collisions isdirectly related to the underlying property thatdistinguishes plasma from gas So violent are thecollisions between plasma particles that electronsare knocked away from their atoms As a result,plasma does not have the atomic structure typi-cal of a gas; rather, it is composed of positive ionsand electrons Plasma particles are thus electri-cally charged, and, therefore, greatly influenced

by electrical and magnetic fields

Formed at very high temperatures, plasma isfound in stars and comets’ tails; furthermore, thereaction between plasma and atomic particles inthe upper atmosphere is responsible for the auro-

ra borealis or “northern lights.” Though notfound on Earth, plasma—ubiquitous in otherparts of the universe—may be the most plentifulamong the four principal states of matter

Q U A S I - S T A T E S Among the states of matter discussed by physicists are sever-

quasi-al terms that describe the structure in which ticles are joined, rather than the attraction andrelative movement of those particles “Crys-talline,” “amorphous,” and “glassy” are all terms

par-to describe what may be individual states of ter; so too is “colloidal.”

mat-A colloid is a structure intermediate in sizebetween a molecule and a visible particle, and ithas a tendency to be dispersed in another medi-um—as smoke, for instance, is dispersed in air.Brownian motion describes the behavior of mostcolloidal particles When one sees dust floating in

a ray of sunshine through a window, the lightreflects off colloids in the dust, which are driven

of Matter

back and forth by motion in the air otherwise

imperceptible to the human senses

D A R K M A T T E R The number of states

or phases of matter is clearly not fixed, and it is

quite possible that more will be discovered in

outer space, if not on Earth One intriguing

can-didate is called dark matter, so described because

it neither reflects nor emits light, and is therefore

invisible In fact, luminous or visible matter may

very well make up only a small fraction of the

mass in the universe, with the rest being taken up

by dark matter

If dark matter is invisible, how doastronomers and physicists know it exists? By

analyzing the gravitational force exerted on

visi-ble objects when there seems to be no visivisi-ble

object to account for that force An example is

the center of our galaxy, the Milky Way, which

appears to be nothing more than a dark “halo.” In

order to cause the entire galaxy to revolve around

it in the same way that planets revolve around the

Sun, the Milky Way must contain a staggering

quantity of invisible mass

Dark matter may be the substance at theheart of a black hole, a collapsed star whose mass

is so great that its gravitational field prevents

light from escaping It is possible, also, that dark

matter is made up of neutrinos, subatomic

parti-cles thought to be massless Perhaps, the theory

goes, neutrinos actually possess tiny quantities of

mass, and therefore in huge groups—a mole

times a mole times a mole—they might possess

appreciable mass

In addition, dark matter may be the decidingfactor as to whether the universe is infinite The

more mass the universe possesses, the greater its

overall gravity, and if the mass of the universe is

above a certain point, it will eventually begin to

contract This, of course, would mean that it is

finite; on the other hand, if the mass is below this

threshold, it will continue to expand indefinitely

The known mass of the universe is nowhere near

that threshold—but, because the nature of dark

matter is still largely unknown, it is not possible

yet to say what effect its mass may have on the

total equation

A “ N E W ” F O R M O F M A T T E R ?

Physicists at the Joint Institute of Laboratory

Astrophysics in Boulder, Colorado, in 1995

revealed a highly interesting aspect of atomic

behavior at temperatures approaching absolute

zero Some 70 years before, Einstein had

predict-ed that, at extremely low temperatures, atomswould fuse to form one large “superatom.” Thishypothesized structure was dubbed the Bose-Einstein Condensate (BEC) after Einstein andSatyendranath Bose (1894-1974), an Indianphysicist whose statistical methods contributed

to the development of quantum theory

Cooling about 2,000 atoms of the elementrubidium to a temperature just 170 billionths of

a degree Celsius above absolute zero, the cists succeeded in creating an atom 100 microm-eters across—still incredibly small, but vast incomparison to an ordinary atom The super-atom, which lasted for about 15 seconds, cooleddown all the way to just 20 billionths of a degreeabove absolute zero The Colorado physicistswon the Nobel Prize in physics in 1997 for theirwork

physi-In 1999, researchers in a lab at Harvard versity also created a superatom of BEC, andused it to slow light to just 38 MPH (60.8km/h)—about 0.02% of its ordinary speed

Uni-Dubbed a “new” form of matter, the BEC maylead to a greater understanding of quantummechanics, and may aid in the design of smaller,more powerful computer chips

States and Phases and In Between

At places throughout this essay, references havebeen made variously to “phases” and “states” ofmatter This is not intended to confuse, butrather to emphasize a particular point Solids,liquids, and gases are referred to as “phases,”

because substances on Earth—water, forinstance—regularly move from one phase toanother This change, a function of temperature,

is called (aptly enough) “change of phase.”

There is absolutely nothing incorrect inreferring to “states of matter.” But “phases ofmatter” is used in the present context as a means

of emphasizing the fact that most substances, atthe appropriate temperature and pressure, can besolid, liquid, or gas In fact, a substance may even

be solid, liquid, and gas

An Analogy to Human Life

The phases of matter can be likened to the

phas-es of a person’s life: infancy, babyhood, hood, adolescence, adulthood, old age The tran-sition between these stages is indefinite, yet it is