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Level I Course Manual Section 7Publ No 1 560 009 C

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7. Basic Heat Transfer

What will you learn

Definition of temperature and its measurement

Thermodynamics

Heat and temperature

Heat transfer: conduction, convection, radiation

Thermography provides us with a powerful tool for non-contact temperature

measurement, usually surface temperature measurement. But we also need abasic understanding of how the surface is being heated and what might liebehind that temperature.We have already gone through the basics of heat radiation. This chapter willdeal with the three modes for heat transfer, i.e. conduction, convection andradiation. Firstly, however, we need to consider some basic concepts.

EnergyThe concepts of energy, heat and temperature are very central in physics andeven more so in thermography.

The first law of thermodynamics states that the sum of the total energy contentsin a closed system is constant.

This law is more commonly known as the energy principle. In straightforwardterms the law states that energy cannot be created or destroyed. It can betransformed into other forms, but it cannot disappear.

These are examples of various forms of energy:

Energy of position, potential energy

Energy of movement, kinetic energy

Chemical energy, like coal, woods, oil, etc.

Electrical energy

For example we know that chemical energy (petrol) can be transformed intokinetic energy, i.e. the movement of a car.

All matter with a temperature above absolute zero (0 Kelvin or 273C) radiatesenergy. And we know that energy cannot be created. Hence if a body radiatesenergy, that radiated energy must come from the body itself. It is called theinternal energyof the body.


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Heat and temperatureHeat is thermal energy per time unit. A flow of thermal energy will occur when

there is a difference in temperature. Thermal energy is transferred from awarmer object to a colder object. This is the second law of thermodynamics. Itsays that:

Heat will spontaneously flow from hotter to colder, thereby transferring thermalenergy from one body or place to another.

Temperature represents the energy quantity per molecule, or an average of aquantity of molecules. Temperature is therefore a measure of the state that thesubstance is in.

Modes of heat transferHeat transfer can take place in three ways, by conduction, by convection, andby radiation. The three modes of heat transfer can occur at the same time, andindependently of each other.

Take the example of a house in winter. The temperature inside the wall is 20 Cwhereas the harsh winter weather means that on the exterior the wall has atemperature of -10 C. That temperature difference will cause heat to flow fromhot to cold. The bigger the temperature difference the stronger the heat flow.The house is losing heat all the time through the walls, windows, by ventilation

etc. If we do not go on heating the house there will, after a few days or hoursdepending on the quality of the insulation, be no temperature differencesbetween the inside and the outside of the house.When the temperature differences have evened out, we have reached what iscalled thermal equilibrium. At thermal equilibrium, no more thermal energy willbe transferred, i.e. there will be no heat flow.

ConductionThe thermal conductivityof the wall decides the amount of heat, which will floatthrough the wall. The higher the thermal conductivity the lower the thermalresistance, which is the inverse of thermal conductivity. The amount of heat,which will, driven by the temperature difference, float through the wall isdescribed by Fouriers law for heat conduction:


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Q = *(Ti To) / l

where Q heat flow, W/m2 thermal conductivity, W/mT

l length of the conducting material, mTi , To indoor and outdoor temperatures in C (or K)

ConvectionFurthermore, we can see a certain temperature difference between the innerwall and the air temperature in the room. So is the case with the outer wall andthe external temperature (Tair,o ). These temperature differences are called

transition thermal resistance. That means the temperature difference, which isnecessary to drive the heat from the air in the room into the wall or from theouter wall into the external air. This effect is a typical case ofconvection, i.e. theair is moving along the wall thereby losing a part of its energy to the wall. Thecorresponding equation is very similar to Fouriers law of heat conduction but isinstead called Newtons cooling law.

Q = (Twall Tair)

where Q heat flow, W/m2 sec

convective heat transfer coefficient, W/m2T

Twall, Tair Wall and air temperatures in C (or K).

The same goes for the transition from the outside wall to the outside air. Theheat exchange between the wall and the air takes place by the molecules in theair hitting the molecules in the wall.

The heat flow always goes from hotter to colder, no matter if it goes from air towall or from the wall to the air.

We often hear about the so-called wind chill effect. The table below shows theinfluence of the wind speed on the cooling convection effect. Note forexample, that the cooling effect increases with a factor 3 when the windincreases from 3 to 15 m/s.

It is common to replace the sum of the partial thermal conductivities by onefactor for the conductivity of the whole wall, the k value. Consider a wall in ahouse.

The wall consists of three different layers, each with its thickness l and itsconductivity , hence the thermal resistance of each layer is l/.


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If we for the wall, which is sketched in the figure above, put in a k valueaccording to the Swedish building standard for a normal one-family house, i.e.

0.2, which is a high degree of insulation, we will get:

Q = 0.2(20 (-10)) = 6 W/m2

No normal house has that k value everywhere. The windows, for example, havea much higher thermal conductivity. For simplification, if a house has a totalradiating area of 400 m2, this relatively big house will lose about 2,400 Wthrough the walls into the cold outside air. Compare this with the power of anormal electrical radiator, which is in the order of 1,500 to 2,000 W, and we can

see that such an insulation saves a lot of heating power.

Convection appears in the transition from the wall to the air. Another example ofconvection is the cooling effect that cold water has on a swimmer. The fasteryou swim, the more heat is lost to the water. Should you fall into very coldwater, stay calm and move as little as possible in order to lose less heat to thewater which obviously will increase the possibility of survival.

This example is about heat transfer from a body to a liquid whereas it was froma gas to a body in the example with the house. A common technical word forgases andliquids is fluids. Convection can only take place if fluids are involved.

Furthermore, we all know that the cooling effect of the wind increases with thewind speed. Therefore convection is divided into natural convection and forcedconvection, e.g. when a fan is applied. Forced convection is very often used forcooling the electronics boards in todays computers.

RadiationIn the thermographic world we use the radiation from the objects in order tomeasure their temperatures. As has been said before, all matter radiates heat,

at least as long as they have a temperature, which is above the absolute zero,i.e. 273 C. The whole chapter Infrared theory is devoted to the nature ofradiation.


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Convection:

Heat from stove causes the

water and the air in the room

to circulate

Conduction:

Heat travels from

hot end of poker to

cooler handle

Radiation:

Heat travels through

space as em waves

to warm the cat.

Milling Machine

*>149.0F

*


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Camera Spot 1

180.1

*>232.1F

*


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i n f r a m e t r i c s

Camera Spot 1

96.9

*>158.2F

*


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Thermal Inversion

Time Vs. Temperature for Wet Vs. Dry Roofing

Interpreting images: If you can determine the primary source of heat transferfor a given situation, you can then assess the above factors and find the

probable source the anomaly.For example: in measuring the temperature of a loose or corroded electricalconnection, see picture above, a large portion of the heat is radiating from thesurface. Some heat is being conducted down the wire, which creates a distinctpattern of bleeding or smearing. The remaining heat is lost to convection by theair in the room and radiation.If this connection is outside and exposed to a wind it will cool down and may notaccurately indicate the severity of the problem. If the target had a low emissivity(shiny or polished metal), it may not be possible to do IR analysis at all. Thetarget would behave like an IR mirror. Assuming the surface is dirty or corroded(is a good radiator) then the conduction heat pattern gives us good clues about

the source of the problem.

Transient heating: Some targets heat and cool in cycles. At some points in thecycle there may not be enough of a temperature difference to do thermography.For outside targets that rely on solar heat loading, the time of day is an

important factor. In most climates, the target and the environment will reachthermal equilibrium at two times of day. Once the sun has heated everything tothe same temperature there is no thermal contrast. In the same manner, aftersundown the target and environment will cool off to the same temperature.

Some targets are best evaluated in a steady state condition and not in theirtransient state. For electrical surveys the components should be under sufficientload, at least 30%, for a sufficient amount of time to stabilize. For mechanicalequipment surveys it is important to make sure the equipment has been runninglong enough to thermally stabilize. Steam traps cycle. Their cycling times needto be considered when interpreting a thermogram.

Diagram 2


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The above graph shows the changes in temperature of a roof that has damagedwet insulation. Notice that there is a greater temperature difference at noon.

Although there is a greater delta-T at that point the wet (cooler) insulation isgoing to be difficult to see against the hot black roof. For optimum contrast it isdesirable to view the target when it is hotter than the foreground.

Methodical thermography Determine the source of the heat: Anomalies or defects that are the

same temperature as their surroundings will not be visible. In this casethe whole scene will look only a few shades of gray. Be sure to considertransient effects and the structure of the target. The camera can onlysee the surface.

Examine the structure of the whole target: What components arebetween the heat source and the surface (ex: solid insulation or an airgap)? When the source of the heat is indirect temperature differencesmust be taken much more seriously. A few degree delta-T on thesurface of a component with a protective shield may represent hundredsof degrees in internal temperature rise. A small temperature rise on alow emissivity target should also be taken seriously, as a correctedreading may be much higher. Remember that heat is moving in threedimensions and the camera is only showing the front two-dimensionalsurface.

Determine the factors that effect heat loss or gain: For example heatis lost by conduction down a wire, or convection to the environment bywind.

Take into account any factors between the target surface and thedetector: Some examples are moisture, covers, filters, windows andlong distances where atmospheric attenuation may be an issue.

*>98.8F

*


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Reflections: The lower the emissivity of a target the greater the possibility therewill be reflections. If the source of the reflection is hot enough (furnaces) it may

also load heat onto the surface of the target. This true increase in temperaturemust me accounted for blocked since it may result in an artificailly high delta-T.If methodical thermography is applied common sense can determine if a hotspot is a true spot. Metal conducts heat well. If an isolated hot spot is found ona metal part, and no conduction pattern is seen, then the hot spot would besuspect. Further investigation is needed. A true hot spot should appear in thecamera as a progressive cooling or smearing pattern. To verify if you are seeinga hot spot vs. a reflection, the operator should try to change the relative angle tothe target in an arc. Ideally measurements are taken perpendicular to thecamera lens but if moving off axis a few degrees results in a reflection freeimage, this may be a preferable scenario. Changing your angle changes your

background.

Summary: Thermal images are created by the detection of radiation comingfrom the surface of a target. The heat flow patterns created are due toconduction, convection.

The camera operator should have knowledge of the target in order to begin toassess the cause or effects of hot/cold spots. Determining acceptance criteria isa challenge to a thermographer because there are so many factors involved inheat transfer. A small change in one factor can have dramatic effects the IRsignature of the target and thus on the interpretation of the temperature reading.

It is always up to the company or operator to determine what level of accuracyis needed and balance that with practical considerations. On the other hand, abad call can be costly. A complete and accurate assessment of a targetinvolves much more than just taking a temperature reading. The more anoperator understands about heat the more complete the results will be.

BACKGROUND

AMBIENT

FOREGROUND


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SOURCE: MICROSOFT

ENCARTA Multimedia Encyclopedia

Temperature

The sensation of warmth or coldness of a substance on contact is determinedby the property known as temperature. Although it is easy to compare therelative temperatures of two substances by the sense of touch, it is impossibleto 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 inseveral physical properties, which may be measured with precision. As thetemperature varies, a substance expands or contracts, its electrical resistivitychanges, and in the gaseous form, it exerts varying pressure. The variation in astandard property usually serves as a basis for an accurate numericaltemperature scale (see below).

Temperature depends on the average kinetic energy of the molecules of asubstance, and according to kinetic theory 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 thetemperature termed absolute zero.

Temperature, in physics, is the property of systems that determines whetherthey are in thermal equilibrium. The concept of temperature stems from the ideaof measuring relative hotness and coldness and from the observation that theaddition of heat to a body leads to an increase in temperature as long as nomelting 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 identicaland thermal equilibrium is reached. Thus, temperatures and heat, althoughinterrelated, refer to different concepts, temperature being a property of a bodyand heat being an energy flow to or from a body by virtue of a temperaturedifference.

Temperature changes have to be measured in terms of other property changesof a substance. Thus, the conventional mercury thermometer measures theexpansion of a mercury column in a glass capillary, the change in length of thecolumn being related to the temperature change. If heat is added to an idealgas contained in a constant-volume vessel, the pressure increases, and thetemperature change can be determined from the pressure change by Gay-Lussac's law, provided the temperature is expressed on the absolute scale.


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Temperature ScalesFive different temperature scales are in use today: the Celsius scale, knownalso as the centigrade scale, the Fahrenheit scale, the Kelvin scale, theRankine scale, and the international thermodynamic temperature scale. The

Heat Flow between Two Gases

Two identical gases at different temperatures are kept apart by a barrier. Thearrows in the boxes represent the speed of the molecules. The gas at the highertemperature is composed of molecules which move at a higher average speed.When the barrier is removed, the gases mix and the individual gas molecules

collide with each other. The molecules in the higher temperature gas slow downand its temperature decreases. The molecules in the lower temperature gasspeed up and its temperature increases. The final temperature of the gas iscalled the equilibrium temperature.

Microsoft Illustration

"Heat Flow between Two Gases," Microsoft (R) Encarta. Copyright (c) 1993Microsoft Corporation. Copyright (c) 1993 Funk & Wagnall's Corporation


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centigrade scale, with a freezing point of 0C and a boiling point of 100C, iswidely used throughout the world, particularly for scientific work, although it wassuperseded officially in 1950 by the international temperature scale. In the

Fahrenheit scale, used in English-speaking countries for purposes other thanscientific work and based on the mercury thermometer, the freezing point ofwater is defined as 32F and the boiling point as

212F. In the Kelvin scale, the most commonly used thermodynamictemperature scale, zero is defined as the absolute zero of temperature, that is, -273.15C, or -459.67F. Another scale employing absolute zero as its lowestpoint is the Rankine scale, in which each degree of temperature is equivalent toone degree on the Fahrenheit scale. The freezing point of water on the Rankinescale is 492R, and the boiling point is 672R.

In 1933 scientists of 31 nations adopted a new international temperature scalewith additional fixed temperature points, based on the Kelvin scale andthermodynamic principles. The international scale is based on the property ofelectrical resistivity, with platinum wire as the standard for temperature between-190 and 660C. Above 660C, to the melting point of gold, 1063C, a standardthermocouple, which is a device that measures temperature by the amount ofvoltage produced between two wires of different metals, is used; beyond thispoint temperatures are measured by the so-called optical pyrometer, whichuses the intensity of light of a wavelength emitted by a hot body for the purpose.

In 1954 the triple point of waterthat is, the point at which the three phases ofwater (vapor, liquid, and ice) are in equilibriumwas adopted by internationalagreement as 273.16 K. The triple point can be determined with greaterprecision than the freezing point and thus provides a more satisfactory fixedpoint for the absolute thermodynamic scale. In cryogenics, or low-temperatureresearch, temperatures as low as 0.003 K have been produced by thedemagnetization of paramagnetic materials. Momentary high temperaturesestimated to be greater than 100,000,000 K have been achieved by nuclearexplosions.

One of the earliest temperature scales was that devised by the German

physicist Gabriel Daniel Fahrenheit. According to this scale, at standardatmospheric pressure, the freezing point (and melting point of ice) is 32F, andthe boiling point is 212F. The centigrade, or Celsius scale, invented by theSwedish astronomer Anders Celsius, and used throughout most of the world,assigns a value of 0C to the freezing point and 100C to the boiling point. Inscientific work, the absolute or Kelvin scale, invented by the Britishmathematician and physicist William Thomas, 1st Baron Kelvin, is most widelyused. In this scale, absolute zero is at -273.16C, which is zero K, and thedegree intervals are identical to those measured on the centigrade scale. Thecorresponding absolute Fahrenheit or Rankine scale, devised by the British


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engineer and physicist William J. M. Rankine (1820-72), places absolute zero at-459.69F, which is 0R, and the freezing point at 491.69R. A more consistentscientific temperature scale, based on the Kelvin scale, was adopted in 1933.

Effects of TemperatureTemperature plays an important part in determining the conditions in whichliving matter can exist. Thus, birds and mammals demand a very narrow rangeof body temperatures for survival and must be protected against extreme heator cold. Aquatic species can exist only within a narrow temperature range of thewater, which differs for various species. Thus, for example, the increase intemperature of river water by only a few degrees as a result of heat dischargedfrom power plants may kill most of the native fish.

The properties of all materials are also markedly affected by temperaturechanges. At arctic temperatures, for example, steel becomes very brittle andbreaks easily, and liquids either solidify or become very viscous, offering highfrictional resistance to flow. At temperatures near absolute zero, many materialsexhibit strikingly different characteristics. At high temperatures, solid materialsliquefy or become gaseous; chemical compounds may break up into theirconstituents.

The temperature of the atmosphere is greatly influenced by both the land andthe sea areas. In January, for example, the great landmasses of the northernhemisphere are much colder than the oceans at the same latitude, and in Julythe situation is reversed. At low elevations the air temperature is alsodetermined largely by the surface temperature of the earth. The periodictemperature changes are due mainly to the sun's radiant heating of the landareas of the earth, which in turn convect heat to the overlying air. As a result ofthis phenomenon, the temperature decreases with altitude, from a standardreference value of 15.5C (60F) at sea level (in temperate latitudes), to about -55C (about -67F) at about 11,000 m (about 36,000 ft). Above this altitude, thetemperature remains nearly constant up to about 33,500 m (about 110,000 ft).

Thermometer

A thermometer is an instrument used to measure temperature. The mostcommonly used thermometer is the mercury-in-glass type, which consists of auniform-diameter glass capillary that opens into a mercury-filled bulb at oneend. The assembly is sealed to preserve a partial vacuum in the capillary. If thetemperature increases, the mercury expands and rises in the capillary. Thetemperature may then be read on an adjacent scale. Mercury is widely used formeasuring ordinary temperatures; alcohol, ether, and other liquids are alsoemployed for this purpose.


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Outdoor Thermometer

A red-dyed alcohol thermometer measures an outside air temperature of about6C (about 43F). In a thermometer, an expanding fluid such as alcohol ormercury is trapped within a closed glass rod. As the fluid expands or contracts,it is measured by marks calibrated for given temperatures. The scale may bemarked for either the Celsius or Fahrenheit temperature scales or both.

The invention of the thermometer is attributed to Galileo, although the sealedthermometer did not come into existence until about 1650. The modern alcoholand mercury thermometers were invented by the German physicist GabrielFahrenheit, who also proposed the first widely adopted temperature scale,named after him, in which 32F is the freezing point of water and 212F is itsboiling point at standard atmospheric pressure. Various temperature scaleshave been proposed since his time; in the centigrade, or Celsius, scale, devisedby the Swedish astronomer Anders Celsius and used in most of the world, thefreezing point is 0, the boiling point is 100.

Types of ThermometersA wide variety of devices are employed as thermometers. The primaryrequirement is that one easily measured property, such as the length of themercury column, should change markedly and predictably with changes intemperature. The variation of that property should also remain fairly linear withvariations in temperature. In other words, a unit change in temperature shouldlead to a unit change in the property to be measured at all points of the scale.


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The electrical resistance of conductors and semiconductors increases with anincrease in temperature. This phenomenon is the basis of the resistance

thermometer in which a constant voltage, or electric potential, is applied acrossthe thermistor, or sensing element. For a thermistor of a given composition, themeasurement of a specific temperature will induce a specific resistance acrossthe thermistor. This resistance can be measured by a galvanometer andbecomes a measure of the temperature.

Various thermistors made of oxides of nickel, manganese, or cobalt are used tosense temperatures between -46 and 150C (between -50 and 300F).Similarly, thermistors employing other metals or alloys are designed for use athigher temperatures; platinum, for example, can be used up to 930C (1700F).With proper circuitry, the current reading can be converted to a direct digital

display of the temperature.

Very accurate temperature measurements can be made with thermocouples, inwhich a small voltage difference (measured in millivolts) arises when two wiresof dissimilar metals are joined to form a loop, and the two junctions havedifferent temperatures. To increase the voltage signal, several thermocouplesmay be connected in series to form a thermopile. Since the voltage depends onthe difference of the junction temperatures, one junction must be maintained ata known temperature; otherwise an electronic compensation circuit must bebuilt into the device to measure the actual temperature of the sensor.

Thermistors and thermocouples often have sensing units less than 1/4 cm (lessthan 1/10 in) in length, which permits them to respond rapidly to temperaturechanges and also makes them ideal for many biological and engineering uses.

The optical pyrometer is used to measure temperatures of solid objects attemperatures above 700C (about 1300F), where most other thermometerswould melt. At such high temperatures, solid objects radiate sufficient energy inthe visual range to permit optical measurement by exploiting the so-called glowcolor phenomenon. The color at which hot objects glow changes from dull redthrough yellow to nearly white at about 1300C (about 2400F). The pyrometer

contains a light bulb type of filament controlled by a rheostat (dimmer switch)that is calibrated so that the colors at which the filament glows correspond tospecific temperatures. The temperature of a glowing object can be measured byviewing the object through the pyrometer and adjusting the rheostat until thefilament blends into the image of the object. At this point the temperatures of thefilament and the object are equal and can be read from the calibrated rheostat.Another temperature-measuring device, used mainly in thermostats, relies onthe differential thermal expansion between two strips or disks made of differentmetals and either joined at the ends or bonded together.


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Special-Purpose ThermometersThermometers may also be designed to register the maximum or minimum

temperature attained. A mercury-in-glass clinical thermometer, for example, is amaximum-reading instrument in which a trap in the capillary tube between thebulb and the bottom of the capillary permits the mercury to expand withincreasing temperature, but prevents it from flowing back unless it is forcedback by vigorous shaking. Maximum temperatures reached during the operationof tools and machines may also be estimated by special paint patches thatchange color when certain temperatures are reached.

Accuracy of MeasurementAccurate measurement of temperature depends on the establishment of

thermal equilibrium between the thermometric device and its surroundings; thatis, when at equilibrium no heat is exchanged between the thermometer and thematerial it touches or material in its vicinity. A clinical thermometer, therefore,must be inserted long enough (more than one minute) to reach near-equilibriumwith the human body to yield an accurate reading. It should also be inserteddeep enough, and have sufficient contact with the body, to indicate temperatureaccurately. These conditions are almost impossible to achieve with an oralthermometer, which generally indicates a body temperature lower than thatgiven by a rectal thermometer. Insertion times can be significantly reduced withsmall, rapidly reacting thermometers such as thermistor devices.

Any thermometer indicates only its own temperature, which may not agree withthe actual temperature of the object to be measured. In measuring the airtemperature outside a building, for example, if one thermometer is placed in theshade and one in the sun, only a few centimeters away, the readings on the twoinstruments may be quite different, although the air temperature is the same.The thermometer in the shade may lose heat by radiation to cold building walls.Its reading, therefore, will be slightly below the true air temperature. On theother hand, the thermometer placed in the sun will absorb the sun's radiantheat. As a result, the indicated temperature may be significantly above the trueair temperature. To avoid such errors, accurate temperature determinations

require the shielding of the thermometer from hot and cold sources to or fromwhich heat might be transferred by radiation, conduction, or convection.

Absolute ZeroAbsolute zero is the lowest temperature theoretically possible. It ischaracterized by a complete absence of heat. Absolute zero is approximately-273.16C (-459.69 F), or zero degree on the Kelvin scale (0 K).


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The concept of absolute zero temperature was first deduced from experimentswith gases; when a fixed volume of gas is cooled, its pressure decreases with

its temperature. Although this experiment cannot be conducted below theliquefaction point of the gas, a plot of the experimental values of pressureversus temperature can be extrapolated to zero pressure. The temperature atwhich the pressure would be zero is the absolute zero temperature. Thisexperimental concept of a gas-thermometer temperature scale and of absolutezero was subsequently shown to be consistent with the theoretical definitions ofabsolute zero.

Absolute zero cannot be reached experimentally, although it can be closelyapproached. Special procedures are needed to reach very low, or cryogenic,temperatures (Cryogenics). Liquid helium, which has a normal boiling point of

4.3 K (-268.9 C), can be produced by cryostats, extremely well-insulatedvessels, based on a design by the American mechanical engineer SamuelCollins (1898-1984). If the helium is then evaporated at reduced pressures,temperatures as low as 0.7 K can be obtained. Lower temperatures require theadiabatic (no heat transfer) demagnetization of paramagnetic substances(substances of low magnetizability), such as chrome alum, while they are beingsurrounded with a liquid helium bath. The method, which was first developed in1937 by the Canadian-American chemist William Giauque, utilizes a magneticfield that initially aligns the ionic magnets of the material. If the magnetic field isremoved, the magnets again assume their random orientation, reducing thethermal energy of the material and thus its temperature. Temperatures as lowas 0.002 K have been reached with the demagnetization of paramagnetic salts,and the demagnetization of atomic nuclei has yielded temperatures as low as0.00001 K.

Temperature measurements at values close to absolute zero also presentspecial problems. Gas thermometers can only be used up to the liquefactionpoint of helium. At lower temperatures, electric and magnetic measurementsmust be used to determine the effective temperature.

The concept of absolute zero temperature is also important in theoretical

considerations. According to the third law of thermodynamics, the entropy, orstate of disorder, of a pure crystal is zero at absolute zero temperature; this is ofconsiderable importance in analyzing chemical reactions and in quantumphysics.


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If any such system is placed in contact with an infinite environment that exists atsome certain temperature, the system will eventually come into equilibrium with

the environmentthat is, reach the same temperature. (The so-called infiniteenvironment is a mathematical abstraction called a thermal reservoir; in realitythe environment need only be large relative to the system being studied.)


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Temperatures are measured with devices called thermometers.A thermometer

contains a substance with conveniently identifiable and reproducible states,such as the normal boiling and freezing points of pure water. If a graduatedscale is marked between two such states, the temperature of any system canbe determined by having that system brought into thermal contact with thethermometer, provided that the system is large relative to the thermometer.

First Law of ThermodynamicsThe first law of thermodynamics gives a precise definition of heat, anothercommonly used concept.

When an object is brought into contact with a relatively colder object, a processtakes place that brings about an Carnot equalization of temperatures of the twoobjects. To explain this phenomenon, 18th-century scientists hypothesized thata substance more abundant at higher temperature flowed toward the region at alower temperature. This hypothetical substance, called caloric, was thought tobe a fluid capable of moving through material media. The first law ofthermodynamics instead identifies caloric, or heat, as a form of energy. It canbe converted into mechanical work, and it can be stored, but is not a materialsubstance. Heat, measured originally in terms of a unit called the calorie, and

Carnot EngineMicrosoft Illustration

The idealized Carnot engine wasenvisioned by the French physicist NicolasLonard Sadi Carnot, who lived during theearly 19th century. The Carnot engine istheoretically perfect, that is, it converts themaximum amount of energy intomechanical work. Carnot showed that theefficiency of any engine depends on thedifference between the highest and lowesttemperatures reached during one cycle.The greater the difference, the greater theefficiency. An automobile engine, forexample, would be more efficient if thefuel burned hotter and the exhaust gascame out of the cylinder at a lowertemperature.


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work and energy, measured in ergs, were shown by experiment to be totallyequivalent. 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, becauseenergy cannot be created or destroyedsetting aside the later ramifications ofthe equivalence of mass and energythe amount of heat transferred into asystem plus the amount of work done on the system must result in acorresponding increase of internal energy in the system. Heat and work aremechanisms by which systems exchange energy with one another.

In any machine some amount of energy is converted into work; therefore, nomachine can exist in which no energy is converted into work. Such ahypothetical machine (in which no energy is required for performing work) is

termed a perpetual-motion machine of the first kind. Since the input energymust now take heat into account (and in a broader sense chemical, electrical,nuclear, and other forms of energy as well), the law of energy conservationrules out the possibility of such a machine ever being invented. The first law issometimes given in a contorted form as a statement that precludes theexistence of perpetual-motion machines of the first kind.

The equivalence of heat and work was explained by the German physicistHermann Ludwig Ferdinand von Helmholtz and the British mathematician andphysicist William Thomson, 1st Baron Kelvin, by the middle of the 19th century.Equivalence means that doing work on a system can produce exactly the sameeffect as adding heat; thus the same temperature rise can be achieved in a gascontained in a vessel by adding heat or by doing an appropriate amount of workthrough a paddle wheel sticking into the container where the paddle is actuatedby falling weights. The numerical value of this equivalent was first demonstratedby the British physicist James Prescott Joule in several heating and paddle-wheel experiments between 1840 and 1849.

That performing work or adding heat to a system were both means oftransferring energy to it was thus recognized. Therefore, the amount of energyadded by heat or work had to increase the internal energy of the system, which

in turn determined the temperature. If the internal energy remains unchanged,the amount of work done on a system must equal the heat given up by it. This isthe first law of thermodynamics, a statement of the conservation of energy. Notuntil the action of molecules in a system was better understood by thedevelopment of the kinetic theory could this internal energy be related to thesum of the kinetic energies of all the molecules making up the system.


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From the second law, it follows that in an isolated system (one that has nointeractions with the surroundings) internal portions at different temperatures

will always adjust to a single uniform temperature and thus produce equilibrium.This can also be applied to other internal properties that may be differentinitially. If milk is poured into a cup of coffee, for example, the two substanceswill continue to mix until they are inseparable and can no longer bedifferentiated. Thus, an initial separate or ordered state is turned into a mixedor disordered state. These ideas can be expressed by a thermodynamicproperty, called the entropy (first formulated by Clausius), which serves as ameasure of how close a system is to equilibrium, that is, to perfect internaldisorder. The entropy of an isolated system, and of the universe as a whole,can only increase, and when equilibrium is eventually reached, no more internalchange of any form is possible. Applied to the universe as a whole, this principle

suggests that eventually all temperature in space becomes uniform, resulting inthe so-called heat death of the universe.

Locally, the entropy can be lowered by external action. This applies tomachines, such as a refrigerator, where the entropy in the cold chamber isbeing reduced, and to living organisms. This local increase in order is, however,only possible at the expense of an entropy increase in the surroundings; heremore disorder must be created.

This continued increase in entropy is related to the observed nonreversibility ofmacroscopic processes. If a process were spontaneously reversible, that is, ifafter having undergone a process both it and all the surroundings could bebrought back to their initial state, the entropy would remain constant in violationof the second law. While this is true for macroscopic processes, and thereforecorresponds to daily experience, it does not apply to microscopic processes,which are believed to be reversible. Thus, chemical reactions betweenindividual molecules are not governed by the second law, which applies only tomacroscopic ensembles.

From the promulgation of the second law, thermodynamics went on to otheradvances and applications in physics, chemistry, and engineering. Most

chemical engineering, all power-plant engineering, and air-conditioning and low-temperature physics are just a few of the fields that owe their theoretical basisto thermodynamics and to the subsequent achievements of such scientists asMaxwell, the American physicist Willard Gibbs, the German physical chemistWalther Hermann Nernst, and the Norwegian-born American chemist LarsOnsager (1903-76).


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could not manage. A complete solution of these equations, furthermore, wouldtell 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 transientto be important.

Statistical methods were devised therefore to obtain averages of themechanical variables of the molecules in a system and to provide the grossfeatures of the system. These gross features turn out to be, precisely, themacroscopic thermodynamic variables. The statistical treatment of molecularmechanics is called statistical mechanics, and it anchors thermodynamics tomechanics.

Viewed from the statistical perspective, temperature represents a measure of

the average kinetic energy of the molecules of a system. Increases intemperature reflect increases in the vigor of molecular motion. When twosystems are in contact, energy is transferred between molecules as a result ofcollisions. The transfer will continue until uniformity is achieved, in a statisticalsense, which corresponds to thermal equilibrium. The kinetic energy of themolecules also corresponds to heat andtogether with the potential energyarising from interaction between moleculesmakes up the internal energy of asystem.

The conservation of energy, a well-known law of mechanics, translates readilyto the first law of thermodynamics, and the concept of entropy translates intothe extent of disorder on the molecular scale. By assuming that all combinationsof molecular motion are equally likely, thermodynamics shows that the moredisordered the state of an isolated system, the more combinations can be foundthat could give rise to that state, and hence the more frequently it will occur. Theprobability of the more disordered state occurring overwhelms the probability ofthe occurrence of all other states. This probability provides a statistical basis fordefinitions 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, anotion from classical physics. In terms of quantum mechanics, residualmolecular motion will exist even at absolute zero. An analysis of the statisticalbasis of the third law goes beyond the scope of the present discussion.


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Freezing PointThe freezing point is the temperature at which a liquid congeals into the solid

state at a given pressure.

The freezing point of a pure (unmixed) liquid is essentially the same as themelting point of the same substance in its solid form and may be regarded asthe temperature at which the solid and liquid states of the substance are inequilibrium. If heat is applied to a mixture of liquid and solid substance at itsfreezing point, the temperature of the substance remains constant until it hasbecome completely liquefied, because the heat is absorbed not in warming thesubstance but in providing the latent heat of fusion. Similarly, if heat isabstracted from a mixture of liquid and solid substance at its freezing point, thesubstance will remain at the same temperature until it has become completely

solid, because heat is given off by the substance in its change from the liquid tothe solid state. Hence, the freezing point or melting point of a pure substancemay also be defined as the temperature at which freezing or melting continuesonce it has commenced.

All solids melt when heated to their melting points, but most liquids can remainliquid even though cooled below their freezing points. A liquid may remain inthis supercooled state for some time. This phenomenon is explained bymolecular theory, which conceives the molecules of a solid as being wellordered and the molecules of a liquid as being disordered. To solidify, a liquidmust have a nucleus (a point of molecular orderliness) around which thedisordered molecules can crystallize. The formation of a nucleus is a matter ofchance, but once a nucleus forms, the supercooled liquid will solidify rapidly.The freezing point of a solution is lower than the freezing point of the puresolvent before introduction of the solute (substance dissolved).

The amount that the freezing point is lowered depends on the molecularconcentration of the solute and on whether the solution is an electrolyte.Nonelectrolytic solutions have higher freezing points for a given concentration ofsolute than do electrolytes. The molecular weight of an unknown or unidentifiedsubstance may be determined by measuring the amount by which the freezing

point of a solvent is lowered when a known amount of the unidentifiedsubstance is dissolved in it. This process of determining molecular weights iscalled cryoscopy.

In mixed substances and alloys, the freezing point of the mixture may be muchlower than the freezing points of any of its individual components.

The freezing point of most substances is increased by increase of pressure. Insubstances, however, that expand on freezing (for example, water) pressurelowers the freezing point. An example of this effect can be observed if a heavyobject is placed on a block of ice. The area immediately underneath the object


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will begin to turn to liquid and will refreeze, without any change in temperature,when the object is removed. This process is known as regulation.

Boiling PointThe boiling Point is the temperature at which the vapor pressure of a liquidslightly exceeds the pressure of the atmosphere above the liquid. Attemperatures below the boiling point (b.p.), evaporation takes place only fromthe surface of the liquid; during boiling, vapor forms within the body of the liquid;and as the vapor bubbles rise through the liquid, they cause the turbulence andseething associated with boiling. If the liquid is a single substance or anazeotropic solution (a mixture that has a constant b.p.), it will continue to boil as

heat is added without any rise in temperature; that is, boiling occurs at constanttemperature regardless of the amount of heat applied to the liquid.

When the pressure on a liquid is increased, the b.p. goes up. Water at 1atmosphere pressure (760 torr, or about 14.7 lb/sq in) boils at 100 C (212 F),but when the pressure is 217 atmospheres (164,920 torr, or 3200 lb/sq in), theb.p. reaches its maximum, 374 C (705 F). Above this temperature (the criticaltemperature of water), liquid water is identical to saturated steam.

If the pressure on a liquid is reduced, the b.p. is lowered. At higher elevations,where air pressure is less, water boils below 100 C. In Denver, Colorado,which is 1.6 km (1 mi) above sea level, the b.p. of water averages 94 C (201F). When the pressure on a sample of water falls to 4.55 torr (0.088 lb/sq in),boiling occurs at 0 C (32 F), which is the normal freezing point.

Boiling points cover a wide temperature range. The lowest b.p. is that of helium, 2689. C (-452 F). The highest is probably that of tungsten, about 5900 C(10,650 F). The boiling points given in the separate articles on the variouselements and compounds apply at normal pressure unless specifically stated

otherwise.

Heat and TemperatureA different sensation is experienced when a hot or a cold body is touched,leading to the qualitative and subjective concept of temperature. The addition ofheat to a body leads to an increase in temperature (as long as no melting orboiling occurs), and in the case of two bodies at different temperatures broughtinto contact, heat flows from one to the other until their temperatures becomethe same and thermal equilibrium is reached. To arrive at a scientific measure


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of temperature, scientists used the observation that the addition or subtractionof heat produced a change in at least one well-defined property of a body. The

addition of heat, for example, to a column of liquid maintained at constantpressure increased the length of the column, while the heating of a gas confinedin a container raised its pressure. Temperature, therefore, can invariably bemeasured by one other physical property, as in the length of the mercurycolumn in an ordinary thermometer, provided the other relevant propertiesremain unchanged. The mathematical relationship between the relevantphysical properties of a body or system and its temperature is known as theequation of state. Thus, for an ideal gas, a simple relationship exists betweenthe pressure, p, volume V, number of moles n, and the absolute temperature T,given by pV nRT= , where R is the same constant for all ideal gases. Boyle's law,

named after the British physicist and chemist Robert Boyle, and Gay-Lussac's

law or Charles's law, named after the French physicists and chemists JosephLouis Gay-Lussac and Jacques Alexandre Csar Charles, are both contained inthis equation of state.

Until well into the 19th century, heat was considered a massless fluid calledcaloric, contained in matter and capable of being squeezed out of or into it.Although the so-called caloric theory answered most early questions onthermometry and calorimetry, it failed to provide a sound explanation of manyearly 19th-century observations. The first true connection between heat andother forms of energy was observed in 1798 by the Anglo-American physicistand statesman Benjamin Thompson, Count von Rumford, who noted that theheat produced in the boring of cannon was roughly proportional to the amountof work done. In mechanics, work is the product of a force on a body and thedistance through which the body moves during its application.

HeatHeat is the transfer of energy from one part of a substance to another, or fromone body to another by virtue of a difference in temperature. Heat is energy intransit; it always flows from a substance at a higher temperature to thesubstance at a lower temperature, raising the temperature of the latter and

lowering that of the former substance, provided the volume of the bodiesremains constant. Heat does not flow from a lower to a higher temperatureunless another form of energy transfer, work, is also present.Until the beginning of the 19th century, the effect of heat on the temperature ofa body was explained by postulating the existence of an invisible substance orform of matter termed caloric. According to the caloric theory of heat, a body ata high temperature contains more caloric than one at a low temperature; theformer body loses some caloric to the latter body on contact, increasing thatbody's temperature while lowering its own. Although the caloric theorysuccessfully explained some phenomena of heat transfer, experimentalevidence was presented by the American-born British physicist Benjamin


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Thompson (later known as Count von Rumford) in 1798 and by the Britishchemist 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 PrescottJoule, in a series of highly accurate experiments, provided conclusive evidencethat heat is a form of energy in transit and that it can cause the same changesin a body as work.

Heat UnitsHeat is measured in terms of the calorie, defined as the amount of heatnecessary to raise the temperature of 1 g of water at a pressure of 1 atm from15to 16C. This unit is sometimes called the small or gram calorie to

distinguish it from the large calorie, or kilocalorie, equal to 1000 cal, which isused in nutrition studies. In mechanical engineering practice in the UnitedStates and Great Britain, heat is measured in British thermal units, or Btu. OneBtu is the quantity of heat required to raise the temperature of 1 lb of water 1Fand is equal to 252 cal. Mechanical energy can be converted into heat byfriction, and the mechanical work necessary to produce 1 cal is known as themechanical equivalent of heat. It is equal to 4.1855 107 ergs/cal or 778 ft-lbBtu. According to the law of conservation of energy, all the mechanical energyexpended to produce heat by friction appears as energy in the objects on whichthe work is performed. This fact was first conclusively proven in a classicexperiment performed by Joule, who heated water in a closed vessel by meansof rotating paddle wheels and found that the rise in water temperature wasproportional to the work expended in turning the wheels.

If heat is converted into mechanical energy, as in an internal-combustionengine, 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 engineis perfectly efficient.


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Latent Heat

A number of physical changes are associated with the change of temperature ofa substance. Almost all substances expand in volume when heated andcontract when cooled. The behavior of water between 0 and 4C (32 and39F) constitutes an important exception to this rule. The phase of a substancerefers to its occurrence as either a solid, liquid, or gas, and phase changes inpure substances occur at definite temperatures and pressures. The process ofchanging from solid to gas is referred to as sublimation, from solid to liquid asmelting, and from liquid to vapor as vaporization. If the pressure is constant,these processes occur at constant temperature. The amount of heat required toproduce a change of phase is called latent heat, and hence, latent heats ofsublimation, melting, and vaporization exist. If water is boiled in an open vessel

at a pressure of 1 atm, the temperature does not rise above 100C (212F), nomatter how much heat is added. The heat that is absorbed without changing thetemperature of the water is the latent heat; it is not lost but is expended inchanging the water to steam and is then stored as energy in the steam; it isagain released when the steam is condensed to form water. Similarly, if amixture of water and ice in a glass is heated, its temperature will not changeuntil all the ice is melted. The latent heat absorbed is used up in overcoming theforces 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 at100C, 541 cal are needed.

Specific HeatThe heat capacity, or the measure of the amount of heat required to raise thetemperature of a unit mass of a substance one degree is known as specificheat. If the heating process occurs while the substance is maintained at aconstant volume or is subjected to a constant pressure the measure is referredto as a specific heat at constant volume or at constant pressure. The latter isalways larger than, or at least equal to, the former for each substance. Because1 cal causes a rise of 1C 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 isnot necessary to distinguish between the constant-volume and constant-pressure specific heats, as they are approximately equal. Generally, the twospecific heats of a substance depend on the temperature.


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CalorimetryCalorimetry is the science of measuring a quantity of heat (expressed in

calories), as distinct from thermometry, the science of measuring the intensity ofheat (expressed as temperature). A calorimeter is the instrument used tomeasure the amount of heat; one widely used type consists of an insulatedcontainer of water, a stirring device, and a thermometer. A heat source isplaced in the calorimeter, the water is stirred until equilibrium is reached, andthe rise of temperature is noted by reading the thermometer.

Because the heat capacity of the calorimeter is known (or can be measured byusing a standard heat source), the amount of heat liberated can be readilycalculated. When the heat source is a hot object of known temperature, thespecific and latent heat may be measured as the object cools. Latent heat,

which is not associated with a change in temperature, is the heat evolved orabsorbed by a substance as it changes from one state to another, as from liquidto solid or vice versa. When the heat source is a chemical reaction, such as theburning of a fuel, the reacting substances are placed in a heavy steel vesselcalled a bomb. The bomb is placed within the calorimeter, and the reaction isstarted by ignition with an electric spark.


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Transfer of HeatThe physical methods by which energy in the form of heat can be transferred

between bodies are conduction and radiation. A third method, which alsoinvolves the motion of matter, is called convection. Conduction requires physicalcontact between the bodies or portions of bodies exchanging heat, but radiationdoes 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 adifferent temperature and is always accompanied by the motion of the liquid orgas. The science dealing with the transfer of heat between bodies is called heattransfer.

Heat TransferHeat Transfer, in physics, is the process by which energy in the form of heat isexchanged between bodies or parts of the same body at different temperatures.Heat is generally transferred by convection, radiation, or conduction. Althoughthese three processes can occur simultaneously, it is not unusual for onemechanism to overshadow the other two. Heat, for example, is transferred byconduction through the brick wall of a house, the surfaces of high-speed aircraftare heated by convection, and the earth receives heat from the sun by radiation.

ConductionThis is the only method of heat transfer in opaque solids. If the temperature atone 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 thesolid matter, which transport energy if a temperature difference is applied. Thistheory helps to explain why good electrical conductors also tend to be goodheat conductors. Although the phenomenon of heat conduction had beenobserved for centuries, it was not until 1882 that the French mathematicianJean Baptiste Joseph Fourier gave it precise mathematical expression in what

is now regarded as Fourier's law of heat conduction. This physical law statesthat the rate at which heat is conducted through a body per unit cross-sectionalarea is proportional to the negative of the temperature gradient existing in thebody.

The proportionality factor is called the thermal conductivity of the material.Materials such as gold, silver, and copper have high thermal conductivities andconduct heat readily, but materials such as glass and asbestos have values ofthermal conductivity hundreds and thousands of times smaller, conduct heatpoorly, and are referred to as insulators. In engineering applications it isfrequently necessary to establish the rate at which heat will be conducted


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RadiationThis 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 generallyapplied to all kinds of electromagnetic-wave phenomena. Some radiationphenomena can be described in terms of wave theory, and others can beexplained in terms of quantum theory. Neither theory, however, completelyexplains all experimental observations. The German-born American physicistAlbert Einstein conclusively demonstrated (1905) the quantized behavior ofradiant energy in his classical photoelectric experiments. Before Einstein'sexperiments the quantized nature of radiant energy had been postulated, andthe German physicist Max Planck used quantum theory and the mathematicalformalism of statistical mechanics to derive (1900) a fundamental law of

radiation. The mathematical expression of this law, called Planck's distribution,relates the intensity or strength of radiant energy emitted by a body to thetemperature of the body and the wavelength of radiation. This is the maximumamount of radiant energy that can be emitted by a body at a particulartemperature. Only an ideal body (blackbody,) emits such radiation according toPlanck's law. Real bodies emit at a somewhat reduced intensity. Thecontribution of all frequencies to the radiant energy emitted by a body is calledthe emissive power of the body, the amount of energy emitted by a unit surfacearea of a body per unit of time. As can be shown from Planck's law, theemissive power of a surface is proportional to the fourth power of the absolutetemperature. The proportionality factor is called the Stefan-Boltzmann constantafter two Austrian physicists, Joseph Stefan (1835-93) and Ludwig Boltzmann,who, in 1879 and 1884, respectively, discovered the fourth power relationshipfor the emissive power. According to Planck's law, all substances emit radiantenergy merely by virtue of having a positive absolute temperature. The higherthe temperature, the greater the amount of energy emitted. In addition toemitting, all substances are capable of absorbing radiation. Thus, although anice cube is continuously emitting radiant energy, it will melt if an incandescentlamp is focused on it because it will be absorbing a greater amount of heat thanit is emitting.

Opaque surfaces can absorb or reflect incident radiation. Generally, dull, roughsurfaces absorb more heat than bright, polished surfaces, and bright surfacesreflect more radiant energy than dull surfaces. In addition, good absorbers arealso good emitters; good reflectors, or poor absorbers, are poor emitters. Thus,cooking utensils generally have dull bottoms for good absorption and polishedsides for minimum emission to maximize the net heat transfer into the contentsof the pot. Some substances, such as gases and glass, are capable oftransmitting large amounts of radiation. It is experimentally observed that theabsorbing, reflecting, and transmitting properties of a substance depend uponthe wavelength of the incident radiation. Glass, for example, transmits largeamounts of short wavelength (ultraviolet) radiation, but is a poor transmitter of


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long wavelength (infrared) radiation. A consequence of Planck's distribution isthat 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 expressionof this observation and states that the wavelength of maximum energy,expressed in microns (millionths of a meter), multiplied by the Kelvintemperature of the body is equal to a constant, 2878. Most of the energyradiated by the sun, therefore, is characterized by small wavelengths. This fact,together with the transmitting properties of glass mentioned above, explains thegreenhouse effect. Radiant energy from the sun is transmitted through the glassand enters the greenhouse. The energy emitted by the contents of thegreenhouse, however, which emit primarily at infrared wavelengths, is nottransmitted out through the glass. Thus, although the air temperature outside

the greenhouse may be low, the temperature inside the greenhouse will bemuch higher because there is a sizable net heat transfer into it.

In addition to heat transfer processes that result in raising or loweringtemperatures of the participating bodies, heat transfer can also produce phasechanges such as the melting of ice or the boiling of water. In engineering, heattransfer processes are usually designed to take advantage of thesephenomena. In the case of space capsules reentering the atmosphere of theearth at very high speed, a heat shield that melts in a prescribed manner by theprocess called ablation is provided to prevent overheating of the interior of thecapsule. Essentially, the frictional heating produced by the atmosphere is usedto melt the heat shield and not to raise the temperature of the capsule.


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Vacuum bottleA vacuum Bottle, also known as a vacuum flask, is a container, usuallycylindrical, and usually made of two glass walls with a near-vacuum in between.The bottle is used in the home and in scientific and industrial research tomaintain liquids, and sometimes solids, at near-constant temperatures. Anefficient flask can keep its contents at the desired hot or cold temperature for aslong as three days. This is achieved by preventing the transfer of heat between

Solar Heating

The suns rays warm the water flowing through copper tubing inside the solarcollector on the roof of this house. The hot water then flows from the solarcollector to a heat exchanger. Inside the heat exchanger, cold water is warmedby the hot water flowing from the solar collector. The warmed water then flows

through pipes to hot water faucets in the house. The cooled water in the heatexchanger flows back to the solar collector to be warmed again by the sunsrays.

Microsoft Illustration

"Solar Heating," Microsoft (R) Encarta. Copyright (c) 1993 MicrosoftCorporation. Copyright (c) 1993 Funk & Wagnall's Corporation


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the bottle and its surroundings. The walls are usually made of glass because itis a poor conductor of heat; its surfaces are usually lined with a reflective metal

to reduce the transfer of heat by radiation. The near-vacuum condition betweenthe container walls, moreover, greatly reduces the transfer of heat byconvection. The whole fragile flask rests on a shock-absorbing spring within ametal or plastic container, and the air between the flask and the containerprovides further insulation.

Vacuum bottles, sometimes referred to as thermos bottles, were first calledDewar flasks after the British chemist Sir James Dewar, who invented the firstone in 1892 to aid him in his work with liquid gases.

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