One has to be careful when defining temperature not to confuse it with heat. Heat is a form of energy. Temperature is something different. We could begin with a technical definition, but I would prefer to start with a question. How hot is it? The answer to this question (or a question like this) is a measurement of temperature. The hotter something is, the higher its temperature. Therefore I would like to propose the following informal definition …
Temperature is a measure of hotness.
Don't go looking for "hotness" in any dictionary (except maybe a slang dictionary). There is no such word. I made it up. Despite this fact, I believe that most speakers of English will understand this neologism. Unfortunately this won't do for scientific purposes. Quantities in science are typically defined operationally (through the process by which they are measured) or theoretically (in terms of the theories of a specific discipline). We will begin with a theoretical definition of temperature and end with an operational definition.
Let's review what you should already know.
The archetypal example of this is the rock at the top of a hill. Due to its height above the bottom of the hill it possesses gravitational potential energy. Give it a push and it will start to roll. If we assume the ideal situation of a closed system where no energy is lost on the way down, then the rock's initial potential energy will equal its final kinetic energy.
Now take the archetypal example one step further. Assume the rock crashes into a wall. Neither the rock nor the wall are made of rubber, so the rock comes to a halt. Now it appears as if we have violated the law of conservation of energy. The kinetic energy is lost and nothing has come along to replace it. Where has the energy gone?
The answer to this question is inside the rock (and inside the wall). The energy has been transformed from the external energy visible as the motion of the rock as a whole to the internal energy of the motion of the invisible parts that make up the rock (and the wall). The two energies are identical in size, but different in appearance. External energy is visible because it is organized. The translational kinetic energy of a rock is due to coordinated motion. All the parts move forward together. The rotational energy is also coordinated. The parts all rotate together around the center of mass. In contrast, the internal kinetic energy of a rock is invisible since the pieces are so small and numerous and their motion is completely uncoordinated. Their motions are statistically random with a mean value of zero making the energy invisible to macroscopic beings like us. Potential energy can also exist in external and internal forms. I won't provide you with an example here but I will say that external potential energy is relatively obvious. Internal potential energy is somewhat obscure.
If you believe that objects can have internal energy, then it shouldn't be much of a stretch to believe that they can exchange this energy. This is known as thermal contact. The irreducible bits and pieces of objects responsible for carrying the internal energy are known as atoms — from the Greek "α τομή" [a tomi] meaning "can't be cut" — but belief in atoms is not a necessity. It just makes life easier. (Surprisingly, much of thermal physics and thermodynamics was worked out before atoms were generally considered real.) Since we are dealing with large numbers of atoms in uncoordinated motion, there will be times and places where the transfer of internal energy will run in one direction and different times and places where the transfer of internal energy will be in the opposite direction. Since the numbers are so unimaginably large, we really don't care about what happens to any one atom. All that we can observe in such cases is the net or overall transfer of internal energy. This is known as heat. If the net exchange of internal energy is zero; that is, if no heat flows from one region to another; then the whole system is said to be in thermal equilibrium.
Since heat is defined as internal energy in transit from one place to another, nothing can be said to have heat or store heat. Instead we say that heat flows from one place to another. The direction is indicated by the sign in front of the number. Use "+" when heat flows into a place and "−" when it flows out. Heat can travel left, right, up, down, forward, or backward, but that's not usually the way it's described. Heat is a form of energy and energy is scalar, therefore specific directions and angles and all the rest of that vector stuff should be ignored (initially, anyway).
Heat is a form of energy and the unit of energy is the joule, therefore heat should be measured in joules. Before this fact was known, however, heat had its own special units; like the calorie and the british thermal unit (Btu), for example. These units still pop up from time to time. But enough about heat. Let's get back to temperature. What is it?
Two regions have the same temperature when there is no net exchange of internal energy between them.
In it's most primitive sense, temperature is what determines the direction of heat flow. The net transfer of internal energy between regions in thermal contact is out from the region with the higher temperature and into the region with lower temperature. In more concise terms, heat flows from hot to cold. That's the theoretical definition of temperature.
Temperature is measured with a thermometer. The basic operating principle behind all thermometers is that there is some quantity, called a thermometric variable, that changes in response to changes in temperature. The relationship between temperature and the thermometric variable may be direct or inverse or it may be determined by a polynomial or power function. In any case, it is the thermometric variable that gets measured. There is no way to measure temperature directly.
|liquid in glass||volume|
|constant volume gas||pressure|
|bimetallic strip||coil pitch|
Once we've settled on the thermometric variable to be measured, the next step is to decide on a temperature scale. Not because "units matter" (as every physics teacher says when they subtract points from students who forgot to include them in the answer) but rather because temperature has no meaning without values defined as standard. In thermometry, what we need are fixed points: reproducible experiments based on natural phenomena that occur at a definite temperature under a proscribed set of conditions. Actually, we need at least two fixed points and a defined range of numbers (called a fundamental interval) between the lower fixed point and the upper fixed point. The other reason that the operational definition of temperature is so tightly bound with temperature scales is that the early science of thermometry is tied up with the invention and construction of thermometers.
The first thermometer was constructed in what is now northern Italy in the Seventeenth Century by either Santorio Santorio (1561-1636) also known by his latin name Sanctorus, Galileo Galilei (1564-1642) the man who basically invented the scientific method, or Giovanfrancesco Sagredo (1571-1620) an instrument maker who is sometimes called a "disciple" of Galileo. All three men built what are known as liquid in glass thermometers, which consist of a glass reservoir of liquid attached to a narrow glass tube. When temperature increases, the liquid expands and rises up in the tube. When temperature decreases, the liquid contracts and falls back down the tube. The height of the column is therefore related to the temperature in a simple linear fashion. Galileo did not put a scale on his device, so what he invented is better called a thermoscope since all it can do is show changes in temperature, not really measure them. Sanctorus added a scale to an air in glass thermoscope and thus could be credited with inventing thermometer, but … Air in glass devices respond to pressure changes as well as temperature changes and pressure was not something that was well understood at the time. Sagredo added a scale to his thermometer with 360 divisions imitating the Classical division of the circle. Ever since then, temperature units have been called "degrees" whether or not there were 360 of them in the fundamental interval.
Robert Hooke (1635-1703) of London was the first to suggest using the freezing point of water as a lower fixed point. Ole Rømer (1644-1710) of Copenhagen assigned a value of 7.5° to the freezing point and 60° to the boiling point of water so that normal body temperature would wind up as 22.5° or three times the freezing point. In the days when thermometers were graduated by hand, such tricks were commonly built into temperature scales.
In any case, normal body temperature is not the kind of fixed point that satisfies the needs of serious thermometry. There is just too much variation in the concept of "normal" as it applies to human beings. Different people can have different body temperatures and still be considered healthy and everyone's body temperature varies over the course of the day. We are coldest in the early morning and hottest in the middle of the afternoon. Such a variable number just doesn't cut it as a fixed point. Some other failed ideas for fixed points include …
The longest lived of the temperature scales is the work of Daniel Gabriel Fahrenheit (1686-1736). Fahrenheit was born to a German family living in Danzig, Prussia (now Gdansk, Poland). When he was 15 he lost both of his parents to mushroom poisoning and was apprenticed to a local merchant who later moved him to The Netherlands. Fahrenheit did not enjoy this arrangement and basically skipped out on his master. Apprenticeships are less like the semester internships modern college students deal with and are more like seven years of indentured servitude. During his period as a runaway and for a few years after, Fahrenheit traveled throughout The Netherlands, Denmark, Germany, Sweden, and Poland; acquired technical skills like glassblowing and instrument making; and learned Dutch, French, English, and thermal physics.
When he was 28 he astounded the scientific community by constructing a pair of thermometers that gave the same readings. What astounds me is that anyone would have found this act astounding, but apparently no one had ever done it before. Sagredo's now historic 360 degree thermometer assigned 0° to a snow and salt mixture, 100° to snow, and 360° to the hottest summer day. Thermometers of the kind first built in northern Italy were calibrated to unreproduceable fixed points. This meant that thermometers manufactured in 1650 gave different results from those manufactured in 1651 and thermometers manufactured in Florence gave different results from those manufactured in Venice.
Fahrenheit settled on three fixed points, which he detailed in a paper presented before the Royal Society of London in 1724. (Emphasis has been added to certain keywords.)
Hujus scalæ division tribus nititur terminis fixis, qui arte sequentimodo parari possunt; primus illorum in informa parte vel initio scalæ reperitur, & commixtione glaciei, aquæ, & salis Armoniaci vel etiam maritimi acquiritur; huic mixturæ si thermometron imponitur, fluidum ejus usque ad gradum, qui zero notatur, descendit. Melius autem hyeme, quam æstate hoc experimentum succedit. The division of the scale depends on three fixed points, which can be determined in the following manner. The first is found in the uncalibrated part or the beginning of the scale, and is determined by a mixture of ice, water and ammonium chloride or even sea salt. If the thermometer is placed in this mixture, its liquid descends as far as the degree that is marked with a zero. This experiment succeeds better in winter than in summer. Secundus terminus obtinetur, si aqua & glacies absque memoratis salibus commiscentur, imposito thermometro huic mixturæ, fluidum ejus tricesimum secundum occupat gradum, & terminus initii congelationis a me vocatur; aquæ enim stagnantes tenuissima jam glacie obducuntur, quando hyeme liquor thermometri hunce gradum attingit. The second point is obtained if water and ice are mixed without the aforementioned salts. When the thermometer is placed in this mixture, its liquid reaches the 32nd degree. I call this freezing point. For still waters are already covered with a very thin layer of ice when the liquid of the thermometer touches this point in winter. Terminus tertius in nonagesimo sexto gradu reperitur; & spiritus usque ad hunc gradum dilatatur, dum thermometrum in ore vel sub axillis hominis in statu sano viventis tam diu tenetur donec perfectissime calorem corporis acquisivit. The third point is situated at the 96th degree. Alcohol expands up to this point when it is held in the mouth or under the armpit of a living man in good health until it has completely acquired his body heat. Daniel Gabriel Fahrenheit Translation by J. Holland
After Fahrenheit's death these fixed points were changed so that the scale bearing his name now has only two, more sensible fixed points. The normal freezing point of water stayed at the 32℉ but the saltwater and body heat points were dropped in favor of an upper fixed point of 212℉ at the normal boiling point of water. This divided the fundamental interval up into 180 degrees, which was an easy number to work with. Dividing an interval up into halves or thirds (or powers of halves and thirds) is not that bad. It's fifths that are the real challenge. The factors of 96 are 2, 2, 2, 2, 2, 3; which is devoid of the terrible fives. The factors of 180 are 2, 2, 3, 3, 5; which still includes a five, but at least there's only one. The factors of 100 are 2, 2, 5, 5; which has twice as many fives in it and thus twice the headaches.
René Antoine Ferchault de Réaumur (1683-1757) France. Anders Celsius (1701-1744) Sweden. Centigrade scale has also be claimed by Daniel Ekström, Mårten Strömer, Christian of Lyons,.
Since there are one hundred degrees between the two reference points, the the names degree centigrade and centesimal degree were used as well as the name degree Celsius. In 1948 these alternate names were dropped and degree Celsius was chosen as the official name. This was done to honor Celsius for his work in designing the original system and to avoid inconsistent use of the prefix centi. The name "centigrade" implies that there is a unit called the "grade".
William Thomson a.k.a. Lord Kelvin (1824-1907) Ireland-Scotland suggests the first absolute temperature scale. Rudolph Julius Emanuel Gottlieb a.k.a. Clausius (1822-1888) Germany suggested that the scale be modified so that the size of one degree on Thomson's scale was the same as one centigrade degree.
Let me tell ya somethin'.
|T[℃] =||T[K] − 273.15||⇔||T[K] =||T[℃] + 273.15|
|T[℃] =||(T[℉] − 32)||5||⇔||T[℉] =||9||T[℃] + 32|
|event, location, phenomena, process
|~1032||planck temperature, upper limit of temperature|
|~1016||hottest laboratory experiment (Fermilab)|
|~109||core of hottest stars|
|~107||core of the sun|
|~106||solar corona (the sun's atmosphere)|
|25,000||surface of blue stars|
|6500||D65 standard white hot (effective)|
|6000||center of earth|
|5800||surface of the sun|
|3500||surface of red stars|
|4900||2700||3000||incandescent light bulb|
|1250||680||950||dull red hot|
|930||500||770||incipient red heat|
|850||460||730||mean surface temperature on Venus|
|840||450||720||daytime surface temperature on Mercury|
|574.5875||301.4375||574.5875||fahrenheit and kelvin scales coincide|
|530||280||550||very hot home oven|
|451||233||506||paper burns (according to Bradbury)|
|136||58||331||hottest surface temperature on earth (Libya 1922)|
|108||42||315||New York City record high|
|100||37.778||310.928||nothing of importance|
|98.6||37.0||310.2||human body (traditional US)|
|98.2||36.8||309.9||human body (revised)|
|96||human body (according to Fahrenheit)|
|300||quick approximation of room temperature|
|59||15||288||mean surface temperature on earth|
|32.018||0.01||273.16||water triple point|
|0||−18||255||ice-water-salt mixture (according to Fahrenheit)|
|−13||−25||248||New York City record low|
|−37.9019||−38.8344||234.3156||mercury triple point|
|−40||−40||233||fahrenheit and celsius scales coincide|
|−56||−49||220||mean surface temperature on Mars|
|−108||−78||195||sublimation point of dry ice|
|−128||−89||184||coldest surface temperature on earth (Vostok 1983)|
|−300||−180||90||nighttime surface temperature on Mercury|
|−308.8196||−189.3442||83.8058||argon triple point|
|54.3584||oxygen triple point|
|50||mean surface temperature on Pluto|
|24.5561||neon triple point|
|13.8033||hydrogen triple point|
|2.73||mean temperature of the universe|
|2.174||helium I – helium II λ point (0.050 atm)|
|~1||coldest point in space (Boomerang Nebula)|
|0.95||helium freezes (26 atm)|
|10−10||coldest laboratory experiment (Helsinki Tech)|