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What
is Temperature? |
DEFINITION:
(For this website usage):
Temperature: A measure proportional to
the average translational kinetic energy associated
with the disordered microscopic motion of atoms and molecules. The flow of heat
is from a high temperature region toward a lower temperature region.
-Courtesy
of the references cited below but mostly based on the operational definition on
the HyperPhysics pages at Georgia State University by R. Nave. and the discussion
in "Traceable Temperatures" Second Ed. by Nicholas and White-See the
discussions and definitions below for our rationale. |
Looking
at textbooks and related references and even websites for a definition of temperature
can be very frustrating. Many talk around the subject and never get to the point
or never state a clear definition. Oh, yes, we can find information about the
fact that temperature is not heat and what temperature scales are. We know it's
related to heat and there is a difference between them. What is it? It's got to
be on the Web.....
Bottom line. It is. However it is like finding the needle
in a haystack; just like many topics on the web. Search engines do not help much;
it takes digging by someone who understands the subject. A serious group of
experts at The American Society for Testing and Materials (ASTM) publishes standards
about devices and information on how to calibrate, test and specify temperature
sensors. But their standard on Teminology, ASTM E-344, is notably missing a definition
for "Temperature"! Surprise! Is it just us, or have you, too, noticed
that there seems to be a real difficulty in defining or agreeing the meaning of
the word and concept of "Temperature". |
(Bottom
line: It is not a simple concept, despite all the work done on it over the past
few hundred years. Yes, it can be defined, but not easily. In many cases simplified
explanations, not true definitions, are used. See the HyperPhysics Web site at
Georgia State University for the clearest and best explained definition we have
seen to date. Their definition appears at the bottom of this page.)
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Temperature-Do
we know what it is?
Here are some samples of definitions
that we have found: - "Temperature is the degree of 'hotness' of a body:
more precisely it is the potential for heat transfer. In our everyday lives, we
are aware of different temperatures through the sensation of touch, but how hot
or cold something feels is subjective. We can say that the kettle is hotter than
the ice-cream, but not by how much. Measurement, on the other hand, must be objective
and a thermometer is used."
National
Physical Laboratory, Teddington, Middlesex, UK, TW11 0LW © Crown Copyright
2001. Reproduced by permission of the Controller of HMSO. - "What
is Temperature?"
"In a qualitative manner, we can describe the temperature
of an object as that which determines the sensation of warmth or coldness felt
from contact with it. It is easy to demonstrate that when two objects of the same
material are placed together (physicists say when they are put in thermal contact),
the object with the higher temperature cools while the cooler object becomes warmer
until a point is reached after which no more change occurs, and to our senses,
they feel the same. When the thermal changes have stopped, we say that the two
objects (physicists define them more rigorously as systems) are in thermal equilibrium
. We can then define the temperature of the system by saying that the temperature
is that quantity which is the same for both systems when they are in thermal equilibrium."
From
The Popular Website "About
Temperature" (since 21 Nov 1995).
"About
Temperature" Disponible en espanol - Temperature, when measured
in Kelvin degrees, is a number that is directly proportional to the average kinetic
energy of the molecules in a substance. So, when the molecules of a substance
have a small average kinetic energy, then the temperature of the substance is
low.
From The Physics and Mathematics Web Site "Zona
Land"
NOTE:
This website also
has an interesting page that graphically illustrates the relationship between
molecular motion and the temperature
of a gas. - "Temperature can be defined in macroscopic
terms, using concepts of thermodynamics, as an intrinsic property of matter that
quantifies the ability of one body to transfer thermal energy (heat) to another
body..."Temperature can also be defined on a microscopic scale as proportional
to the random kinetic energy of an assemblage of molecules or atoms."
"Industrial
Temperature Measurement" T.W Kerlin and R.L. Shepard, The
Instrument Society of America, Research Triangle Park, NC, 1982 (ISBN 0-87664-622-4) - "Temperature:
A convenient operational definition of temperature is that it is a measure of
the average translational kinetic energy associated
with the disordered microscopic motion of atoms and molecules. The flow of heat
is from a high temperature region toward a lower temperature region.
"The
details of the relationship to molecular motion are described in kinetic theory.
The temperature defined from kinetic theory is called the kinetic temperature.
Temperature is not directly proportional to internal
energy since temperature measures only the kinetic energy part of the internal
energy, so two objects with the same temperature do not in general have the same
internal energy (see water-metal example). Temperatures are measured
in one of the three standard temperature scales (Celsius, Kelvin, and Fahrenheit).
"A More General View of Temperature: When a high temperature object
is placed in contact with a low temperature object, then energy will flow from
the high temperature object to the lower temperature object, and they will approach
an equilibrium temperature. "When the details of this common-sense scenario
are examined, it becomes evident that the simple view of temperature embodied
in the commonly used kinetic temperature approach has some significant problems.
"The above illustration summarizes the situation when the kinetic temperature
gives a reasonable general description of the nature of temperature. For monoatomic
gases acting like point masses, a higher temperature simply implies higher average
kinetic energy. Faster molecules striking slower ones at the boundary in elastic
collisions will increase the velocity of the slower ones and decrease the velocity
of the faster ones, transferring energy from the higher temperature to the lower
temperature region. "With time, the molecules in the two regions approach the
same average kinetic energy (same temperature) and in this condition of thermal
equilibrium there is no longer any net transfer of energy from one object to the
other. The concept of temperature is complicated by internal degrees of freedom
like molecular rotation and vibration and by the existence of internal interactions
in solid materials which can include collective modes. " The internal motions
of molecules affect the specific heats of gases, with diatomic hydrogen being
the classic case. Collective modes affect the specific heats of solids, particularly
at low temperatures. Complications such as these have led to the adoption of a
different approach to the concept of temperature in the study of thermodynamics.
"Schroeder's proposal for a theoretical definition of temperature is:
"*
Temperature is a measure of the tendency of an object to spontaneously give
up energy to its surroundings. When two objects are in thermal contact, the one
that tends to spontaneously lose energy is at the higher temperature."(Thermal
Physics, Ch 1.) "The kinetic temperature for monoatomic ideal gases described
above is consistent with this definition of temperature for the simple systems
to which it applies. In that case the equilibrium reached is one of maximum entropy,
and the rate of approach to that state will be proportional to the difference
in temperature between the two parts of the system. Noting that the equilibrium
state of a collection of particles will be the state of greatest multiplicity,
then one can define the temperature in terms of that multiplicity (entropy)
as follows: "Temperature is expressed as the inverse of the rate of
change of entropy with internal energy, with volume V and number of particles
N held constant. This is certainly not as intuitive as molecular kinetic energy,
but in thermodynamic applications it is more reliable and more general." From
the website HyperPhysics
at Georgia State Univerity, author R. Nave. - "For most materials,
temperature can be considered to be a measure of the density of heat in a body."...
"Kelvin
was also able to show that this definition (The ratio of heat taken in at a high
temperature to the heat taken out at a lower temperature for an ideal, Carnot,
heat engine depends on the ratio of a function of the temperatures) leads to an
equation for ideal gases of the form PV=constant xT so
that Kelvin's definition of temperature is equivalent to the the gas scale originally
proposed by Amontons, and implemented by Chappuis in 1889..... "When applied
to an ideal box they obtained (by considering a material model based on the movement
and collisions of individual atoms in a closed box, Kelvin and others were able
to show that thermal equilibrium requires the mean kinetic energy of the atoms
to be the same) the result PV= constant x (mv^2/2)
where (mv^2/2) is the average kinetic energy of each atom in the gas. Comparison
of this equation with Equation (1.3) (the previous equation) shows that the temperature
is proportional to the average kinetic energy of each atom...."Note that the total
kinetic energy of molecular gases is higher than that for monatomic gases because
they can rotate and vibrate; in that case the temperature is proportional to the
mean translational kinetic energy." "Traceable
Temperatures" Second Edition, by J. V. Nicholas and D. R. White, John Wiley
& Sons, LTD, Chichester, 2001
ISBN 0-471-49291-4 .
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SUMMARY:
Temperature can be defined in several ways, many of which are not completly
accurate, but the first definition of Number 5, above, seems to almost fit
our understanding and work reasonably well as an "Operational Definition"
for the term. However, the detail provided in the last reference (Number 6)
gives a more complete understanding and shows that the word "proportional"
really belongs in the definition. At least that's the opinion of this website
editor and publisher. If you have another view, please let us know and, if
it is persuasive, we will post here also. Just feedback on our special website for feedback and commentary. | |
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