- Kinetic theory
temperatureof an ideal monatomic gasis a measure related to the average kinetic energyof its atoms as they move. In this animation, the size of heliumatoms relative to their spacing is shown to scale under 1950 atmospheres of pressure. These room-temperature atoms have a certain, average speed (slowed down here two trillion fold).] Kinetic theory (or kinetic theory of gases) attempts to explain macroscopicproperties of gases, such as pressure, temperature, or volume, by considering their molecular composition and motion. Essentially, the theory posits that pressure is due not to static repulsion between molecules, as was Isaac Newton's conjecture, but due to collisions between molecules moving at different velocities. Kinetic theory is also known as the kinetic-molecular theory or the collision theory.
The theory for ideal gases makes the following assumptions:
* The gas consists of very small particles, each of which has a
massor weight in SI units, kilograms.
* The number of molecules is large such that statistical treatment can be applied.
* These molecules are in constant, random motion. The rapidly moving particles constantly collide with each other and with the walls of the container.
* The collisions of gas particles with the walls of the container holding them are perfectly elastic.
interactions among molecules are negligible. They exert no forces on one another except during collisions.
* The total
volumeof the individual gas molecules added up is negligiblecompared to the volume of the container. This is equivalent to stating that the average distanceseparating the gas particles is relatively large compared to their size.
* The molecules are perfectly spherical in shape, and elastic in nature.
* The average
kinetic energyof the gas particles depends only on the temperature of the system.
* Relativistic effects are negligible.
* Quantum-mechanical effects are negligible. This means that the inter-particle distance is much larger than the
thermal de Broglie wavelengthand the molecules can be treated as classical objects.
* The time during collision of molecule with the container's wall is negligible as comparable to the time between successive collisions.
* The equations of motion of the molecules are time-reversible.In addition, if the gas is in a container, the collisions with the walls are assumed to be instantaneous and elastic.
More modern developments relax these assumptions and are based on the
Boltzmann equation. These can accurately describe the properties of dense gases, because they include the volume of the molecules. The necessary assumptions are the absence of quantum effects, molecular chaosand small gradients in bulk properties. Expansions to higher orders in the density are known as virial expansions. The definitive work is the book by Chapman and Enskog but there have been many modern developments and there is an alternative approach developed by Grad based on moment expansions.Fact|date=November 2007 In the other limit, for extremely rarefied gases, the gradients in bulk properties are not small compared to the mean free paths. This is known as the Knudsen regime and expansions can be performed in the Knudsen number.
The kinetic theory has also been extended to include inelastic collisions in
granular matterby Jenkins and others.Fact|date=November 2007
Pressureis explained by kinetic theory as arising from the force exerted by gas molecules impacting on the walls of the container. Consider a gas of "N" molecules, each of mass "m", enclosed in a cuboidal container of volume "V". When a gas molecule collides with the wall of the container perpendicular to the "x" coordinate axis and bounces off in the opposite direction with the same speed (an elastic collision), then the momentumlost by the particle and gained by the wall is:
where "vx" is the "x"-component of the initial velocity of the particle.
The particle impacts the wall once every 2"l/vx" time units (where "l" is the length of the container). Although the particle impacts a side wall once every 1"l/vx" time units, only the momentum change on one wall is considered so that the particle produces a momentum change on a particular wall once every 2"l/vx" time units.:
forcedue to this particle is:
The total force acting on the wall is:
where the summation is over all the gas molecules in the container.
The magnitude of the velocity for each particle will follow:
Now considering the total force acting on all six walls, adding the contributions from each direction we have:
where the factor of two arises from now considering both walls in a given direction.
Assuming there are a large number of particles moving sufficiently randomly, the force on each of the walls will be approximately the same and now considering the force on only one wall we have:
The quantity can be written as , where the bar denotes an average, in this case an average over all particles. This quantity is also denoted by where is the root-mean-square velocity of the collection of particles.
Thus the force can be written as:
Pressure, which is force per unit area, of the gas can then be written as:
where "A" is the area of the wall of which the force exerted on is considered.
Thus, as cross-sectional area multiplied by length is equal to volume, we have the following expression for the pressure
where "V" is the volume. Also, as "Nm" is the total mass of the gas, and mass divided by volume is density
where ρ is the density of the gas.
This result is interesting and significant, because it relates pressure, a
macroscopicproperty, to the average (translational) kinetic energyper molecule (1/2"mvrms"2), which is a microscopicproperty. Note that the product of pressure and volume is simply two thirds of the total kinetic energy.
Temperature and kinetic energy
ideal gas law,:Eq.(3)1is one important result of the kinetictheory: The average molecular kinetic energy is proportional tothe absolute temperature.
From Eq.(1) and Eq.(3)1,we have:Thus, the product of pressure andvolume per mole is proportional to the average(translational) molecular kinetic energy.
Eq.(1) and Eq.(4)are called the "classical results", which could also be derived from
statistical mechanics; for more details, see [ [http://clesm.mae.ufl.edu/wiki.pub/index.php/Configuration_integral_%28statistical_mechanics%29 Configuration integral (statistical mechanics)] ] .
Since there are
degrees of freedom(dofs)in a monoatomic-gas system with particles,the kinetic energy per dof is:In the kinetic energy per dof,the constant of proportionality of temperature is 1/2 times
Boltzmann constant. This result is relatedto the equipartition theorem.
As noted in the article on
heat capacity, diatomicgases should have 7 degrees of freedom, but the lighter gases actas if they have only 5.
Thus the kinetic energy per kelvin (monatomic
ideal gas) is:
* per mole: 12.47 J
* per molecule: 20.7 yJ = 129 μeV
At standard temperature (273.15 K), we get:
* per mole: 3406 J
* per molecule: 5.65 zJ = 35.2 meV
Number of collisions with wall
One can calculate the number of atomic or molecular collisions with a wall of a container per unit area per unit time.
Assuming an ideal gas, a derivation [ [http://www.chem.arizona.edu/~salzmanr/480a/480ants/collsurf/collsurf.html Collisions With a Surface ] ] results in an equation for total number of collisions per unit time per area:
RMS speeds of molecules
From the kinetic energy formula it can be shown that
with "v" in m/s, "T" in kelvins, and "R" is the
gas constant. The molar mass is given as kg/mol. The most probable speed is 81.6% of the rms speed, and the mean speeds 92.1% (distribution of speeds).
Daniel Bernoullipublished "Hydrodynamica", which laid the basis for the kinetic theory of gases. In this work, Bernoulli positioned the argument, still used to this day, that gases consist of great numbers of molecules moving in all directions, that their impact on a surface causes the gas pressure that we feel, and that what we experience as heatis simply the kinetic energyof their motion. The theory was not immediately accepted, in part because conservation of energyhad not yet been established, and it was not obvious to physicists how the collisions between molecules could be perfectly elastic.
Other pioneers of the kinetic theory (which were neglected by their contemporaries) were
Mikhail Lomonosov(1747), [Lomovosov 1758] Georges-Louis Le Sage(ca. 1780, published 1818), [Le Sage 1780/1818] John Herapath(1816) [Herapath 1816, 1821] and John James Waterston(1843), [Waterston 1843] which connected their research with the development of mechanical explanations of gravitation. In 1856 August Krönig(probably after reading a paper of Waterston) created a simple gas-kinetic model, which only considered the translational motion of the particles. [Krönig 1856]
Rudolf Clausius, according to his own words independently of Krönig, developed a similar, but much more sophisticated version of the theory which included translational and contrary to Krönig also rotational and vibrational molecular motions. In this same work he introduced the concept of mean free pathof a particle. [Clausius 1857] In 1859, after reading a paper by Clausius, James Clerk Maxwellformulated the Maxwell distributionof molecular velocities, which gave the proportion of molecules having a certain velocity in a specific range. This was the first-ever statistical law in physics. [Mahon 2003] In his 1873 thirteen page article 'Molecules', Maxwell states: “we are told that an 'atom' is a material point, invested and surrounded by 'potential forces' and that when 'flying molecules' strike against a solid body in constant succession it causes what is called pressureof air and other gases.” [Maxwell 1875] In 1871, Ludwig Boltzmanngeneralized Maxwell's achievement and formulated the Maxwell–Boltzmann distribution. Also the logarithmic connection between entropyand probabilitywas first stated by him.
In the beginning of twentieth century, however, atoms were considered by many physicists to be purely hypothetical constructs, rather than real objects. An important turning point was
Albert Einstein's (1905) [Einstein 1905] and Marian Smoluchowski's (1906) [Smoluchowski 1906] papers on Brownian motion, which succeeded in making certain accurate quantitative predictions based on the kinetic theory.
title =Ueber die Art der Bewegung, welche wir Wärme nennen
journal =Annalen der Physik
title =Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen
journal =Annalen der Physik
author= Herapath, J.
title =On the physical properties of gases
journal =Annals of Philosophy
title=On the Causes, Laws and Phenomena of Heat, Gases, Gravitation
journal= Annals of Philosophy
title =Grundzüge einer Theorie der Gase
journal =Annalen der Physik
author=Le Sage, G.-L.
chapter=Physique Mécanique des Georges-Louis Le Sage
title=Deux Traites de Physique Mécanique
place=Geneva & Paris
chapter=On the Relation of the Amount of Material and Weight
editor= Henry M. Leicester
journal= Mikhail Vasil'evich Lomonosov on the Corpuscular Theory
place = Cambridge
publisher=Harvard University Press
title=The Man Who Changed Everything – the Life of James Clerk Maxwell
author=Maxwell, James Clerk
title =Zur kinetischen Theorie der Brownschen Molekularbewegung und der Suspensionen
journal =Annalen der Physik
author = Waterston, John James
year = 1843
title = Thoughts on the Mental Functions (reprinted in his "Papers", 3, 167, 183.)
The Mathematical Theory of Non-uniform Gases : An Account of the Kinetic Theory of Viscosity, Thermal Conduction and Diffusion in GasesSydney Chapman, T. G. Cowling
* [http://www.math.umd.edu/~lvrmr/History/EarlyTheories.html Early Theories of Gases]
* [http://www.lightandmatter.com/html_books/0sn/ch05/ch05.html Thermodynamics] - a chapter from an online textbook
* [http://physnet.org/modules/pdfmodules/m156.pdf "Temperature and Pressure of an Ideal Gas: The Equation of State"] on [http://www.physnet.org Project PHYSNET] .
* [http://www.ucdsb.on.ca/tiss/stretton/chem1/gases9.html Introduction] to the kinetic molecular theory of gases, from The Upper Canada District School Board
* [http://comp.uark.edu/~jgeabana/mol_dyn/ Java animation] illustrating the kinetic theory from University of Arkansas
* [http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/ktcon.html Flowchart] linking together kinetic theory concepts, from HyperPhysics
* [http://www.ewellcastle.co.uk/science/pages/kinetics.html Interactive Java Applets] allowing high school students to experiment and discover how various factors affect rates of chemical reactions.
* [http://www.bustertests.co.uk/answer/molecular-kinetic-theory/ Molecular kinetic theory fundamentals]
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Look at other dictionaries:
kinetic theory — n. the theory that the minute particles of all matter are in constant motion and that the temperature of a substance is dependent on the velocity of this motion, increased motion being accompanied by increased temperature: according to the… … Universalium
kinetic theory — n. the theory that the minute particles of all matter are in constant motion and that the temperature of a substance is dependent on the velocity of this motion, increased motion being accompanied by increased temperature: according to the… … English World dictionary
kinetic theory — noun Date: 1864 either of two theories in physics based on the fact that the minute particles of a substance are in vigorous motion: a. a theory that the temperature of a substance increases with an increase in either the average kinetic energy… … New Collegiate Dictionary
kinetic theory — noun (physics) a theory that gases consist of small particles in random motion • Syn: ↑kinetic theory of gases • Topics: ↑physics, ↑natural philosophy • Hypernyms: ↑scientific theory … Useful english dictionary
kinetic theory — theory that states that all bodies are composed of many tiny particles in motion … English contemporary dictionary
kinetic theory — noun the body of theory which explains the physical properties of matter in terms of the motions of its constituent particles … English new terms dictionary
kinetic theory of gases — Physics. a theory that the particles in a gas move freely and rapidly along straight lines but often collide, resulting in variations in their velocity and direction. Pressure is interpreted as arising from the impacts of these particles with the … Universalium
kinetic theory of heat — noun a theory that the temperature of a body increases when kinetic energy increases • Hypernyms: ↑kinetic theory, ↑kinetic theory of gases * * * Physics. a theory that the temperature of a body is determined by the average kinetic energy of its… … Useful english dictionary
kinetic theory of gases — noun (physics) a theory that gases consist of small particles in random motion • Syn: ↑kinetic theory • Topics: ↑physics, ↑natural philosophy • Hypernyms: ↑scientific theory … Useful english dictionary
kinetic theory of gases — kinetinė dujų teorija statusas T sritis fizika atitikmenys: angl. gas kinetic theory; kinetic theory of gases vok. kinetische Gastheorie, f rus. кинетическая теория газов, f pranc. théorie cinétique des gaz, f … Fizikos terminų žodynas