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| {{About|kinetic theory of gases|branch of classical mechanics|Kinematics}}
| | Oscar is how he's called and he totally enjoys this title. My day occupation is a meter reader. What I love performing is doing ceramics but I haven't produced a dime with it. North Dakota is our beginning location.<br><br>My web blog - std testing at home ([http://guestgame.com/members/olenv66cisbfj/activity/624169/ click the next web site]) |
| [[Image:Translational motion.gif|thumb|right|300px|The [[temperature]] of an ideal [[monatomic]] [[gas]] is a measure of the average [[kinetic energy]] of its atoms. The [[Bohr radius|size]] of [[helium]] atoms relative to their spacing is shown to scale under 1950 [[Atmosphere (unit)|atmospheres]] of pressure. The atoms have a certain, average speed, slowed down here two [[1000000000000 (number)|trillion]] fold from room temperature.]]
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| :''This article applies to gases; see also [[Kinetic theory of solids]]''
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| The '''kinetic theory''' of gases describes a gas as a large number of small particles ([[atom]]s or [[molecule]]s), all of which are in constant, [[randomness|random]] [[motion (physics)|motion]]. The rapidly moving particles constantly collide with each other and with the walls of the container. Kinetic theory explains [[macroscopic]] properties of gases, such as pressure, temperature, viscosity, thermal conductivity, and volume, by considering their molecular composition and motion. The theory posits that gas pressure is due to the impacts, on the walls of a container, of molecules or atoms moving at different velocities.
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| While the particles making up a gas are too small to be visible, the jittering motion of pollen grains or dust particles which can be seen under a microscope, known as [[Brownian motion]], results directly from collisions between the particles and gas molecules. As pointed out by [[Albert Einstein]] in 1905, this experimental evidence for kinetic theory is generally seen as having confirmed the existence of atoms and molecules.
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| ==Assumptions==
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| The theory for ideal gases makes the following assumptions:
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| * The gas consists of very small particles known as molecules. This smallness of their size is such that the total [[volume]] of the individual gas molecules added up is negligible compared to the volume of the smallest open ball containing all the molecules. This is equivalent to stating that the average distance separating the gas particles is large compared to their [[atomic radius|size]].
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| * These particles have the same [[mass]].
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| * The number of molecules is so large that statistical treatment can be applied.
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| * These molecules are in constant, [[randomness|random]], and rapid motion.
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| * The rapidly moving particles constantly collide among themselves and with the walls of the container. All these collisions are perfectly elastic. This means, the molecules are considered to be perfectly spherical in shape, and elastic in nature.
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| * Except during collisions, the [[interaction]]s among molecules are [[negligible]]. (That is, they exert no [[force]]s on one another.)
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| :This implies:
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| ::1. [[Special relativity|Relativistic]] effects are negligible.
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| ::2. [[Quantum mechanics|Quantum-mechanical]] effects are negligible. This means that the [[mean inter-particle distance| inter-particle distance]] is much larger than the [[thermal de Broglie wavelength]] and the molecules are treated as [[classical mechanics|classical]] [[physical body|objects]].
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| ::3. Because of the above two, their dynamics can be treated classically. This means, the equations of motion of the molecules are time-reversible.
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| * The average [[kinetic energy]] of the gas particles depends only on the [[thermodynamic temperature| absolute temperature]] of the [[system]].
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| * The time during collision of molecule with the container's wall is negligible as compared to the time between successive collisions.
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| * Because they have mass, the gas molecules will be affected by gravity.
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| 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 chaos]] and small gradients in bulk properties. Expansions to higher orders in the density are known as [[virial expansion]]s. An important book on kinetic theory is that by Chapman and Cowling.<ref>Chapman, S., Cowling, T.G. (1939/1970).</ref> An important approach to the subject is called Chapman–Enskog theory.<ref>Kauzmann, W. (1966). ''Kinetic Theory of Gases'', W.A. Benjamin, New York, pp. 232–235.</ref> There have been many modern developments and there is an alternative approach developed by Grad based on moment expansions.<ref>Grad 1949</ref>
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| 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]].
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| ==Properties==
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| === {{anchor|Pressure and Kinetic Energy}}Pressure and kinetic energy===<!-- This section is linked from [[Pressure]] -->
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| [[Pressure]] is explained by kinetic theory as arising from the force exerted by molecules or atoms impacting on the walls of a container. Consider a gas of ''N'' molecules, each of mass ''m'', enclosed in a cuboidal container of volume ''V''=''L''<sup>3</sup>. 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 [[momentum]] lost by the particle and gained by the wall is:
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| :<math>\Delta p = p_{i,x} - p_{f,x} = p_{i,x} - (-p_{i,x}) = 2 p_{i,x} = 2 m v_x\,</math>
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| where ''v<sub>x</sub>'' is the ''x''-component of the initial velocity of the particle.
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| The particle impacts one specific side wall once every
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| :<math>\Delta t = \frac{2L}{v_x}</math>
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| (where ''L'' is the distance between opposite walls).
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| The [[force]] due to this particle is:
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| :<math>F = \frac{\Delta p}{\Delta t} = \frac{m v_x^2}{L}.</math>
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| The total force on the wall is
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| :<math>F = \frac{Nm\overline{v_x^2}}{L}</math>
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| where the bar denotes an average over the ''N'' particles.
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| Since the assumption is that the particles move in random directions, we will have to conclude that if we divide the velocity vectors of all particles in three mutually perpendicular directions, the average value along each direction must be equal. (This does not mean that each particle always travel in 45 degrees to the coordinate axes.)
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| <math> \overline{v_x^2} = \overline{v^2}/3 </math>.
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| We can rewrite the force as
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| :<math>F = \frac{Nm\overline{v^2}}{3L}.</math>
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| This force is exerted on an area ''L''<sup>2</sup>. Therefore the pressure of the gas is
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| :<math>P = \frac{F}{L^2} = \frac{Nm\overline{v^2}}{3V}</math>
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| where ''V''=''L''<sup>3</sup> is the volume of the box.
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| The ratio ''n''=''N''/''V'' is the [[number density]] of the gas (the mass density ''ρ''=''nm'' is less convenient for theoretical derivations on atomic level). Using ''n'', we can rewrite the pressure as
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| :<math> P = \frac{n m \overline{v^2}}{3}.</math>
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| This is a first non-trivial result of the kinetic theory because it relates pressure, a [[macroscopic]] property, to the average (translational) [[kinetic energy]] per molecule <math>{1 \over 2} m\overline{v^2}</math> which is a [[microscopic]] property.
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| ===Temperature and kinetic energy===
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| From the [[ideal gas law]]
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| {{NumBlk|:|<math>\displaystyle PV = N k_B T ,</math>|{{EquationRef|1}}}}
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| where <math>\displaystyle k_B</math> is the [[Boltzmann constant]] and <math>\displaystyle T</math> the
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| [[Thermodynamic temperature|absolute]] [[temperature]],
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| and from the result <math>P = {Nm\overline{v^2}\over 3V} </math>, we have <math>PV = {Nm\overline{v^2} \over 3} </math>
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| and, thus, <math>k_B T = {m\overline{v^2}\over 3} ;</math>
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| then the temperature <math>\displaystyle T</math> takes the form
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| {{NumBlk|:|<math> \displaystyle T = {m\overline{v^2}\over 3 k_B}</math>|{{EquationRef|2}}}}
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| which leads to the expression of the kinetic energy of a molecule
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| :<math> \displaystyle \frac {1} {2} m\overline{v^2} = \frac {3} {2} k_B T.</math>
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| The kinetic energy of the system is N times that of a molecule <math> K= \frac {1} {2} N m \overline{v^2} </math>
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| The temperature becomes
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| {{NumBlk|:|<math> \displaystyle T = \frac {2} {3} \frac {K} {N k_B}.</math>|{{EquationRef|3}}}}
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| Eq.({{EquationNote|3}})
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| is one important result of the kinetic
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| theory:
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| ''The average molecular kinetic energy is proportional to
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| the absolute temperature''.
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| From Eq.({{EquationNote|1}}) and
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| Eq.({{EquationNote|3}}),
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| we have
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| {{NumBlk|:|<math>
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| \displaystyle
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| PV
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| =
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| \frac
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| {2}
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| {3}
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| K.
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| </math>|{{EquationRef|4}}}}
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| Thus, the product of pressure and
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| volume per [[Mole (unit)|mole]] is proportional to the average
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| (translational) molecular kinetic energy.
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| Eq.({{EquationNote|1}}) and Eq.({{EquationNote|4}})
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| are called the "classical results",
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| which could also be derived from [[statistical mechanics]];
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| for more details, see
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| .<ref>
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| [http://clesm.mae.ufl.edu/wiki.pub/index.php/Configuration_integral_%28statistical_mechanics%29 Configuration integral (statistical mechanics)]
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| </ref>
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| Since there are
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| <math>\displaystyle 3N</math>
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| [[Degrees of freedom (physics and chemistry)|degrees of freedom]] in a monatomic-gas system with
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| <math>\displaystyle N</math>
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| particles,
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| the kinetic energy per degree of freedom per molecule is
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| {{NumBlk|:|<math>
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| \displaystyle
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| \frac
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| {K}
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| {3 N}
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| =
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| \frac
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| {k_B T}
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| {2}
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| </math>
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| |{{EquationRef|5}}}}
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| In the kinetic energy per degree of freedom,
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| the constant of proportionality of temperature
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| is 1/2 times
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| [[Boltzmann constant]]. In addition to this, the temperature will decrease when the pressure drops to a certain point.
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| This result is related
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| to the [[equipartition theorem]].
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| As noted in the article on [[heat capacity]], diatomic
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| gases should have 7 degrees of freedom, but the lighter gases act
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| as if they have only 5.
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| Thus the kinetic energy per kelvin (monatomic [[ideal gas]]) is:
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| * per mole: 12.47 J
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| * per molecule: 20.7 yJ = 129 μeV.
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| At [[Standard conditions for temperature and pressure|standard temperature]] (273.15 K), we get:
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| * per mole: 3406 J
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| * per molecule: 5.65 zJ = 35.2 meV.....
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| ===Collisions with container===
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| One can calculate the number of atomic or molecular collisions with a wall of a container per unit area per unit time.
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| Assuming an ideal gas, a derivation<ref>[http://www.chem.arizona.edu/~salzmanr/480a/480ants/collsurf/collsurf.html Collisions With a Surface<!-- Bot generated title -->]</ref> results in an equation for total number of collisions per unit time per area:
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| ::<math>A = \frac{1}{4}\frac{N}{V} v_{avg} = \frac{n}{4} \sqrt{\frac{8 k_{B} T}{\pi m}} . \,</math>
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| This quantity is also known as the "impingement rate" in vacuum physics.
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| ===Speed of molecules===
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| From the kinetic energy formula it can be shown that
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| :<math>v_\mathrm{rms} = \sqrt {{3 k_{B} T}\over{m}}</math>
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| with ''v'' in m/s, ''T'' in kelvins, and ''m'' is the molecule mass (kg). The most probable speed is 81.6% of the rms speed, and the mean speeds 92.1% ([[isotropy|isotropic]] [[Maxwell-Boltzmann distribution#Distribution of speeds|distribution of speeds]]).
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| See: [[Root-mean-square speed]]
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| ===Transport properties===
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| The kinetic theory of gases deals not only with gases in thermodynamic equilibrium, but also very importantly with gases not in thermodynamic equilibrium. This means considering what are known as 'transport properties', such a viscosity and thermal conductivity.
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| ==History==
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| In approximately 50 [[before common era|BCE]], the Roman philosopher [[Lucretius]] proposed that that apparently static macroscopic bodies were composed on a small scale of rapidly moving atoms all bouncing off each other.<ref>{{cite doi|10.1098/rstl.1867.0004}}</ref> This [[Epicureanism|Epicurean]] atomistic point of view was rarely considered in the subsequent centuries, when [[Aristotle]]an ideas were dominant.
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| [[Image:HYDRODYNAMICA, Danielis Bernoulli.png|thumb|upright|Hydrodynamica front cover]]
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| In 1738 [[Daniel Bernoulli]] published ''[[Hydrodynamica]]'', which laid the basis for the kinetic theory of gases. In this work, Bernoulli posited 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 [[heat]] is simply the [[kinetic energy]] of their motion. The theory was not immediately accepted, in part because [[conservation of energy]] had not yet been established, and it was not obvious to physicists how the collisions between molecules could be perfectly elastic.
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| Other pioneers of the kinetic theory (which were neglected by their contemporaries) were [[Mikhail Lomonosov]] (1747),<ref>Lomonosov 1758</ref> [[Georges-Louis Le Sage]] (ca. 1780, published 1818),<ref>Le Sage 1780/1818</ref> [[John Herapath]] (1816)<ref>Herapath 1816, 1821</ref> and [[John James Waterston]] (1843),<ref>Waterston 1843</ref> 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.<ref>Krönig 1856</ref>
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| In 1857 [[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 path]] of a particle.
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| <ref>Clausius 1857</ref>
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| In 1859, after reading a paper by Clausius, [[James Clerk Maxwell]] formulated the [[Maxwell distribution]] of 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.<ref>Mahon 2003</ref> 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 [[pressure]] of air and other gases.”<ref>Maxwell 1875</ref>
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| In 1871, [[Ludwig Boltzmann]] generalized Maxwell's achievement and formulated the [[Maxwell–Boltzmann distribution]]. Also the [[logarithm]]ic connection between [[entropy]] and [[probability]] was first stated by him.
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| In the beginning of the 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)<ref>Einstein 1905</ref> and [[Marian Smoluchowski]]'s (1906)<ref>Smoluchowski 1906</ref>
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| papers on [[Brownian motion]], which succeeded in making certain accurate quantitative predictions based on the kinetic theory.
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| ==See also==
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| {{Statistical mechanics}}
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| * [[BBGKY hierarchy|Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy of equations]]
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| * [[Boltzmann equation]]
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| * [[Collision theory]]
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| * [[Critical temperature]]
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| * [[Gas laws]]
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| * [[Heat]]
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| * [[Maxwell-Boltzmann distribution]]
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| * [[Mixmaster dynamics]]
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| * [[Thermodynamics]]
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| * [[Vlasov equation]]
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| ==References==
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| * {{Citation
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| | author=Clausius, R.
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| | title =Ueber die Art der Bewegung, welche wir Wärme nennen
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| | journal =Annalen der Physik
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| | volume =176
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| | pages =353–379
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| | year =1857
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| |url=http://gallica.bnf.fr/ark:/12148/bpt6k15185v/f371.table
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| |doi=10.1002/andp.18571760302|bibcode = 1857AnP...176..353C
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| | issue=3 }}
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| * de Groot, S. R., W. A. van Leeuwen and Ch. G. van Weert (1980), Relativistic Kinetic Theory, North-Holland, Amsterdam.
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| * {{Citation
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| | author=Einstein, A.
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| * {{Citation
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| * {{Citation
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| * {{Citation
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| | chapter-url=http://resolver.sub.uni-goettingen.de/purl?PPN521099943}}
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| * Liboff, R. L. (1990), Kinetic Theory, Prentice-Hall, Englewood Cliffs, N. J.
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| * {{Citation
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| | year= 1758/1970
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| |chapterurl=http://www.archive.org/details/mikhailvasilevic017733mbp}}
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| * {{Citation|author=Mahon, Basil
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| * Williams, M. M. R. (1971), Mathematical Methods in Particle Transport Theory, Butterworths, London.
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|
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| ==Endnotes==
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| {{Reflist|2}}
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| ==Further reading==
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| *Sydney Chapman and T.G. Cowling (1939/1970). ''The Mathematical Theory of Non-uniform Gases: An Account of the Kinetic Theory of Viscosity, Thermal Conduction and Diffusion in Gases'', (first edition 1939, second edition 1952), third edition 1970 prepared in co-operation with D. Burnett, Cambridge University Press, London.
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| *J.O. Hirschfelder, C.F. Curtiss, and R.B. Bird (1964). ''Molecular Theory of Gases and Liquids'', second edition (Wiley).
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| *R.L. Liboff (2003). ''Kinetic Theory: Classical, Quantum, and Relativistic Descriptions'', third edition (Springer).
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| == External links ==
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| * [http://www.math.umd.edu/~lvrmr/History/EarlyTheories.html Early Theories of Gases]
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| * [http://www.lightandmatter.com/html_books/0sn/ch05/ch05.html Thermodynamics] - a chapter from an online textbook
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| * [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].
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| * [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
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| * [http://comp.uark.edu/~jgeabana/mol_dyn/ Java animation] illustrating the kinetic theory from University of Arkansas
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| * [http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/ktcon.html Flowchart] linking together kinetic theory concepts, from HyperPhysics
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| * [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.
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| {{DEFAULTSORT:Kinetic Theory}}
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| [[Category:Gases]]
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| [[Category:Thermodynamics]]
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| [[Category:Concepts in physics]]
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