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Kinetic theory of gases
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missing image!
 Translational motion.gif 
upright=1.4The temperature of an ideal monatomic gas is proportional to the average kinetic energy of its atoms. The size of helium atoms relative to their spacing is shown to scale under 1950 atmospheres of pressure. The atoms have a certain, average speed, slowed down here two trillion fold from room temperature.
The kinetic theory of gases describes a gas as a large number of submicroscopic particles (atoms or molecules), all of which are in constant, rapid, random motion. The randomness arises from the particles' many collisions with each other and with the walls of the container.Kinetic theory of gases explains the 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 results from particles' collisions with the walls of a container at different velocities.Kinetic molecular theory defines temperature in its own way, in contrast with the thermodynamic definition.Under an optical microscope, the molecules making up a liquid are too small to be visible. However, the jittery motion of pollen grains or dust particles in liquid are visible. Known as Brownian motion, the motion of the pollen or dust results from their collisions with the liquid's molecules. Translational motion.gif 
upright=1.4The temperature of an ideal monatomic gas is proportional to the average kinetic energy of its atoms. The size of helium atoms relative to their spacing is shown to scale under 1950 atmospheres of pressure. The atoms have a certain, average speed, slowed down here two trillion fold from room temperature.
History
In approximately 50 BCE, the Roman philosopher Lucretius proposed that apparently static macroscopic bodies were composed on a small scale of rapidly moving atoms all bouncing off each other.JOURNAL, Maxwell, J. C., 1867, On the Dynamical Theory of Gases, Philosophical Transactions of the Royal Society of London, 157, 49, 10.1098/rstl.1867.0004, This Epicurean atomistic point of view was rarely considered in the subsequent centuries, when Aristotlean ideas were dominant.missing image!
 HYDRODYNAMICA, Danielis Bernoulli.png 
upHydrodynamica front cover
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.BOOK, The Quantum Dice, L.I Ponomarev, I.V Kurchatov, 1 January 1993, CRC Press, 9780750302517, {{rp36â€“37}}Other pioneers of the kinetic theory (which were neglected by their contemporaries) were Mikhail Lomonosov (1747),Lomonosov 1758 GeorgesLouis 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 KrÃ¶nig (probably after reading a paper of Waterston) created a simple gaskinetic model, which only considered the translational motion of the particles.KrÃ¶nig 1856In 1857 Rudolf Clausius, according to his own words independently of KrÃ¶nig, developed a similar, but much more sophisticated version of the theory which included translational and contrary to KrÃ¶nig also rotational and vibrational molecular motions. In this same work he introduced the concept of mean free path of a particle.Clausius 1857 In 1859, after reading a paper on the diffusion of molecules by Rudolf Clausius, Scottish physicist James Clerk Maxwell formulated the Maxwell distribution of molecular velocities, which gave the proportion of molecules having a certain velocity in a specific range.See:  HYDRODYNAMICA, Danielis Bernoulli.png 
upHydrodynamica front cover
 Maxwell, J.C. (1860) "Illustrations of the dynamical theory of gases. Part I. On the motions and collisions of perfectly elastic spheres," Philosophical Magazine, 4th series, 19 : 19â€“32.
 Maxwell, J.C. (1860) "Illustrations of the dynamical theory of gases. Part II. On the process of diffusion of two or more kinds of moving particles among one another," Philosophical Magazine, 4th series, 20 : 21â€“37. This was the firstever statistical law in physics.BOOK, The Man Who Changed Everything â€“ the Life of James Clerk Maxwell, Mahon, Basil, Wiley, 2003, 0470861711, Hoboken, NJ, 52358254, Maxwell also gave the first mechanical argument that molecular collisions entail an equalization of temperatures and hence a tendency towards equilibrium.JOURNAL, Gyenis, Balazs, 2017, Maxwell and the normal distribution: A colored story of probability, independence, and tendency towards equilibrium, Studies in History and Philosophy of Modern Physics, 57, 53â€“65, 1702.01411, 2017SHPMP..57...53G, 10.1016/j.shpsb.2017.01.001, 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."Maxwell 1875
Assumptions
The theory for ideal gases makes the following assumptions: 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 size.
 These particles have the same mass.
 The number of molecules is so large that statistical treatment can be applied.
 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.
 Except during collisions, the interactions among molecules are negligible. (That is, they exert no forces on one another.)
This implies:
1. Relativistic effects are negligible.
2. Quantummechanical effects are negligible. This means that the interparticle distance is much larger than the thermal de Broglie wavelength and the molecules are treated as classical objects.
3. Because of the above two, their dynamics can be treated classically. This means that the equations of motion of the molecules are timereversible.
 The average kinetic energy of the gas particles depends only on the absolute temperature of the system. The kinetic theory has its own definition of temperature, not identical with the thermodynamic definition.
 The elapsed time of a collision between a molecule and the container's wall is negligible when compared to the time between successive collisions.
 There are negligible gravitational force on molecules.
Equilibrium properties
{{anchorPressure and Kinetic Energy}}Pressure and kinetic energy
In kinetic model of gases, the pressure is equal to the force exerted by the atoms hitting and rebounding from a unit area of the gas container surface. Consider a gas of N molecules, each of mass m, enclosed in a cube of volume V = L3. When a gas molecule collides with the wall of the container perpendicular to the x axis and bounces off in the opposite direction with the same speed (an elastic collision), the change in momentum is given by:
Delta p = p_{i,x}  p_{f,x} = p_{i,x}  (p_{i,x}) = 2 p_{i,x} = 2 mv_x,
where p is the momentum, i and f indicate initial and final momentum (before and after collision), x indicates that only the x direction is being considered, and v is the speed of the particle (which is the same before and after the collision).The particle impacts one specific side wall once every
Delta t = frac{2L}{v_x},
where L is the distance between opposite walls.The force due to this particle is
F = frac{Delta p}{Delta t} = frac{m v_x^2}{L}.
The total force on the wall is
F = frac{Nmoverline{v_x^2}}{L},
where the bar denotes an average over the N particles.Since the motion of the particles is random and there is no bias applied in any direction, the average squared speed in each direction is identical:
overline{v_x^2} = overline{v_y^2} = overline{v_z^2}.
By Pythagorean theorem in three dimensions the total squared speed v is given by
overline{v^2} = overline{v_x^2} + overline{v_y^2} + overline{v_z^2},
overline{v^2} = 3overline{v_x^2}.
Therefore:
overline{v_x^2} = frac{overline{v^2}}{3},
and the force can be written as:
F = frac{Nmoverline{v^2}}{3L}.
This force is exerted on an area L2. Therefore, the pressure of the gas is
P = frac{F}{L^2} = frac{Nmoverline{v^2}}{3V},
where V = L3 is the volume of the box.In terms of the kinetic energy of the gas K:
PV = frac{2}{3} times{K}.
This is a first nontrivial result of the kinetic theory because it relates pressure, a macroscopic property, to the (translational) kinetic energy of the molecules Nfrac{1}{2} moverline{v^2}, which is a microscopic property.Temperature and kinetic energy
Rewriting the above result for the pressure as PV = {Nmoverline{v^2}over 3} , we may combine it with the ideal gas law{{NumBlk:displaystyle PV = N k_B T ,{{EquationRef1}}}}where displaystyle k_B is the Boltzmann constant and displaystyle T theabsolute temperature defined by the ideal gas law, to obtain
k_B T = {moverline{v^2}over 3} ,
which leads to simplified expression of the average kinetic energy per molecule,The average kinetic energy of a fluid is proportional to the root meansquare velocity, which always exceeds the mean velocity  Kinetic Molecular Theory
displaystyle frac {1} {2} moverline{v^2} = frac {3} {2} k_B T.
The kinetic energy of the system is N times that of a molecule, namely K= frac {1} {2} N m overline{v^2} .Then the temperature displaystyle T takes the form{{NumBlk: displaystyle T = {moverline{v^2}over 3 k_B}{{EquationRef2}}}}which becomes{{NumBlk: displaystyle T = frac {2} {3} frac {K} {N k_B}.{{EquationRef3}}}}Eq.({{EquationNote3}})is one important result of the kinetictheory:The average molecular kinetic energy is proportional to the ideal gas law's absolute temperature.From Eq.({{EquationNote1}}) andEq.({{EquationNote3}}),we have{{NumBlk:
displaystyle
PV
=
frac
{2}
{3}
K.
PV
=
frac
{2}
{3}
K.
4}}}}Thus, the product of pressure andvolume per mole is proportional to the average(translational) molecular kinetic energy.Eq.({{EquationNote1}}) and Eq.({{EquationNote4}})are called the "classical results",which could also be derived from statistical mechanics;for more details, see:Configuration integral (statistical mechanics) {{webarchiveurl=https://web.archive.org/web/20120428193950weblink date=20120428 }}Since there aredisplaystyle 3Ndegrees of freedom in a monatomicgas system withdisplaystyle Nparticles,the kinetic energy per degree of freedom per molecule is{hide}NumBlk:
displaystyle
frac {K} {3 N} = frac {k_B T} {2} 
5{edih}}}In the kinetic energy per degree of freedom,the constant of proportionality of temperatureis 1/2 times Boltzmann constant or R/2 per mole. In addition to this, the temperature will decrease when the pressure drops to a certain point.{{why?date=June 2014}}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 diatomic gases actas if they have only 5. Monatomic gases have 3 degrees of freedom.Thus the kinetic energy per kelvin (monatomic ideal gas) is 3 [R/2] = 3R/2:
Collisions with containerOne 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 derivationCollisions With a Surface. results in an equation for total number of collisions per unit time per area:
N^* = frac{1}{4}frac{N}{V} v_text{avg} = frac{n}{4} sqrt{frac{8 k_{B} T}{pi m}}.
This quantity is also known as the "impingement rate" in vacuum physics.Speed of moleculesFrom the kinetic energy formula it can be shown that
v_text{p} = sqrt{2 cdot frac{k_B T}{m}},
bar v = frac {2}{sqrt{pi}} v_p = sqrt{frac {8}{pi} cdot frac{k_B T}{m}},
v_text{rms} = sqrt{frac{3}{2}} v_p = sqrt{{3} cdot frac {k_B T}{m}},
where v is in m/s, T is in kelvins, and m is the mass of one molecule of gas. The most probable (or mode) speed v_text{p} is 81.6% of the rms speed v_text{rms}, and the mean (arithmetic mean, or average) speed bar v is 92.1% of the rms speed (isotropic distribution of speeds).See:
Transport propertiesThe 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.Viscosity and kinetic momentumIn books on elementary kinetic theoryBOOK, Sears, F.W., Salinger, G.L., 1975, Thermodynamics, Kinetic Theory, and Statistical Thermodynamics, AddisonWesley Publishing Company, Inc., Reading, Massachusetts, USA, 3, 10, 286â€“291, 9780201068948, one can find results for dilute gas modeling that has widespread use. Derivation of the kinetic model for shear viscosity usually starts by considering a Couette flow where two parallel plates are separated by a gas layer. The upper plate is moving at a constant velocity to the right due to a force F. The lower plate is stationary, and an equal and opposite force must therefore be acting on it to keep it at rest. The molecules in the gas layer have a forward velocity component u which increase uniformly with distance y above the lower plate. The nonequilibrium flow is superimposed on a MaxwellBoltzmann equilibrium distribution of molecular motions.Let sigma be the collision cross section of one molecule colliding with another. The number density C is defined as the number of molecules per (extensive) volume C = N/V . The collision cross section per volume or collision cross section density is C sigma , and it is related to the mean free path l by
quad l = frac {1} {sqrt{2} C sigma}
Notice that the unit of the collision cross section per volume C sigma is reciprocal of length. The mean free path is the average distance traveled by a molecule, or a number of molecules per volume, before they make their first collision.Let u_{0} be the forward velocity of the gas at an imaginary horizontal surface inside the gas layer. On the average, a molecule that crosses the surface makes its last collision before crossing at a distance equal to twothirds of the mean free path (i.e. 2l/3 ) away from the surface. At this distance above and below the surface, the forward momentum of the molecule is respectively
quad p_{x}^{pm} = m left( u_{0} pm frac {2}{3} l {d u over dy} right)
where m is the molecular mass. The molecular flux Phi includes all molecules arriving at one side of an element of the surface within the gas layer. The incoming molecules are coming from all directions at the one side of the surface and with all speeds. This molecular flux (i.e. the number flux) is related to the average molecular speed bar v by
quad Phi = frac {1}{4} bar v C
Notice that the forward velocity gradient du/dy can be considered to be constant over a distance of mean free path. Next we multiply by the total flux to get the change of momentum per unit time and per unit area, that is carried by the molecules crossing from either above or below the surface area. This gives the equation
quad Phi p_{x}^{pm} = frac {1}{4} bar v C cdot m left( u_{0} pm frac {2}{3} l {d u over dy} right)
The net rate of momentum per unit area that is transported across the imaginary surface is thus
quad tau = Phi p_{x}^{+}  Phi p_{x}^{} = frac {1}{3} bar v C m cdot l {d u over dy}
The defining equation for the (shear) viscosity eta of the gas is
quad frac {F} {A} = eta {d u over dy}
Combining the above kinetic equation with defining equation for (shear) viscosity by tau = F/A gives the equation for shear viscosity, which is usually denoted eta_{0} when it is a dilute gas:
quad eta_{0} = frac {1} {3} bar v C m l
Combining this equation with the equation for mean free path gives
quad eta_{0} = frac {1} {3 sqrt{2} } frac {m cdot bar v} {sigma}
From statistical thermodynamics for gases we have equations relating average molecular speed to most likely speed and further to temperature. These statistical results gives the average (equilibrium) molecular speed as
quad bar v = frac{2}{sqrt{pi}} v_{p} = 2 sqrt{frac{2}{pi} cdot frac {k_{B}T}{m_{}}}
where v_{p} is the most probable speed, k_{B} is the Boltzmann constant. We note that
quad k_{B} cdot N_{A} = R quad text{and} quad M = m cdot N_{A}
and insert the velocity in the viscosity equation above. This gives the well known equation for shear viscosity for dilute gases:
quad eta_{0} = frac {2} {3 sqrt{pi} } cdot frac {sqrt{m k_{B} T}} { sigma }
frac {2} {3 sqrt{pi} } cdot frac {sqrt{MRT}} { sigma cdot N_{A} }and M is the molar mass. The equation above presupposes that the gas density is low (i.e. the pressure is low). This implies that the kinetic translational energy dominates over rotational and vibrational molecule energies. The viscosity equation further presupposes that there is only one type of gas molecules, and that the gas molecules are perfect elastic and hard core particles of spherical shape. This assumption of elastic, hard core spherical molecules, like billiard balls, implies that the collision cross section of one molecule can be estimated by
quad sigma = pi left( 2 r right)^2 = pi d^2
The radius r is called collision cross section radius or kinetic radius, and the diameter d is called collision cross section diameter or kinetic diameter of a molecule in a monomolecular gas. There are no simple general relation between the collision cross section and the hard core size of the (fairly spherical) molecule. The relation depends on shape of the potential energy of the molecule. For a real spherical molecule (i.e. a noble gas atom or a reasonably spherical molecule) the interaction potential is more like the LennardJones potential or Morse potential which have a negative part that attracts the other molecule from distances longer than the hard core radius. The radius for zero LennardJones potential is then appropriate to use as estimate for the kinetic radius.Local nomenclature list:{{div colcolwidth=30em}}
See also{{Statistical mechanics}}
Notes{{Reflist2}}References
 bibcode = 1857AnP...176..353C issue=3 }}
 bibcode = 1905AnP...322..549E issue=8 }}
 bibcode = 1856AnP...175..315K issue=10 }}
 bibcode = 1906AnP...326..756V issue=14 }}
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