- This page is about the physical properties of gas as a state of matter. For the uses of gases, and other meanings, see Gas (disambiguation).
A gas is a state of matter, consisting of a collection of particles (molecules, atoms, ions, electrons, etc. History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J ) without a definite shape or volume that are in more or less random motion.
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Physical characteristics
Due to the electronic nature of the aforementioned particles, a "force field" is present throughout the space around them. Originally a term coined by Michael Faraday to provide an intuitive paradigm but theoretical construct (in the Kuhnian sense for the behavior of electromagnetic fields Interactions between these "force fields" from one particle to the next give rise to the term intermolecular forces. In Physics, Chemistry, and Biology, intermolecular forces are forces that act between stable Molecules or between functional groups of Dependent on distance, these intermolecular forces influence the motion of these particles and hence their thermodynamic properties. Here is a partial list of thermodynamic properties of Fluids T Temperature *\rho Density It must be noted that at the temperatures and pressures characteristic of many applications, these particles are normally greatly separated. This separation corresponds to a very weak attractive force. As a result, for many applications, this intermolecular force becomes negligible.
A gas also exhibits the following characteristics:
- Relatively low density and viscosity compared to the solid and liquid states of matter. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different Viscosity is a measure of the resistance of a Fluid which is being deformed by either Shear stress or Extensional stress. A solid' object is in the States of matter characterized by resistance to Deformation and changes of Volume. Liquid is one of the principal States of matter. A liquid is a Fluid that has the particles loose and can freely form a distinct surface at the boundaries of
- Will expand and contract greatly with changes in temperature or pressure, thus the term "compressible". Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface
- Will diffuse readily, spreading apart in order to homogeneously distribute itself throughout any container.
Macroscopic
When analyzing a system, it is typical to specify a length scale. In Physics, length scale is a particular Length or Distance determined with the precision of one order (or a few orders of magnitude A larger length scale may correspond to a macroscopic view of the system, while a smaller length scale corresponds to a microscopic view. Macroscopic is commonly used to describe physical objects that are measurable and observable by the Naked eye. Microscopic is a term used to describe objects smaller than those that can easily be seen by the naked Eye and which require a lens or Microscope to see
On a macroscopic scale, the quantities measured are in terms of the large scale effects that a gas has on a system or its surroundings such as its velocity, pressure, or temperature. Mathematical equations, such as the Extended hydrodynamic equations, Navier-Stokes equations and the Euler equations have been developed to attempt to model the relations of the pressure, density, temperature, and velocity of a moving gas. The Navier–Stokes equations, named after Claude-Louis Navier and George Gabriel Stokes, describe the motion of viscous Fluid substances such
Pressure
The pressure exerted by a gas uniformly across the surface of a container can be described by simple kinetic theory. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface Kinetic theory (or kinetic theory of gases) attempts to explain Macroscopic properties of Gases such as pressure temperature or volume by considering The particles of a gas are constantly moving in random directions and frequently collide with the walls of the container and/or each other. These particles all exhibit the physical properties of mass, momentum, and energy, which all must be conserved. A physical property is any aspect of an object or substance that can be measured or perceived without changing its identity. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object In Classical mechanics, momentum ( pl momenta SI unit kg · m/s, or equivalently N · s) is the product In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός In Physics, a conservation law states that a particular measurable property of an isolated Physical system does not change as the system evolves In classical mechanics, Momentum, by definition, is the product of mass and velocity. Classical mechanics is used for describing the motion of Macroscopic objects from Projectiles to parts of Machinery, as well as Astronomical objects Kinetic energy is one half the mass multiplied by the square of the velocity. The kinetic energy of an object is the extra Energy which it possesses due to its motion
The sum of all the normal components of force exerted by the particles impacting the walls of the container divided by the area of the wall is defined to be the pressure. In Mathematics, given a vector at a point on a Surface, that vector can be decomposed uniquely as a sum of two vectors one Tangent to the surface called The pressure can then be said to be the average linear momentum of these moving particles. In Classical mechanics, momentum ( pl momenta SI unit kg · m/s, or equivalently N · s) is the product A common misconception is that the collisions of the molecules with each other is essential to explain gas pressure, but in fact their random velocities are sufficient to define this quantity.
Temperature
The temperature of any physical system is the result of the motions of the molecules and atoms which make up the system. Thermodynamic temperature is the absolute measure of Temperature and is one of the principal parameters of Thermodynamics. In Physics the word system has a technical meaning namely it is the portion of the physical Universe chosen for analysis In statistical mechanics, temperature is the measure of the average kinetic energy stored in a particle. Statistical mechanics is the application of Probability theory, which includes mathematical tools for dealing with large populations to the field of Mechanics The methods of storing this energy are dictated by the degrees of freedom of the particle itself (energy modes). For information on degrees of freedom in other sciences see Degrees of freedom. A quantum mechanical system or particle that is bound, confined spacially can only take on certain discrete values of energy as opposed to classical particles which These particles have a range of different velocities, and the velocity of any single particle constantly changes due to collisions with other particles. The range in speed is usually described by the Maxwell-Boltzmann distribution. The Maxwell–Boltzmann distribution is a Probability distribution with applications in Physics and Chemistry.
Specific Volume
When performing a thermodynamic analysis, it is typical to speak of intensive and extensive properties. Specific volume (v is the volume occupied by a unit of mass of a material In the Physical sciences an intensive property (also called a bulk property) is a Physical property of a system that does not depend on the Properties which depend on the amount of gas are called extensive properties, while properties that do not depend on the amount of gas are called intensive properties. Specific volume is an example of an intensive property because it is the volume occupied by a unit of mass of a material, meaning we have divided through by the mass in order to obtain a quantity in terms of, for example,
. Notice that the difference between volume and specific volume differ in that the specific quantity is mass independent.
Density
Because the molecules are free to move about in a gas, the mass of the gas is normally characterized by its density. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different Density is the mass per volume of a substance or simply, the inverse of specific volume. For gases, the density can vary over a wide range because the molecules are free to move. Macroscopically, density is a state variable of a gas and the change in density during any process is governed by the laws of thermodynamics. Given that there are many particles in completely random motion, for a static gas, the density is the same throughout the entire container. Fluid statics (also called hydrostatics) is the Science of Fluids at rest and is a sub-field within Fluid mechanics. Density is therefore a scalar quantity; it is a simple physical quantity that has a magnitude but no direction associated with it. In Physics, a scalar is a simple Physical quantity that is not changed by Coordinate system rotations or translations (in Newtonian mechanics or It can be shown by kinetic theory that the density is proportional to the size of the container in which a fixed mass of gas is confined.
Microscopic
On the microscopic scale, the quantities measured are at the molecular level. Microscopic is a term used to describe objects smaller than those that can easily be seen by the naked Eye and which require a lens or Microscope to see Different theories and mathematical models have been created to describe molecular or particle motion. A few of the gas-related models are listed below.
Kinetic theory
Kinetic theory attempts to explain macroscopic properties of gases by considering their molecular composition and motion. Kinetic theory (or kinetic theory of gases) attempts to explain Macroscopic properties of Gases such as pressure temperature or volume by considering
Brownian motion
Brownian motion is the mathematical model used to describe the random movement of particles suspended in a fluid often called particle theory. This article is about the physical phenomenon for the stochastic process see Wiener process. Particle physics is a branch of Physics that studies the elementary constituents of Matter and Radiation, and the interactions between them
Since it is at the limit of (or beyond) current technology to observe individual gas particles (atoms or molecules), only theoretical calculations give suggestions as to how they move, but their motion is different from Brownian Motion. The reason is that Brownian Motion involves a smooth drag due to the frictional force of many gas molecules, punctuated by violent collisions of an individual (or several) gas molecule(s) with the particle. The particle (generally consisting of millions or billions of atoms) thus moves in a jagged course, yet not so jagged as we would expect to find if we could examine an individual gas molecule.
Intermolecular forces
- See also: Intermolecular force
As discussed earlier, momentary attractions (or repulsions) between particles have an effect on gas dynamics. The Van der Waals equation is an Equation of state that can be derived from a special form of the potential between a pair of molecules (hard-sphere repulsion In Physics, Chemistry, and Biology, intermolecular forces are forces that act between stable Molecules or between functional groups of Gas dynamics is a branch of dynamics concerned with studying the motion of gases In physical chemistry, the name given to these "intermolecular forces" is the "Van der Waals force". Physical chemistry, is the application of Physics to macroscopic microscopic atomic subatomic and particulate phenomena in chemical systems It is mostly defined as a large
Simplified models
An equation of state (for gases) is a mathematical model used to roughly describe or predict the state of a gas. In Physics and Thermodynamics, an equation of state is a relation between state variables More specifically an equation of state is a thermodynamic At present, there is no single equation of state that accurately predicts the properties of all gases under all conditions. Therefore, a number of much more accurate equations of state have been developed for gases under a given set of assumptions. The "gas models" that are most widely discussed are "Real Gas", "Ideal Gas" and "Perfect Gas". Each of these models have their own set of assumptions to, basically, make our lives easier when we want to analyze a given thermodynamic system.
Real gas
Real gas effects refers to an assumption base where the following are taken into account:
- Compressibility effects
- Variable heat capacity
- Van der Waal forces
- Non-equilibrium thermodynamic effects
- Issues with molecular dissociation and elementary reactions with variable composition. Real gas effects refers to an assumption base where the following are taken into account Compressibility effects Variable heat capacity Van der The compressibility factor ( Z) is a useful thermodynamic property for modifying the Ideal gas law to account for the Real gas behaviour Specific heat capacity, also known simply as specific heat, is the measure of the heat energy required to increase the Temperature of a unit quantity Non-equilibrium thermodynamics is a branch of Thermodynamics concerned with studying Time -dependent Thermodynamic systems irreversible transformations Dissociation in Chemistry and Biochemistry is a general process in which ionic compounds ( complexes, Molecules, or Salts) separate An elementary reaction is a Chemical reaction in which one or more of the Chemical species which react directly to form products in a single Reaction step
For most applications, such a detailed analysis is excessive. An example where "Real Gas effects" would have a significant impact would be on the Space Shuttle re-entry where extremely high temperatures and pressures are present. NASA 's Space Shuttle, officially called the Space Transportation System ( STS) is the Spacecraft currently used by the United States
Ideal gas
An "ideal gas" is a simplified "real gas" with the assumption that the compressibility factor Z is set to 1. The compressibility factor ( Z) is a useful thermodynamic property for modifying the Ideal gas law to account for the Real gas behaviour So the state variables follow the ideal gas law. The ideal gas law is the Equation of state of a hypothetical Ideal gas, first stated by Benoît Paul Émile Clapeyron in 1834
This approximation is more suitable for applications in engineering although simpler models can be used to produce a "ball-park" range as to where the real solution should lie. An example where the "ideal gas approximation" would be suitable would be inside a combustion chamber of a jet engine. specific --->A jet engine is a Reaction engine that discharges a fast moving jet of Fluid to specific --->A jet engine is a Reaction engine that discharges a fast moving jet of Fluid to It may also be useful to keep the elementary reactions and chemical dissociations for calculating emissions. Exhaust gas is Flue gas which occurs as a result of the Combustion of fuels such as Natural gas, Gasoline /petrol Diesel, Fuel
Perfect gas
By definition, A perfect gas is one in which intermolecular forces are neglected. Gas while Ideal gas and this article are constructed -->By definition a perfect gas is one in which intermolecular forces are neglected So, along with the assumptions of an Ideal Gas, the following assumptions are added:
- Neglected intermolecular forces
By neglecting these forces, the equation of state for a perfect gas can be simply derived from kinetic theory or statistical mechanics.
This type of assumption is useful for making calculations very simple and easy to do. With this assumption we can apply the Ideal gas law without restriction and neglect many complications that may arise from the Van der Waals forces.
Along with the definition of a perfect gas, there are also two more simplifications that can be made although various textbooks either omit or combine the following simplifications into a general "perfect gas" definition. For sake of clarity, these simplifications are defined separately.
Thermally perfect
- The gas is in Thermodynamic equilibrium
- Not chemically reacting
- Internal energy, Enthalpy, and Specific Heat are functions of Temperature only. Gas while Ideal gas and this article are constructed -->By definition a perfect gas is one in which intermolecular forces are neglected In Thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium Mechanical equilibrium, and In Thermodynamics, the internal energy of a Thermodynamic system, or a body with well-defined boundaries, denoted by U, or sometimes In Thermodynamics and molecular chemistry, the enthalpy (denoted as H, h, or rarely as χ) is a quotient or description of Specific heat capacity, also known simply as specific heat, is the measure of the heat energy required to increase the Temperature of a unit quantity
e = e(T) h = h(T) de = CvdT dh = CpdT
This type of approximation is useful for modeling, for example, an axial compressor where temperature fluctuations are usually not large enough to cause any significant deviations from the Thermally perfect gas model. Axial compressors are rotating aerofoil based compressors in which the working fluid principally flows parallel to the axis of rotation Heat capacity is still allowed to vary, though only with temperature and molecules are not permitted to dissociate.
Calorically perfect
Finally, the most restricted gas model is one where all the above assumptions apply and we also apply:
- Constant Specific Heats
e = CvT h = CpT
Although this may be the most restrictive model, it still may be accurate enough to make reasonable calculations. Gas while Ideal gas and this article are constructed -->By definition a perfect gas is one in which intermolecular forces are neglected For example, if a model of one compression stage of the axial compressor mentioned in the previous example was made (one with variable Cp, and one with constant Cp) to compare the two simplifications, the deviation may be found at a small enough order of magnitude that other factors that come into play in this compression would have a greater impact on the final result than whether or not Cp was held constant. (compressor tip-clearance, boundary layer/frictional losses, manufacturing impurities, etc. )
Historical Synthesis
Boyle's Law was perhaps the first expression of an equation of state. Boyle's law (sometimes referred to as the Boyle-Mariotte law) is one of several Gas laws and a special case of the Ideal gas law. In 1662 Robert Boyle, an Irishman, performed a series of experiments employing a J-shaped glass tube, which was sealed on one end. Mercury was added to the tube, trapping a fixed quantity of air in the short, sealed end of the tube. Then the volume of gas was carefully measured as additional mercury was added to the tube. The pressure of the gas could be determined by the difference between the mercury level in the short end of the tube and that in the long, open end. Through these experiments, Boyle noted that the gas volume varied inversely with the pressure. In mathematical form, this can be stated as: pV = constant.
This law is used widely to describe different thermodynamic processes by adjusting the equation to read pVn = constant and then varying the n through different values such as the specific heat ratio, γ. A thermodynamic process may be defined as the energetic evolution of a Thermodynamic system proceeding from an initial state to a final state Ideal gas relations For an ideal gas the heat capacity is constant with temperature
In 1787 the French physicist Jacques Charles found that oxygen, nitrogen, hydrogen, carbon dioxide, and air expand to the same extent over the same 80 kelvin interval. In Thermodynamics and Physical chemistry, Charles's law is a gas law and specific instance of the Ideal gas law, which states that Jacques Alexandre César Charles ( November 12, 1746 – April 7, 1823) was a French inventor scientist mathematician and balloonist
In 1802, Joseph Louis Gay-Lussac published results of similar experiments, indicating a linear relationship between volume and temperature: V1 / T1 = V2 / T2
In 1801 John Dalton published the Law of Partial Pressures: The pressure of a mixture of gases is equal to the sum of the pressures of all of the constituent gases alone. The expression Gay-Lussac's law is used for each of the two relationships named after the French chemist Joseph Louis Gay-Lussac and which concern the properties of Gases Joseph Louis Gay-Lussac (also Louis Joseph Gay-Lussac, December 6, 1778 – May 9, 1850) was a French chemist In Chemistry and Physics, Dalton's law (also called Dalton's law of partial pressures) states that the total Pressure exerted by a John Dalton FRS (6 September 1766 &ndash 27 July 1844 was an English Chemist, Meteorologist and Physicist. Mathematically, this can be represented for n species as: Pressuretotal = Pressure1 + Pressure2 + . . . + Pressuren
Special Topics
Compressibility
The compressibility factor (Z) is used to alter the ideal gas equation to account for the real gas behavior. The compressibility factor ( Z) is a useful thermodynamic property for modifying the Ideal gas law to account for the Real gas behaviour It is sometimes referred to as a "fudge-factor" to make the ideal gas law more accurate for the application. Usually this Z value is very close to unity.
Reynolds Number
In fluid mechanics, the Reynolds number is the ratio of inertial forces (vsρ) to viscous forces (μ/L). In Fluid mechanics and Heat transfer, the Reynolds number \mathrm{Re} is a Dimensionless number that gives a measure of the Ratio It is one of the most important dimensionless numbers in fluid dynamics and is used, usually along with other dimensionless numbers, to provide a criterion for determining dynamic similitude.
Viscosity
As we saw earlier: Pressure acts perpendicular (normal) to the wall. Viscosity is a measure of the resistance of a Fluid which is being deformed by either Shear stress or Extensional stress. The tangential (shear) component of the force that is left over is related to the viscosity of the gas. As an object moves through a gas, viscous effects become more prevalent.
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic, stochastic property changes. In Fluid dynamics, turbulence or turbulent flow is a fluid regime characterized by chaotic Stochastic property changes This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time.
Boundary Layer
Particles will, in effect, "stick" to the surface of an object moving through it. In Physics and Fluid mechanics, a boundary layer is that layer of Fluid in the immediate vicinity of a bounding surface This layer of particles is called the boundary layer. At the surface of the object, it is essentially static due to the friction of the surface. The object, with its boundary layer is effectively the new shape of the object that the rest of the molecules "see" as the object approaches. This boundary layer can separate from the surface, essentially creating a new surface and completely changing the flow path. The classical example of this is a stalling airfoil. For other uses see Stall. In Aerodynamics, a stall is a sudden reduction in the lift forces generated by an Airfoil
Maximum Entropy Principle
As the total number of degrees of freedom approaches infinity, the system will be found in the macrostate that corresponds to the highest multiplicity. The principle of maximum entropy is a postulate about a universal feature of any Probability assignment on a given set of Propositions ( Events hypotheses In Statistical mechanics, a microstate describes a specific detailed microscopic configuration of a system that the system visits in the course of its thermal fluctuations
Thermodynamic Equilibrium
Equilibrium thermodynamics applies if the energy change within a system occurs on a timescale large enough for a sufficient number of molecular collisions to occur so that the energy transfer between molecules and between energy modes to allow the new energy value to be distributed in equilibrium among the molecules. In Thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium Mechanical equilibrium, and (For typical systems, this is on the order of a few nanoseconds)
Etymology
The word "gas" was invented by Jan Baptist van Helmont, perhaps as a Dutch pronunciation re-spelling of "chaos". Jan Baptist van Helmont (bapt January 12, 1579 &ndash December 30, 1644) was an Early modern period Flemish Chaos (derived from the Ancient Greek, Chaos) typically refers to Unpredictability, and is the antithesis of Cosmos. [1]
See also
References
- John D. Anderson. In Physics, thermodynamics (from the Greek θερμη therme meaning " Heat " and δυναμις dynamis meaning " John D Anderson Jr (born October 1, 1937) is the Curator of Aerodynamics at the National Air and Space Museum at the Smithsonian Institution Modern Compressible Flow: Third Edition New York, NY : McGraw-Hill, 2004. ISBN 007-124136-1
- Philip Hill and Carl Peterson. Mechanics and Thermodynamics of Propulsion: Second Edition Addison-Wesley, 1992. ISBN 0-201-14659-2
- John D. Anderson. Fundamentals of Aerodynamics: Fourth Edition New York, NY : McGraw-Hill, 2007. ISBN-13: 978-0-07-295046-5 ISBN-10: 0-07-295046-3
- National Aeronautics and Space Administration (NASA). Animated Gas Lab. Accessed February, 2008.
- Georgia State University. HyperPhysics. Accessed February, 2008.
- Antony Lewis WordWeb. Accessed February, 2008.
- Northwestern Michigan College The Gaseous State. Accessed February, 2008.
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