Gas Totally Explained

Everything about Gases totally explained

: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.) without a definite shape or volume that are in more or less random motion.

Physical characteristics

Due to the electronic nature of the aforementioned particles, a "force field" is present throughout the space around them. Interactions between these "force fields" from one particle to the next give rise to the term intermolecular forces. Dependent on distance, these intermolecular forces influence the motion of these particles and hence their thermodynamic properties. 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.
Will expand and contract greatly with changes in temperature or pressure, thus the term "compressible".
Will diffuse readily, spreading apart in order to homogeneously distribute itself throughout any container.

Macroscopic

When analyzing a system, it's typical to specify a length scale. A larger length scale may correspond to a macroscopic view of the system, while a smaller length scale corresponds to a microscopic view.
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.

Pressure

The pressure exerted by a gas uniformly across the surface of a container can be described by simple kinetic theory. 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. In classical mechanics, Momentum, by definition, is the product of mass and velocity. Kinetic energy is one half the mass multiplied by the square of the velocity.
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. The pressure can then be said to be the average linear momentum of these moving particles. 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. In statistical mechanics, temperature is the measure of the average kinetic energy stored in a particle. The methods of storing this energy are dictated by the degrees of freedom of the particle itself (energy modes). 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.

Specific Volume

When performing a thermodynamic analysis, it's typical to speak of intensive and extensive properties. Properties which depend on the amount of gas are called extensive properties, while properties that don't depend on the amount of gas are called intensive properties. Specific volume is an example of an intensive property because it's the volume occupied by a unit of mass of a material, meaning we've divided through by the mass in order to obtain a quantity in terms of, for example,

extstyle frac = Pressure_1 + Pressure_2 + ... + Pressure_n

Special Topics

Compressibility

The compressibility factor (

Z

) is used to alter the ideal gas equation to account for the real gas behavior. 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 (v_sρ) to viscous forces (μ/L). 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. The tangential (shear) component of the force that's 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. 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. This layer of particles is called the boundary layer. At the surface of the object, it's 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.

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.

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. (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 Flemish Belgian pronunciation re-spelling of "chaos".

Further Information

Get more info on 'Gases'.

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