In physics, 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 Edit

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. 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 Edit

When analyzing a system, it is 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.