Physics Definitions in Alphabetical Order

• Collective Behaviour: Collective behaviour emerges within a plasma wherein intricate motions are influenced not just by local conditions but also by the state of the plasma in distant regions. Owing to both short- and long-range electromagnetic forces present in an ionized plasma, numerous charged particles act as a unified entity rather than distinct individual particles. This results in intricate particle motions, giving rise to phenomena like plasma waves or instabilities, phenomena not typically exhibited by a single charged particle [Chen, 1984; Chouduri, 1998].
• Collisionless Plasma: A plasma where the density is sufficiently low to render Coulomb collisions negligible when compared to the prevailing, and therefore dominating, long-range electromagnetic forces of external fields and collective behaviour. Coulomb collisions, typically perceived as physical collisions, denote the short-range electrostatic force between two charged particles, leading to deflections in their direction of motion [Chen, 1984; Chouduri, 1998].
• Equation of State: An equation establishing an empirical (derived from experimental data) or theoretical (derived from physical principals) relation among various thermal properties (temperature, pressure, volume, or density)  of a substance for a given state of matter (solid, liquid, gas, or plasma) or under particular conditions (due to, for example, phase transitions or intermolecular forces). One of the best-known examples of an equation of state is the Ideal Gas Law, expressed as , where is the pressure, is the volume, is the number of particles, is Boltzmann’s constant, and is the temperature [Schroeder, 2000].
• Equipartition Theorem: In a system in thermal equilibrium at sufficiently high temperatures, where quantum mechanical effects have minimal influence, each quadratic degree of freedom contributes an average of thermal energy, where is Boltzmann’s constant and is the temperature. A degree of freedom corresponds to the various ways in which energy (translational, rotational, or vibrational) can be stored, while a quadratic degree of freedom represents energy dependent on the square of a general coordinate (like the energy of a harmonic oscillator, caused by a spring force for example, which is proportional to the square of the displacement from the equilibrium position). Assuming that a particle moves freely, its kinetic energy, expressed as , with and the mass and speed of the particle, respectively, can be equated to . This gives the interpretation that the particle’s root-mean-squared speed is due to its random thermal motions [Schroeder, 2000]. Therefore, in a plasma, the definition of temperature is regarded as a measure of the thermal kinetic energy of a specific species [Chen, 1984; Chouduri, 1998]. See Temperature for a further discussion of this.
• Debye Shielding: The process by which a concentration of charge, an electric field, or a potential difference within a plasma is shielded or screened across a specific distance by a rearrangement of particles within the plasma. This rearrangement stems from the attraction between opposite charges, resulting in the nullification of electric fields due to an increased number of particles with a certain charge within a specific region, thereby reducing the overall electric field. The distance over which electric fields are significantly attenuated (decreased exponentially) is called the Debye length and the electric field will therefore have a negligible effect a few Debye lengths away. This collective behaviour among particles implies that the effect of external electric fields or local charge concentrations within a plasma can be diminished on a global scale [Chen, 1984; Chouduri, 1998].
• Magnetohydrodynamics (MHD): A mathematical description of a plasma, conceptualising it as a fluid dominated by a single particle species (assumed to be the heavier ions, although it is possible to have MHD equations for a plasma with multiple ion species) and slow motions. It is derived by combining the hydrodynamic fluid equations with Maxwell’s equations, governing electric and magnetic fields, under certain simplifying conditions. The specific assumptions are that the plasma is quasi-neutral with singly charged ions, that the Larmor radius (the radius of particles gyrating around the magnetic field) is much smaller than the length scale over which quantities change so that viscosity can be neglected, and that the ions are much more massive than the electrons so that fast motions can be neglected. These assumptions stem from the plasma being quasi-neutral and having collective behaviour, as the more mobile electrons readily adapt to and follow the heavier ions, limiting the fastest motions describable by MHD to motions governed by ion movements, and by extension, the types of waves or instabilities [Chen, 1984; Chouduri, 1998].
• Plasma: In its simplest definition, plasma represents the fourth state of matter, composed of an ionised gas containing positively charged ions, negatively charged electrons, and neutral atoms. The word ‘plasma’ originates from a Greek word meaning ‘something moulded or fabricated,’ which might be a misnomer given the definition here. However, due to the presence of charged particles within a plasma, its motions are shaped by electric and magnetic fields, together with the way that the charged particles influence one another on a global scale. In a more technical definition, the ionised gas is presumed to exhibit quasi-neutrality and collective behaviour, with the additional criteria that it must be dense enough for the Debye length to be much smaller than the spatial extent of the gas, that the number of particles in the Debye sphere (the sphere with the Debye length as radius) must be large enough for collective behaviour to occur, and that the Coulomb collision rate must be sufficiently small for the gas to behave like plasma and not a neutral fluid [Chen, 1984; Chouduri, 1998]. See Collective Behaviour, Debye Shielding, and Quasi-neutrality for explanations of these terms.
• Quasi-neutrality: On average, a plasma can be considered neutral (comprising equal amounts of positive and negative charges) because of the collective behaviour exhibited by a large number of particles shielding potential short-term local non-uniformities caused by external fields or localised disturbances [Chen, 1984; Chouduri, 1998].
• Temperature: Several definitions are possible, with the operational definition relying on measurements made by a thermometer, indicating how hot or cold a substance is with respect to some reference point. From the mathematical viewpoint in thermodynamics, temperature can be interpreted as a measure of the tendency for heat to flow spontaneously. This implies that if two objects are in thermal contact (capable of exchanging heat), the object that spontaneously loses energy is at a higher temperature, and if the two objects are in thermal contact for long enough, they will reach thermal equilibrium, a state where there is no spontaneous net flow of heat and both objects are at the same temperature [Schroeder, 2000]. More specifically in the context of plasma, temperature is understood as a measure of the thermal kinetic energy arising from the random motions of particles from a specific species. A distinction is made between the temperature of different species since they reach thermal equilibrium by colliding with one another, but the collective behaviour of particles and external fields can cause deviations from thermal equilibrium among particle species [Chen, 1984; Chouduri, 1998]. See the Equipartition Theorem as to how this definition arises.

References

Chen, F.F. 1984. Introduction to plasma physics and controlled fusion. vol. 1, 2nd ed. Plenum, NY.

Choudhuri, A.R. 1998. The physics of fluids and plasmas: an introduction for astrophysicists. Cambridge University Press, UK.

Griffiths, D.J. 1999. Introduction to electrodynamics. 3rd ed. Prentice Hall, Upper Saddle River, New Jersey.

Schroeder, D.V. 2000. An introduction to thermal physics. 1st ed. Pearson.