electron pressure

The term electron pressure is used for the pressure contribution of electrons within a material, e.g., within a star. Sometimes the phrase is meant to be short for electron degeneracy pressure (aka degenerate electron pressure), a particular type of electron pressure.

The pressure of a (non-degenerate) gas that includes more than one kind of particle can be broken down into the pressure contribution of each type, and in a low density plasma, electron pressure is the contribution of free electrons as particles, e.g., within a Maxwell-Boltzmann distribution. If the plasma is neutral and its atoms are fully ionized, the free-electron-count is equivalent to the sum of the atomic numbers of the ions. If the atoms are not fully ionized, that fact must be accommodated, using the Saha equation or some function that estimates it in the appropriate regime. The ideal gas law applies, counting the free electrons as one of the types of particles. The electrons repel each other and attract positive ions (Coulomb force), but if sufficiently spread, that force is merely the mechanism of their elastic collisions (the free electrons nearing an ion are often slung back away like long-period comets nearing the Sun rather than recombining with the ion). However, if the plasma is sufficiently dense, a correction term in the equation of state (EoS) that estimates the effects of the Coulomb force can be useful.

At extreme matter densities, e.g., within white dwarfs, electron degeneracy becomes a factor: the electrons are sufficiently confined that per the Pauli exclusion principle, they must be at different energy levels to avoid duplicate quantum numbers at too close quarters, and for a confining pressure to further increase the density requires the energy to boost some to higher energy levels. This energy-requirement is manifest as a resistance to further confinement (overcoming such resistance is work, i.e., the conversion of energy), the resistance termed electron degeneracy pressure, an outward pressure that must be overcome. When this degeneracy is partial (at densities where just a bit of such degeneracy is sufficient), a correction term can be incorporated in the EoS that estimates the added effect.