### equation of state

**(EoS)**
(equation relating temperature, pressure, and volume)

An **equation of state** (abbreviated **EoS**) relates
temperature, pressure, and volume of a substance.
**Boyle's law**, from the 1600s is the original example:
that given a constant temperature, the pressure and volume
are inversely proportional.
Equations of state are examples of
**constitutive equations** (or **constitutive relations**),
equations describing the behavior of a material,
The term *equation of state* is commonly used in some
fields, often for gases. The other terms are also
commonly used for solids, such as the effects of stress,
e.g., when and how it bends.

An equation of state of a gas, in effect, depends
upon its partition function.

The terms **soft** versus **hard** (or **stiff**) refer to
a characteristic of the equation of state:
*soft* means pressure varies weakly
with density and *hard* or *stiff* means pressure varies
strongly with density.

(*mathematics,physics*)
**Further reading:**

http://en.wikipedia.org/wiki/Equation_of_state

**Referenced by pages:**

degeneracy

electron pressure

GW170817

ideal gas law

Lane-Emden equation

mean molecular weight (μ)

metallic hydrogen

neutron degenerate matter

partition function (Z)

partial differential equation (PDE)

Schwarzschild radius (R_{S})

specific heat

stellar structure

Tillotson equation

theory of figures (TOF)

Tolman-Oppenheimer-Volkoff limit (TOV)

Wigner crystal

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