Ehrenfest Relations

thermodynamic formulas describing the relation between macroscopic characteristics of the phases of a thermodynamic system during a second-order phase transition (for example, the transition of helium from the liquid to the superfluid state and the transition of a ferromagnetic to a paramagnetic):

where p, T, and V are the pressure, temperature, and volume of the system and ΔC_p, Δα_p, and ΔB_T are the changes in specific heat, the isobaric thermal expansion coefficient, and the isothermal compressibility coefficient during the phase transition. The relations were derived by P. Ehrenfest in 1933.