# Isentropic change of state

isentropic and isothermal change
in state in the pV diagram

In thermodynamics , a process or a flow is called isentropic if the entropy does not change: ${\ displaystyle S}$

{\ displaystyle {\ begin {aligned} S = \ mathrm {const.} & \ Leftrightarrow {\ frac {\ mathrm {d} S} {\ mathrm {d} t}} = 0 \ end {aligned}}}

As Isentropic lines are called the same entropy . Since entropy and potential temperature are directly related to each other, the term isentropic is also used synonymously for lines of equal potential temperature.

An adiabatically reversible process is always isentropic, but this does not apply to the reverse.

The isentropic change of state of ideal gases can be described by Poisson's equations:

${\ displaystyle p (v) = p_ {0} \ cdot {\ left ({\ frac {v_ {0}} {v}} \ right) ^ {\ kappa}}}$
${\ displaystyle T (v) = T_ {0} \ cdot {\ left ({\ frac {v_ {0}} {v}} \ right) ^ {\ kappa -1}}}$
${\ displaystyle T (p) = T_ {0} \ cdot {\ left ({\ frac {p} {p_ {0}}} \ right) ^ {\ frac {\ kappa -1} {\ kappa}}} }$

there are:

The index 0 indicates the initial state, the variables without an index are the variables .