# Proper name for a thermodynamic process with constant internal energy $U$

Back in the day I learned that a few special thermodynamical processes have special names.

For example, if one keeps $P$ constant, the process is called isobaric, if one keeps $T, V$ or $S$ constant, one gets, correspondingly, isothermic, isochoric or isentropic processes. Similarly, if one keeps $\dfrac{\mathrm{d} \ln P}{\mathrm{d} \ln \rho}$ constant during the process, it is called polytropic, and if $\delta Q = 0$ at any time, the process is called adiabatic.

Now, the question: what is the process called, if one keeps internal energy $U$ constant?

• @Danu, "isentropic" is typically used for hydrodynamical flows, rather than processes. Though I do agree that adiabatic is a bit more general (I add a specification). – Alexey Bobrick Nov 11 '13 at 18:42
• When the energy is kept constant, the process is called isoenergetic (or, if you prefer, iso-energetic). – Georg Sievelson Nov 11 '13 at 19:16
• Notice that if there is some subtlety and you keep a constant internal energy $U=\text{cte}$ but not a constant energy $E=U+E_{\text{m}}$, by modifying the mechanical energy $E_{\text{m}}$, you should refrain from using standard names like isoenergetic and explain precisely what happens. – Georg Sievelson Nov 11 '13 at 19:21
• Dear @GeorgSievelson, thank you, this is exactly what I have been looking for! For completeness, would the term be suitable for flows, which keep specific internal energy constant (remembering your other comment, though)? – Alexey Bobrick Nov 11 '13 at 19:25
• I would also gladly consider it as an answer, if you can apply a comment->answer transformation to your reply. – Alexey Bobrick Nov 11 '13 at 19:26

Notice that if there is some subtlety and you keep a constant internal energy $U=\text{cte}$ but not a constant energy $E=U+E_{\text{m}}$, by modifying the mechanical energy $E_{\text{m}}$, you should refrain from using standard names like isoenergetic and explain precisely what happens.