  We argue that, within the realm of gauge-gravity duality, for a large class
of systems in a steady-state there exists an effective thermodynamic
description. This description comes equipped with an effective temperature and
a free energy, but no well-defined notion of entropy. Such systems are
described by probe degrees of freedom propagating in a much larger background,
e.g. $N_f$ number of ${\cal N} =2$ hypermultiplets in ${\cal N}=4$ $SU(N_c)$
super Yang-Mills theory, in the limit $N_f \ll N_c$. The steady-state is
induced by exciting an external electric field that couples to the
hypermultiplets and drives a constant current. With various stringy examples,
we demonstrate that an open string equivalence principle determines a unique
effective temperature for all fluctuations in the probe-sector. We further
discuss various properties of the corresponding open string metric that
determines the effective geometry which the probe degrees of freedom are
coupled to. We also comment on the non-Abelian generalization, where the
effective temperature depends on the corresponding sector of the fluctuation
modes.
