  Weakly magnetized shock waves are paramount to a large diversity of
environments, including supernova remnants, blazars, and binary-neutron-star
mergers. Understanding the distribution of energy between electrons and ions
within these astrophysical shock waves spanning a wide spectrum of velocities
is a long-standing challenge. In this study, we present a unified model for the
downstream electron temperature within unmagnetized shock waves. Encompassing
velocities from Newtonian to relativistic, we probe regimes representative of
the gradual deceleration of the forward shock in the late gamma-ray burst
afterglow phase, such as GRB 170817A. In our model, heating results from an
ambipolar electric field generated by the difference in inertia between
electrons and ions, coupled with rapid electron scattering in the decelerating
turbulence. Our findings demonstrate that the electron temperature consistently
represents $10\%$ of the incoming ion kinetic energy in the shock front frame
over the full range of shock velocities.
