  The review deals with a novel approach (MNEQT) to nonequilibrium
thermodynamics (NEQT) that is based on the concept of internal equilibrium
(IEQ) in an enlarged state space involving internal variables as additional
state variables. The IEQ-macrostates are unique in the enlarged state space and
have no memory just as EQ macrostates are in the EQ state space. The approach
provides a clear strategy to identify the internal variables for any model
through several examples. The MNEQT deals directly with system-intrinsic
quantities, which are very useful as they fully describe irreversibility.
Because of this, MNEQT solves a long-standing problem in NEQT of identifying a
unique global temperature T of a system, thus fulfilling Planck's dream of a
global temperature for any system, even if it is not uniform such as when it is
driven between two heat baths; T has the conventional interpretation of
satisfying the Clausius statement that the exchange macroheat flows from hot to
cold, and other sensible criteria expected of a temperature. The concept of the
generalized macroheat dQ converts the Clausius inequality into the Clausius
equality dS=dQ/T, which also covers macrostates with memory, and follows from
the extensivity property. The equality also holds for a NEQ isolated system.
The novel approach is extremely useful as it also works when no internal state
variables are used to study nonunique macrostates in the EQ state space at the
expense of explicit time dependence in the entropy that gives rise to memory
effects. To show the usefulness of the novel approach, we give several examples
such as irreversible Carnot cycle, friction and Brownian motion, the free
expansion, etc.
