According to the Physics For Scientists
and Engineers book, the Second Law of Thermodynamics
states “The entropy of an isolated system (or group of
systems) never decreases. The entropy either increases,
until the system reaches equilibrium, or, if the system
began in equilibrium, stays the same.” This is where
physics plays a part. Basically when two “systems” with
different temps intermingle, heat energy is shifted
impulsively from hotter, to colder.
The picture on the left indicates the efficiency of a heat system working between two energy levels. The picture on the right indicates how entropy is defined, where entropy explains the amount of unavailable energy in a system.
Images courtesy of: http://www.engineeringtoolbox.com