Second Law of Thermodynamics (Clausius Statement) (Heat cannot spontaneously flow from cold to hot body)

When to use this

Clausius statement: It is impossible for heat to flow spontaneously from a colder body to a hotter body without external work. Equivalent to Kelvin-Planck statement: no cyclic process can convert ALL heat entirely into work — some heat must be rejected. Both statements are equivalent. Violation of one = violation of the other. Defines the direction of natural processes and entropy increase.