Heat will not spontaneously flow from a colder body to a hotter body. Establishes the concept of entropy (S) as a measure of the disorder or randomness in a system. It implies that natural processes tend to move toward states of higher entropy.
Entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.
Thermal Reservoir
Aka. heat reservoir. A body that can either absorb or release heat at constant temperature. Isothermal heat transfer device.
Sink
A thermal reservoir that absorbs heat.
Source
A thermal reservoir that releases heat.
Kelvin-Planck Statement
It is impossible for any device that operates on a cycle to exchange heat with a single reservoir and produce a net amount of work. Explains the operational constraints of a heat engine.
Clausius Statement
It is impossible to construct a system which will transfer heat from a cooler body to a hotter body without work being done on the system by surrounding.
Establishes the operational constraints of heat pumps and refrigerators.