Second law of thermodynamics
The second law of thermodynamics is a physical law based on universal empirical observation concerning heat and energy interconversions. A simple statement of the law is that heat always flows spontaneously from hotter to colder regions of matter. Another statement is: "Not all heat can be converted into work in a cyclic process."
Nicolas Léonard Sadi Carnot in the traditional uniform of a student of the École Polytechnique
Rudolf Clausius
James Clerk Maxwell
In thermodynamics, heat is the thermal energy transferred between systems due to a temperature difference. In colloquial use, heat sometimes refers to thermal energy itself. Thermal energy is the kinetic energy of vibrating and colliding atoms in a substance.
A glowing-hot metal bar showing incandescence, the emission of light due to its temperature, is often recognized as a source of heat
Rudolf Clausius
A red-hot iron rod from which heat transfer to the surrounding environment will be primarily through radiation.
Joseph Black