However, historically, when the modern theory of heat was developed, heat was regarded as internal kinetic energy of molecular movement, and as recently as 1922 , Max Planck described temperature as degree of heat, or state of heat, and distinguished temperature, or degree of heat from quantity of heat. Planck was also clear that heat radiation was something arising from particles, rather than from a geometric surface. Heat was therefore a force in a body that could do work, and there was no need for a separate term to describe thermal energy.

In ordinary language the word heat has a broader meaning. This can lead to confusion if the diversity of usage of words is forgotten.

Thermodynamically, energy can be produced or transferred as heat by thermal conduction, by thermal radiation, by friction and viscosity, and by chemical dissipation.

Heat transfer by conduction and by radiation from a hotter to a colder body is spontaneous. The second law of thermodynamics requires that the transfer of energy from one body to another with an equal or higher temperature can only occur with the aid of a heat pump by mechanical work, or by some other similar process in which entropy is increased in the universe in a manner that compensates for the decrease of entropy in the cooled body, due to the removal of the heat from it. For example, energy may be removed against a temperature gradient by spontaneous evaporation of a liquid.

The engineering discipline of heat transfer recognizes heat transfer by conduction, by convective circulation, by net mass transfer, and by radiation.

In physics, especially in calorimetry, and in meteorology, the concepts of latent heat and of sensible heat are used.

A related and potentially confusing term is thermal energy, loosely defined as the energy of a body that increases with its temperature.

Adjective

warm (comparative wärmer, superlative am wärmsten)

1. warm, hot

Antonyms

• kalt, kühl

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