Heat

Lore & Background

In ordinary language, 'heat' or 'warmth' refers to either thermal energy or temperature, or the human perception of these. Speculation on heat as a separate form of matter has a long history, involving the phlogiston theory, the caloric theory, and fire. The caloric theory was replaced around the end of the eighteenth century by the 'mechanical' theory of heat, which is accepted today. Early modern scientists such as Francis Bacon, Galileo Galilei, Robert Hooke, and Robert Boyle described heat as motion of the constituent particles of objects. No clear distinction was made between heat and temperature until the mid-18th century, nor between the internal energy of a body and the transfer of energy as heat until the mid-19th century.

Reader's Guide

Heat can enter or leave a body by thermal conduction, by electromagnetic radiation, or through friction due to macroscopic mechanical movement. When heat is generated by friction, it may end up partly in each of the two engaged bodies. By assuming conservation of heat in a transfer, calorimetry measures it by its effect on the states of interacting bodies, for example, by the amount of ice melted or by change in temperature of a body. The standard unit for the rate of heating is the watt (W), defined as one joule per second. Heat flux is defined as rate of heat transfer per unit cross-sectional area (watts per square metre).

Did You Know?

• The symbol for heat was introduced by Rudolf Clausius and Macquorn Rankine.
• Heat released by a system into its surroundings is by convention a negative quantity; when a system absorbs heat from its surroundings, it is positive.