heat

Form of energy possessed by a substance by virtue of the vibrational movement (kinetic energy) of its molecules or atoms. Heat energy is transferred by conduction, convection, and radiation. It always flows from a region of higher temperature (heat intensity) to one of lower temperature. Its effect on a substance may be simply to raise its temperature, or to cause it to expand, melt (if a solid), vaporize (if a liquid), or increase its pressure (if a confined gas).

Measurement
Quantities of heat are usually measured in units of energy, such as joules (J) or calories (cal). The specific heat of a substance is the amount of heat that is needed to raise the temperature of 1 kg of the substance by 1 K (°C).

Conduction, convection, and radiation
Conduction is the passing of heat along a medium to neighbouring parts with no visible motion accompanying the transfer of heat; for example, when the whole length of a metal rod is heated when one end is held in a fire. Convection is the transmission of heat through a fluid (liquid or gas) in currents; for example, when the air in a room is warmed by a fire or radiator. Radiation is heat transfer by infrared rays. It can pass through a vacuum, travels at the same speed as light, and can be reflected and refracted; for example, heat reaches the Earth from the Sun by radiation. For the transformation of heat, see thermodynamics.