heat

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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.

The yellow of the emblem is said to denote Angola's natural wealth. Red is said to stand for the blood spilt by the freedom fighters. Black represents Africa. Effective date: 11 November 1975. >>