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The methods by which heat is transmitted can be classified into three distinct types known as convection, conduction, and radiation. In any actual case of heat transmission, a combination of these methods may be operating simultaneously, and the principal problem is to determine the rate at which heat flows from the source at higher temperature to the source at lower temperature.
We may distinguish between the three processes of heat transfer by considering whether a medium is required for the transfer of heat, and whether that medium is at rest or in motion. In the process of conduction, thermal energy is transmitted by a medium which is at rest. The process of convection requires a moving medium. In general, a fluid transports the energy. In processes of natural convection, the density differences between the heated fluid and its cooler neighborhood generate buoyant forces which cause the heated fluid to move. Heat energy is delivered to a fluid in one region of the container and becomes internal energy of the fluid. The fluid is set in motion, and the internal energy is liberated as heat in some other portion of the container. Radiation requires no medium. The energy reaching us from the sun and stars comes in the form of radiation through the vacuous space between the sun and the earth and between the stars and the earth. Radiation may be transmitted through a substance, as in the transmission of sunlight through the air or through a windowpane, and, in so doing, changes may take place in the substance and in the character of the radiation.