Heat

Heat is a form of energy associated with the random motion of atoms and molecules. In general heat tends to "flow" from a region of high temperature to a region of low temperature. In an isolated system, heat will tend to disperse throughout the entire system until all points equilibrate at the same temperature.

The amount of heat required to heat a material from an initial temperature, T₀, to a final temperature, T_f depends on the the [heat capacity]? of that material according to the relationship:

Δ H = ∫(T=T₀, T=T_f) C_p(T) dT

In a sense, the heat capacity is a measure of a material's ability to store heat.

As mentioned previously, heat tends to move from a high temperature region to a low temperature region. This heat transfer may occur by any of three mechanisms, conduction, convection, and radiation.

Conduction? is the most common means of heat transfer in a solid. On a microscopic scale, conduction occurs as hot, rapidly moving or vibrating atoms and molecules interact with neighboring atoms and molecules, transfering some of their energy (heat) to these neighboring atoms.

Convection? is usually the dominant form of heat transfer in liquids and gases. In convection, heat transfer occurs by the movement of hot or cold portions of the fluid. For example, when water is heated on a stove, hot water from the bottom of the pan rises, heating the water at the top of the pan. Two types of convection are commonly distinguished, free convection, in which gravity and buoyancy forces drive the fluid movement, and forced convection, where a fan, stirrer, or other means is used to move the fluid.

Radiation is the final means of heat transfer. Radiative heat transfer is the only form of heat transfer that can occur in the absense of any form of material and as such is the only means of heat transfer through a vacuum. Thermal radiation is a direct result of the movements of atoms and molecules in a material. Since these atoms and molecules are composed of charged particles (protons and electrons), their movements result in the emission of electromagnetic radiation, which carries energy away from the surface. At the same time, the surface is constantly bombarded by radiation from the surroundings, resulting in the transfer of energy to the surface. Since the amount of emitted radiation increases with increasing temperature, a net transfer of energy from higher temperatures to lower temperatures results.

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