Heat Transfer
Heat transfer is the net passage of energy as a result of temperature differences. This energy is transferred in the direction of decreasing temperature until thermal equilibrium (equality of temperatures) is achieved. The basic mechanisms involved in this process include radiation (the transfer of energy in the form of electromagnetic waves) and conduction (the transfer of kinetic energy). Heat transfer in fluids can occur at a faster rate, because large masses of a fluid can be displaced and can mix with other fluid masses of different temperatures. This process is considered a distinct mechanism called convection. In many heat transfer processes, radiation and convection or conduction work together, although one is often dominant.
Every object emits electromagnetic radiation in a wave spectrum related to its own temperature. An object cooler than its surroundings will absorb more energy in the form of radiation than it emits. This radiation can pass through both free space and transparent media. Heat transfer by radiation helps sustain life on Earth energy received from the Sun is an example of this process.
The electromagnetic radiation associated with heat transfer is sometimes referred to as blackbody radiation (where "blackbody" is an ideal emitter and absorber) or as thermal radiation. Thermal radiation is often associated with infrared radiation, although more thermal energy is received from the visible potion of the Sun's spectrum than from the infrared portion.
The molecules of a hotter material move faster and therefore have higher kinetic energy than the molecules of a cooler material. When molecules collide with slower neighboring molecules, kinetic energy is transferred from one molecule to another. The rate of heat transfer is high for metals (which, therefore, are said to have higher conductivity) and quite low for gases like air.
The process of convection occurs when groups of molecules are displaced to the vicinity of slower or faster molecules and mix with them. Forced convection occurs when hotter or cooler parts of a fluid are moved by way of forces other than gravity, such as a pump. Natural or free convection occurs when fluids are heated from below (like a pot on a kitchen stove) or cooled from above (like a drink with ice cubes on top). Hotter portions of the fluid expand, become lighter, and move upwards, while cooler, heavier portions descend. Convection can be many times faster than conduction alone. Vertical and horizontal convection plays a major role in the distribution of heat on Earth through the movements of atmospheric and oceanic masses.
This is the complete article, containing 412 words
(approx. 1 page at 300 words per page).
