Heat Transfer
Heat Transfer
Heat transfer, in thermal physics, 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. When such a heat transfer occurs, from a hot to a cold body, it is based on the second law of thermodynamics. German mathematical physicist Rudolf Clausius (1822–1888) stated this law as “Heat cannot of itself pass from a colder to a hotter body” and, more technically as, “The entropy of an isolated system not at equilibrium will tend to increase over time, approaching a maximum value.”
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.
Radiation
Every object emits electromagnetic radiation in a wave spectrum related to its own temperature. An object that is 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.
Conduction and convection
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.
See also Thermodynamics.