Conduction
Conduction is the transfer of heat from higher to lower temperature through molecular collisions or free electrons, without bulk motion of the material. Fourier's law, formulated by Fourier (1822), states that heat flux is proportional to the temperature gradient (q = −k·dT/dx). The constant k is thermal conductivity: high for metals, low for air and insulation. This is why a metal handle feels colder than wood in winter.
Convection
Convection is heat carried by a moving fluid (liquid or gas). Heat transfer between a solid surface and a fluid is expressed by Newton's law of cooling, q = h(Ts − T∞), where h is the convection heat transfer coefficient.
- Natural convection: flow caused by heated fluid rising because of density differences (water in a pot, air around a heater).
- Forced convection: a fan or pump moves the fluid (electric fans, computer cooling). h is generally larger.
Radiation
Radiation is heat carried by the electromagnetic waves every body emits according to its temperature, so it needs no medium. The energy emitted by a blackbody is proportional to the fourth power of absolute temperature (the Stefan-Boltzmann law); for real surfaces it is multiplied by emissivity ε (0 to 1), giving q = εσT⁴. Sunlight crossing the vacuum of space to reach Earth is the classic example.
Thermal resistance and combined modes
In real problems the three modes occur together. Steady one-dimensional conduction, such as heat loss through a wall, can be treated like an electric circuit using thermal resistance (R = L/kA): the resistances of several layers and of surface convection are added in series to find the total heat flow. A vacuum flask reduces all three modes with a vacuum layer (blocking conduction and convection) and silvering (blocking radiation).
Related concepts
Heat transfer builds on the laws of thermodynamics, which deal with energy conservation and entropy, but differs in dealing with rates. Many explanations of the Mpemba effect, the claim that hot water freezes first, also focus on differences in heat transfer conditions such as evaporation, convection and contact with the container.
See also: Boiling Point and Pressure, Automotive Lighting Technology (LED and Laser Headlamps)
References
- Fourier, J. (1822). Théorie analytique de la chaleur. Paris: Firmin Didot.
- Incropera, F. P., DeWitt, D. P., Bergman, T. L., & Lavine, A. S. (2007). Fundamentals of heat and mass transfer (6th ed.). Hoboken, NJ: Wiley.
- Callen, H. B. (1985). Thermodynamics and an introduction to thermostatistics (2nd ed.). New York: Wiley.