Thermal radiation

From The Book of THoTH (Leaves of Wisdom)

Thermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature. Infrared radiation from a common household radiator or electric heater is an example of thermal radiation, as is the light emitted by a glowing incandescent light bulb. Thermal radiation is generated when heat from the movement of charged particles within atoms is converted to electromagnetic radiation.

The emitted wave frequency of the thermal radiation is a probability distribution depending only on temperature, and for a genuine black body is given by Planck’s law of radiation. Wien's law gives the most likely frequency of the emitted radiation, and the Stefan-Boltzmann law gives the heat intensity.

Properties

There are three main properties that characterize thermal radiation:

• Thermal radiation, even at a single temperature, occurs at a wide range of frequencies. How much of each frequency is given by Planck’s law of radiation.
• The main frequency (or color) of the emitted radiation increases as the temperature increases. For example, a red hot object radiates most in the long wavelengths of the visible band, which is why it appears red. If it heats up further, the main frequency shifts to the middle of the visible band, and the spread of frequencies mentioned in the first point make it appear white. We then say the object is white hot. This is Wien's law.
• The total amount of radiation, of all frequencies, goes up very fast as the temperature rises. An object at the temperature of a kitchen oven (about twice room temperature in absolute terms) radiates 16 times as much power per unit area. An object the temperature of the filament in an incandescent bulb (roughly 3000 K, or 10 times room temperature) radiates 10,000 times as much per unit area. Mathematically, the total power radiated rises as the fourth power of the absolute temperature, the Stefan-Boltzmann law.

External links

• http://sol.sci.uop.edu/~jfalward/heattransfer/heattransfer.html - Website on heat transfer
• http://panda.unm.edu/courses/finley/p262/ThermalRad/ThermalRad.html - Website on thermal radiation
• http://www.du.edu/~etuttle/weather/atmrad.htm - Website on thermal radiation in Earth's atmosphere

Applications

• [1] How to use radiant heating and cooling for conditioning occupants and building surfaces.
• [2] How radiant heating works for building applications with sustainable and energy efficient methods.