# Black Body Radiation and Enrgy Dissipation of Human Body (Where Does All the Energy from Foods Go?)

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Black Body Radiation and Enrgy Dissipation of Human Body (Where Does All the Energy from Foods Go?)
When solid objects are heated, they emit radiation. At normal temperature, we are not aware of this radiation because the intensity is low. At higher temperatures, there is sufficient infrared radiation that we can feel the "heat" if we are close to them. At even higher temperature, the objects actually glow, like the heating element of toaster or red-hot electric stove burner.

Since they are radiating energy, there must be a rate at which they are radiating. They may radiate 10 Joules per second or maybe 100 Joules per second. This rate is related to the temperature of the object. It's obvious that higher the temperature, they higher will be the rate of radiation. What may not be obvious is that the rate of radiation is proportional to the fourth power of temperature. Also, the bigger the object, higher the radiation. So the rate is proportional to their surface area. And the equation for this relation (Called Stefan-Boltzmann Equation) is,

Here is a universal constant known as Stefan-Boltzmann constant. The factor is called emissivity, is a number between 0 and 1. This depends on the characteristics of surface of radiation material.

The color of the light emitted by the hot object is also related to temperature. As the temperature increases, the electromagnetic radiation reaches a peak at higher frequency. An ideal black body would release all kinds of radiation when heated but the peak radiation depends on the temperature. It is found experimentally that the wavelength at the peak of the spectrum, is related to Kelvin temperature by

This is known as Wien's Law.

Let me clear it with an example. Assume that Sun act as a black body and emits light whose peak intensity occurs in the visible spectrum at around 500nm. Putting this value on that equation gives Sun's surface temperature which is 6000K.

Intensity as a Function of Wavelength at Different Temperatures

As you can see, as the temperature increases the intensity

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