home  | más sobre termo | otros temas de Física | lecciones del maestro Ciruela | tonterías |  @ 
   NO ME SALEN
   (APUNTES TEÓRICOS DE BIOFÍSICA DEL CBC)
  Heat
  Radiation - Stefan–Boltzmann’s law

  

nomesalen

 

Radiation - Stefan–Boltzmann’s law

Heat can flow as radiation. When it flows in this way, it can do it in a material medium or in the absolute void. Radiation that transports heat needs no matter to move.

I'm talking about the most common type of radiation, which is present in the entire universe, and is called electromagnetic radiation. This image represents the continuous spectrum of electromagnetic radiation from the most energetic and penetrating rays (on top) to the most soft and harmless rays (at the bottom). The column on the left represents the length of the wave.
 

The maximum energy corresponds to the smallest wavelength; at that end are Gamma Rays. Luckily there are very few in the universe.

Next, there we have X-rays. They are so penetrating that they pass through our bodies and serve to make transparent photographs (radiographies or bone scan). They are not harmless: you should not expose yourself to X-rays without an express need.

Ultraviolet light is still penetrating. It does not penetrate our whole body but is capable of producing lesions on our skin.

Then, we have a narrow range of wavelengths, which is for us the most beautiful: the visible light. It goes from violet to red.

With less energy, the infrared waves are the following, and the "thermal" or far infrared section begins, which is the wavelength at which the heat is transmitted.

Finally, we have the range of microwaves and (beyond) the TV, FM and AM radio waves.
 

 

Everything that exists in nature radiates energy, in all directions: people, clouds, atmospheric gases, vegetables, metallic objects… they all radiate energy.

The intensity of the energy that an object radiates depends basically on its (absolute) temperature. In particular, energy radiation in the form of heat is described by Stefan-Boltzmann's Law

    
  Q = σ . ε . A . T4  
Δt
 

 

where T is the absolute temperature of the body radiating; Sigma is the Boltzmann constant, σ = 5,67 x 10-8 W/m2 K4; epsilon, ε, is the emissivity factor of the body, and is a number (without units) which takes values between 0 and 1; represents the surface property of the body, which makes it more opaque or more reflective to radiation. The emissivity of a mirror is 0 and the emissivity of the black body is 1. The emissivity of human skin of any hue is more or less the same, with a value close to 1 (the color difference only affects the visible and ultra visible range, but not the thermal range); A is the area of the body exposed to radiate or receive radiation.

As can be seen, radiation is strongly dependent on body temperature, as it depends on the fourth power of absolute temperature.

In addition you probably have realized that the radiation (quotient between the caloric energy and the considered interval of time) must be measured in a unit of power.

    
[ QN ] = W        (watt)  
Δt
   

or any other quotient between units of energy and units to time, for example: calories per minute.

In general, bodies radiate and absorb radiant energy simultaneously. The net balance, or the net power, can be roughly obtained by:

   
  QN = σ . ε . A . (Ta4 – Te4)  
Δt
   

where Ta is the absolute temperature of the environment in which the body is located (absorption temperature) and Te is the absolute temperature of the body surface (emission temperature).

   

Curious facts:

    
  • Electromagnetic waves travel at the speed of light ... (although it would be more correct to say: light travels at the speed of electromagnetic waves).
  • Waves with higher energy (and penetration into living organisms) are often called ionizing waves, because one of the damaging consequences they have is that they separate electrical charges from molecules -usually neutral-, producing pairs of ions that alter biological functioning. There is another type of biological damage that consists on breaking or altering the information chains of our cells, DNA, and we are not always able to repair it correctly.
  • The longer wavelength, the less frequency. The product between them is constant, and that constant is the speed of the electromagnetic wave, that is, light speed: c ≈ 300.000.000 m/s.
  • The boundaries of the region of electromagnetic waves capable of transmitting heat are diffuse, and extend - even with low efficiency - towards the ends of the wavelengths.
  • Many times the heat produced by electromagnetic radiation does not depend on the type of wavelength but rather on the interaction between it and matter. For example: a large part of solar radiation heats the earth's surface and is returned to space as infrared radiation.
    

Captious questions:

    
    • Does our skin have heat radiation sensors?
    • Why does our skin synthesize that pigment called melanin that generates the tanning of bathers?
    • If all of our species descended from the black African race, Why did the populations that migrated to boreal latitudes lose pigmentation?
    • Why it is calorically nice to sunbathe behind a glass, but the skin does not tan?
    • ¿Cuál es la relación entre el cáncer de piel y la exposición a la radiación ultravioleta?
    • What are really creams-sunscreen filters, and what is the meaning of the filter indicator number?
    • Why sometimes we enter places where even when the temperature is not low we feel uncomfortable ... we say that the place is not "warm" and we prefer another place with more people, more furniture, more things on the walls, Hardwood floors or carpets?
    • Why are hot water thermos made with silver glass?
 
     
Some rights reserved. Reproduction permitted if quoting the source. I thank Rafael Mac Donough for sending an erratum. Last updated on Dec-16. Translated by Esteban Djeordjian. Buenos Aires, Argentina.