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   NO ME SALEN
   (APUNTES TEÓRICOS DE BIOFÍSICA DEL CBC)
  Heat transmission - Convection

  

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Heat is energy flowing from a body to another. Occurs spontaneously in a unique direction: from the hotter body to the colder one. To be more specific, heat flows from the body with higher temperature to the body with lower temperature.

Heat transmission is the name given to different processes through which heat flows from one body to another. Basically, there are three types of heat transmission, which are described below.

If heat flows through a material, it’s called conduction, which will be next lesson’s topic.

But heat can also flow without a material, it’s to say, it could also flow through void. This kind of transmission is called radiation, and will be approached after conduction.

And finally, heat can also take a cab: it can flow with the body, and leave it under certain conditions. This mechanism is called convection.

Notice the greatest difference between convection and the other two types of heat transmission: matter displacement.  The first two process described do not involve matter movement from a place to another. However, convection is possible due bodies movement.
 

CONVECTION
Fluids are the most common and efficient heat transmitters because their own versatility to move. The most common ‘trigger’ for this process is the buoyancy difference between fluids with lower density inside fluids with higher density.

For example, hot air is less dense that cold air, so if we have a mass of hot air inside a much bigger mass of cold air, which wraps up it, hot air will raise, like a cork would raise if it is put at the bottom of a bathtub full of water.


  
 

Cumulonimbus are typical cloud formations of stormy summer days. Clouds enter an ascending stream of warm air which took heat from the earth surface. When they find atmospheric layers of low pressure they expand taking the characteristic form of fungus.

 

 

Fluid currents, when raising, take heat from the lower levels, and transport it to higher levels.

For convection there are no simple laws or generalizations, so we are happy just by roughly understanding the process, and generally (at least in the first Physics courses) we are not interested in its measure.

    
   

 

Curious facts:

    
  • Cooling systems of explosion engines use forced convection, through which the transporter (water) is boosted by a mechanic bomb. That takes away the heat (and cools) the block, and expels it to the atmosphere (like the radiator).
  • Plate tectonics is the mechanism that describes earth’s surface dynamics and explains the most important facts about its orography: volcanism, formation of mountains, earthquakes, and etcetera. Plate’s triggers are convection movements over the earth mantle, due its plastic and fluid interior, which raises and sinks constantly, dragging the plates laterally.
    
 
   

Captious Questions:

   
  • Why heaters go at floor level while air conditioner equipments are installed on the ceiling?
  • Why in the single-door refrigerators (those with the freezer inside) the freezer is at the top and not at the bottom?
  • What are convection cells?
  • How does a modern hot air balloon work?
  • What causes the fungus form of the gases of an atomic explosion?
  • Why the water in the ponds keeps liquid even when the ambient temperature can be well below 0 °C?
  
     

Some rights reserved. Reproduction permitted if quoting the source. Last updated on Dec-16. Translated by Esteban Djeordjian. Buenos Aires, Argentina.