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Today, I was reading an article How Thermoses Work It goes on to explain all three processes of heat transfer:

  • Conduction
  • Radiation
  • Convection

In Convection, It states,

If it weren't for convection your soup would stay hot a lot longer, because it turns out that air is a pretty poor heat conductor.

What does it have to do with air being poor conductor for the soup to stay hot a lot longer?

In other words, if air were a good conductor of heat, would your soup stay hot for longer?

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1  
The reason is that air just isn't very dense. –  Ron Maimon Dec 5 '11 at 1:25
    
Just for fun, anybody want to tackle the issue of evaporative cooling? Turning liquid water into vapor takes quite a bit of energy and plays a major role in cooling wet objects (soup, sweating people ect.) –  user3823992 Feb 23 at 5:44

5 Answers 5

Well, the argument is not very well put:

Because the air is a bad conductor of heat the soup stays hot longer: only the first layer of air touching the soup gets hot fast, and heat is not transmitted efficiently to the bulk of air.

For soup, in contrast to thermos, evaporation cooling should also be considered. Convection by continuously replacing the contacting layer of air increases the heat transfer to the bulk of air by conduction and at the same time the rate of evaporation increases, increasing cooling. So the soup cools faster than if there were no convection.

If air were a good conductor of heat, the soup would cool fast, as in a metal plate on a metal surface.

You could go through a read of the wiki article.

Edit: Georg's comment makes me add that of course the soup would be also cooling because it will be radiating with the corresponding to its temperature black body spectrum. Convection increases the rate of heat loss over the loss through radiation.

To address the title, which differs from the questions in the content:

Why is air a poor conductor of heat?

It is mainly the very low density of gases that make them bad conductors of heat. In liquids and solids atoms and molecules are densely packed and transfer of energy has much smaller distances to happen. In a gas molecules have to scatter off molecules to exchange energy in larger distances, so the probability of transfer is much smaller.

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Remember the "debate" on radiation from sun-heated streets? I do :=) –  Georg Dec 4 '11 at 17:56

Air is a bad conductor because, to conduct heat current molecules should absorb heat and transmit it to neighbor by vibrating. In case of air molecules near the hot surface absorbs the heat and start vibrating, but neighbor molecule is so far that this vibration should be very high and so the heat energy required is high for small conduction to start.

Convection helps air in conduction by making molecules in air mobile so that the transfer of heat is not solely depend on the vibration of molecule, but another (or that molecule itself) can travel to transfer heat.

So , "If air were a good conductor of heat" then soup will not stay hot for longer because this time convection+conduction will both help to transfer heat away from soup.

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""nd transmit it to neighbor by vibrating in case of air molecules near the hot surface absorbs the heat and start vibrating but neighbor molecule is so far that this vibration should be very high and so the heat energy required is high for small conduction to start."" This is nonsense. Look up vibrational states of nitrogen and oxygen. –  Georg Dec 4 '11 at 17:58
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Georg I used vibration as a general term not that "molecular vibration", may be that was confusing i was trying to make it simpler to understand.Apologies.. –  p.j Dec 5 '11 at 8:07

Because conduction is the transfer of heat through a substances as a result of neighbouring vibrating particles, The particles in air are far apart. Heat does not travel easily by conduction through air.

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Convection requires the hot air just above your soup that has been heated up to rise as its density reduces after being heated, allowing the colder air around the rising hotter air to replace the hotter air and the same thing will happen again. As the convection currents (the rising of the hot air and the sinking of the cold air) keep taking place just above your soup, the previously hot soup will start to lose thermal energy (heat) and your soup will cool down despite the fact that air is a poor conductor of heat. In other words, cooling of your soup only takes place in convection due to the movement of air and not the heating up of air itself. As a result, if air was a good conductor of heat, it would not make a difference.

Hope this helps.

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Thermal conductivity in the linear regime (i.e. no convection) is determined by the rate of energy transfer by diffusion of molecules from the hot regime (high average kinetic energy) to the cold regime (low average kinetic energy). This rate is proportional to the density of molecules, the mean velocity, the mean free path, and the specific heat per particle, kappa\sim n c_p v l. This might suggest that the heat conductivity of a metal is larger because the density is bigger. This is not true, because the mean free path goes inversely as density, l \sim 1/(n\sigma), where \sigma is the scattering cross section. The main reason that metals are good conductors is that the velocity of electrons in a metal is much bigger than the velocity of air molecules (because electrons are lighter, and because at low T the electron velocity at the Fermi surface is still large because of the Pauli principle).

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