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What make air a good insulator?

  1. Apr 22, 2012 #1
    I am curious to know, what make air in itself a good insulator? Just to clarify I am not talking about materials that trap the air and stop the convection current from moving, from a hot surface to a cold one.

    Is it because of the thermal capacity of air, which I believe is something like 1.0084 kg/c. My reason for thinking this is because air is classed as a fluid.

    So when I look at something like an island surround by water it take the water long to lose it heat that the island due to have a higher heat capacity.
     
  2. jcsd
  3. Apr 22, 2012 #2

    DaveC426913

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    Gold Member

    Yup. Water has a very high heat capacity; air has a low capacity.
     
  4. Apr 22, 2012 #3
    Thermal conductivity, like diffusivity, and viscosity is a transport phenomenon.

    In the case of thermal conductivity, what is being transported is (average) kinetic energy, measured by the local temperature.

    In the case of diffusivity, what is being transported is particles, measured by the local concentration.

    In the case of viscosity, what is being transported is (average) momentum, measured by the flow velocity.

    All these quantities are proportional, or, at least, correlated to one another for a given substance. In the kinetic theory of gases, a crucial quantity on which they depend is the mean free path. I will not give any derivation of these quantities, but merely point out some links:

    Einstein equation for the diffusion coefficient. As you can see, it depends on the viscosity.

    Similar correlations may be found between thermal conductivity and viscosity of gases. A search on Google reveals many papers published on this subject.

    Molar specific heat capacity of a gas has a very simple form:

    [tex]
    c_n \equiv \frac{C}{n} = \frac{f}{2} \, R
    [/tex]

    But, if you want to find the specific heat capacity (per unit mass), you need to divide by the molar mass:
    [tex]
    c_m \equiv \frac{C}{m} = \frac{C}{n} \, \frac{n}{m} = \frac{f}{2} \, \frac{R}{M}
    [/tex]
    Thus, the more massive the molecules (atoms) of the gas, the smaller the specific heat capacity.

    For mixtures, you need to take the averaged sum of the above expression.

    One more thing, there is another mechanism for heat transfer in highly mobile fluids, and especially gases, namely, through convection. This means a macroscopic flux of a volume of a fluid between different points. As the volume of fluid moves, it carries its internal energy content, and exchanges it with its surrounding. In this mechanism, heat capacity may play a significant role.
     
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