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Question 2.171: Thermos Bottle. (a) State and justify how the thermal conduc...

Thermos Bottle.

(a) State and justify how the thermal conductivity of an ideal gas depends on its density at fixed temperature.

(b) A thermos (Dewar) bottle is constructed of two concentric glass vessels with the air in the intervening space reduced to a low density. Why can it act as an insulating container even though the vacuum is not perfect?

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(a) The mean speed of the air molecules is constant at fixed temperature.
However the greater the gas density is, the more frequent will be the collisions and the transmission of energy will become faster. Hence the thermal conductivity is higher for greater gas density.
(b) As the air density is low, the thermal conductivity is also low. It can therefore enhance heat insulation.

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