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Does the temperature of a vacuum chamber drop if left for 24 hours or more, since some in space in quite cold somewhere and quite hot how does vacuum become cold near to absolute zero.

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  • $\begingroup$ What happens to the contents of a vacuum flask? $\endgroup$ – Solar Mike Jun 17 at 4:57
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    $\begingroup$ The wording of your question is a little confusing. Strictly speaking, temperature is a property of matter (although we can say that blackbody radiation in thermal equilibrium with a lump of matter has the same temperature as the matter). So a perfect vacuum doesn't have a temperature. $\endgroup$ – PM 2Ring Jun 17 at 5:04
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As pointed out by the comments, vacuum itself does not have temperature, since temperature is defined in terms of the kinetic energy of the particles. However, if you put an object (let's say an idealized blackbody) inside the vacuum, its temperature is not simply zero. The temperature will be actually related to the relation of incoming and outgoing radiation. This is the case since electromagnetic radiation can indeed travel through vacuum. In a situation where there is no appreciable source of radiation such as a star, and the object is left by itself inside the vacuum, its temperature will decrease until it reaches about 3 K, the temperature of cosmic background radiation. At such temperature, the object will reach equilibrium.

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@AlonsoPerezLona already explains it, I just want to clarify even further in terms of thermodynamics.

It's all about thermodynamic equilibrium. Everything exchanges heat through vacuum via thermal radiation, so if you have a vacuum chamber with walls at different temperatures, these walls will eventually equilibrate. And if you put something in a chamber, it will equilibrate to the same temperature, too.

For a vacuum chamber at equilibrium, you can say that the vacuum itself has a temperature. It's not actually empty, it's full of thermal radiation (and photons are just as valid particles as everything else). Away from all stars, the outer space has temperature of about 2.7K (cosmic background), and if you are near a star, you will feel equilibrate at some kind of average, so that outgoing radiation will match the received radiation - for our Earth, this equilibrium sets the average temperature somewhere around 10-15°C.

This is the reason a good vacuum flask has mirror walls. Vacuum doesn't stop heat exchange (only stops exchange through thermal conduction), but mirror walls slow down the radiation exchange. Eventually, anything in a vacuum flask will reach the average temperature of its walls.

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