vacuum temp

[gbfish]

can you measure the temperature inside a vacuum and what would it be?

[Daave]

i dont know for sure, but in your double glazed windows there is a vacuum inbetween the sheets of glass...

and it normally feels colder than the room the glass is in


hmmm, youve got me wondering now!

[shammydodger]

The temperature of something is merely a measure of how much energy it has. This energy is mainly the kinetic energy of the atoms (or molecules) that make up this "something", which are whizzing about. Each of these atoms/molecules makes its own partial contribution to the total energy of the system. In a total vacuum there is, by definition, nothing in the system. Therefore, nothing to contribute any energy to the system. The temperature, therefore, would be zero Kelvin. Of course, it is impossible to create a total vacuum, so that temprature is only approachable though not achievable. However, as one evacuates a system, the temperature will decrease towards zero as the population of atoms/molecules decreases towards a total vacuum. These temperatures can be (and indeed have been) measured.

There is a standard experimental apparatus in O Level physics that incorporates a mechanical alarm clock inside a bell jar. The purpose is to demonstrate the need for air to transmit sound waves. As the bell jar is evacuated of air, the ringing alarm gets quieter and quieter. One could, if one wished, incorporate a thermometer into this apparatus to show that, as the vacuum increases, the temperature inside the bell jar does indeed decrease.

[Daave]

just one question, who is kelvin?

[shammydodger]

http://scienceworld.wolfram.com/biography/Kelv in.html

[mjd]

Thank you for your explanation shammydodge but isn't the temperature decrease just a consequence of the ideal gas law (P*V/T = constant).

Surely if after partial evacuation the bell jar was left for a period then the temperature inside would reach equalibrium with ambient. The converse also being true, increase the pressure & the temperature rises, but if left for a time then the gas would cool. High pressure does not equate to hot not low pressure to cold.

[shammydodger]

mjd

The temperature decrease isn't a "consequence" of any law. It's a consequence of the decreased energy within the system. The ideal gas law (PV = nRT) merely describes the behaviour of such systems.

And when explaining such behaviour, it is critical to understand what is meant by "the system". In this example, the system is the environment inside the bell jar and nothing else.

[john91]

There isn't a vacuum in double glazed windows, its either air or sometimes nitrogen.

[shammydodger]

mjd

Further to my earlier reply to you, I should also point out that the gas law which you refer to applies only to a fixed mass of gas. Obviously, in the case of creating a (partial) vacuum, the mass of gas is not fixed.