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Name: Ed F.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 7/12/2004


Question:
When teaching about a vacuum I often ask the question "what is the temperature of a vacuum." Often the students will answer "is it absolute zero?" Of course that is not 100% true. My explanation is that if you put a thermometer in a vacuum it will continue to read its temperature unless heat is transferred by radiation. Can the temperature reading of a thermometer in a vacuum ever go down?


Replies:
- A thermometer "in a vacuum" will be in thermal equilibrium with the electromagnetic radiation field surrounding it. You cannot get away from this even if the thermometer is in outer space. It will equilibrate with the radiation field to which it is exposed (not just visible but from the longest wavelength microwave through X-rays and gamma rays. This mode of energy exchange occurs even in a "perfect vacuum" since it requires no medium to transport the energy. So the thermometer will record the average energy field. Of course you have to generalize what you mean by the "thermometer". A typical mercury in glass type may not be appropriate for the measurement.

Vince Calder


Temperature is a property of matter (it is related to how much energy is stored in the mater in the form of motion -- vibrations) and hence does not apply to a vacuum.

Heat can be transferred by conduction, convection, or radiation. By putting the thermometer in a (perfect) vacuum you have removed the opportunities for conduction and convection. Radiation is the only path for energy transfer and hence temperature change. If the surroundings of the vacuum container are cooler than the thermometer it will loose energy to its surroundings by radiation -- though very slowly.

Greg Bradburn



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