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:
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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.
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