Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 18/07/2015 07:50:02
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harshad gulannavar asked the Naked Scientists:
How much energy is required to create absolute vacuum in one cubic meter.
What do you think?
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To condense every last particle of gas in a one cubic meter chamber a cold sink would have to be maintained for an indefinite time and or a getter fired as in CRT,s despite thermal leakage.
To remove EVERY particle would take an infinite time and energy.
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It is not possible to create "absolute vacuum" in a box of any size.
Given a box that was substantially larger than one cubic meter, and ignoring quantum effects for the moment, one could reduce the number of particles in the box to the point where a given cubic meter of it probably has no particle in it at any given point in time (but this would take a lot of energy, and is of limited utility).
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Atmospheric pressure at sea level is about 10.2 tonnes per square meter (http://extraconversion.com/pressure/bars/bars-to-tonnes-per-square-meter.html), or about 100kN of force on a square meter.
In "Physics Land" (ie everything behaves in an ideal manner), assume that you have a square tube with a 1m x 1m hole, open at one end, and a perfectly-fitting square plug that can move along the square tube. To produce a cubic meter of vacuum, you just need to pull the plug out by 1 meter.
Now, Energy=Force x Distance=100x103N x 1m = 100kJ.
Of course, as other respondents have noted, there is no such thing as a perfect seal, moving a plug is likely to scrape off atoms (and larger lumps), and there is no material which has zero vapour pressure. So the above is a "best possible" answer.
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It would be interesting to know how much power is expended per cubic meter to maintain the near perfect vacuum in the LHC
As far as I can ascertain the refrigeration system of the LHC consumes 170 KW to maintain 9000 cubic meters of vacuum which equate to about 19w/m^3 of course the initial pump down from atmospheric pressure would have required a considerable amount of energy.
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Atmospheric pressure at sea level is about 10.2 tonnes per square meter (http://extraconversion.com/pressure/bars/bars-to-tonnes-per-square-meter.html), or about 100kN of force on a square meter.
In "Physics Land" (ie everything behaves in an ideal manner), assume that you have a square tube with a 1m x 1m hole, open at one end, and a perfectly-fitting square plug that can move along the square tube. To produce a cubic meter of vacuum, you just need to pull the plug out by 1 meter.
Now, Energy=Force x Distance=100x103N x 1m = 100kJ.
Of course, as other respondents have noted, there is no such thing as a perfect seal, moving a plug is likely to scrape off atoms (and larger lumps), and there is no material which has zero vapour pressure. So the above is a "best possible" answer.
Nice response, Evan. I was going to post the exact same thing. Great minds think alike! :)