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What is the temperature in vacuum

  1. Jul 7, 2011 #1

    kelvin490

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    hi, I have learnt that temperature defined by man is a measurement of average kinetic energy of particles, how about that in a space that is no particles at all or just a very few number of them? What readings will we get if we put a temperature-measuring device in a highly vacuumized container?

    Thank you.
     
  2. jcsd
  3. Jul 7, 2011 #2

    Drakkith

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    You will get a measurement of the temperature of the EM radiation hitting your detector. In space we have measured the Cosmic Microwave Background at 2.725 k. However, even in space there are particles, just like you said. The temperature of these vary in the extreme from location to location. Also, you have more than the CMB in space. The EMR from other stars is much higher in temp than the CMB. It simply varies enormously.
     
  4. Jul 8, 2011 #3
    So does that mean if I was sitting in a vacuum, I wouldn't lose any heat?
     
  5. Jul 8, 2011 #4

    cjl

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    You would lose heat through radiation.
     
  6. Jul 8, 2011 #5

    phinds

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    The CMB and other stuff would radiate TO you, but a pretty small amount, and you would radiate to THEM, but a much larger amount, and you would die of freezing. I've heard that this would occur pretty quickly and you would likely NOT die from lack of oxygen, if you started off with a lungful. Actually, I think it's more like you would go into shock very quickly and your heart would stop before you froze.

    Any way you cut it, it would be a really bad idea.
     
  7. Jul 8, 2011 #6

    Pengwuino

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    Also, if you really are in a vacuum and you neglect the CMBR, there is no temperature because temperature is a thermodynamic phenomena which really needs a large number of particles to make sense.
     
  8. Jul 8, 2011 #7

    Drakkith

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    Negative, you will die from lack of oxygen long before you freeze. Remember, your body is producing heat and the only way to get rid of it is through radiation, which is much less effective than convection/conduction.
     
  9. Jul 8, 2011 #8

    cjl

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    I believe you would only lose heat at a rate of a few hundred watts, so you'd pretty much definitely asphyxiate first. Definitely a bad idea though...
     
  10. Jul 8, 2011 #9

    kelvin490

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    Why we will radiate to the space at a much slower rate than that we absorb radiation?
     
  11. Jul 8, 2011 #10

    Pengwuino

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    You won't. Your body radiates at a much higher temperature than the CMB.
     
  12. Jul 8, 2011 #11

    Drakkith

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    Specifically referring to the CMB, it is at such a low energy that only a very very low temperature would be required to emit light of that frequency/spectrum. You and I are MUCH higher than the temperature of an object that would emit something similar to the CMB. So not only do we emit a higher frequency of light, aka infrared, we also emit more photons than we absorb.

    If you moved away from the shadow of the earth or spacecraft or whatever, and into direct sunlight, you would begin to heat up.
     
  13. Jul 9, 2011 #12

    kelvin490

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    Can we say that on earth our energy loss in radiation are mostly balanced by the energy of air particles around us (and other forms of energy)?
     
  14. Jul 9, 2011 #13

    phinds

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    On a cold day, no, we radiate more than the surroundings. On a hot day, no, we radiate less than the surroundings. On a Goldilocks day, yes, it exactly balances.
     
  15. Jul 9, 2011 #14

    Drakkith

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    What do you mean by balanced? Depending on the temperature of the air your body will lose or gain heat because of it. If the temperature of the air is say, 20 degrees F, the heat loss via the air is probably much greater than radiation.
     
  16. Jul 9, 2011 #15
    Not so. The amount we radiate is a function of our mean skin temperature (about 33°C), our radiative coefficient (about 0.95), and our mean skin area. It is independent of our surroundings. Net radiation, of course is another matter. However, the question was on how much we radiate.

    I gave my students a little problem on this once, but I can't find my notes. I seem to recall that the average human being radiates some 22,000 Calories (kilocalories) per 24-hour day.
     
  17. Jul 9, 2011 #16

    Drakkith

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    I think that is what phinds meant klimatos.
     
  18. Jul 10, 2011 #17

    phinds

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    I can't figure how you interpreted what I said as meaning that I thought the amount we radiate differs based on the surroundings. What I said was that we radiate MORE or LESS than our surroundings (or rarely, the same amount) which was a direct answer to the question, which was about BALANCE, not absolute amount. It seems to me that you misunderstood both the question I was answering and my answer.
     
  19. Jul 10, 2011 #18
    I appear to have done just that. I apologize. Sometimes I pay too much attention to the wording and not enough to the sense of a statement.
     
  20. Jul 10, 2011 #19

    phinds

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    No problem. I've done the same.
     
  21. Jul 11, 2011 #20
    What exactly are you talking about when you say we would radiate heat? When I think of heat transfer, I usually think conduction where a particle vibrating so fast hits another particle which is vibrating at a different speed and energy is transferred. If you are in a vacuum, how is the heat leaving your body?
     
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