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If a black object warms faster during the day, does it cool faster at night?

  1. Jun 29, 2010 #1
    no text
     
  2. jcsd
  3. Jun 29, 2010 #2

    russ_watters

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    Yes!

    .........................
     
  4. Jun 29, 2010 #3
    lol so this is related to black-body radiation? Does this actually translate to real-world examples, say I have a car with black upholstery that gets hot fast during the day, will it cool down faster at night than a car with white upholstery? Or is it more complicated than that?
     
  5. Jun 29, 2010 #4

    Why is that?
     
  6. Jun 29, 2010 #5

    russ_watters

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    No, it is no more complicated than that - you have it exactly right. Have you ever noticed condensation or frost on the outside of your car windows in the morning....but not necessarily on the hood or sides of the car? Ever wonder why....? Now you know!

    This is also a substantial problem for people like me who use reflecting telescopes. The telescope tube blocks radiation from the ground from hitting the mirror, but allows the mirror to radiate heat to the atmosphere. As a result, the interior of the telescope will get cooler much faster than the air outside and the mirror (or corrector plate covering the front of the tube) will gather condensation long before you start seeing it on the ground.
     
  7. Jun 29, 2010 #6
    Thanks!
     
  8. Jun 29, 2010 #7
    I think there are two distinct mechanisms here.

    Black body will lose more energy on radiation towards the sky at night, at the same temperature. But radiation may not be the primary cooling mechanism, especially for large and relatively cool objects. The cooling rate of an object with characteristic dimension ~1 m at room-temperature due to black-body radiation is on the order of 1 degree per hour. It will probably cool faster because of heat exchange with the atmosphere.
     
  9. Jun 29, 2010 #8
    I still dont understand how the color of an object affects its cooling at night. The power it puts off as a black body is proportional to its temperature, not its color. Is it simply because it gets hotter during the day that it cools down faster at night?

    confused about this
     
  10. Jun 29, 2010 #9

    Power is proportional to emissivity times temperature to the fourth power. Emissivity is higher for dark-colored objects, lower for bright colored and reflective objects.

    From thermodynamics standpoint, if you were to put your object inside a black-body cavity, you'd expect it to come to equilibrium at the same temperature as walls of the cavity. Otherwise you could build a perpetual motion machine that draws energy from the difference in temperatures of the object and the cavity.

    Since light-colored objects absorb less than 100% of incident radiation, it follows that they must also emit less.
     
  11. Jun 29, 2010 #10

    russ_watters

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    Yes, however an object can only cool below ambient via radiation.
     
  12. Jun 29, 2010 #11

    russ_watters

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    Further clarification: "black" doesn't just mean the color we can see, but also has to be applied to infrared to adequately deal with radiation. An object that is black in visible light color may or may not be "black" in the infrared.
     
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