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B Cooler objects able to increase the temperature of warmer objects?

  1. Nov 25, 2017 #1
    Not sure this should go here (you guys need a thermodynamics section). But there is quite the debate going on about cooler objects being able to increase the temperature of warmer objects here:
    https://wattsupwiththat.com/2017/11/24/can-a-cold-object-warm-a-hot-object/comment-page-1

    They are claiming that the IR from the colder object is absorbed by molecules of the warmer object, thereby thermalizing the molecules more, increasing the temp of the warmer object.

    Would like some feedback here about this.

    Thanks.
     
  2. jcsd
  3. Nov 25, 2017 #2

    Dale

    Staff: Mentor

    A cold object cannot warm a warm object without work being done.
     
  4. Nov 25, 2017 #3
    Yes I know that. The point in that debate is IR from the cooler object must be being absorbed by the warmer object. The question is, what happens to the IR photons from the cooler object when they come in contact with the atoms in the hotter object?
     
  5. Nov 25, 2017 #4
    Of course it can. The temperatur of an object in thermal equilibrium can be increased by any additional heat source, including colder objects.
     
  6. Nov 25, 2017 #5
    [sigh]

    No it cant. Entropy forbids a colder object from giving energy to a hotter object. Why is this such an issue? This is basic thermodynamics.
     
  7. Nov 25, 2017 #6

    anorlunda

    Staff: Mentor

    Heat flows from warm to cold. The warm object will send more IR to the cold one than it receives.
     
  8. Nov 25, 2017 #7
    These are the arguments I've been dealing with.

    Warm to cold is a NET flow of energy.
    Cold IR photons get absorbed by the warmer atoms which get "thermalized".

    So what happens to the warmer atoms when they encounter IR photons from a colder object? I say nothing happens. If anything, the atom immediately re-emits that energy.
     
  9. Nov 25, 2017 #8

    anorlunda

    Staff: Mentor

    Your over thinking it. Thermodynamics is the study of averages of many particles. You can not apply it one photon at a time.

    Just believe what @Dale said.
     
  10. Nov 25, 2017 #9

    russ_watters

    User Avatar

    Staff: Mentor

    The article doesn't say that (and "thermalizing" isn't a word). It says a warm object will be kept warmer by exchanging radiation with a cold object that is blocking an even colder object than if the cold object weren't there....which is kinda a "duh".

    Here's the point of the article:
     
  11. Nov 25, 2017 #10

    Dale

    Staff: Mentor

    Do you have a reference for this?
     
  12. Nov 25, 2017 #11
    Thermalizing is the term Willis used. I know it's not a word. And if you read through the comments, Willis et al state categorically that the colder objects emitting IR which when interacting with a warmer object, will increase the energy of that warmer object, and hence it's temperature.

    It is clear reading the comments that people do not understand the difference between energy, heat and temperature. One person even objected to the official definition of heat!!!
     
  13. Nov 25, 2017 #12

    russ_watters

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    Staff: Mentor

    No, he doesn't, unless he's posting under an alias in the comments section -- it looks to me like you are reading through comments to the article (and "thermalz..." is used there several times). Please quote exactly what you are referring to.
    No, that isn't how this works. In general, you need to quote exactly what you are referencing, but for this case, we're not going to argue by proxy with the comments section to an article. That's a waste of our time.
    You're skipping part of the article's claim, per my previous post.
    I really don't care -- is that what this is about? A proxy argument with the people commenting on that article? We don't do that here. We'll help you understand what the author is saying, but that's it. If you have a question of your own, ask it yourself.
     
  14. Nov 25, 2017 #13

    Dale

    Staff: Mentor

    This is correct, and it doesn’t matter if the mechanism of heat transfer is through thermal radiation or through thermal conduction. In both cases on a microscopic level there is energy going both ways and on a statistical level the energy goes from hot to cold.

    They can be absorbed, reflected, scattered, or transmitted, based on the spectral characteristics of the object. IR photons are not particularly different from other photons.
     
  15. Nov 26, 2017 #14
    Of course that's what happens. Just let’s do the math for a simple example: A spherical black body with the radius ##r## (e.g. Earth), heated by a heat source with the power ##P## (e.g. absorbed light from the Sun). In the dynamic equilibrium the body will emit as much energy as it absorbs from the heat source. Assuming a homogeneous temperature distribution this results in the equilibrium temperature

    [itex]T_{eq}^4 = \frac{P}{{4 \cdot \pi \cdot \sigma \cdot r^2 }}[/itex]

    according to the Stefan–Boltzmann law. That’s the warm object.

    Now let’s see what happens if we add another black body as additional heat source – let’s say a spherical shell (e.g. the atmosphere) with the temperature ##T_{add}## around the first body. This shell will also emit heat radiation. If this radiation hits the first body, it will be absorbed, resulting in the additional heat

    [itex]P_{add} = 4 \cdot \pi \cdot r^2 \cdot \sigma \cdot T_{add}^4[/itex]

    and therefore in the new equilibrium temperature

    [itex]\frac{P}{{4 \cdot \pi \cdot \sigma \cdot r^2 }} + T_{add}^4 > T_{eq}^4[/itex]

    no matter if ##T_{add}## is above, equal or below ##T_{eq}##.

    Claiming that this is not possible is a usual argumentation in climate change denial. I can’t believe that something like this is supported in this forum!
     
  16. Nov 26, 2017 #15

    mfb

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    2016 Award

    Staff: Mentor

    To thermalize is a word.

    There are two different things discussed here, and the lack of separation between them leads to the confusion.

    * If you look at an existing system and add a new object to it, that can increase the temperature of an existing object, even if the new object is colder than the existing object.
    * The net power flow between these objects (any pair of objects in fact) will be from hot to cold.

    Both statements are true, and they don't contradict each other.

    As an example, the presence of Mars increases the temperature of Earth a tiny bit, as some light from it reaches Earth. The heat Earth emits is independent of the existence of Mars, but the heat it gets is increased from Mars.
     
  17. Nov 26, 2017 #16

    Dale

    Staff: Mentor

    It is this power source that warms the object, not the colder object. This is work, as I mentioned above.

    Here you areassuming a cold reservoir at 0 K.


    Yes, if you have two different systems, one with a cold reservoir at 0 K and the other with a cold reservoir at a higher temperature then the equilibrium temperature will be higher for the system with the warmer cold reservoir. This is not an example of a cold reservoir increasing the temperature of a hot reservoir without work. Without P the temperature will drop to the temperature of the cold reservoir, not increase.

    I stay out of climate change arguments. What I said is correct, regardless of what other groups of people may misuse the concepts. Please do not attempt any “guilt by association” here.
     
  18. Nov 26, 2017 #17
    No, the object is warmed by the power source and by the colder object. Claiming that the colder object doesn't warm the warmer object is simply wrong. Any heat source contributes to the warming regardless of its temperature.

    The absorbed sunlight provides heat only. Tere is no work involved.

    That doesn't justify your general claim that "A cold object cannot warm a warm object without work being done."

    It would be correct if you would say that a cold object cannot warm a warm object without work being done if no additional heat sources and no heat sinks are involved. But you didn't do that.
     
  19. Nov 26, 2017 #18

    mfb

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    2016 Award

    Staff: Mentor

    It depends on your perspective. You can also say it cools, but it cools less than whatever else it blocks e.g. an even colder object).
     
  20. Nov 26, 2017 #19
    Objects radiate energy to the surroundings and absorb energy from the surroundings. Because of temperature differences the hot object will be a net radiator and cool down and the cold object a net absorber and heat up.
     
  21. Nov 26, 2017 #20
    Yes, I could say that, but we are talking about warming and not about cooling and it is no matter of perspective that any heat source contributes to the warming.
     
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