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Thread: Dark colored objects and heat

  1. #1 Dark colored objects and heat 
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    Most people reading this probably know that dark objects absorb light (and its heat energy) while light/white objects reflect it and stay cooler. This means my black roof makes for a very hot attic. My question is, do the dark/black objects also absorb more heat in the absence of light? For instance, there is no direct sunlight inside my attic - but it's a scorching 110 degrees in there during the summer... if I have 2 plastic bins in the attic, one white - one black, does the black one get hotter because it absorbs heat/energy waves more than the white bin?

    I wouldn't think so, but I was in my attic the other day thinking about it while doing some work.


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    I think so, but I'm not an expert in this matter. I think that black material is black because it absorbs energy, so absorbing the light makes it black. So if it also absorbs heat energy, this explains why black objects are hotter. So in the absence of light, it makes sense that it would also absorb heat.

    One interesting thing that I thought of is that it doesn't absorb heat energy because it's black, it's black because it absorbs energy


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    That's not really how color works. It's true that a black object is black because it absorbs light of all visible colors, but we can't see how it reacts to light outside the visible spectrum. Some black objects might be really good reflectors in the infrared range, for example.

    Heat is transfered in three different ways. Radiation, conduction and convection. Radiation is where hot objects give off light (look up Black Body Radiation) and other objects absorb light. Conduction is where a hot object is touching a cooler object and transfers heat. Convection is the transfer of heat through a fluid (like air). Of these, only radiation has anything to do with color.
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    your two boxes will be at 110F-the air temperature.
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    There will be some heat transfer by radiation from the hot ceiling to the objects inside, but it will be in the long infra-red wavelengths. Most non-metallic surfaces have emittances close to unity in the long infra-red, no matter what color they are in the visible spectrum. So, they will end up at the same temperature. I think they might be a little wee bit warmer than the air, though - not enough to notice, probably.
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    Quote Originally Posted by MagiMaster
    That's not really how color works. It's true that a black object is black because it absorbs light of all visible colors, but we can't see how it reacts to light outside the visible spectrum. Some black objects might be really good reflectors in the infrared range, for example.
    Or the opposite, an object may reflect visible light while being a good infrared absorber/emitter. In the past, a white lead-based paint was sometimes used for household radiators for this very reason.
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  8. #7 Re: Dark colored objects and heat 
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    Quote Originally Posted by ducati321
    Most people reading this probably know that dark objects absorb light (and its heat energy) while light/white objects reflect it and stay cooler. This means my black roof makes for a very hot attic. My question is, do the dark/black objects also absorb more heat in the absence of light? For instance, there is no direct sunlight inside my attic - but it's a scorching 110 degrees in there during the summer... if I have 2 plastic bins in the attic, one white - one black, does the black one get hotter because it absorbs heat/energy waves more than the white bin?

    I wouldn't think so, but I was in my attic the other day thinking about it while doing some work.
    If the object is truly black it will absorb all wavelengths.

    In the absence of all light there is nothing to absorb.

    But if there is infrared radiation, and if the object absorbs all wavelengths then it will absorb the infrared that is commonly associated with heat, assuming that it is colder than the surroundings.

    Black objects are also efficient radiators, so if they are warmer than their surroundings they lose heat by radiation more readily that objects of other colors.
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    Ok , so what is the temperature of the two boxes, I maintain its 110F, that is the original question you know.
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    Quote Originally Posted by fizzlooney
    Ok , so what is the temperature of the two boxes, I maintain its 110F, that is the original question you know.
    You gave no initial or boundary conditions.

    The temperature is 12, scale to be decided later.

    If you wait a long time and don't change the conditions, things in your attic will eventually equilibrate and everythig will be at the same temperature.
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    Guys, I'm right about this one.

    There is no equilibrium. The attic is probably vented so there will be some cooler outside air coming in and settling at floor level. Even if there is no ventilation, there will be some flow of heat down through the floor into the cooler living spaces below. This means there will be a temperature gradient with the hottest air at the peak and the coolest down on the floor where the bins are sitting. If the bins are cooler than the roof, there will be a net flow of heat by radiation from the roof to the bins, this heat being dissipated to the surroundings by conduction and convection. For conduction and convection to occur there must be a temperature gradient between the bins and the surrounding air.
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    There are a lot of interesting comments here...

    I'm going to take away from this that without any direct contact with light - the boxes would maintain the same temperature, whereas if they were both exposed to direct sunlight, the black box would get hotter.

    thanks for humoring me folks
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  13. #12  
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    Quote Originally Posted by ducati321
    There are a lot of interesting comments here...

    I'm going to take away from this that without any direct contact with light - the boxes would maintain the same temperature, whereas if they were both exposed to direct sunlight, the black box would get hotter.

    thanks for humoring me folks
    Then you don't understand heat transfer.
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    It's a balance between emissivity and absorption. Some black colors are actually cooler to wear as clothing in the summer because they radiate as a black body radiator more energy than other colors. If the absorb only the visible spectrum, but radiate the entire spectrum...
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    Quote Originally Posted by Wild Cobra
    It's a balance between emissivity and absorption. Some black colors are actually cooler to wear as clothing in the summer because they radiate as a black body radiator more energy than other colors. If the absorb only the visible spectrum, but radiate the entire spectrum...
    Anything that radiates a given wavelength well, will absorb that wavelength equally well.
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  16. #15 Re: Dark colored objects and heat 
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    Quote Originally Posted by ducati321
    Most people reading this probably know that dark objects absorb light (and its heat energy) while light/white objects reflect it and stay cooler.

    let me help you with that

    black bodies (all mass absorbs) the energy or you could not see it

    the black (mass that absorbs most all wavelengths) is not refracting or returning it (them colors) for you to see

    White on the other had is refracting, most all of them.

    all heat is just of em (light) upon the mass resonating. (it is not so much the colors we see, as the mass/energy combination that governs what 'color' ....'heat'....'energy'... the mass can retain.

    bet you take a black piece of iron and can make it white HOT






    This means my black roof makes for a very hot attic. My question is, do the dark/black objects also absorb more heat in the absence of light? For instance, there is no direct sunlight inside my attic - but it's a scorching 110 degrees in there during the summer... if I have 2 plastic bins in the attic, one white - one black, does the black one get hotter because it absorbs heat/energy waves more than the white bin?

    I wouldn't think so, but I was in my attic the other day thinking about it while doing some work.
    you just keep that tinker tinking................... "yes" that energy is associating and often the isolated mass will be hotter than its immediate evironment, because the energy IS associating upon THAT mass (the elements within)

    think of colors; for example of white light, the light is absorbed, then you see the refracted portion


    ie.... are you running on 98.6 for the most part, ever wonder why?

    you take in energy (upon mass/calories) and them reactions (not reductions) are releasing energy and the other structures are capturing them (resonant energy transfer)........

    far more complex to this conversation but the consistancy of the ideology DOES APPLY
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  17. #16 Re: Dark colored objects and heat 
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    Quote Originally Posted by Bishadi
    Quote Originally Posted by ducati321
    Most people reading this probably know that dark objects absorb light (and its heat energy) while light/white objects reflect it and stay cooler.

    let me help you with that

    black bodies (all mass absorbs) the energy or you could not see it

    the black (mass that absorbs most all wavelengths) is not refracting or returning it (them colors) for you to see

    White on the other had is refracting, most all of them.

    all heat is just of em (light) upon the mass resonating. (it is not so much the colors we see, as the mass/energy combination that governs what 'color' ....'heat'....'energy'... the mass can retain.

    bet you take a black piece of iron and can make it white HOT






    This means my black roof makes for a very hot attic. My question is, do the dark/black objects also absorb more heat in the absence of light? For instance, there is no direct sunlight inside my attic - but it's a scorching 110 degrees in there during the summer... if I have 2 plastic bins in the attic, one white - one black, does the black one get hotter because it absorbs heat/energy waves more than the white bin?

    I wouldn't think so, but I was in my attic the other day thinking about it while doing some work.
    you just keep that tinker tinking................... "yes" that energy is associating and often the isolated mass will be hotter than its immediate evironment, because the energy IS associating upon THAT mass (the elements within)

    think of colors; for example of white light, the light is absorbed, then you see the refracted portion


    ie.... are you running on 98.6 for the most part, ever wonder why?

    you take in energy (upon mass/calories) and them reactions (not reductions) are releasing energy and the other structures are capturing them (resonant energy transfer)........

    far more complex to this conversation but the consistancy of the ideology DOES APPLY
    This is just wrong. You don't even know the difference between "refracting" and "reflecting".
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  18. #17  
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    Quote Originally Posted by Janus
    Quote Originally Posted by Wild Cobra
    It's a balance between emissivity and absorption. Some black colors are actually cooler to wear as clothing in the summer because they radiate as a black body radiator more energy than other colors. If the absorb only the visible spectrum, but radiate the entire spectrum...
    Anything that radiates a given wavelength well, will absorb that wavelength equally well.
    Yes, I stand corrected on that part. Still, temperature plays an effect on the spectra. My point was that a visually black item could actually absorb less solar radiation than something that is white. The black absorbs the 400 to 700 nanometer range, whereas a white would reflect it. Now if that is all the black absorbs, but the white also absorb much of the 200-400 and 700-2000 nanometer range, then the white absorbs more energy. There are fabrics and dyes engineered to do similar things. Such things have miltary applications as well for invisibility to IR. Anyway, the white would receive more energy than the black. Then if the black has larger spectral properties in the 30 to 60 micrometer range, it will emit heat better than the white. The relative difference is very notable.
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  19. #18  
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    Quote Originally Posted by Wild Cobra
    Quote Originally Posted by Janus
    Quote Originally Posted by Wild Cobra
    It's a balance between emissivity and absorption. Some black colors are actually cooler to wear as clothing in the summer because they radiate as a black body radiator more energy than other colors. If the absorb only the visible spectrum, but radiate the entire spectrum...
    Anything that radiates a given wavelength well, will absorb that wavelength equally well.
    Yes, I stand corrected on that part. Still, temperature plays an effect on the spectra. My point was that a visually black item could actually absorb less solar radiation than something that is white. The black absorbs the 400 to 700 nanometer range, whereas a white would reflect it. Now if that is all the black absorbs, but the white also absorb much of the 200-400 and 700-2000 nanometer range, then the white absorbs more energy. There are fabrics and dyes engineered to do similar things. Such things have miltary applications as well for invisibility to IR. Anyway, the white would receive more energy than the black. Then if the black has larger spectral properties in the 30 to 60 micrometer range, it will emit heat better than the white. The relative difference is very notable.
    There may a be a terminology problem here.

    I think that when you say "black" or "white" you mean sometehing that looks black or white to the human eye.

    In most physics contexts a "black" object is something that absorbs all wavelenghts, visible or not, and a "white" object is one that reflects all wavelengths.
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  20. #19  
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    Quote Originally Posted by DrRocket
    Quote Originally Posted by Wild Cobra
    Quote Originally Posted by Janus
    Quote Originally Posted by Wild Cobra
    It's a balance between emissivity and absorption. Some black colors are actually cooler to wear as clothing in the summer because they radiate as a black body radiator more energy than other colors. If the absorb only the visible spectrum, but radiate the entire spectrum...
    Anything that radiates a given wavelength well, will absorb that wavelength equally well.
    Yes, I stand corrected on that part. Still, temperature plays an effect on the spectra. My point was that a visually black item could actually absorb less solar radiation than something that is white. The black absorbs the 400 to 700 nanometer range, whereas a white would reflect it. Now if that is all the black absorbs, but the white also absorb much of the 200-400 and 700-2000 nanometer range, then the white absorbs more energy. There are fabrics and dyes engineered to do similar things. Such things have miltary applications as well for invisibility to IR. Anyway, the white would receive more energy than the black. Then if the black has larger spectral properties in the 30 to 60 micrometer range, it will emit heat better than the white. The relative difference is very notable.
    There may a be a terminology problem here.

    I think that when you say "black" or "white" you mean sometehing that looks black or white to the human eye.

    In most physics contexts a "black" object is something that absorbs all wavelenghts, visible or not, and a "white" object is one that reflects all wavelengths.
    You are correct. I was referring to visual black and white. I think if someone follows with the wavelengths I listed, that's apparent.

    Isn't what you speak of the difference between "black" and "black body?"
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  21. #20  
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    Quote Originally Posted by Wild Cobra

    Isn't what you speak of the difference between "black" and "black body?"
    Generall a "black" object is a "black body", the terms being interchangeable.

    The problem with an object that is just visually black is precisely what you observed. it does not tell you much about the absorption or reflection outside of visual wavelengths and hece you don't know much about the behavior in terms of radiative heat transfer without additional information.

    The problem is not that what you said is wrong. It is correct. It is just that precise language for the application of the mathematics of heat transfer requires knows the behavior of the material at all wavelengths. So in that context "black" means absorbs everything. If you want to talk about something that does not, then that needs to be explicitly specified and the absorption spectrum clearly identified.
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  22. #21  
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    Quote Originally Posted by DrRocket
    Quote Originally Posted by Wild Cobra

    Isn't what you speak of the difference between "black" and "black body?"
    Generall a "black" object is a "black body", the terms being interchangeable.

    The problem with an object that is just visually black is precisely what you observed. it does not tell you much about the absorption or reflection outside of visual wavelengths and hece you don't know much about the behavior in terms of radiative heat transfer without additional information.

    The problem is not that what you said is wrong. It is correct. It is just that precise language for the application of the mathematics of heat transfer requires knows the behavior of the material at all wavelengths. So in that context "black" means absorbs everything. If you want to talk about something that does not, then that needs to be explicitly specified and the absorption spectrum clearly identified.
    Well, I'm old school. Must be the new-fangled way of speaking. To me, you must specify black body if that is what you mean. Black is too nonspecific.

    Must be something like writing:

    OK, i c u 8 that apple.

    I make several mistakes myself, but you will not convince me that black is assumed to mean black body. To me, it's just people accepting laziness.
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  23. #22  
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    Both bins will equilibrate to a temp 0f 110 F under the conditions initially stated .
    The end.
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  24. #23 Re: Dark colored objects and heat 
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    Quote Originally Posted by DrRocket
    Quote Originally Posted by Bishadi
    Quote Originally Posted by ducati321
    Most people reading this probably know that dark objects absorb light (and its heat energy) while light/white objects reflect it and stay cooler.

    let me help you with that

    black bodies (all mass absorbs) the energy or you could not see it

    the black (mass that absorbs most all wavelengths) is not refracting or returning it (them colors) for you to see

    White on the other had is refracting, most all of them.

    all heat is just of em (light) upon the mass resonating. (it is not so much the colors we see, as the mass/energy combination that governs what 'color' ....'heat'....'energy'... the mass can retain.

    bet you take a black piece of iron and can make it white HOT






    This means my black roof makes for a very hot attic. My question is, do the dark/black objects also absorb more heat in the absence of light? For instance, there is no direct sunlight inside my attic - but it's a scorching 110 degrees in there during the summer... if I have 2 plastic bins in the attic, one white - one black, does the black one get hotter because it absorbs heat/energy waves more than the white bin?

    I wouldn't think so, but I was in my attic the other day thinking about it while doing some work.
    you just keep that tinker tinking................... "yes" that energy is associating and often the isolated mass will be hotter than its immediate evironment, because the energy IS associating upon THAT mass (the elements within)

    think of colors; for example of white light, the light is absorbed, then you see the refracted portion


    ie.... are you running on 98.6 for the most part, ever wonder why?

    you take in energy (upon mass/calories) and them reactions (not reductions) are releasing energy and the other structures are capturing them (resonant energy transfer)........

    far more complex to this conversation but the consistancy of the ideology DOES APPLY
    This is just wrong. You don't even know the difference between "refracting" and "reflecting".

    perhaps you are not up to speed


    basic wiki

    In fact, reflection of light may occur whenever light travels from a medium of a given refractive index into a medium with a different refractive index. In the most general case, a certain fraction of the light is reflected from the interface, and the remainder is refracted. Solving Maxwell's equations for a light ray striking a boundary allows the derivation of the Fresnel equations, which can be used to predict how much of the light reflected, how much is refracted in a given situation. Total internal reflection of light from a denser medium occurs if the angle of incidence is above the critical angle


    no matter how you skin it, the energy is absorbed and re-emitted



    Light waves incident on a material induce small oscillations of polarisation in the individual atoms, causing each atom to radiate a weak secondary wave (in all directions like a dipole antenna). All of these waves add up to specular reflection (following Hero's equi-angular reflection law) and refraction. Light–matter interaction in terms of photons is a topic of quantum electrodynamics, and is described in detail by Richard Feynman in his popular book QED: The Strange Theory of Light and Matter.


    thought other might wish to read a weee bit more to over come your ranting
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    At a fine enough level, yes, all radiation is absorbed and then reemitted, but that does not mean that reflection and refraction are the same thing.
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  26. #25  
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    Quote Originally Posted by Wild Cobra
    Well, I'm old school. Must be the new-fangled way of speaking. To me, you must specify black body if that is what you mean. Black is too nonspecific.

    Must be something like writing:

    OK, i c u 8 that apple.

    I make several mistakes myself, but you will not convince me that black is assumed to mean black body. To me, it's just people accepting laziness.
    That is precisely the problem. Black is either a black body or it is completely nonspecific. And being nonspecific there is no way that you can analyze or adequately describe the relevant physics -- you don't know what wavelenghts are absorbed. To do anything quantitative you would need to specify clearly the absorption spectrum.

    So if you do not accept that a black object is a black body, what is your definition of a black object ? Is that definition sufficient to permit answering questions involving heat transfer ?

    White is the same way. It does not mean "appears white to the human eye", it means it reflects all wavelenghts. White light is by definition an equal mixture of all wavelengths. White noise has a flat Fourier transform. They don't really exist, and are just idealizations for the purpose of simplified analysis. A white object reflects all wavelengths incident upon it.

    These are all idealizations, but are at least clearly defined idealizations.
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    Quote Originally Posted by DrRocket
    describe the relevant physics -- you don't know what wavelenghts are absorbed. To do anything quantitative you would need to specify clearly the absorption spectrum.
    so what wavelength is 'heat'?
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    Heat is generally accepted to be the IR band as such
    The International Commission on Illumination (CIE) recommended the division of optical radiation into the following three bands:[4]

    IR-A: 700 nm–1400 nm
    IR-B: 1400 nm–3000 nm
    IR-C: 3000 nm–1 mm
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    Quote Originally Posted by fizzlooney
    Heat is generally accepted to be the IR band as such
    The International Commission on Illumination (CIE) recommended the division of optical radiation into the following three bands:[4]

    IR-A: 700 nm–1400 nm
    IR-B: 1400 nm–3000 nm
    IR-C: 3000 nm–1 mm

    i like it..... you're trying.

    common sense tells you what you posted and you could perhaps make a fine magnetron for heating water, knowing that. (2450) (one wavelength, for the most part, for microwave ovens; it heats the h2o structure causing it to resonate, that changes the temp (resonance of the other mass), through-out the specimen (food))

    but mass at absolute (approximate) cold (BEC- Bose Einstein Condensate) that increases it resonance (state of energy) is from em (light) in one wavelength or another.

    now each group of mass can only retain certain wavelengths and why 'slowing' is often apparent and observed

    when too much (energy) is captured, a structure may release (momentum to mass) from the group. (look up how BEC is cooled and the trap created (colorado univ/MIT)

    my point is, all mass is capturing and releasing em in one or more of her illustrious wavelength or a portion there of (entangled system)

    ie... for a (per se) electron to do anything, it must have 'energy' upon that structure to go anywhere (be causal)

    so what i am sharing is an set of eyes to view a different 'perspective'

    Heat is just "energy upon mass" and the easiest way to observe that is put a hot and cold piece of iron side by side and compare them; what is on that mass to cause that effect?






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    Screw this , Have a nice holiday vacation
    Bye
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  31. #30  
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    Quote Originally Posted by Bishadi
    Quote Originally Posted by DrRocket
    describe the relevant physics -- you don't know what wavelenghts are absorbed. To do anything quantitative you would need to specify clearly the absorption spectrum.
    so what wavelength is 'heat'?
    5
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