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Thread: Why does a vacuum flask (thermos) keep things warm?

  1. #1 Why does a vacuum flask (thermos) keep things warm? 
    Forum Cosmic Wizard icewendigo's Avatar
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    1- Why does a vacuum flask or thermos, keep things warm? I was under the impression that vacuum was cold somehow, like the cold of space?

    2- What is the temperature of rocks exposed to sunlight on the moon? Since theres no atmosphere should they be freezing, or does the unfiltered pure sunlight heat them up more than the coldness of space keeps them cold?


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    Quote Originally Posted by icewendigo View Post
    1- Why does a vacuum flask or thermos, keep things warm? I was under the impression that vacuum was cold somehow, like the cold of space?

    2- What is the temperature of rocks exposed to sunlight on the moon? Since theres no atmosphere should they be freezing, or does the unfiltered pure sunlight heat them up more than the coldness of space keeps them cold?
    There are three basic mechanisms for heat transfer: Conduction, convection and radiation. A vacuum flask seeks to minimize all three. By pumping the air out, there's no medium, so there's no convection or conduction (except through the thin material of the flask itself). By keeping the thing closed and opaque, radiation is minimized, too. That's how it keeps cold things cold and hot things hot.

    Space is "cold" only insofar as you radiate heat away, so unless there's a source of energy to balance that, you'll get cold and stay that way.

    Now for rocks on the moon: Incident light will tend to heat them up, but the rocks will radiate heat (at a rate proportional to the fourth power of temperature), so they won't get infinitely hot; they heat up to a temperature that results in a balance between the heating by incident light, and cooling by radiation. I recall that the peak temperatures on the moon's surface can get well above 100 degrees C. So much for the "coldness of space!"

    The foregoing ignores conduction to and from rocks and soil in contact with the rocks under consideration, of course. In reality conduction is going to be a factor (which would tend to moderate the swings somewhat).


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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by icewendigo View Post
    1- Why does a vacuum flask or thermos, keep things warm? I was under the impression that vacuum was cold somehow, like the cold of space?
    The other thing to bear in mind is that temperature is a measure of the energy (speed, very crudely) of the atoms/molecules making up a material. A vacuum has no atoms/molecules and therefore has no temperature.
    ei incumbit probatio qui dicit, non qui negat
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    Forum Cosmic Wizard icewendigo's Avatar
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    So in addition to photovoltaic solar panels, could an underground moon base with energy infrastructure on the surface, potentially accumulate heat and transfer the heat to deep underground reservoirs? (with pipes circulating a liquid between surface solar furnace and a deep reservoir that could then be used to provide heat during a period without daylight?)
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    Quote Originally Posted by tk421 View Post
    By keeping the thing closed and opaque, radiation is minimized, too.
    The glass bottle inside has a shiny surface which minimizes the radiative transfer of heat between the bottle and the outer metal container.
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    Iuvenis ducis Darkhorse's Avatar
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    As well the material used for the inner lining matters. Stainless steel will keep the liquid hotter longer than plastic due to the steel heating up and reaching equilibrium with the liquid faster than plastic. Less energy is drawn from the liquid to heat the material up.
    Ignorance more frequently begets confidence, than it does knowledge. [Charles Darwin]
    Physical laws are kinda like Pringles. It is hard to break just one law. [Dr. Rocket]
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    Quote Originally Posted by icewendigo View Post
    So in addition to photovoltaic solar panels, could an underground moon base with energy infrastructure on the surface, potentially accumulate heat and transfer the heat to deep underground reservoirs? (with pipes circulating a liquid between surface solar furnace and a deep reservoir that could then be used to provide heat during a period without daylight?)
    Sure, it wouldn't be cheap, and the most effective but yes. Theoretically you can store practically all the incoming energy as heat. It just isn't all so usefull.
    In the information age ignorance is a choice.
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    Quote Originally Posted by Darkhorse View Post
    As well the material used for the inner lining matters. Stainless steel will keep the liquid hotter longer than plastic due to the steel heating up and reaching equilibrium with the liquid faster than plastic. Less energy is drawn from the liquid to heat the material up.
    I don't follow your argument. Steel will heat up faster, because it is more conductive. But for as long as the liquid is being stored, the plastic will also effectively reach an equilibrium before the liquid is consumed, so any difference would be due to the difference in heat capacity. And if anything, the steel heating up will cool the liquid, which draws more, not less, energy from the liquid.
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    Iuvenis ducis Darkhorse's Avatar
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    Quote Originally Posted by Harold14370 View Post
    Quote Originally Posted by Darkhorse View Post
    As well the material used for the inner lining matters. Stainless steel will keep the liquid hotter longer than plastic due to the steel heating up and reaching equilibrium with the liquid faster than plastic. Less energy is drawn from the liquid to heat the material up.
    I don't follow your argument. Steel will heat up faster, because it is more conductive. But for as long as the liquid is being stored, the plastic will also effectively reach an equilibrium before the liquid is consumed, so any difference would be due to the difference in heat capacity. And if anything, the steel heating up will cool the liquid, which draws more, not less, energy from the liquid.
    It is a counter intuitive for sure.

    My point relies on what both you and tk421 have mentioned. In a vacuum there is very little convection (transfer of heat through fluid/gas motion), or conduction (transfer of heat through molecular movement and interaction). There is radiation loss through the vacuum. That loss can be reduced by changing the properties of the material used as a liner.

    However conduction does happen in the liner material until the material reaches the same temperature as the liquid. Since the liner wall is a different temperature than the liquid heat will flow from the liquid to the wall in an attempt to reach equilibrium. The amount of energy required to raise the temperature of a material is known as it's heat capacity. Here are the values for plastic and steel.

    Plastic = 2.3027
    Steel = 0.466

    My stainless steel mug weighs in at 300g with the top off, so let say the actual inside material weighs about 150g. If we assume that the liner is at room temperature 30C and the coffee is at 90C we are looking at a 60C difference.

    20.724 kj = .15(kg) 2.3027 (kJ/kg0C) 60 (oC)
    4.194 kj = .15(kg) 0.466 (kJ/kg0C) 60 (oC)

    So to raise the temperature of the plastic mug to the temperature of the coffee we need to expend 16.5 kj more than when using a steel mug.

    Once that material is up to the same temperature we can consider the radiation factor. In order to calculate the value of heat transfered through radiation we need to consider the emissivity of the material (how efficient the material transfers heat) and the absolute material temperature (which doesn't really matter here).

    An emissivity value of 1 would be a perfect black body that transfers heat with no loss, a value of 0 would be a perfect heat reflector.

    Plastic has an emissivity value of anywhere from 0.89 to 0.95 resulting in a value about 1.01 kW/m^2
    Stainless steel has an emissivity value of 0.59 resulting in .65 kW/m^2

    So stainless steel reflects more radiation than plastic, however this is not just because stainless steel is shinier than plastic, it relates to the internal structure of the material as well. For example gravel has an emissivity value of .28 resulting in .31 kW/m^2.

    Stainless steel heats up quicker than plastic therefore it reaches it's max radiation value more rapidly, however that radiation value is less than the radiation value for plastic so once the plastic heats up it radiates more energy.

    Moral of the story, use stainless steel mugs, and preheat your cup or drink your coffee faster. :-)
    Ignorance more frequently begets confidence, than it does knowledge. [Charles Darwin]
    Physical laws are kinda like Pringles. It is hard to break just one law. [Dr. Rocket]
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