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mass vs heat storage ability

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  • rohart
    replied
    Electrons are bound to atomic nuclei with forces that are massively greater than the forces that hold atoms in position in solid structures. Electrons couldn't care less if a substance gets hotter. Having said that, a photon will be absorbed by an electron and shift it up an energy level, which will add a small amount of momentum to the atom, and cause it to vibrate in position a little more.

    It's the amplitude of the vibration of atoms about their equilibrium positions that we measure as temperature.

    Brownian motion is the motion of minute visible particles - like particles of smoke or liquid borne dust or large bacteria - as they are buffetted around by collisions with atoms or molecules of the medium in which they are suspended. Large particles would be buffetted so often it would even out and you'd see nothing. Small particles may get hit just that bit more on one side than the other for us to see a little movement, jiggling about. This is Brownian motion. It's not the bulk gas or liquid we're seeing - it's the jiggling about of the larger particles. In a gas, the atoms or molecules are slamming into each other and bouncing off all the time at high speed, and far too fast to see. We see the effect of a larger particle being hit unevenly and jiggling about.

    Now having drooled all that out, I'll qualify it all by saying it's old science from the 60's and 70's, but I believe I can get away with it as I haven't had to refer to anything of quantum size. Everything I've mentioned is pretty basic large scale stuff.

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  • ed_h
    replied
    While the "planetary model" of the atom with its electrons in orbits around the nucleus may still be a useful picture for some things, it's pretty outdated and electrons don't really do that.

    Ed

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  • J Tiers
    replied
    Water has a high heat capacity in part because the molecules have so many vibrational modes.

    Other materials may be limited due to crystal lattice structure, etc.

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  • Erich
    replied
    As temp goes up the atoms vibrate more vigorously in the crystal lattice

    Absolute zero temp is the temperature where all atomic vibration stops.

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  • grzdomagala
    replied
    Hi Darryl

    Start with article about "ideal gas". It is a simplified model of heat and temperature of gas. This model assumes that atoms are infinitely small and hard particles. Calculations based on this model are useful only on limited range of temperatures and pressures - but it's value lies in fact that it's simple and gives some "intuitive feel". If you're interested in effects of temperature on electrons - im afraid the only way is to study quantum mechanics - but as far i know these effects become interesting if you are operting at very high temperatures (plasma) or with semiconductors that react very strong to such changes (it's funny that our computer technology is based on such minute changes )


    Wysłane z mojego GT-N7100 przy użyciu Tapatalka

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  • darryl
    replied
    Let me know if I'm just being an idiot here- but I'm trying to grasp the concept of what actually happens when matter increases in temperature. There is an expansion in almost every material, but what is actually going on in the atoms or molecules- do the electrons speed up as they travel longer distances in their orbits, or do the atoms vibrate to a greater extent (brownian motion?). Seems to me that if the electrons travel around the nucleus of an atom at a fixed rate (speed of light?) then they can't traverse a larger orbit without speeding up. If they are circulating at less than the speed of light, then it would seem that at a certain temperature limit they would achieve the speed of light- after which a further increase in temperature would have the effect of changing the entire dynamic.

    It seems to me that Brownian motion would have the electrons changing their orbital speed constantly, and at a pretty high frequency also as the atoms jiggle around.

    If we just look at expansion and contraction, do the atoms of a material space themselves farther apart of come closer together, or does the size of the atom change with temperature changes?

    Personally, I don't like the idea of limits- so when it comes to something heating up to beyond millions of degrees, or cooling to absolute zero- first of all why is there an absolute zero? Can matter cool below that temperature by some mechanism? And is there any limit to how how matter can get- is there a point at which a fundamental change in known physics occurs?

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  • Forrest Addy
    replied
    There is a difference between heat and temperature. Temperature is a measure of the effect of heat (energy increase or decrease) on a substance. Different substances are affected by heat to differing degrees which is listed in table of specific heats, among other things.

    Water has a high specific heat - the highest, I think, of common substances

    The specific heats of steel Vs aluminum are considerably different as mentioned above.

    You think about one aspect of a material's response to heat - change in temperature - and you have to consider rates of expansion, melting point, latent heat of fusion, eutectic triple point, emissivity, thermal conductivity, etc. Materials science is fun when it's not maddening.
    Last edited by Forrest Addy; 06-12-2017, 03:17 AM.

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  • wombat2go
    replied
    Yes,
    in your context
    By weight Al has 2 times the heat capacity of steel.

    By volume, steel has 1.5 times the heat capacity of Al

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  • darryl
    replied
    According to the one link that CalM posted, aluminum has twice the capacity to absorb heat as does steel, with silicon about in the middle between them. Lithium has almost four times the capacity of aluminum to absorb heat. I find that interesting. Lithium is an interesting material in more ways than one. Perhaps its ability to make a high capacity cell is related to its specific heat. Just trying to understand things-

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  • wombat2go
    replied
    Hi Darryl.
    I would agree with your comment except I don't understand it enough after many years.
    As I understand, science has caught up with the mechanics of the specific heat for pure elements and crystals,
    but maybe not with accuracy sufficient for industry.
    With metal "mixtures", and "alloys" including the carbon steels,
    we seem to be still in the days of having to measure it by empirical research.

    The reference I added to post #7 is accurate to +/- 2% in my experience, for steels up to about 0.4%C
    In USA, you can search on NASA and NIST.
    You might find enthalpy functions and graphs there.
    A step change in enthalpy ( for example by the heat of phase change of carbon steel at 727 Celsius )
    is differentiated to be an impulse in specific heat.
    Is it so? ... I am not saying.

    Hope you can study and let us know what you find!

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  • CalM
    replied
    http://www.engineersedge.com/materia...tals_13259.htm

    Water
    http://www.engineeringtoolbox.com/wa...es-d_1508.html

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  • darryl
    replied
    So there is a difference- you can't just calculate based on weight, though off-hand it would seem that you could. From what I've just been reading, heat is absorbed into a material according to how the atomic and/or molecular structure accepts it. I'm beginning to understand it now.

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  • wombat2go
    replied
    At 20 Celsius:
    Mass specific heats C:
    Steel C = 464 Watt.second per kilogram.Kelvin
    Aluminum C = 934 Watt.second per kilogram.Kelvin

    For steel, for example, it takes 464 Watt applied for 1 second to raise 1 kilogram by 1 Celsius.

    In furnace heating, we often use volume flow, so we use the volume specific heats:

    Steel Cy = 3.65e6 Watt.second / metre-cubed.Kelvin
    Aluminum Cy = 2.52e6 Watt.second / metre-cubed.Kelvin

    The mass and volume specific heats are related by the densities:
    Steel ( 0.3%C) d = 7860 kilogram/metre-cubed
    Aluminum d = 2697 kilogram/metre-cubed

    The specific heat of steels in solid state, varies with temperature
    For example steel at forging temperature of 1230 celsius has a volume specific heat of:
    Cy = 5.10e6 Watt.second / metre-cubed.Kelvin

    Ref for Cy:
    Davies E. J. "Conduction and induction heating" 1990 Peregrinus Pages 346~347
    Last edited by wombat2go; 06-11-2017, 10:49 PM. Reason: Add ref

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  • CalM
    replied
    Water wins if you want more from less.

    Water is not so good if you want less from more. ;-)

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  • J Tiers
    replied
    Different materials have different heat capacities, meaning the energy necessary to raise the temperature one degree.

    So the materials you mention should have different heat capacity. I do not recall what the difference is offhand for those.

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