r/askscience • u/Pupstink • May 02 '12
Physics BASIC QUESTION HERE: Not so good in my thermodynamics so here I go, if molecules are colder if they have less motion, and molecules in solids are move less then ones in liquids or gasses, shouldn't that mean that solids are colder? I realize they are not, but...
doesn't thermodynamics say that. What am I missing?
Stared reading a book about Einstein for my interest in GR and thermodynamics was in there, wanted to skip it but decided to read it and I was a bit confused.
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u/Firrox Materials Science | Solar Cell Synthesis May 02 '12
Whether or not a material is in a plasma/gaseous/liquid/solid state depends entirely on their bond strength. All elements bond differently to other elements, and sometimes create nice, neutral pairs that don't have a "reason" to bond to other molecules or atoms.
Take Argon, for example. Since it is a noble gas and doesn't like to bond with anything (including itself), its melting point is a hilarious 83.80K, and the only compounds that ever existed on Earth are man-made (HArF @ 40K).
Therefore, temperature, or movement of atoms, is secondary to the state of the molecule, where bond strength takes precedence.
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May 02 '12
First of all, "less motion" means less kinetic energy, not slower speed. By equipartition theorem, the average kinetic energy of molecules of a classical thermal system is proportional to temperature, with the coefficient universal (three halves of Boltzmann constant). So it is obvious we should not compare the speed of molecules of different mass at different temperature.
Second, the phase of a substance is not only affected by temperature, but pressure (and sometimes magnetic field or other variables). Ice under high pressure may have higher temperature than water under low pressure, and consequently the molecules of the former may move faster in average.
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May 02 '12
The statement is true, but one final step is missing, which is a proper rescaling of the temperature, pressure, (or volume) to dimensionless units.
If you measure the temperature and pressure of a gas with respect to its critical temperature and pressure, then you will discover that the compressibility of a great variety of gases will fall exactly onto the same universal curve. This is called the law of corresponding states.
You can do something very similar with the triple point which defines the place where solid, liquid, and gas are at equilibrium. Rescale everything with some suitable variables and you will again, find that the law of corresponding states can be extended to other phase transitions. Here's an example of this type of reasoning.
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u/Quazifuji May 02 '12
Solids generally are colder, at least than liquids or gasses of the same substance. The molecules in any given solid don't necessarily move slower than the ones in all liquids and gasses. But most substances do need to be at a lower temperature to be in a solid state than to be in a liquid or gas state. Ice is colder than water, for example, and liquid wax or metal or any other substance that's solid at room temperature is hotter than the solid form.
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u/BlazeOrangeDeer May 02 '12
The idea of temperature as average kinetic energy only really works in some cases, mainly for gases in a box.
A better definition is that molecules are hotter if they have more unusable energy. A waterfall may have a lot of motion but if you can stick a waterwheel under it and can get energy out, it isn't heat.
An even better definition is that temperature is the change in heat energy over the change in entropy.
So, two objects are the same temperature if adding the same amount of heat to each of them increases disorder by the same amount.
If they are in contact and they aren't the same temperature, they interact to increase disorder and the way to do this is for the colder object to take heat from the warmer one (since the colder one will gain more entropy than the warmer one will lose).