Hi, I would like to know what more qualified individuals think about the physics of this experiment. Two steel plates about an inch thick pressed together vertically with a sprung force of 100N per plate. The temperature at the surface of one plate is sustained at -200'C while the surface of the other plate is sustained at 200'C. Are there any affects on the sprung force?
One plate expands by: \(\frac{\Delta V}{V} = \int_{0}^{+200}\alpha_V(T)\,dT\) The other plate contracts by: \(\frac{\Delta V}{V} = \int_{0}^{-200}\alpha_V(T)\,dT\) As long as the coefficient \(\alpha(T)\) is identical on both interval \([0,+200]\) and \([-200,0]\) there is no effect on the sprung force. If the coefficient behaves in a non-homogeneous way, then there is going to be either a net contraction or a net expansion of the two-plate assembly, hence, there is going to be a force that "aids" or "counters" the external force.
Thanks for that Tach, let's assume absolute perfect coefficients and that there is no push or pull in either direction due to contraction/expansion, even if there is the equipment is able to compensate for it and maintain 100N per plate. What about thermodynamics? Does preventing energy from transferring through convection defer energy else where I wonder. Heat allows atoms to move more quickly and easily, but perhaps the atoms in this condition gain even more energy because they are in a state of suspension. The overall distance they can move is reduced because they are in a convection push of war (cannot think of a better analogy there, convection compression if there is such a thing), however they can still move and when they move the velocity at which they move is increased. These atoms are at only perhaps 0'C but they are highly energised, doesn't the conservation of energy mean that preventing convection (kinetic energy) results in potential energy?
It sounds like you're asking whether there's a direct relation between the spring force and the temperature of the things they're pushing together. There isn't. I wouldn't have thought of the heat\(\rightarrow\)expansion\(\rightarrow\)pressure effect if Tach hadn't pointed it out, so I won't make any generalizations about indirect effects. But in terms of direct effects, you don't need more spring potential energy to "counteract" the increased kinetic energy or anything like that. Think of it this way: if you just have a metal plate on a table and you heat it up, you don't suddenly need springs to hold it together. Similarly, if you already have springs in place when you heat it up, you don't need any more spring force to keep things the same.
Yes I realised yesterday that using a sprung force wouldn't work because there would have to be movement to change the force but the plates aren't just going to form a gap. So it's pressure I wanted to measure, for that the plates would be clamped together and the clamps would have pressure sensors built into them to measure any change in pressure. Would there be any change in pressure considering what I said in the my previous post about kinetic energy turning into potentional energy?
I don't think so. Surely Tach's previous answer has this right: if the coefficient of thermal expansion is the same over both temperature ranges, then the cold plate will contract by the same amount as the hot one expands and there will be no change in pressure. I don't understand where this idea about converting atomic scale kinetic energy into potential energy comes from. As the material is solid, the atoms are vibrating about fixed positions in the crystal lattice. In a vibration there is constant interconversion of kinetic and potential energy. At higher energy levels, the amplitude of the vibration is greater, as the kinetic energy is sufficient to overcome the attractive bonding between the atoms to a greater degree. This is what causes expansion when solids get hotter. If this tendency to expand were forcibly restricted, as for example in a welded railway rail, then as it is heated the metal would be increasingly put into a state of compression. But in your set up one plate gets cold by the same amount as the other is heated, doesn't it?
The steel is a solid and is static however the kinetic convection energy moves and flows always, that is the law of thermodynamics. Forget about any expansion and contraction, let's assume we can set the condition so that there is equal expansion and cotraction resulting in no change in pressure on the sensors from expansion and contraction. I'm really only interested in the affects on pressure from manipulating the flow of kinetic energy, am struggling to think of an anology but I will try.
There is no convection in this process. Convection only occurs in fluids. This setup is solid. Do you perhaps mean conduction? There will be plenty of that of course, from the hot plate to the cold one.
It is somewhat difficult to understand what you are trying to say. Are you talking about heat transfer through convection? Which law is that? Are you asking what will be the affect on pressure due to heat transfer by convection? If that is what you are asking, then it would have no effect based on your conditions that the expansion and contraction were equal resulting in no pressure change.
@exchemist, yep conduction for solids my mistake. I said affects on pressure from manipulating the flow of kinetic energy. Also using the term potential energy is not right. I meant more that manipulating the flow of kinetic energy might affect pressure potentially.
The reason that the pressure would change is due to the expansion or contraction of the material due to the temperature. The heat flow itself has no affect on the presures, it is the response of the material to the change in temperature that results in a pressure change. The CTE (Coefficient of Thermal Expansion) of the material will determine magnatude of the expansion (hence the pressure change in your scenario) per degree of temperature change. Increasing the rate of heat transfer will only increase the rate of pressure change because the temperature will increase faster.
OK, so conduction then, from the hot plate to the cold one, right? Well, as I said before, the only kinetic energy involved in this is that due to the vibration of the atoms. Those in the hot plate vibrate more vigorously than those in the cold one, and at the junction between the two, the collisions between them transfer kinetic energy from the hot to the cold. However this kinetic energy is absorbed in stimulating greater degrees of vibration in the atoms of the colder plate, thus warming it up. So yes there is a flow of thermal energy. But this creates no macro level force, or pressure, between the plates, because the amplitude of vibration of the atoms is only a few fractions of a nanometre. If the hot plate were hot enough to boil off some of the atoms, then yes a pressure would be created of course, by the gas thus produced.
Well you mentioned from hot to cold which is essentially what I'm talking about, the flow of conduction. Energy flows between atoms until there is greater equilibrium and in nature the flow is usually unrestricted or unsprung. If I consider snooker balls as atoms and line 5 balls up 10cm apart then make a perfectly and impossibly straight shot with the cue ball through the 5 balls, the energy flows through the balls, kinetic energy is reduced as each ball strikes the next and the final ball rolls off outwards in the same direction the cue ball was hit (flow) at a reduced speed relative to the speed of the original cue ball. A bit dull and boring but this is what is most commonly seen in nature i.e. one direction, in one end out the other. That's all obvious but what happens if the flow of kinetic energy is restricted. What if the flow is artificially restricted so that each ball provides more and more resistance to kinetic energy to the point where the final ball hardly moves at all (-200'C). Wouldn't this change the direction of the flow in which the kinetic energy travels, if the energy is turned back on itself while the flow of energy is being sustained from the hot plate, wouldn't this create pressure somewhere in between? Hard to compare this to the snooker ball analogy because it's not really possible to keep supplying sustained kinetic energy but if the final ball is tucked up against a cushion, the second to last ball will bounce back off it and the flow of kinetic energy will reverse. If there was more kinetic energy coming from the other direction, well some ball at some point is going to experience pressure and if there is enough pressure there is force, maybe even explosive force.
That is simply called an insulating material. If the temperature was not effeciently transfered to the next atom then the rate of heat transfer would be slower. Look up heat capacity. No it would simply result in a large difference in temperature (\(\Delta T\)) over a small distance. It would result in stress in the material (pressure) but again only due to the difference in temperature. Again this is due to a difference in temperature in an insulating material. If you take a glass that is at about 100C and plunge it into cold water the pressure differences will cause the glass to shatter - due to the large differences in temperature through the glass.
OK, but what is there to prevent the thermal energy being transmitted? You say the temperature of the cold plate is maintained at -200C. Well then, since heat is entering all the time from the hot plate, this means the cold one must be being refrigerated - which means heat (energy of vibrating atoms) is being conducted OUT of the cold plate. So there is no impediment to heat flow. And if you disconnected the refrigeration, all that would happen is that the cold plate would heat up until it was the same temperature as the hot one. And then the heat flow would stop, as there would no longer be any temperature gradient to drive it. There is no mechanical way to stop atoms vibrating. You can virtually stop them by cooling close to absolute zero, but this means you have to help the residual thermal energy leave the system rather than obstructing the heat flow.
@origin, shattering glass is interesting. It's the tension within the glass that causes it to shatter, it's not caused by any external pressures like gas for example. What if the material is strong enough to withstand the tension, that tension will take the path of least resistance right, it's like torque, it doesn't appear to do anything, it's just there as potential energy. The path of least resistance in this case is between the plates. @exchemist, the condition is extreme and sustained, some energy flows out of the system and that's why I suggested -200'C instead of something closer to 0K. Of course heat is all relative, can say things back to front and inside out, it's all correct depending on your point of view. I'm trying to keep it simply in terms of kinetic energy and flow. The tension that breaks the hot glass in cold water is perhaps caused by a reverberation if you like of kintetic energy working against the kinetic energy coming from the opposite direction, the atoms are compressed/pressurised to the point where they might break apart from one another.
The glass breaks because of differential expansion, nothing to do with reverberation of energy. Hot glass takes up more space than cold glass, so if you have a sharp change in temperature across a piece of glass, there is a compression of the hot part and a tension of the cold part. The compression is fine, as glass is fine under compression, but it has very weak tensile strength, so it cracks. Metals have far better tensile strength so they just tend to bend.
65H, lets look at the basics and see if that helps. Temperature is the measure of the translational kenetic energy of the atoms. Heat is transfered by the this vibrational energy being translated to adjacent atoms. In general (but no in every case) the larger the translational vibrations of the atoms the more a material expands. The faster the vibrations can be transfered between the atoms of a material the the higer the heat transfer rate. Materials with a low heat transfer rate can develope a large delta T with continous heat input. If the stress due to the temperature differences in the material is too high the material will fail - in the case of a brittle material like glass the material will shatter in the case of a ductile material it will warp and bend. What you are talking about whether you realize it or not is simply the effect of stress due to expansion from temperature in a material.
Shattering glass doesn't help this scenario, this test isn't for thermal shock or tensile strength. Let's assume the steel is strong enough and expansion/contraction is not going to cause a failure or affect the results hugely. If there is any pressure caused by what I've suggested, well it should be significant considering the extreme differences in temperature. What about the snooker ball scenario, does that not apply to particle physics? If convection is movement of energy through atoms, well that sounds a lot like a wave in water. Eventually that wave will come back towards the source and at some point two waves will collide and cause a big splash except in a solid there's no way to release the energy from this collision hence perhaps a build up of pressure. If conduction energy does not act like waves I would appreciate learning why it doesn't. There are some articles online on the subject like this one (cannot post links yet) but I'd rather not spend the money and it could be a waste.
