Timeline for Why does Truesdell think entropy is an undefined object?
Current License: CC BY-SA 3.0
13 events
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| Aug 20 at 0:03 | comment | added | FlatterMann | @hyportnex Where thermodynamics throws a wrench is that our perfectly irreversible process requires a vacuum at T=0. So, yeah, in reality there are always two such processes at work, even in vacuum. One radiative heat transfer from T1 to T2 and another one from T2 to T1. Some of the energy always comes back because of that. I agree that the usual textbook definitions suffer from not reflecting that well, but that's what Carnot invented his cycles for, isn't it? | |
| Aug 19 at 23:52 | comment | added | FlatterMann | @hyportnex Physics is not what a philosopher says physics is. Physics is the rational description of nature. Perfectly irreversible processes exist and they are actually the most simple processes of all. A ray of light in vacuum is such a process. The flow of electricity in a pair of conductors is a close approximation. What does not exist is perfect reversibility. That was always an imaginary concept, although I agree that too many physicists are holding on to it religious for all the wrong reasons (mostly educational ones). | |
| Aug 19 at 23:02 | comment | added | hyportnex | @Flatterman "By definition an entropy measurement would require some form of reversible heat flow" that is your definition. Others may define it differently and Truesdell is certainly in the "other" camp, but whenever/wherever you can measure heat flow through a conductor you can also measure said flow between two infinitesimally close temperatures, and there you will have measured entropy flow as well. As an experimentalist, have you ever seen heat flow without entropy flow such that local entropy flux is different from the local heat flux divided by the local temperature? | |
| Aug 19 at 22:30 | comment | added | FlatterMann | @hyportnex By definition an entropy measurement would require some form of reversible heat flow. I have no clue where the reversible heat flow is supposed to be in an electric heater. No matter how long I wait, there is no energy, neither electric nor heat coming out of the heater device ever again. It's a close to ideal implementation (up to heat conduction in the insulating materials which can be made negligible) of an irreversible process. | |
| Aug 19 at 13:03 | comment | added | hyportnex | @FlatterMan" Precision calorimetry on the bench" is the one that measures "heat" $Q'$ at one "temperature" $T'$, that is just thermostatic entropy, calorique, $Q'/T'$. Then the difference between entropy measurement and calorimetric heat is a calibration factor. The place where you need the 1st law is in the "conductor" between the source and the sink which is not a law but a constitutive definition of what a heat conductor is. | |
| Apr 9, 2023 at 23:12 | comment | added | FlatterMann | @hyportnex A force can be measured directly by letting it accelerate a mass. A good high school physics teacher will show this experiment using an air table or similar device. Newton's second law is a prescription of measurement. So is the first law of thermodynamics: it equates all forms of energy to an equivalent amount of heat energy. We take a lump of hot metal and we throw it in a water bath. Then we measure the temperature. We restart the experiment with cold water and we are now heating the water bath with an electric heater to the same temperature. Precision calorimetry on the bench. | |
| Apr 9, 2023 at 22:55 | comment | added | hyportnex | @FlatterMann I do not understand how you read Truesdell's comment as equating force with deformation for quote "...as good as the explanation of force as being a push or pull in the amount of the mass times the acceleration produced". Unless you mean this "Just as force can never be measured directly — for all measurements of force presume some special mechanical constitutive equation" but then how else but using a constitutive relation you could measure it directly? True, "heat" is not a dead-end but to me calorique is a more primitive and simpler concept, I follow Carnot wherever he goes. | |
| Apr 9, 2023 at 22:39 | comment | added | FlatterMann | Heat is not a dead end and while entropy (as a state function) is hard to measure, heat is so easy to measure, with reasonably high precision, that you can do it in a high school physics experiment (and we do). I hope that Truesdell's ideas about physics are not severely limited by what he knew (or didn't know) about experimental physics. What concerns me about the quote is that he is equating force with deformation rather than acceleration. Even that is simply false at the high school physics level. | |
| Jun 4, 2020 at 16:03 | history | edited | CommunityBot |
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| Mar 27, 2018 at 16:09 | comment | added | hyportnex | I do not think that by "you will run around in a circle" Truesdell means that all possible definitions are circular, instead by having a well-formulated definition will get you nowhere. So he writes that we should think of entropy where we would say "heat", and also "While we can never define entropy except in cases where we do not need it, and while we cannot measure it directly, we can, in time, get used to it...". In the next edition of this essay that was published with the title "Rational Thermodynamics" he calls it "calory" to avoid confusion :). | |
| Mar 27, 2018 at 14:41 | history | edited | hyportnex | CC BY-SA 3.0 |
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| Mar 27, 2018 at 14:31 | comment | added | Jerome | Unless I am mistaken, that seems to explain why "heat" is a dead end. But effectively says that entropy is the right concept we should use instead of heat (which I agree with by the way). My question is why is entropy a circular definition or too special to mean anything? | |
| Mar 27, 2018 at 14:09 | history | answered | hyportnex | CC BY-SA 3.0 |