How does the second law of thermodynamics disprove all  machines?
122590
Fermi in his lecture asserts:     The second law of thermodynamics rules out the possibility of constructing a perpetuum mobile of the second kind.   So, this means there can be no machine which just transforms all the heat energy gained by cooling surrounding bodies into mechanical work.  But how does actually the Second Law prohibit this?   The hot reservoir provides heat energy to the system. Does it cause a decrease in entropy of the universe(system + hot reservoir)? How? In order to receive heat wouldn't the system have to be cooler than the reservoir? If so, then entropy increases as the heat energy gets expelled from the reservoir at a higher temperature than the temperature the system receives the heat energy. Is it so?  I'm not getting how the Second Law nullifies the existence of this machine. Could anyone please explain how the Law prohibits the perpetual machine of the second kind?
62637
According to Peierls and Landau, 2D crystals were thermodynamically unstable. They can't exist! Of course, this theory was disapproved in 2004 (example: graphene).  What is the general definition of stability of a general system?  What is the thermodynamics' stability?
73894
The oil, vinegar and other liquids in homemade salad dressing separate into layers after sitting for a while, making the mixture become more organized as time evolves.  Why doesn't this violate the 2nd law of thermodynamics?  I assume that the answer is that since the separation is due to gravity, the effect is due to an external force and so the system isn't closed which is necessary for the 2nd law, but I'm not sure.  If that's the answer, then what happens if I consider the whole system include the salad dressing, gravitational field and whatever mass is generating the gravitational field?  The entropy of the salad dressing seems to decrease and so the entropy of some other component of this system must be increasing, but it's hard to see what this other component would be.
82614
From      The temperature of an ideal monatomic gas is a measure of the average kinetic energy of its atoms.    Now if I remove all the particles from the box shown below will the temperature be zero?
57360
Eddington wrote     The law that entropy always increases holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations — then so much the worse for Maxwell's equations. If it is found to be contradicted by observation — well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.   and Einstein wrote     [classical thermodynamics] is the only physical theory of universal content which I am convinced will never be overthrown, within the framework of applicability of its basic concepts.   Why did they say that? Is it a very deep insight they had, or is it something one can be convinced of quite easily? Or even, is it trivial?
77623
I was solving a true or false question regarding total mechanical energy and the following was the problem.     It is possible for a moving object to have negative total mechanical energy.   This is my claim.  Total mechanical energy is defined as   $$M=K+U$$  where $K=\frac{1}{2}mv^2$ and $U=mgh$.  No matter which direction the object is moving $K$ must be positive due to the $v^2$. (I am assuming that mass is also never negative.)  Potential energy is also positive because $g=9.8$ and height is a distance, which is also non-negative.  And yet, the problems says that the answer is true, as in the total mechanical energy is allowed to be negative.  What am I not seeing here ?
112876
Measurement of a quantum observable (in an appropriate, old-fashioned sense) necessarily involves coupling to a system with a macroscopically large number of degrees of freedom. Entanglement with this "apparatus" takes care of the decoherence. It is often said (I can provide references upon request) that the remaining problem is the one of "selection", and this is the point where one invariably invokes something philosophically radical, like many-worlds interpretation.  In the above (pretty standard) context, I am trying to make sense of the following observation. Looking at the measuring system from a statistical mechanics point of view, it seems that triggering a particular macroscopic outcome requires spontaneous symmetry breaking via a (thermodynamically) irreversible transition of the "apparatus" from a metastable to a higher entropy final state. My attitude is that "statistical mechanics point of view" is not far from "decoherent large quantum system".  So, the question is:  Is it fair to say that statistical irreversibility ("the second law") and quantum measurement irreversibility (the "wave-function collapse") are necessarily linked? Can this link be made more concrete (e.g., traced in details in a particular model)? Can you give references to approaches to the measurement problem that explore this connection?
54786
My book uses the argument that the multiplicities of a few macrostates in a macroscopic object take up an extraordinarily large share of all possible microstates, such that even over the entire lifetime of the universe, if each microstate had an equal chance of being accessed, fluctuations away from equilibrium would never occur.  My question to this statistical proof is this: In the real world, is there really an infinitesimal but nonzero chance that macroscopic systems could access some of the more unlikely macrostates (e.g. heat transfer from a cold object to a hot object)?
104874
I'm in a mechanical physics class, and I'm having a hard time understanding what the quantity of work represents. How can I understand it conceptually?
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How is it proved to be always true? It's a fundamental principle in Physics based on all of our currents observations of multiple systems in the universe. Is it always true to all systems? Because we haven't tested or observed them all. Would it be possible to discover/create a system that could lead to a different result? How are we 100% sure that energy is always conserved? Finally, why did we conclude it's always conserved? What if a system keeps doing work over and over and over with time?
47439
A  takes a pressurized input stream, most typically of a gas, and creates two output streams with a temperature differential. .  Both linked sources are scarce with information about how and why this works. Now, I have two questions:   Why does it work, specifically why should the situation in the vortex lead to a transfer of thermal energy from the inner stream to the outer one? How efficient can it be?  How do you define the efficiency of a device that may be closer to Maxwell's Demon than to a heat pump? My feeling is that any analysis should not only take into account the sum of input energy (thermal and mechanical energy instream) and sum output (thermal energies and pressures of both gas streams), but also the Temperature differential that is created - since that contains an ability to create work.* If course it's pointless to create heat from high grade 8mech.) energy to transform it back to mech. energy - but it gives an idea on the worth of the output.
16767
prevents a universal axiomatic system for math. Is there any reason to believe that it also prevents a  for physics?    Edit:  I haven't before seen a formulation of Gödel that included time. The formulation I've seen is that any axiomatic systems capable of doing arithmetic can express statements that will be either 1) impossible to prove true or false or 2) possible to prove both true and false.  This leads to the question: Are theories of (nearly) everything, axiomatic systems capable of doing arithmetic? (Given they are able to describe a digital computer, I think it's safe to say they are.) If so, it follows that such a theory will be able to describe something that the theory will be either unable to analyse or will result in an ambiguous result. (Might this be what forces things like the Heisenberg uncertainty principle?)
55209
Newton's 2nd law of motion can't be applied for mass-varying systems. Another force, known as Thrust must come to play. It can be measured using law of conservation of linear momentum. $$\text{Thrust} M.\dfrac{dv}{dt} = v_{rel} .\dfrac{dm}{dt}.$$   where $M$ is the mass of the system;   $\frac{dm}{dt}$ is the rate at which the mass changes;    &amp;    $v_{rel}$ is the relative velocity of the exhausted mass w.r.t to the main system.    This is what my book writes. Now, the left hand side of the eqn. represents force on mass $M$ whereas the right hand side represents the force due to variable mass. How can they be equal? During the application of thrust, $M$ does change and the equation had to be $$ (M - dm). \dfrac{dv}{dt} = v_{rel} .\dfrac{dm}{dt}$$ But that is not the book's equation. Why is mine wrong? What is the intuition?
3
Does TG () prove that ZFC2 (second-order ZFC) has a model? Does it at least prove the consistency of ZFC2?
121349
Why is latent heat required for a system to change its state? (I.e. liquid water to ice) Why the "extra" energy?
118603
I'm pretty sure that you can't take a magnifying glass and make it focus to a point that is hotter than the surface of your light source. For example, when you're outside trying to fry ants with your magnifying glass, it's impossible to get it hotter than 5,000 C (the temperature of the surface of the sun). My dad was arguing with me about it because he didn't see why this was true. My easiest argument was that the 2nd law of thermodynamics prevents this from happening because heat can't flow passively from a place of lower energy/entropy to a place of higher energy/entropy. He didn't buy it, saying that there wasn't anything preventing the light from focusing to a hotter point.  So I was wondering, are there some laws of optics that prevent this from happening? Or alternatively, is there a way to show that you could build a perpetual motion machine from this? Any help is appreciated.
107185
My question pertains to a possible inconsistency between the first law of thermodynamics and the general expansion of space itself.   First, the knowns:   Space itself has, and continues to expand - the acceleration aspect is not relevant to the discussion Empty space in a vacuum has intrinsic energy - some refer to this as dark energy Energy cannot be created or destroyed as per the first law of thermodynamics   Concisely, what is the source of energy that is imbued into this "new" space?  A few speculations:   The energy for this process is taken from something other source that already exists inside or outside the known universe. The energy for this process is created from nothing, thus violating the first law. If space itself is quantized, perhaps the planck length itself is expanding across the universe, giving the appearance that new space is created, but actually it is just the space "particles" themselves that are expanding.  At some critical point this expanding space would have observable consequences for our own scale of existence.   These ponderings can also be applied to the big bang theory - either energy was created at some point (from nothing) or existence of anything has been an infinite cycle into the past. Either way we are dealing with infinites. Create something from nothing and you will find infinites cropping into your calculations.