I read a news article about how they were able to create a negative temperature, below absolute zero, and my question is how does this work?  I know that there are different definitions of temperature, and for the negative temperature, it's referring to entropy, the disorder.  Can someone explain to me how is the negative temperature created and why is it in some situations when we add energy to a system, entropy does not increase?
18170
Could someone provide me with a mathematical proof of why, a system with an absolute  (such that of a spin system) is  than any system with a positive temperature (in the sense that if a negative-temperature system and a positive-temperature system come in contact, heat will flow from the negative- to the positive-temperature system).
77048
Is  a ?  If not, what is it then?  If yes why, don't we teach kids this basic example?    UPDATE: I probably meant a regular commonplace fire of the usual temperature. That should simplify the answer.
61903
Combining the first and second law of thermodynamics we can get the following equation  $$TdS=dU-P_{ext}dV$$  First question: Is this equation applicable for irreversible processes such that that $dS≠\dfrac{dQ}{T}$?  Second question:If the system temperature $T_{sys}$ is smaller than the surrounding temperature $T_{sur}$, which temperature should we put in the equation?   I have this question because sometimes people use $T_{sur}$ instead of $T_{sys}$ (e.g. , ) but the equation is supposed to describe changes in the system.
22605
Sorry to ask the long way, but trying to make the question clear so that I can get a clear answer.   Why does it make any physical sense for a body to have negative potential energy?   If a body A is at infinite distance from another body B, it has zero gravitational potential energy, but if it is at "zero" distance from another body B, it has negative infinite potential energy, and acquires infinite kinetic energy in the process of getting there.  Please don't tell me that the zero point of potential energy is arbitrary, and it could just as well have zero potential energy when it is at zero distance from another body B, and positive infinite potential energy when it is at infinite distance from the other body B. How can it have this? When body A is at infinite distance from the other body B, then it is not influenced by the other body B at all, it is as if the other body B does not exist at all, so then the potential energy in body A must be zero, as there is no gravitational field of the other body B to cause any potential energy in body A.  But then the question is if the potential energy contained in the body A is zero at infinite distance from the other body B, then how is it possible for this zero potential energy to be reduced further and get changed to kinetic energy as the body A accelerates towards the other body B, so that the potential energy becomes negative? As I see it, if the body A never had any potential energy to begin with, then this not present energy cannot change into kinetic energy, otherwise we would be creating kinetic energy out of nothing.   Why does it make any physical sense to say that the body is in "potential energy debt", ie the potential energy is reduced to negative when it was zero to begin with?
75454
For historical reasons, I understand that a proton's charge was assigned the label of "positive", the electron's charge was assigned the label of negative, and, crucially, equations of current in E &amp; M are written with the notion that current is the flow of positive charge despite the rarity of actual flows of protons in real systems (which I understand are mostly biological systems).   If the possibility existed to instantaneously and at no cost, retcon these associations such that protons have a negative charge, electrons have a positive charge, and current remains the flow of positive charge, would there be any beneficial or detrimental outcomes?  For instance, would there be:   new interpretations of physical laws that would provide different or better insights into any relevant subfield of Physics significant simplifications to notation in any field of Physics or Engineering significant increases in understanding in the fields of electricity, electronics, electrical engineering, radiation/radiative transport, etc. a noticeably easier learning curve for any subfield of physics dealing with the concept of charge   Additionally, is there a permutation of positive charge, negative charge, and flow of current definition that would improve upon the one I suggested?
87810
By the fluctuation-dissipation theorem (detailed-balance for Langevin equation), $$\sigma^2 = 2 \gamma k_B T$$ where $\sigma$ is the variance of noise, $\gamma$ is a friction coefficient, $k_B$ is Boltzmann's constant, and $T$ is temperature. So in principle, one can have $\gamma\neq 0$ while $T=0$ and $\sigma=0$.  Is it indeed possible to experimentally achieve a system whose temperature and noise approach zero, but whose friction coefficient $\gamma$ does not approach zero?   If yes, what would be an example of such a system? What is the physical source of friction for such a system? If not, why not? Is there some sort of "quantum" correction to the fluctuation-dissipation theorem that rules out such zero-noise, non-zero friction systems?
13179
I'm having some trouble understanding the concept of negative work. For example, my book says that if I lower a box to the ground, the box does positive work on my hands and my hands do negative work on the box. So, if work occurs when a force causes displacement, how does negative work happen? Are my hands displacing anything?
76862
According to this potentialcurve and the other curve(?) the potential energy is minimal when the intermolecular forces equal zero.     Why? How should I interprete negative potential energy in this context?
105990
How can we use the idea of negative frequency for the sake of simplifying mathematics if negative frequency does not exist (to my knowledge) in nature ? For example, when plotting the spectra of a Fourier series.
45818
In , temperatures of millions of degrees (Celsius, Kelvin, it doesn't really matter at that point) are mentioned. But, what does it mean exactly? What is measured, and how? As I more or less expected,  the  mentions that the official definition  (the one I was told in elementary school, either between freezing and boiling points of water for Celsius, or between absolute zero and triple point of water for Kelvin) doesn't really work above 1300K. So, questions:   For &quot;earthly temperatures&quot;, how is temperature measured consistently? Why would linearity of the measuring device be assumed, in particular to extrapolate beyond the initial (0-100, or 0-273.16) range? What guarantees that two thermometers, working under different principles (say, mercury and electricity), that agree at 0C and 100C, will agree at 50C?  What do statements like &quot;the temperature in the Sun's core is $1.5\times10^7K$ mean? Or, even, what do the &quot;7000K&quot; in the Earth's core mean?
27047
Imagine a circuit consisting of just a battery and conducting wires which have zero dissipativity so that there is no loss of energy( Zero resistance). If the wires are connected to both terminals the current will flow in the direction of lower potential. Can we say that there has been a voltage drop in the current in going from positive terminal to negative terminal? I asked this question because  voltage drop happens across resistor and I don't understand why it should occur only there. Because  I think, considering a positive test charge its electric potential should decrease  as it travel from higher potential to lower potential  whether any dissipation occurs or not.Does voltage drop refers to loss in total energy of electron i.e. because in this case KE + Electric potential will be zero and the total energy of electrons remains unchanged and thus we can says that voltage drop is zero. Is voltage drop(assumed here to mean loss in electric potential) equal to work output under normal conditions. I think no - because electric potential difference gets converted into kinetic energy of electrons out of which only some gets converted into heat or is it the case that in a resistor entire kinetic energy gets converted into another energy such as heat? Is electric field inside circuit constant? If yes how? Will not the shape of circuit matter?
93868
Can a negative charge ionize air? Adding to this question, I have studied that positive charge ionizes air but never studied that negative charge ionize air. If possible, please explain how does it ionize air?
9684
Is it possible to create matter? In a recent discussion I had, it was suggested that with enough energy in the future, "particles" could be created.   It seems like this shouldn't be possible due to conservation but perhaps I could be wrong. Would any of you Physics masters care to elaborate?  (Note... I will understand the basics, I am by no means an expert.)
24667
I don't really know much about entropy but I learned recently that the entropy in the universe is constantly increasing and that perhaps someday we will reach maximum entropy and the universe will end in heat death. In order for this to be true there are two prerequisites:   Heat must constantly be created. There must be no way to destroy heat (at least no way that turns it into a more useful form of energy).   1 is certainly true. Every process creates some heat. When a person walks his/her feet hit the ground with kinetic energy and the friction creates heat.   But I don't see how 2 can be true. For one thing, heat never even leaves its place of origin (earth, another planet or a star). In order for heat to travel in requires air (or some other conductor/convector). Since there is no air in space there is also no heat. Heat leaves things only in the form of electromagnetic radiation (light) (in the case of earth, infrared).   So conversion from heat to light is certainly possible and is even found in nature. Now how do we get from light to other forms of energy? Well, photosynthesis. Solar panels. So this is possible too. If so, why will we ever run out of useful energy? Even if the rest of the universe does, can't we still reharness our own energy by converting the heat to light and light to other forms of energy?  Suppose the sun died (and leave aside the fact that it would first destroy earth with a supernova), can't we just put up a huge solar panel around earth to pick up all the infrared and convert it back to useful energy?
76828
The nature of the experiment has been discussed , but my main question is this: is there any theory that has predicted the results of this experiment or are we completely clueless about what's happening? In other words, have we come up with a new hypothetical interaction that could explain the results?
82436
If a gas expands adiabatically will the work done be positive or negative?  I think it will be positive as $\delta W=p dV$ and we have a positive sign due to the work done by the gas is positive as it pushes to expand. But this site has the negative of the answer I obtain.   (See adiabatic processes section)
35930
I was reading  and I got some doubts about a sentence about entropy, specifically when Ethan explains the irreversibility of the conditions of the hot-and-cold room, as in this figure:  In his words:  It's like taking a room with a divider down the middle, where one side is hot and the other is cold, removing the divider, and watching the gas molecules fly around. In the absence of any other inputs, the two halves of the room will mix and equilibrate, reaching the same temperature. No matter what you did to those particles, including reverse all of their momenta, they'd never reach the half-hot and half-cold state ever again.  My question is: Is the spontaneous evolution from the equilibrium temperature (right side of the image) to the half-hot and half-cold state (left side) physically and theoretically impossible/forbidden, or is it simply so astronomically unlikely (from a statistical perspective) that in reality it never happens? The article seems to suggest the former, but I was under the impression of the latter.