Why do we always talk of heat flow from a hotter region to a coder region? Would it be wrong to say that cold flows from one place to another?
121253
Conventionally, heat flows from hot to cold. How legitimate is it to redefine things in terms of "cold" rather than "heat" eg Cold flows from cold to hot?
5860
When we try to establish a relation between the pressure and temperature in adiabatic process we come across a equation..  $dU = dq - PdV$  $dq=0$ (Adiabatic process) and,  $dU=C_v.dT$ (Heat capacity at constant volume)  Therefore, $C_v.dT = -PdV$$\tag1$  In this equation we are using heat capacity defined at constant volume but their still is some work done by the system (i.e, $PdV$ is not $0$ or $dV$ not equals to $0$).  The first part of the equation $(1)$ is implying that the volume is constant but the second part is implying that the volume is not constant (if it was there would be no work done).  Then why there is this contradiction ?
101030
According to thermodynamics every adiabatic system and (with no external energy added) will reach thermodynamic equilibrium or an ergodic state (2 law of thermodynamics entropy can not decrease in a closed system). State at which Temperature (or mean kinetic energy of the particles) will be same everywhere. So the answer to the question should be simple and straightforward $dT/dh = 0$. But is it the case? Let's open  page on Wikipedia and will see that adiabatic system will always create and support gradient $$dT/dh &lt; 0$ and equal to some constant. One can argue that this is because this system do not reach equilibrium and do not relax much. But look at calculation, if some additional relaxation took place then the gradient must decay and reach 0 at some time, but it is not the case. One can argue and say that ok gradient is there but there is no contradiction with second law. But a thought experiment conducted by Maxwell with two columns of different gases. He showed that if the calculations are correct, then gradients of different magnitude will be generated in these columns. And therefore, in a system where these two gas columns are isolated everywhere except at the top, there will certainly be a flow of heat from a colder body to a hotter one. Also, because gradient is exists Boltzmann distribution are mistaken. It is simply striking how two such contradictory statements, namely the universality of the second law of thermodynamics and the gradient in the gas column in the gravitational field, are able to coexist, this is pure schizophrenia. I also made a simple computational model which clearly shows that adibatic lapse rate you can find it on  or in the answer of this question. Here I list some findings which I made while discussed this question and made my model:  If you consider collisions of perfect gas particles it always leads to just velocity exchange (Newton's cradle as analogy), this statement can be easily proved mathematically, because masses are the same and collision are rigid and radius of the particle is negligible. This is the real reason why you should not take into account perfect gas collisions because it does not introduce into the model any new properties.  Despite the fact that Boltzmann and others derived their distributions for an ideal gas, implying the property of ergodicity of the system, in reality there is no energy mixing for the ideal gas model and particle collisions will not help at all here (see the previous paragraph). In reality, a certain entity is needed that would mix the energies of the particles and I introduced such an entity, and just after that the gradient manifested itself in all its glory.
94427
It was very cold outside, this morning, when I took the car that slept in the snow, with a simple cloth on the windshield. I entered the vehicle, drove a kilometer or so. The air inside was so cold I could see my breath (or maybe I forgot to brush my teeth). Warm air was blowing on the windshield from the inside and suddenly…  The glass in front of my became opaque, starting from the bottom (the place where the hot air was blowing) and freezing up, up, up until the whole screen was filled in about 5 seconds. It reminded my a little bit of how some baterias spread.  When I tried to remove the mist, I realised it was ice in the car. My windshield instant froze.  I know the theory behind supercooling: a very cold and still liquid can freeze when moved. I'm not sure what happened here, but there were some pretty big turns in the road before it froze. It really looked like the hot air froze the place and not the movement.  Any idea about what happened ?
34919
Reading  first,   Divergence (div) is “flux density”—the amount of flux entering or leaving a point. Think of it as the rate of flux expansion (positive divergence) or flux contraction (negative divergence). If you measure flux in bananas (and c’mon, who doesn’t?), a positive divergence means your location is a source of bananas. You’ve hit the Donkey Kong jackpot.   Wikipedia has:   In physical terms, the divergence of a three dimensional vector field is the extent to which the vector field flow behaves like a source or a sink at a given point. It is a local measure of its "outgoingness"—the extent to which there is more exiting an infinitesimal region of space than entering it.   What puzzles me is, what's the direction?  Doesn't divergence need a direction too?  For example, say you have this 2d vector field  $$ F(x,y) = (x,y) $$  This field is everywhere "spreading" from the origin:    The divergence is:  $$ \nabla \cdot (x,y) = 2 $$  Ie everywhere positive.  Everywhere is a "source".  At every point, if you draw an infinitesimally small circle, as the Wikipedia definition has, no matter how small it is, that circle will see more "stuff" leaving it than entering it -- ie everywhere is a source.  But that only makes sense if you are talking as measured using a vector from the origin.  But nowhere in the definitions of divergence do I see this last bolded comment mentioned, making me uneasy.  Take a vector field with values in the opposite direction:  $$ F(x,y) = (-x,-y) $$    The divergence everywhere is  $$ \nabla \cdot (-x,-y) = -2 $$  Ie everywhere negative.  Everywhere is a "sink".  Ie the field "slows down" at every point.  If you are talking about every point, and the direction of flow is actually towards the origin.  Then and only then, is the function everywhere "slowing down".  So like, what exactly is the divergence at a point?  Does it really make sense?  Am I right about having to consider the direction as towards the origin in order for the "sources/sinks" idea to make sense?  Where is this stated?
85892
I have seen people use both forms below. Which is correct? If both are, in which situation is each better used?  I am a software engineer based in New York. I am a software engineer based out of New York.
49172
This is a sort of a generalized question and not just referring to the flow of current. This includes fluids and many other such entities.  But why does this flow occur. For example if I consider current, then the definition of potential at any point is the work done by external agent in bringing a unit positive charge from infinity to that point. How can we deduce from this definition that the current will flow from higher to lower potential. In fluids, the fluid flows from higher point to lower point. Why so (referring, again to potential)?  Please avoid any analogies in answering the question.
24116
The title question is rather illustrative. I suppose the real question would be:  Is heat cumulative?  Put back into an example:   If I have a lit candle right beneath an iron bar, assuming the candle will remain lit indefinitely, and that the heat-losing rate is below the heat-getting rate. Will the bar eventually reach the needed temperature for it to melt?  If the answer is no:  Once the iron bar reached the max temp the candle can get it to. Where does all of the energy (heat) go after?  EDIT:   The question "is heat cumulative?" can be ignored as it is out of place and is misleading. Althrough the answer for it is "yes", it doesn't mean the answer for the general question is also "yes".  The point is not if it is actually possible to melt iron with a candle. Iron and candle are mere parts of the illustration, their properties are irrelevant.  A better phrasing of the main question would be:  Could a heating object contained in a perfectly closed system push the temperature of the system above its own temperature?
33913
I'm writing a paper, please can you tell me which sentence is the right?   The Pseudo-code is outlined in Fig 2 and we elaborate it in the following section.  The Pseudo-code is outlined in Fig 2 and we elaborate on it in the following section.
36568
So I started off with electrostatics and everything seemed nice and mathematical and justified and then "DC circuits" happened!  I just cannot understand the model of electron flow in electrical circuits.  Here are my specific doubts-:  1) If potential difference across a tiny cross section of conducting wire is zero, then why on earth does electron flow across that cross section at all? Never mind potential difference across the whole circuit.   2) Is there a constant electric field across a wire connected to a battery? If yes then how is potential difference across a zero resistance wire constant?   Shouldnt it be increasing?  Doesnt it violate ohms law?  If no,  then why do electrons flow at all?  Please take time to consider these doubts and relieve me of my frustration.  I havs searched through the net for this but every answer seems like beating around the bush.  All of the 4 books I have consulted do not address these facts to my satisfaction.   Frankly I think nobody understands this.
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.
73219
I was wondering about three things:   effect. Human body perceives surrounding temperature as colder, with increasing air flow:      This is understandable, as thermal transfer "consists" of convection, diffusion and radiation.   I assume that in real world example, we are being "chilled" by fan due to our skin  not being 100% dry, and thus wind speed increases convection. Is this assumption correct?   Imagine hypothetical situation with no precondition- human is standing in vast open space somewhere on earth, naked. Air temperature is exactly 36.6℃, there is no wind and for the sake of experiment I assume that sun does not shine, so basically I assume that this is near thermal equilibrium (human body generates energy from metabolic processes). I also assume that this man's body has 36.6℃ on whole surface. Will perceived temperature to this man heavily dependent on air humidity? Why, most probably, will he/she feel that it is very hot? Same situation, but air humidity is 100%, basically a sauna. Yet, wind is also extremely strong, say 110 km/h or 70 mph. Man should feel cold according to wind chill, but how it is possible(to feel cold in such conditions), since fluid (air) that has contact with his/her skin has same temperature as his body? (Assume convection/ would be relatively small because there is no thermal gradient, and diffusion would be relatively small because of high air humidity).
58324
Is there an established name for the region between two parallel planes? I can use sheet, layer, lamina... but I'd like to know whether there is an established name for it.
5227
With the following circuits as examples :    and    How will the current I know how much to flow? Would any other wave travel first in the circuit and then come back and say so much current should flow?
29679
I spent some time studying about temperatures and color of objects. It turns out that as we heat something it turns to red, then yellowish white and if we heat it more it turns to bluish-white.  Like we can say a blue star is hotter than a red star. But why isn't it the same with flames?  Blue flame isn't always hotter than a red flame. It's just the chemistry of it all.   I mean I know the electrons jump from one orbit to another and gets into excited state when heated.  But the questions in my mind are:   If one object appears blue and another red, that doesn't always mean that the blue object is hotter than red. Is that correct? If yes then how is it so? I am sorry but I'm bit confused over it. With the "Color Temperature" concept on Wikipedia. They say 1,700 K to match a flame. But 15,000–27,000 K for a clear blue poleward sky.   This confuses me. The sky appears blue, does that mean its hot? But it isn't right? It's colder. I am not sure if I'm able to frame it properly. It's a bit confusing to me.  Is this something like a glowing object vs reflecting object? The sun is white but Earth is blue. But sun is hotter than the earth. It's the sun radiating light but earth is just scattering it.  But the surface temperature of sun is 5778K while the Wikipedia states our sky is about 15000K-27000... I know there's something I'm missing here. I'm hoping someone can tell me what is it.
56175
Here was my argument against this, the second law of thermodynamics, in effect says that, there is no heat engine that can take all of some energy that was transferred to it by heat and do work on some object. So, if we can not take a 100% the thermal energy of an object, and use it to do work, what about the thermal energy that is rejected to the environment, can we use all of that energy to do work on an object? No, if energy is supposed to be the ability to do work, well that’s a contradiction.
46410
I get confused on the low-level physics of electricity from time to time. It came up in "," and I don't totally get it.  How fast does electricity flow? Is the speed of an electron different in say a resistor than in a wire? Does it matter? Or are the effects of the electron the only important thing, with lower levels of abstraction not useful in practice?  I know there are already materials on this topic, and I have read some of them. I think having the question on this site might inspire some interesting answers to the age-old question.  Bonus points for:   Identifying and clearing up common misconceptions Explaining in a way that someone with a high school diploma could understand, without oversimplifying it so much that its incorrect