# Tagged Questions

Covers the study of (mostly homogeneous) macroscopic systems from a heat/energy/entropy point of view. Maybe combine with [tag:statistical-mechanics].

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### Why does the tongue stick to a metal pole in the winter?

since the Christmas season is here, I would like to ask a question about the movie, "A Christmas Story." In one of the subplots of the movie, Ralphie's friends were betting each other that their ...
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### Canonical Distribution (Partition Function)

For the canonical distribution $$w_{n}=e^{(F-E_{n})/T},$$ is the sum $$Z=\sum_{n}e^{E_{n}/T}$$ a sum over energies or a sum over states? Perhaps this is a silly question, but Landau and Lifshitz ...
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### How is the Horizon Problem really a problem?

I always thought the uniformity in the temperature of the CMB was supposed to be expected, since it's a much more probable initial condition for the universe. I finally found someone explaining what I ...
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### Why does heat added to a system at a lower temperature cause higher entropy increase?

Entropy is defined in my book as $\Delta\ S = \frac{Q}{T}$. To derive the formula it says that entropy should be directly proportional to the heat energy as with more energy the particles would be ...
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### Heat transfer between the bulk of the fluid inside the pipe and the pipe external surface

In an article from Wikipedia about heat transfer coefficients in the section Combining heat transfer coefficients there is an equation which describes the rate of heat transfer between the bulk of the ...
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### Calculate the rise in temperature from directed radiation [closed]

For example I have a 1 watt laser and direct it to a sheet of metal (copper), if I were to direct it for say a time interval of 1 minute what would be the change in temperature? I can predict that it ...
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### Heat Transfer in Cylindrical Coordinates

Lets say one has an infinitely long cylinder with some boundary heat terms on $r=r_0$ of the form $T(r=r_0, \phi,z)=T_0(\phi,z)$. What is the general solution for this type of equation? The general ...
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### Why is molar specific heat at constant volume of a monatomic ideal gas a constant?

I thought specific heat varies depending on the substance. Why is it always $(3/2) R$?
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### Grand Canonical Partition Function

I'm looking over posted lecture notes for a course, and this derivation of the Grand Canonical Partition function eludes me. It goes like this: occupation numbers $n_{α}=0,1,…$, Total particle number ...
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### If the specific heat capacity depend upon the temperature, what formula we should use instead of $Q=mc\Delta T$

Recently, I learned that specific heat capacity is not constant in different temperatures and it depends on the temperature.If I have diagram like this (consider that the red part is a quarter of ...
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### Acceleration of a container in thermodynamics

How will accelerating a container with an ideal gas in it affect the conditions? My initial thoughts are that the ideal gas will collect at the opposite end of acceleration, which means that the ...
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### Change in entropy adiabatic expansion

I think that an adiabatic expansion of a gas should cause the entropy to increase. On the other hand we have for adiabatic processes that $dQ = 0$ and therefore $dS= 0$, which is why I thought that ...
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### efficiency of a stirling engine with spring

I have used the concept of stirling engine with a spring instead of any gas to make an engine....i want to calculate the efficiency of it...here's a pic:
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### Will heating up an object increase its mass? [duplicate]

According to the $E=mc^2$ equation, will an object whose thermal energy (temperature) rises also weigh more? And by the same token, will the mass of an object decrease as its temperature approaches ...
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### Why does room temperature water and metal feel almost as cool as each other?

From what I've read about heat, temperature and conductivity, I understand that the reason water at room temperature feels colder than most other things at the same temperature (like wood, air, cotton)...
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### Why is Cv greater than Cp during anomolous expansion of water?

Well, in general $c_p> c_v$ for most cases but for water having anomolous expansion from $0^{\circ}\mathrm{C}$ to $4^{\circ}\mathrm{C}$ the order is just opposite, $c_v >c_p$, why?
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### Does heat at atomic scale present like macroscopic kinetic energy?

Heat energy is described as the kinetic energy of a molecule or atom, but how does that energy present itself? I keep hearing that it's "random", akin to a randomly jittering atom in space, but I'd ...
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### Does leaving snow on my hot tub cover save me money?

Snow is supposedly such a good insulator, that some animals dig snow caves in which to hibernate through the winter. However, my hands seem to feel more comfortable being exposed to winter air than to ...
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### What is the relationship between liquid-gas expansion pressure and surface area to volume ratio?

If a set volume of liquid is heated above its boiling point, how does the pressure that that volume of gas generates relate to the surface area of the medium in which it is enclosed in. For ...
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### Light Polarizer and the Second Law of Thermodynamics

I have stumped myself with a thought experiment of my own devising. Suppose I take a beam of wholly depolarised, but otherwise plane wave light. Its von Neumann entropy per photon is $\log(2)$ nats ...
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### Physical significance of temperature

Some books say when heat flows into a monatomic gas at constant volume, all of the added energy goes into an increase in random translational molecular kinetic energy. But when the temperature is ...
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### Why are we using water pipe instead of air pipe to warm our houses? [closed]

In France, most of the houses are using water pipes. I wonder why this choice was originally done.
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### Why does air feel cooler when blown through a small hole and warm through a big one? (for example from a mouth) [duplicate]

I hope the question is clear enough and I'm sure that you can try this thing quite easily yourself. When I blow air from my mouth to my palm through a small opening, I feel cool in my palm, but its ...
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### Change in energy ideal gas

I am supposed to calculate the change in energy upon changing both the temperature from $T_1$ to $T_2$ and the volume from $V_1$ to $V_2$. Now I was wondering whether this solution is correct: We ...
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### What is a heat source and heat sink in relation to thermodynamics? [closed]

Honestly, I have exams in two weeks and this is something I missed. What is a heat source and heat sink and how do they interact? I get that a heat sink is for dissipating heat, and it's obvious that ...
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### Copper Bath Keeping the Water Warm

I was watching a home improvement program the other day and the presenter mentioned that a copper bath kept the water warmer for longer. She didn't say what it was being compared with but I have ...
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### Why Does Air Hold More Water When the Air is Warmer?

I know that when the temperature of the air rises, the maximum amount of Water it can hold before the water condenses to water droplets increases. But why is this - has it got something to do with ...
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### How constant volume gas thermomether keeps the volume constant?

So the level of mercury on the left is being kept down by the pressure of the gas, correct? When the temperature rises, shouldn't the mercury level on the left go down? And when the mercury level go ...
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### Do the number of possible microstates increase as temperature decreases?

Entropy change, $\Delta{S}$, can be found from the $\frac{1}{T} - Q$ graph. When the temperature doesn't change during the dispersal of heat energy in the system, the area under the graph is more, ...
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### resistivity and temperature scan rate [closed]

100 amperes pass through a copper bar of $5$x$5$ mm cross-section. The resistivity of copper is $1.7$x $10^{-8}$ ohm-metres. Its volumetric heat capacity is $3.45$ joules per kelvin per cc. ...
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### why do we use absolute pressure in thermodynamics

One of my university doctors asked a question and I couldn't find an answer for so If anyone can help I will appreciate that. The question is why in thermodynamics we usually work with absolute ...
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### What is the density operator for an isothermal–isobaric ensemble (T,p,N)?

In the microcanonical ensemble $(E,V,N)$, the density operator is $$\hat{\rho}=\frac{\delta(\hat{H}-E\,\hat{I})}{Tr(\delta(\hat{H}-E\,\hat{I}))}$$ Where $\hat{H}$ is the Hamiltonian of the system and ...
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### Residual Entropy - Third Law

I've been told that many systems possess some residual entropy at absolute zero. This would seem to disagree with the 3rd Law of Thermodynamics? How can this be explained physically speaking? I am ...
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### What's wrong with this way of thinking about greenhouses?

It's a clear day, the sun is shining, warming the ground to 29°. The air is at 26°; a breeze is blowing mixing the air so the temperature is fairly uniform: Now I section a bit of this scene off ...
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### Why does NASA use gold to hold heat in even though its conductive? [duplicate]

I heard that NASA uses gold to prevent heat loss even though gold is conductive. How does NASA prevent heat loss with a conductive material?
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### A simple experiment and the Maxwell-Boltzmann distribution

Consider two containers separated by a removable wall, each side of which is a perfect mirror for the gas in the respective container. Also the walls of the containers are ideal mirrors. In each ...
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### Van der Waals expansion?

My thermodynamics textbook says that $$pV_m = \frac{RT}{1-\frac{b}{V_m}}-\frac{a}{V_m}$$ where $b$ and $a$ are van der Waal's coefficients. Then it uses this expansions as follows: (1-x)^{-1}= 1+x+...
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### Coefficient of volume expansion for gases

I often read that at 0 degree (Centigrade), gases expand by 1/273 of its volume at 0 degree for one degree rise in temp. So does this coefficient of expansion ( i.e. 1/273) change with temperature?
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### Cooling a satellite

Satellites are isolated systems, the only way for it to transfer body heat to outer space is thermal radiation. There are solar panels, so there is continuous energy flow to inner system. No airflow ...
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### In supersonic flow, why must area increase in order to have velocity increase?

Based on the area velocity relation: $\frac{dA}{A}=(M^2-1)\frac{du}{u}$ For M > 1, in order to increase u, we must increase A. I'm wondering what the explanation is physically and why it is opposite ...
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### How does the movement of molecules change at the edge of a liquid?

I am thinking about how the velocity of molecules measured from a small region of space might change as the region of inquiry moves closer to the edge of a container. Ultimately I am thinking about MR ...
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### Entropy Change in an irreversible process

I have just started learning thermodynamics and the concept of entropy confuses me. Suppose I have a gas in a cylindrical container fitted with a piston. I take it through an adiabatic irreversible ...
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### If entropy of a closed system decreases by a certain amount, why does not the entropy of the surroundings increase by the same amount? [duplicate]

As said by the Second law of thermodynamics, Energy spontaneously disperses from being localized to being spread out if not hindered from doing so. Now, for systems other than isolated one, and ...