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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Is there any difference between relative motion and motion itself

If relative motion is the change in position of a body with reference to another body, and motion is the change in position of a body with time . Then why is relative motion not associated with the ...
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0answers
9 views

entropy change in a irreversible adiabatic process

Consider I have a gas in insulated cylinder fitted with a piston. I expand the gas inside the cylinder, compress it rapidly and move the piston randomly and finally bring the system to its initial ...
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0answers
10 views

Statistical Definition of Temperature in arbitrary Ensembles

I have seen multiple posts on this page that explained the statistical definition of Temperature as the derivative of the Entropy to the energy: $ \frac{1}{T}:= \frac{\partial S}{\partial U}(U, V, N) ...
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0answers
4 views

How to express Darcy velocity as a function of mobility and capillary pressure?

I have a problem with deriving a certain expression for the Darcy velocity of water, when we have a piston like displacement of oil with water. The expression I am supposed to show is the following: ...
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3answers
56 views

Why would we freeze in space if there is no matter to conduct the heat away from us?

If a near perfect vacuum is assumed to be in space, how can heat leak around from our body for example? Although theoretically energy wishes to spread equally all around, it still needs a medium to ...
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0answers
16 views

Final Temperature and Enthalpy Homework Question

3-36E A spring-loaded piston-cylinder device is initially filled with 0.13 lbm of an R-134a liquid-vapor mixture whose temperature is −30°F and whose quality is 80 percent. The spring constant in the ...
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0answers
19 views

car dashboard problem

I stumbled upon this question while I was driving my car. On my dashboard I have fuel gauge and engine temperature gauge next to each other, look at the pic: http://i.stack.imgur.com/aDgKj.png Fuel ...
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3answers
19 views

Pressure and altitude

I am going to ask a simple question, for sure. The pressure with respect to the altitude is given by this formula Where sea level standard atmospheric pressure p0 = 101325 Pa sea level standard ...
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0answers
21 views

Temperature of air from thermal radiation

I was wondering how radioactive heat transfer from a spherical source would affect the air around it from certain distance. I know convection is the most prevalent form of heat transfer through air, ...
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0answers
25 views

Local temperature equation for a planet

After seeing in science fiction too many planets with two suns that look too much like a geocentric system, I'm trying for my own amusement to understand if it's really possible to have a planet with ...
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0answers
13 views

What's the simplest, first order way to calculate the temperature change in this scenario?

I am trying to model this experiment in a simple (i.e., relatively analytic, non-computational) way. I have a little square of a thin film (10s of nanometers thin) of material A, on a relatively ...
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0answers
29 views

Physics specific heat question [on hold]

The instructions below outline the procedure for a demonstration. Materials: four 100 g metal blocks, each of a different metal, four polystyrene foam cups, each containing 150 g of 10°C water ...
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1answer
48 views

Which requires less energy, complete mechanical breaking or vaporization of matter?

Let's say we have a one kilogram piece of rock. Now if we want to separate each molecule from the other molecules making this piece of rock, which method requires the least energy while neglecting ...
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0answers
11 views

Hagedorn temperature as a control parameter of saddle - node bifurcation

Can we interpret the Hagedorn temperature as a control bifurcation of equilibrium parameter of the saddle - node type?
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0answers
24 views

Number of impacts nitrogen molecules make with the palm of a hand [on hold]

What is the number of impacts one mole air makes with the palm of your hand assuming air is entirely made up of nitrogen?
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0answers
34 views

The Ideal Gas Equation in higher dimensions

Basically I wanted to know whether or not the ideal gas equation, $PV=NkT$ would hold in higher dimensions? If so, how would you go about proving this? I can't see any reason as to why it shouldn't ...
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1answer
42 views

Can two different objects or system of molecules have different temperatures, but having same internal kinetic energy?

If I take an extreme case, where a body has only an internal potential energy with zero internal kinetic energy, does this body have a temperature? Another question related to it: if two objects A and ...
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0answers
22 views

What is collimated/paralell beam of atoms? [on hold]

This question is related to following exercise: "The intensity of the collimated, paralell beam of atoms of potassium was reduced by 3% by a layer of argon at 1 mm thickness and pressure $6,0\cdot ...
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1answer
15 views

Example of a system where we cant measure the total possible microstates of a system but can measure the microstates of most probable state?

I have been reading about entropy and I read that entropy is basically a measure of total possible microstates but there was an approximation that when no. of particles become very large we take ...
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1answer
24 views

Calculate the pressure in a container given moles and volume [on hold]

Say there are two containers with the same volume. The first has a temperature T and pressure P and contains 1 mole of $O_2$. The second has a temperature 2T and contains 1 mole of $He_2$. I'd like to ...
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1answer
29 views

Deriving Boyle's law from only the Gay-Lussac laws

My physics professor during a lecture presumably mathematically derived Boyle's law from the two Gay-Lussac laws for ideal gasses. What he said is also printed in his own textbook. He states that, ...
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0answers
29 views

Heat and mass transfer modes inside rapidly rotating cylinder

My question is about simultaneous heat and mass transfer in rotating cylinder system. System can be detailed as: A rotating cylinder contains wet porous substance in a lump shape. Air stream enters ...
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0answers
31 views

Acceleration of an bubble under the sea [duplicate]

An air bubble arises from the bottom of the sea. Find its acceleration if resistance force is proportional to $\rho Av$ where $\rho$ is density of water, $A$ is cross section area and $v$ is velocity ...
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0answers
32 views

Acceleration of an air bubble under the sea [on hold]

An air bubble arises from the bottom of the sea. Find its acceleration if: a) There is no resistance of water b) Resistance force is proportional to $\rho Av$ where $\rho$ is density of water, $A$ ...
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0answers
20 views

solar energy on a specific location

How to find the amount of solar wattage in any specific place on earh in a monthly manner? by solar wattage I mean the energy recieved from the sun in a squared meters.
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2answers
34 views

volume of the air bubble in the water

How does the depth affect the volume (the radius) of an air bubble in the water, if the temperature and density of the water are constant. Is there any relation combining this? Can I say that ...
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12answers
4k views

Why does hot air rise in a column instead of cold air pressing down?

Ok, this looks like a dumb question or even near trolling, but I really don't understand it. When air is heated over an oven plate, it rises. Obviously, I can check by blowing some smoke in. The ...
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1answer
26 views

What does ''population'' mean in regards to the excited and ground states of an atom?

I have a problem I'm working on. It seems simple enough, but there is a term I'm not familiar with. It asks for the ''relative populations of the first excited and the ground states for helium gas in ...
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0answers
29 views

Thermodynamics of Rubik's cube

I was wondering... Could it be possible to calculate the entropy change while rearranging a Rubik's cube from a given arbitrary arrangement? I tried with 3*3*3=27 microstates...but achieved a pretty ...
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3answers
53 views

Thermodynamics at the stoves, cooking problem

I was wondering about a daily question with respect upon heat, time and thermodynamics "made easy". Supposing I put a pot on the stove (we can say without losing so much that the pot is a cylindrical ...
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0answers
36 views

What is the correct setting for Kirchhoff's Law of Thermal Emission? [on hold]

For example, is the setting free space? Is it a surface? etc. So, what is the correct setting of Kirchhoff's Law of Thermal Emission?
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3answers
75 views

Why does conservation of étendue matter when showing one cannot focus light to arbitrary temperatures?

The latest XKCD What-If post discusses how one cannot start a fire by focusing the light from the moon. The answer is no, for well-established reasons, but the explanation isn't watertight. Maybe ...
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0answers
16 views

Nose-Hoover Barostat

Much can be found about the Nose-Hoover Thermostat. However I seem to be having difficulty finding out details about the Nose-Hoover Barostat, and how it is implemented. Would anyone be able to give ...
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0answers
50 views

Physical interpretation of the chemical potential in Bose and Fermionic gas

I understand that both Fermions and bosons have the chemical potential $\nu <0$ when it is T>0, but still behave classically, the fermions would increase its chemical potential at T=0, whereas the ...
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1answer
28 views

Are isentropic closed rigid thermodynamic systems also isolated?

$\newcommand{\d}{\, \mathrm d}$Currently it seems to me that isentropic closed rigid thermodynamic systems have to be also isolated. Is this correct? The reason why it seems to me like this is the ...
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2answers
52 views

gravitational force and irreversibility

If we place a ball at a certain height it falls and the process is irreversible. Is there any entropy change associated with the falling of ball? If so why?
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1answer
27 views

Is there a difference between solid-to-fluid convection and fluid-to-fluid convection?

I found an online calculator relating to convective heat transfer, but the calculator's description says it's for heat transfer between a solid surface and a moving fluid. However, the definitions ...
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0answers
10 views

What is temperature dependence (Schottky defects)?

The problem requires us to use the following property to determine the temperature dependence: For a system in thermal equilibrium in a heat bath at temperature $T$ the Helmholtz free energy is a ...
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0answers
25 views

Mechanical equivalent of heat => transfer from work W to heat Q

Consider the following experiment to determine the mechanical equivalent of heat $\mu = \frac{\Delta W}{\Delta Q}$ Not seen in the picture is a weight with known mass hanging down the cord. When you ...
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0answers
24 views

Carnot cycle in steam processes

Our professor told us that in steam processes, such as Clausis-Rankine process, Carnot's cycle is impossible to achieve. Can someone explain why?
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0answers
44 views

Does Earth's loss of internal energy to the moon reduce local entropy? [on hold]

[ edit: Since posting this I had 3 very contrary thoughts: 1) If the moon had been orbiting faster than the Earth, to the same degree, it seems like everything would be the same except that ...
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1answer
28 views

Heat pump solution to pollution [on hold]

I have invented a process that converts an air conditioner into a heat pump. How many metric tons of carbon equivalent could be eliminated from entering the atmosphere annually if 1.every air ...
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0answers
20 views

Prove for ideal gas temperature being directly proportional to absolute temperature [closed]

I have some difficulties proving that the ideal gas temperature is directly proportional to absolute temperature defined by the second law of thermodynamics. The ideal gas temp. is defined by the ...
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1answer
54 views

How does the fundamental assumption of statistical physics make sense?

Consider two systems A and B in thermal contact. System A has $N_A=3$ simple harmonic oscillators and the system B has $N_B=3$ simple harmonic oscillators as well. Each system has a number of energy ...
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0answers
26 views

Exhaust liquefaction in internal combustion engines

Does an internal combustion engine provide enough efficiency to still deliver mechanical power, if it is forced to liquify it's own exhaust gases by compression? If gas containers would be added to ...
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1answer
45 views

What is the correct statement of Kirchhoff's Law of Thermal Emission?

There are various quite different statements in textbooks and other science literature as to Kirchhoff's Law of Thermal Emission. So, what is the correct statement of Kirchhoff's Law of Thermal ...
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3answers
133 views

How do we define temperature?

I was watching this video What is Temperature?. It states that when we measure temperature we are measuring $dU\over dS$ at equilibrium. But at equilibrium, how the entropy and the internal energy are ...
73
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3answers
4k views

Where is the flaw in this machine that decreases the entropy of a closed system?

I was thinking about a completely unrelated problem (Quantum Field Theory Peskin & Schroeder kind of unrelated!) when the diagram below sprang into my mind for no apparent reason. After some ...
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9answers
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+50

Does hot air really rise?

"Heat rises" or "warm air rises" is a widely used phrase (and widely accepted phenomenon). Does hot air really rise? Or is it simply displaced by colder (denser) air pulled down by gravity?
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20answers
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Cooling a cup of coffee with help of a spoon

During the breakfast with my colleagues, a question popped into my head: What is the fastest method to cool a cup of coffee, if your only available instrument is a spoon? A qualitative answer would ...