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

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Isothermal Process Question

Question: During an isothermal process, 1,000 joules of heat are removed from a trapped gas. Determine the change in internal energy of the gas. The change in internal energy is zero, but I do not ...
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37 views

Why can't a diesel engine burn petrol? [on hold]

On the Internet, I read that the self ignition temperatures of petrol and diesel are around $260$ and $210^{\circ}{\rm C}$ respectively. The difference is about $70^{\circ}{\rm C}$. The temperature at ...
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1answer
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Calculating the specific heat capacity of water

So for a thermodynamics assignment I'm doing, I need to build a solar oven and then use that to gather data. That bit's fine. The problem I'm having is that I need to use this data to calculate the ...
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Example calculation of increase in accessible states upon reimposing constraints in an isolated system?

Can you show me some example calculations of where the accessible states of a system increase upon reimposing the constraints of an isolated system (like moving the piston in a cylinder back to its ...
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How many calories in a block of wood?

I was recently thinking about the human body and the energy we get from food. I understand we don't have the ability to properly digest certain things like stones and wood and grass, but if we did, I ...
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38 views

Can a solid have a different melting point depending on the orientation of its surface with respect to its crystal lattice?

If we assume the second law of thermodynamics, then it's easy to derive the zeroth law of thermodynamics and then show that since the second law of thermodynamics and zeroth law of thermodynamics are ...
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1answer
28 views

Does aerodynamic heatings at the wall depend on the material of the wall itself?

I am reading "Fundamentals of Aerodynamics" 5th edition, J.D.Anderson. He said: "The slope of the temperature profile at the wall is very important; it dictates the aerodynamic heating to or from ...
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51 views

Does a black hole cool off and obey laws of thermal equilibium?

I have read these questions: Kirchhoff law of thermal radiation Why do dark objects radiate thermal electromagnetic energy faster than light objects? And it made me curious. Nothing gets out of a ...
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2answers
56 views

Discrepancy in general work and pressure-volume work

Consider a massless, frictionless piston fit into an airtight container containing an ideal gas. And let us say that the gas undergoes quasi-static isothermal expansion by lowering the pressure ...
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Ideal gas assumption for the Temperature Jump Distance calculation

I want to calculate the contact conductance between two surfaces that have a thin layer of air (on the order of micrometers) between them. A quick literature review shows that the heat transfer ...
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2answers
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Why does $H_2$ have $C_V$=$7/2 R$ at high temperatures, while the total number of degrees of freedom is 6?

The two hydrogen atoms have 6 degrees of freedom in total. Of them, $3$ contribute to translation, $2 $contribute to rotation and $1$ contribute to the vibration. I know that the vibrations motion is ...
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53 views

Do closed system always have real boundary?

Let's consider a system in uniform air with imaginary boundary. As air flow inside and outside of the imaginary boundary, mass is exchanging. But within that system, mass is necessarily fixed. Can I ...
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48 views

Dark matter pressure

Dark matter is suppose to form large halos of unknown yet type of particle with a specific density density profile. Does the dark matter halo can be viewed as a fluid of particles with density and non-...
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31 views

Heated Rod, steady state with temperature flow

so I have the heated rod problem in two dimensions given by $$u_t=ku_{xx}$$ and I have the initial condition $$u(0,t)=0$$ Now my other given condition is $$u(x,0)=f(x)$$ and I am asked to find my ...
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3answers
63 views

If the moon produced energy like the Sun would it radiate the same energy and light on Earth as the Sun? [on hold]

I would like to know if the moon where to shine like the sun would the Earth get the same energy and light as it is getting now from the Sun?
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Exercise about the zero principle of thermodynamics [on hold]

A box of $40 kg$ falls to the floor from a height of $20m$ and when it reaches the floor it remains at rest. If all the energy that the box initially had in the form of gravitational potential ...
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Statistical mechanics with a freely variable parameter

I would prefer to limit this question to the microcanonical ensemble for ease. I'm particularly interested in the limit that there is no intrinsic cost to introducing particles into a system, in the ...
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15 views

Otto and diesel cycle

In case of Otto cycle we consider that heat is immediately added to air fuel mixture(i.e constant volume heat addition) but in case of diesel cycle we consider heat addition at constant pressure and ...
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2answers
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What is the force escaping gas exerts on its container?

There is a simple problem that I met today, one that I couldn't find a solution for that I was satisfied with. Assuming we have a tank filled with gas at a certain pressure $p$ and temperature $T$, ...
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15 views

Transferring heat from unit area to unit volume

I am trying to make a Peltier AC but before that I want to calculate how effective it will be. If I know the temperature t on the area A of the cold side of Peltier device. How can I calculate the ...
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24 views

Do pure gases mix together quicker than already mixed gases?

Imagine a box split into two compartments by a gas-impermeable barrier. In one half is, say, pure oxygen, and in the other is pure, say, nitrogen. You remove the barrier, and the gases mix, eventually ...
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36 views

A strange heat transfer problem

I saw this problem in a physics textbook: Suppose we have a bunch of identical milk bottles at temperature $t_x$. Drop one of them into a thermal insulated box of water, wait for ...
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14 views

Number and nature of independent variables to descibe a thermodynamic system

In a thermodynamic system which has n ways of doing work, we have in total n+1 vaiables or degrees of freedom to know about the system. However, how many of the n+1 have to be intensive or extensive ...
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Gravitational forces become as strong as other fundamental forces

I was doing some reading on Absolute hot and was wondering what this actually means: ...
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678 views

Classical theory fails to explain quantization of motions?

I understand everything written here. But the last point, I cannot get, at all. How does it point towards Quantization of the two motions, since the energy change is not sudden, but gradual? And if ...
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3answers
53 views

Why is the change of heat non zero in a isothermal process?

I was reading the definition of heat capacity and it says that $$\Delta Q = C \ \Delta T$$ (introduction to statistical physics - K. Huang) So my question is that, if we consider an isothermal ...
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Quasi Static Reversibility Theorem

As far as I know, the definition of a reversible process is simply "a process that can be reversed". Meaning, that for an isolated global system containing the subsystem in question, its thermodynamic ...
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27 views

Peltier Calculation Conundrum [migrated]

I have a peltier (TEC) device capable of 400W Qmax, a 160W thermal load that I'd like to keep at ambient, and this water block running at about 1.5 gpm w/ room temp water. I think this system is right ...
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Confusion about entropy when applied to the whole universe. What are the macrostates?

I'm really confused about the concept of entropy when applied to the whole universe. The often hear that the universe started with very low entropy and as the entropy increases the universe will ...
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2answers
47 views

If efficiency is more important, then why condenser is used in boiler?

What's the real reason behind condenser? some says, Without condenser, back-pressure will be created in turbine. After Expansion in turbine, entropy will be maximum so it must be cooled to reduce ...
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53 views

Will hot air blown on a very hot object cool it faster than cool air? [closed]

An educated friend adamately claims that blowing warm air on a very hot object cools it faster than cool air. Example: heat up a barbeque utensil. In a car put it front of a vent with the heat on. ...
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2answers
85 views

Energy yield of a meteor blast - how to explain the discrepancy?

The energy yield of the Chelyabinsk meteor blast is estimated to be about 500 kT TNT, or $2\cdot 10^{15}$ joules. When compared that to its estimated speed ($\sim 2\cdot 10^4$ m/s) and mass ($1.3\cdot ...
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Why does the sand start to increase slowly when shaking for a long time (first law of thermodynamics) [on hold]

Good day! I have a question. At first, the temperature of sand increases rapidly when we are shaking. However, why does the temperature starts to increase slowly when we shaking for a long time? This ...
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Why does it become difficult to sample configuration during a phase change in Monte Carlo simulation?

I recently read in a paper that Monte Carlo integration is very slow near phase transitions. I couldn't understand the concept properly. The paper is Phys. Rev. B 95, 035105 (2017).
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Why Mercury, Cadmium and Zinc has low melting and boiling points and elements next to them start melting at a bit higher temperatures?

I have been playing with online periodic table and noticed that melting/boiling points are lowest for noble gasses and non metals then it starts for metals on left and creeps from right side of the ...
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2answers
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Sonic Boom in Aircraft and Spacecraft

I would like to know why aircraft, and spacecraft produce a double sonic boom on breaking the sound barrier. A while ago, I thought I got it, as there’s a start and finish point to every vehicle. ...
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1answer
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Does Fick's law of diffusion applies between mediums of different pressures with no membrane?

Imagine a stainless steal container with an unsealed lid that contains normal air, inside a room containing normal air. Now, pump a steady flow of CO2 in that container, bringing it to positive ...
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Why does the cloud in a straight line as if someone draw a line in the sky with a straight edge?

My cousin took one of these pictures(the middle) in the evening in BKK n she confirmed that she took it with her phone. I've been trying to explain it. I thought it had to do with hot air and the ...
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How long should I physically hold a glass of red wine with my hand before heating it up? Let's say the wine is at ambient temperature of 22C? [closed]

Have you guys ever seen sommeliers rant about "boorish" people holding a glass of wine the "wrong" way by holding the cup rather than the stem? I always thought it was ridiculous but I would like ...
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114 views

Why do distinct temperature layers form in bodies of water?

This may not be the correct place to ask a question about limnology (which I just found out is the science of lakes) but it seems like the place most likely to produce a good answer. In my diving ...
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4answers
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How is work transferred to the system recognised?

For example, a potato initially at room temperature $25 \sideset{^{\circ}}{}{\mathrm{C}}$ is baked in an oven that is maintained at $200\sideset{^{\circ}}{}{\mathrm{C}}.$ I made potato as the system ...
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Why there is no chemical potential on this thermodynamic relation?

I was following the book by Landau and Lifshitz, Fluid Mechanics (2nd edition) and got stuck trying to understand one of their arguments. On the chapter about Relativistic Fluid Dynamics it is stated ...
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Usage of pure substance chemical potential for the mixture of gases

There is a known expression relating the change in the Gibbs energy of the ideal gas to the change in pressure under the constant temperature $T$: $$\Delta G = nRTln\frac{p_f}{p_i} \tag{1}$$ where $n$ ...
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Re the Post “Heat in the car during sunny day” [closed]

Recently London Underground system has seen temperatures to 40 degrees. I believe I have identified what is going on. The trains absorb heat in the sun and emitting it in the tunnels. Effectively ...
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Stagnation Pressure Loss through Sudden Area Expansion

I was doing some reading in an textbook on gas turbine propulsion physics, and I came across an interesting formula. Consider a duct with cross-sectional area $A_1$ that encounters a sudden area ...
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49 views

Why is equipartition law called a theorem too, in some books? [closed]

In some books, the equipartition law is called a theorem. But a law is an observation, and cannot be proved. On the other hand, a theorem is something established using earlier assertions. So what ...
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Proof of no. of degrees of freedom for $N$-atom molecule [duplicate]

How can we say that the no. of degrees of freedom for a $N$-atom molecule is $3N$? How to $\mathbb{prove}$ it? I have heard people saying that for each atom, there are 3-independent variables to ...
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118 views

Ideal gas in piston with resistor, isothermal expansion, calculate velocity

I have this problem from Resnick's Physics, 5th ed, problem 31-15, sorry I only have the Spanish version (i'll translate): A resistor R=550 ohm, connected to an external battery, is inside an ...
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Is $H_2O$ the only compound which has a density anomaly? [duplicate]

My question is simple: are there any elements, compounds or materials other than water ($H_2O$) which have a density anomaly characterized by a turning point in the $T-\rho$ curve?
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Regarding assumptions

In most of the theories or derivations we take some assumptions like has is ideal, frictionless piston and many more but these are not applicable in real world. But in our real life situations we ...