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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Dissipation and first law of thermodynamics

Consider the following situation: a certain gas is contained in a well-insulated cylinder with a well-insulated piston head. Now, in this case the piston is not frictionless. In order for the piston ...
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0answers
20 views

Constancy of Coefficients of Additive Integrals Throughout Subsystems of a Closed System

I'm studying Landau and Lifshitz's Statistical Physics, Part 1, 3rd edition and am looking for clarification on the following statement, which appears on page 11 in the section on The Significance of ...
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53 views

Carnot Engine- Cold Reservoir is a black body [closed]

My question: A satellite powered by a Carnot engine uses heat from a nuclear reactor at a fixed temperature T0. Heat is released into outer space via thermal radiation emitted by a set of fins at ...
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0answers
24 views

Ratio of Stagnation to Normal Flow Properties in Incompressible Flows

I know in compressible flows the ratio of stagnation to normal flow variables is a function of the Mach number and usually the specific heat ratio. Does this ratio mean anything for incompressible ...
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1answer
227 views

Physics of boiling an egg - what am I missing? (heat capacity and coagulation question)

Update: 1) I originally posted this because the numbers seemed to make no sense to me (seemed very counterintuitive to only need 60-100ml of water to cook an egg when you usually have to boil it ...
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2answers
118 views

Why does pressure decrease as velocity increases?

Bernoulli's principle states that as velocity increase pressure decreases. But higher the velocity, greater is the temperature and pressure must be high. Can you explain the situation in both the ...
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1answer
52 views

Which will dry faster? [closed]

Assume that you have a wet shirt that you have just washed. You squeeze of the surplus water that can be squeezed out. Method 1: 1)You iron the shirt for 5 minutes and then hang it out to dry out in ...
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21 views

COP and input work in a refrigerator [duplicate]

Where can i find a COP and input work requierd in a standard modern refrigerator? I need a sample refrigerator, so that I can calculate the heat disposed to the surrounding and use that to design an ...
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1answer
187 views

Entropy of the cosmological constant and the laws of thermodynamics?

Convention The convention being used is: $ A_{C} = $ The classical variable Premise Consider the following toy-model universe: A universe with a positive cosmological constant. Basic ...
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3answers
165 views

Why is the net entropy change of an irreversible engine positive?

In a Carnot engine the net entropy changein a cycle is zero. But in an irreversible engine operating between two temperatures the net entropy change in a cycle is positive. As I have understood, this ...
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62 views

If a system has an internal energy equal to 0, does this mean it doesn't exist?

Enthalpy is a measure of energy in a thermodynamic system: It includes the internal energy, which is the energy required to create > a system, and the amount of energy required to make room for it ...
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1answer
79 views

Maximum work obtained by mixing 2 gases

2 boxes containing the same number of moles of 2 ideal identical gases with the same adiabatic index (this is given as gamma), at the same initial temperature Ti but with different volumes, V1 and V2 ...
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1answer
63 views

Concept question about entropy of surroundings

By Clausius's definition $$dS_{surr} = \frac{\delta q_{rev}}{T}$$ but does this mean $$dS_{surr} = \frac{\delta q}{T}$$ where $\delta q$ is the infinitesimal heat absorbed by the surrounding in any ...
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2answers
69 views

When sunlight bounces off the Earth, why isn't the entire spectrum reflected rather than just the infrared portion?

I've read that greenhouse gases absorb and reemit sunlight, and that the infrared portion is what bounces off Earth back to space. When sunlight bounces off the Earth, why isn't the entire spectrum ...
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43 views

Does entropy flatten spacetime?

Forgive me if this is a non-question, but I could not find anything regarding this. I only know parts of GR, and I am not familiar with the math. Since energy and matter density curve spacetime, ...
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16 views

Conduction shape factors - textbooks inconsistent

I'm working through a energy flow problem, and need to use the conduction shape factor for 'Row of N long cylinders against planar surface'. However two different texts give two different formulas for ...
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1answer
61 views

Calorimetry Problem

I was doing a problem in thermodynamics where the net heat is 0. I dont understand why if you have say a copper calorimeter with water at say, 15C and add a mass of copper at a higher temperature say, ...
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32 views

Why internal energy is considered as disordered energy? [closed]

Why internal energy is disordered energy? How can the kinetic energy of a system as a whole ordered?
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1answer
62 views

Specific heat capacity for Adiabatic process

We can define infinite number of specific heat capacities between two temperatures(e.g $c_p ,c_v$) . Could we do that for an adiabatic process. As for adiabatic process heat addition or removal is ...
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1answer
70 views

Direction of time in an insulated room

I am puzzled with thought experiment that resembles/is version of Bolzmann's brain-hyphothesis. I could explain it in following way: Let’s assume that we have isolated system full of some stuff, let’...
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26 views

How can one induce rapid temperature changes in a controlled environment?

What kind of processes are there using which it is possible to change the temperature of a material as fast as possible. You are allowed to use any material you want. Constraints - It should ...
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0answers
174 views

Could a falling office tower cause the melting of its iron or steel support structure? [closed]

Discussion on META: Questions related to geopolitical events Suppose you have an office tower, weighing around 300,000 metric tons, and has a height of ...
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1answer
42 views

Heating water- Is it better to heat all of the water at once?

I am curious if it is more efficient to warm a half cup of water and then add an equal amount of room temperature water or warm a full cup together...assuming the water never boils.
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33 views

How fast does water have to be moving for it to gain a lot of heat? [duplicate]

A random question popped into my head a few minutes ago: objects, when moving fast enough, start to get resistance from the air. Since there is friction between the air and the object [water], there ...
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1answer
54 views

Finding total thermal conductivity (Ksys) for n-number of connected materials

I was playing around with the idea of finding a "total" K value for 2 solid, flush touching pieces of metal. The original problem involved the two touching inline bars with a two different maintained ...
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1answer
54 views

Why must the total change in entropy be 0 for a heat cycle to be as efficient as possible?

If $\Delta S_{total}$ were not zero, then heat flow would take place at different temperatures. It is easy to see why this is true by examining the opposite: that when heat flow takes place at the ...
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126 views

Why does the characteristic curve (V vs I) for a light bulb backbend?

When teaching Ohm's Law, I have students do an exploration of a small, incandescent light bulb with a low frequency (1-2 Hz) sine wave. It's a simple series circuit of source and light bulb, ...
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1answer
77 views

Third Law of Thermodynamics and degenerate ground states

The third law of thermodynamics says that at T=0 the entropy of a system must be 0. I'm confused to how to corresponds to the statistical/quantum mechanical treatment of systems, which seems to say ...
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2answers
287 views

Specific heat capacity and temperature, 0 K?

I've found similar threads like this, but with no clear answer. I understand that the specific heat capacity of a substance increases with temperature, because the vibrational nodes and rotational ...
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1answer
49 views

Microcanonical Ensemble - probability of finding momentum

I was trying to calculate the probability of finding a particle with momentum p in the microcanonical ensemble in a 3-dimensional box. $$\rho_i(\vec p)= \langle\delta(\vec p - \vec p_i)\rangle= \int ...
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120 views

specific heat of water and ice

We know that denser objects have lower specific heat capacity. Since water is more dense than ice, water must have lower specific heat capacity than ice but we know that water's heat capacity is $4....
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74 views

Partial derivatives of Internal energy

If we look at fundamental thermodynamic equation we see that $$dU=\left(\frac{\partial U}{\partial S}\right)dS+\left(\frac{\partial U}{\partial V}\right)dV.$$ We call $\left(\dfrac{\partial U}{\...
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66 views

Flow between two infinite plates [closed]

I recently got set this problem and I was wondering if anyone would be able to give me some hints/intuition on how to solve it. Thanks. An incompressible thermal conducting fluid is contained between ...
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111 views

The formula for calculating work transfer for a reversible adiabatic compression

Should we not be using Pdv for calculating the work done during a reversible adiabatic process ? Why are we using Vdp ? Can some one please explain this ? (P.S. I'm taking an introductory course on ...
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Lens-Mirror systems and conservation of specific intensity

This came out of a discussion I started yesterday and a related discussion I found. I'll recap the problem quickly: Consider two blackbodies, with surface areas $A_1$ and $A_2$ and temperatures $T_1$ ...
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50 views

Car Heating due to the sun: comparison of different contributions

Let's imagine that we need to choose a car in order to minimize the Temperature that the inner part of the car will reach after some hours under the sun. We know that two factors (if we suppose that ...
2
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1answer
166 views

relation between speed and temperature in relativistic gas

What is the correct formulation of the relation between the speed and temperature in the relativistic kinetic gas? I have found one mentioned in problem 3.24 of the third edition of Pathria's ...
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2answers
404 views

Gibbs phase rule and degrees of freedom at the triple point / triple line

The Gibbs phase rule tells me that at a substance's triple point, where there are 3 phases in equilibrium, there should be 0 degrees of freedom. Based on my understanding, that means there should be 0 ...
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1answer
273 views

Heat equation with heat loss to surroundings

I am trying to model one dimensional heat diffusion within a uniform copper rod with insulated ends. In addition, I am including a term that models the heat loss to the surroundings from the lateral ...
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59 views

Is a body of gas cooler in its upper portion as a result of gravity? [closed]

I read an article a couple of hours ago named Hydrostatic Lapse which makes the case for a phenomenon that I thought was well and truly confirmed; that gravity is responsible for the cooling of gas ...
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1answer
68 views

Effect of breeze on a very hot day [duplicate]

If air temperature is well above body temperature eg air temp of 50C then would a wind make the apparent temperature higher or lower. Clearly the wind would help take away heat due to increasing ...
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77 views

Entropy Change for a Thermally Isolated System

Clausius' Theorem states that $\int\frac{dQ}{T}\leq0$ for a closed cycle, with equality for a reversible cycle. Suppose we wish to take our system around a closed cycle such that the path from A to B ...
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86 views

How to relate internal energy to atomic motion?

I am trying to conceptualize how atomic motion leads to the thermodynamically-defined internal energy (denoted as $U$ below) through some broad mathematical relationships. I get that the internal ...
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1answer
50 views

How does the second law of thermodynamics affect the efficiency heat-to-work conversion, at the molecular level?

I understand the second law of thermodynamics in terms of the improbability of the high--and-low-velocity particles of a gas to separate themselves so that at one moment they are at different sides of ...
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0answers
24 views

How do I calculate temperature diffusion in a gas contained in a heatsink?

To simplify the problem, consider a sphere with a diameter of $25\mathrm{mm}$ inside an infinite heatsink at $300\mathrm{K}$. At $t=0$ I instantaneously raise the temperature of the gas in the ...
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0answers
78 views

Is the diffusion coefficient time-dependent?

It is known that in the partial differential equation: $$u_t=au_{xx} $$ within the limits $0<x<1$ and $a>0$, the diffusion coefficient, arises in the mathematical modelling of a process of ...
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3answers
252 views

Entropy change in an irreversible process between 2 equilibrium state

Calculating entropy change in an irreversible process between 2 states requires computing the change in entropy for any reversible process between the 2 same states, but why? If someone could provide ...
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0answers
520 views

Molecular structure correction factor in Mass Flow Controller

This question concerns device called Mass Flow Controller (or Mass Flow Meter). For a video explaining the principle: see https://www.youtube.com/watch?v=G62ma2IFh9o These devices are factory ...
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194 views

Confusing Chemical potential of mixtures

I feel that there are very few textbook that treat the chemical potential of mixtures in an understandable clear way, which is why I wanted to ask here about certain things? Although I do not have a ...
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What does the net force do with this droplet in magnetic field?

Consider a spherical droplet with radious $R_d$ which is coated with an immiscible ferrofluid with volume equal to $2v_f$. While the droplet is in rest on a surface, if I approach two magnets in ...