All Questions
17,220 questions
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Efficiency of a 2-stroke spark-ignition IC engine
Efficiency of a 2-stroke spark-ignition IC engine
The slope of $\eta$ vs $r$ graph is steep at less $r$, so is it possible to use more than one cylinder, with less compression ratio with the nth ones ...
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3
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57
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Why doesn't a metal disk expand in all directions when heated?
The hole in a metal disk gets bigger when heated. This has been explained by the reason that if we take the already cut out piece of the metal and heat it separately it also gets bigger, but the metal ...
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1
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19
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Difference in efficiency of isobaric reversible and irreversible process [closed]
I know that the efficiency of a process depends on the reversibility of the same process. When the process is reversible, the efficiency is the highest. Although I don't understand how this works for ...
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2
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45
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Stefan and Boltzmann law
How does Stefan's law relate to cooling
It tells about power radiated by a body at a certain temperature as related to a black body
But it does not include how temperature of that body changes every ...
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1
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38
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Phase transition and connection with fluid dynamics
In statistical mechanics, when there’s a phase transition, is there a mechanism that actually enforces it? For example, could there be some specific motion of the particles that forces a phase ...
1
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1
answer
29
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From material derivatives to partial derivatives in the wave equation
Consider the Cauchy momentum equation:
$$\rho \frac{d^2 \mathbf{u}}{d t^2} = \nabla \cdot \boldsymbol{\sigma} + \rho \mathbf{f}$$
where $\rho(\mathbf{x},t)$ is the density, $\mathbf{u}(\mathbf{x},t)$ ...
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2
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36
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Why must the total time derivative only be a linear function of velocity? [duplicate]
I'm hung up on page 7 of Landau & Lifshitz Course on Mechanics. They claim,
$$L(v'^2) = L(v^2)+\frac{\partial L}{\partial v^2}2\textbf v\cdot \epsilon \tag{p.7}$$
The second term on the right of ...
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How can one calculate the energy required to heat a substance from 298K to 6000K, factoring in formation enthalpies? [closed]
For the example of hydrogen, is it valid to subtract the enthalpy of $H_2$ at $298K$ from the enthalpy of $H$ (atomic) at $6000K$ (after converting $J/mol$ to $J/kg$ to account for the change in mole ...
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67
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Theoretic advancements in the fundamentals of physics can offer a solution to the heat death? [closed]
Recent Rumination over the potential implications of advances in Fundamental Physics and whether they might provide a way to avoid or mitigate the heat death of the universe have led me to wonder if ...
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Irreversible Processes in Thermodynamics
Here’s the updated text using dollar signs for MathJax formatting instead of parentheses:
Question: Understanding Work in Irreversible Processes in Thermodynamics
I am currently studying irreversible ...
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19
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How Dump Tubes (arrangement with lot of perforation) reduces the pressure of Steam flow. How to derive a scientific basis of designing a dump tube [closed]
Recently I have come across simple yet intriguing device - The steam dump pipe with multiple perforations (orifices).
The dump pipe is typically used to reduce the steam flow pressure. The inlet steam ...
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1
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0
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49
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The Green's function for the four-dimensional solution of the Proca equation is what?
From the Proca equation:
\begin{equation}
\color{red}{\partial_\mu}\color{blue}{\partial^\mu}A^\nu-m^2A^\nu=-4\pi J^\nu\\
\end{equation}
For the metric signature (-+++) and considering the steady-...
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3
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1
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118
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Covariant derivative acting on Dirac delta function
Pardon my naive computational question. In my calculations, I encounter the following expression:
\begin{equation} \label{eq1}
\frac{\delta}{\delta g^{\gamma \epsilon}(z)} \left( g_{\mu \alpha}(x) \...
- 85
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1
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88
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Is the second law of thermodynamics about microstates or macrostates?
The second law of thermodynamics says that entropy is increasing. Since the entropy of a given macrostate depends entirely on the probability distribution of the microstates this appears to be a ...
- 3,992
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1
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50
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Whats the lowest energy substance reaches during adiabatic expansion of carnot engine? [closed]
In carnot engine we know the steps like first isothermal, than adiabatic, then reverse isothermal and adiabatic.
And we know that efficiency increase if the temperature of hot box(heat supllier) is ...
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48
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hypthesis how big bang expand [closed]
"D Hypothesis" – Space-Time Swapping and Energy Conservation Mechanism
1. The Concept of Space-Time Swapping
The core idea of the D Hypothesis is the notion that space-time can be swapped ...
-2
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0
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107
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State-dependent properties of different isolated atoms from hydrogen to copper [closed]
I am an independent researcher and I am working on a theory that could theoretically determine a temperature value for a single particle. With new equations I've derived, predicts the values in these ...
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4
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4
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688
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Heating object in airless environment
Lets assume I had two containers sitting over a fire or some source of heat. Lets say I also placed two of the same objects with a low melting point in these containers (lets also assume this object ...
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1
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1
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68
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Understanding entropy and its connection to probability distributions
Entropy tells us about the "uncertainty" of a probability distribution, i.e. roughly how much information is needed to describe an event that is described by a probability distribution. With ...
- 3,992
2
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36
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Why is tensor product used to model the early and late time excitations of a region of a two-side black hole?
In their paper
(Large $N$ algebras and generalized entropy), Witten et al. studied the excitations of the right side $R$ of a two-side black hole. They consider the algebra $\mathcal{A}_{R,0}$ ...
2
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0
answers
36
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Can a window fan keep heat indoors? [closed]
Can a window fan keep heat indoors when it is colder outside than inside? Suppose a window with 1 unit of area must be open. If the goal is to decrease the rate at which the inside / outside ...
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3
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51
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How are heat capacities of substances measured?
To summarise Wikipedia: take substance at known temperature, add known amount of heat energy to it and measure the change in its temperature. But how do we add a known amount of heat energy?
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1
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Does viscosity increase temperature of a fluid at the cost of macroscopic kinetic energy?
Viscosity on molecular level can be stated as a transfer of momentum from fast moving layers to slow moving layers through random molecular collisions. So can we state that (in case of no external ...
user366132
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34
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What is the mechanism by which placing a spoon in a cup of water being heated in a microwave oven prevents the water from becoming super-heated?
Should I place a spoon in a cup of liquid before heating it in a mircowave? explains why one should do this and the safe ways of doing it, but it doesn't provide the physics behind it.
What is the ...
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2
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81
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Complete ionization temperature of an atom
If you google at which temperature hydrogen and helium are ionized completely, so that there is theoretically only a nucleus left, the given answers are around 10000K and 29000K respectively. Is there ...
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78
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NP-hard problems over honeycomb lattice [closed]
Are there physics-based NP-hard problems for spin models defined on the honeycomb lattice? I was not able to find much on Google Scholar. So far I have the following:
Counting the number of “legal ...
- 238
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40
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Mass Conservation in Kinetic Theory
In chapter 9 (The Boltzmann Equation) of Schwabl's 2006 text 'Statistical Mechanics', the author has the following statement of conservation of mass,
$$ \frac{\partial n}{\partial t} + \nabla \mathrm{...
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36
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Throwing near-boling water into minus 20C air
I have observed my daughter throw a flask of near-boiling water into the air outside at -20C. The water appears to turn into "steam" which actually appears to be ice crystals. She also tried ...
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Maxwell's demon paradox and a Szilard engine
I’m now studying quantum information physics and have a question about Maxwell's demon paradox.
I know an explanation using a Szilard engine but I omit the detailed settings as they are well known.
...
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Relationship between temperature and mass dimension in AdS/CFT
I've been reading Hartnoll et. al.'s paper on holographic superconductors, specifically PRL 2008 https://doi.org/10.1103/PhysRevLett.101.031601. In the paper, they mention "Recall that $T$ (...
- 625
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63
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Four gradient relation
I'm doing an exercise in QFT and I have to calculate the energy-momentum tensor for the Klein-Gordon Lagrangian and by doing it I got the following term:
$$ \frac{\partial \ \partial^{\nu}\phi}{\...
- 141
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Mean-field theory and coarse-grained modelling: Are these the same methodology?
In statistical mechanics, a Mean Field Theory defined as solving the reduced representation of physical system, on the other hand Coarse-grained modelling has similar purposes. Are these two ...
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1
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34
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Work in a two-compartment system
A well-insulated container of fixed volume is divided into two equal compartments by a thermally-conductive, weightless, frictionless piston. Initially, compartment A, on the left, contains 50 mol of ...
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4
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2
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830
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Confusion about reversibility of a carnot engine
I recently posted a question about entropy and the conversation changed topic to bring up an interesting question. @Chemomechanics explained that for a transformation to be reversible it is needed ...
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Are density perturbations possible in a globally homogeneous and isotropic universe such as ours...?
I would like to ask some questions about an interesting paper that was written back in the late 90's (https://arxiv.org/abs/astro-ph/9701131)
There, the authors propose how the universe may evolve ...
- 2,878
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1
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What does breaking or overcoming intermolecular forces mean?
Latent heat is thermal energy that rather than increasing the temperature or the kinetic energy of a substance is used to break intermolecular forces. It is also defined as overcoming intermolecular ...
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92
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Why can I hear when my shower gets hot?
I noticed recently that when I turn on my shower, I can hear the moment that the water heats up. The hissing sound gets slightly louder and its pitch changes. The pressure doesn't noticeably change ...
- 3,195
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I have some questions about heat transfer (heat evaporative minichannel heat sink)
Problem Description
As shown in the diagram, an evaporative minichannel heat sink with 48 minichannels is being analyzed. A glass plate is attached to the top of the minichannels for flow ...
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Nusselt number for developing turbulent flow
I need to develop a uniform method to cool a plate 1 degree, our idea was to let helium flow under it to transfer heat via convection. I concluded by some calculations that the Nusselt number for ...
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0
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Good resource for thermodynamics [duplicate]
I am looking for a good resource (preferably a book) on thermodynamics. I am currently an high schooler -- that pretty much sums up my mathematical qualifications so I was looking for something not ...
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1
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65
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Internal energy of a gas and $p$-$V$ curve
The internal energy is said to be same for 2 gases which have undergone 2 different processes.
The same is often explained by a $p$-$V$ curve. If the initial and final final points of 2 different ...
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Hawking radiation without horizons
According to this Bigthink article which cites this paper all massive objects emit Hawking radiation because they curve spacetime.
But I thought Hawking radiation is the time reversal of a particle ...
- 6,665
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Exercise about entropy
I solved the following problem, but I still have some doubts:
The problem: "Two equal quantities of water at temperatures $T_1$ and $T_2$ respectively, with a mass $m$ and specific heat $c$, are ...
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4
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98
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Zero Kelvin and Negative Kelvin [duplicate]
Basically the word Temperature (in Kelvin) of the body is the average Kinetic Energy of molecules in a body.
Zero Kelvin is understandable that all molecules kinetic energy is Zero
But what is ...
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3
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318
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Taking derivative with respect to quantum canonical ensemble expectation value
Let's say I have a Hamiltonian operator $H(x_a..)$, where $x_a,x_b$ are some parameters of my Hamiltonian.
I can calculate the quantum canonical ensemble expectation value of $\frac{\partial H}{\...
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4
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96
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Effect of movement of container on pressure of gas
Take a case- We have two containers A and B, one of the containers is moving with constant velocity while the other one is stationary. Both containers are identical, have same gas, same number of ...
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3
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What if gas molecules collide inelastically?
I know that inelastic collision ($e=0$) of gas molecules does not make sense, but I wonder, if somehow hypothetically gas molecules made inelastic collision with the container in which it is filled ...
- 608
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Why does the partition function require an integral?
The partition function of a single particle can be given by:
$$Z_1 = \sum_{E_r}^{}e^{-\beta E_r}$$
Where $E_r$ denotes the energy of microstate $r$
But the energy of a particle is given by:
$$E_r = \...
- 474
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43
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Why do airplane air conditioning systems use a turbine to expand air? [closed]
Why do airplane air conditioning systems use a turbine to expand air?
The cooling medium in airplanes is air.
The Joule-Thomson effect causes the temperature to drop during expansion under these ...
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