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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Boundary value problem

Consider the boundary value problem \begin{align} \frac{du}{dt}&= \frac{d^2u}{dx^2} , \\ u(0,t)&=0 \\ u(L,t)&=0 \\ u(x,0)&=f(x) \end{align} I know how to solve it using ...
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Questions regarding entropy

I have some questions regarding entropy: I know that entropy is the measure of the number of specific ways in which a thermodynamic system may be arranged. I know that any natural process occurs in ...
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0answers
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Heat dissipation of a quantum circuit of Hadamard gate and a loop

The following Q&A about reversible computing is available here. It has listed a number of practical scenarios where a reversible circuit can still be dissipating heat. Let's assume that none of ...
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Entropy of this system

We have a system with two energy states $E_0$< $E_1=0$. We also know that state $E_0$ can only take at most $m$ particles. Curently, there are $n<m$ particles in $E_0$. Now, I am supposed to ...
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A model for surface adsorption

I have been trying to model the adsorption of an inert gas on the surface of a clean solid. So far my underlying goal is to express the Gibbs energy as a function of the number $N$ of adsorbed ...
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27 views

Work done in isothermal process [on hold]

So I am mightly confused with work done in thermodynamics. Need some help pls.. What is work done in below two cases (Isothermal) Ireversible 1) Expansion of gas against atmosphere pressure. No ...
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1answer
30 views

Entropy change at varying volumes

I have two questions both related to each other: 1.What is the effect of change in volume (whether increasing or decreasing), of a system, under isothermal conditions? How does this change in ...
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2answers
30 views

Clausius statement of 2nd Law

I'm slightly messed up with Clausius statement of 2nd Law. I've seen at least two versions, which seem to be conceptually different. a) It is impossible to transfer heat from a colder body to a ...
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21 views

Robin boundary conditions for the heat equation

Consider a 3D channel with fluid or gas with walls $\Gamma_1$, inflow part $\Gamma_2$ and outflow part $\Gamma_3$. The temperature is described by the heat equation: $$ \frac{\partial T}{\partial t} ...
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1answer
25 views

Problem Boltzmann distribution [on hold]

I am trying to solve a problem about boltzmann distribution. If(A)=0.74 Uf(V)=0.037 Ia(A)=0.130 I have to find ...
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22 views

Boltzmann Distribution [duplicate]

I am trying to solve a problem about boltzmann distribution. If(A)=0.74 Uf(V)=0.037 Ia(A)=0.130 I have to find ...
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2answers
42 views

Only differences in chemical potential have physical meaning?

Is the statement in the title true? Does the chemical potential of a single pure substance that does not undergo any chemical reactions or phase changes have any physical meaning?
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21 views

maximun work in a reversible process

I often read maximum work done is by a reversible process. Shouldnt the more appropriate sensence should be maximum work done per unit of heat is maximum in a reversible process. ?
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1answer
23 views

Johnson-Nyquist noise for resistive element with temperature gradient

In Nyquist's 1928 paper, Thermal agitation of electric charge in conductors, the voltage noise for a resistor is derived assuming a circuit in thermal equilibrium and one temperature. How does the ...
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1answer
45 views

Energy without temperature

If you know the entropy $S$ of your system. Is there a general way to calculate the internal energy $U$ of your system? So the entropy $S$ is the only thing I know of my system. I have no information ...
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2answers
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Why would the lack of air in a mine in an asteroid prevent you from flooding it?

In Isaac Asimov's I, Robot novel, two scientists stationed in an asteroid where mining activities are performed say "We could flood the mines, if this weren't an airless asteroid" when considering ...
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1answer
23 views

What is the overall rate of heat conduction through two pipes of different dimensions if the thermal conductivity is not constant?

I am trying to work out how I would find the rate of heat transfer through two pipes made of the same material that are joined at their ends but which have different cross-sectional areas and lengths. ...
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in thermodynamics we say pmm is not possible in real life then how the machines in picture work with no external work done

1: http://i.stack.imgur.com/J7b4Q.jpg i want to know how the machine violets the ppm principle with explanation
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30 views

What does negative absolute temperature mean? [duplicate]

I always understood that the concept of absolute temperature stems from the third law of thermodynamics and that the lowest theoretical possible temperature is zero degrees Kelvin. However, ...
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0answers
35 views

Why can thermal shocks damage/break materials?

When an object is heated a lot and then brutally cooled down or vice-versa, it can crack or even break spectacularly. What happens inside that makes it do that? Is it only because the hot core swells ...
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0answers
27 views

Answer to: Is a suit that hides a soldier's heat signature fundamentally possible? [on hold]

I'm new so I'm unable to post an answer to this question: Is a suit that hides a soldier's heat signature fundamentally possible? But I am a Physicist. And yes it is. The simplest answer that I ...
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1answer
56 views

Why doesn't a Brownian ratchet provide free energy?

A Brownian ratchet is described here at Wikipedia. The "why it fails" section reads: Feynman demonstrated that if the entire device is at the same temperature, the ratchet will not rotate ...
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final state when Saturated liquid-vapour mixture contained in a rigid tank is heated

I have this question: Determine the final state when a 2 kg of Saturated liquid-vapour mixture at 50 °C contained in a rigid tank is heated so that the it reaches a single phase for each of the case ...
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What are the physics formulas relevant for this problem? (Latent Heat) [on hold]

I believe that we must use some latent heat formulas to solve this problem. My teacher hasn't covered thermodynamics so I don't know what equations are relevant. You don't have to complete part a. ...
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1answer
29 views

Make a circuit so that it heats water as rapidly as possible

To do this I understand that we need to create the greatest power. What I don't understand is why would we need to use a parallel circuit instead of a series circuit to create the greater power?
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1answer
28 views

Does Nucleation Occur during Phase Equilibrium?

I understand that during phase transition nucleation must occur. I'm wondering, once phase equilibrium is established, does nucleation still occur? For instance, in liquid-vapour equilibrium, does ...
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1answer
35 views

Confusion about the Definition of Boiling Point

From the description of boiling here: Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the ...
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0answers
19 views

How does Brake specific fuel consumption vary with Brake mean effective pressure?

I'm aware that there is a certain relationship between brake specific fuel consumption (BSFC) and brake mean effective pressure (BMEP), but I am wondering if someone could explain how each varies with ...
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6answers
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Is a suit that hides a soldier's heat signature fundamentally possible?

I recently played "Crysis", a game where the protagonist wears a suit that allows the player to hide both himself and his heat signature. Then I watched Iron Man 3, where a kid suggests that Tony ...
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0answers
19 views

Trying to calculate the length of a tube to heat Ar gas

If one were trying to heat Argon gas from ambient (~20C) through a mild steel tube of some diameter 'd' how long would it need to be? Additional unknown includes flow rate of Ar gas so some general ...
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1answer
65 views

Why do vapour cones form around jet fighters?

Apparently this phenomenon has nothing to do with jets breaking the sound barrier and has something to do with the Prandtl-Glauert singularity as described on Wikipedia. But, the Wikipedia article ...
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How do we compute the volumetric efficiency of an engine which is based on forced induction?

The volumetric efficiency of a naturally aspirated engine is computed by: $$n= \frac{V_1}{V_2}$$ With n: volumetric efficiency $V_1$: actual volume of air drawn $V_2$: volume swept by piston ...
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0answers
30 views

Two Limits in Thermodynamics - Reducing Driving Force and Increasing Resistance

I just wanted to clarify what is probably a very obvious idea. Suppose I'm holding a a piston separating two containers at different pressures. If I release the piston, it will naturally move, ...
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1answer
17 views

Latent heat of solidification

What is the heat release mechanism involved in solidification from liquid phase? Is the energy transferred to nucleation centres?
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1answer
27 views

Evaporation of water at room temperature

Could anybody refer me to some literature (textbooks or research, although preferably textbooks!) dedicated to explaining quantitatively the phenomenon of evaporation of fluids? I understand that it's ...
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0answers
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Schwabl states that a change in external parameters cannot increase entropy. If so, how can an adiabatic process be irreversible?

I am working my way through trying to understand statistical physics, and this particular, apparent inconsistency has had me stuck for days. Any help or advice would be hugely appreciated. I was ...
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2answers
42 views

Explanation for negative specific heat capacities in stars?

I've just found out that a negative specific heat capacity is possible. But I have been trying to find an explanation for this with no success. Negative heat capacity would mean that when a system ...
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0answers
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Does entropy increases due to gravity? [closed]

Imagine travelling to the surface of a neutron star where the very intense gravity would make you weigh 140 trillion times more than we do on earth. It's enough to significantly raise the entropy of ...
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4answers
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What is the actual energy content of the Sun?

Beside the energy released by nuclear fusion in the core, the Sun is a hot plasma of hydrogen and helium ranging from thousands to million of degrees. So how does that translate into energy ? I have ...
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2answers
111 views

Why can we say that $\bar{d}Q=TdS$?

When we introduce entropy we do this by saying that: $$\bar{d}Q=TdS.$$ Now I was wondering why this should be true? I know that by looking at a Carnot cycle, we do get this relation for reversible ...
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1answer
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Compute convective heat transfer coefficient for a condensing steam

I have a question about convective heat transfer coefficients. I am trying to figure out how much steam I can condense in this specific heat exchanger. Steam at 9 MPa is condensed in a 1 shell pass, ...
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2answers
37 views

How can we tell if a molecule is in thermodynamic equilibrium from scattering data?

We have a molecule that is emitting/absorbing photons. We know the Hamiltonian and that there are several levels. We count the emitted photons at different angles and frequencies. We can also do ...
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1answer
40 views

Why charge is an extensive property [duplicate]

While reading examples of extensive properties, I cam across electric charge. Can someone please explain to me why electric charge is considered to be an extensive property? We know that extensive ...
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0answers
19 views

joule-thomson effect and linde process

According to the j-t effect the decrease in the temperature is a result of the work done by molecule to expand thus increasing their PE and decreasing their KE, but in the linde process the next step ...
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0answers
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Can the effectiveness of a heat sink be increased with surface texturing?

Some supercapacitors use chemical etching to increase their surface area by several orders of magnitude. Could a similar process be used on a heat sink to increase its surface area and thus its ...
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0answers
17 views

Recommend books on thermodynamics [duplicate]

I want to take on thermo I will be analysing boilers, gas cycles ,ic engines etc. I want to Analyse all of these by using the basic laws gof thermo on my own (if i could) .I have cengel boles book and ...
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2answers
31 views

How much entropy is produced in evaporating water due to irreversible evaporation towards equilibrium (humidity=100%)?

$dS=dH/T_{boil}$ for the increase in entropy by changing a phase at saturation ($T=373\text K$ for $p=1\text {atm}$ for water). However, water also obviously evaporates below boiling point when ...
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1answer
47 views

Calculating Boltzmann constant using semiconductor

My task from the latest laboratory exercise in physics is to calculate Boltzmann constant from known temperature, current and voltage. We were given this circuit: By changing the resistor value, I ...
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0answers
12 views

Thermal resistance of an aluminium plate to ambient?

I have heaters mounted on an aluminium plate inside an enclosure, and I would like to know what the temperature of the plate will be at thermal equilibrium for a given heating power. For this I need ...
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1answer
19 views

Work done in adiabatic reversible process

I was solving a problem on turbine.the steam works on turbine adiabatic reversibly .Is change in enthalpy or is it change in internal energy which equal this work?