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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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?
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2answers
780 views

Is a black body sphere inside a black body shell hotter than the shell?

I am missing something very basic here. Let us assume the shell is at a fixed temperature T by a power generator. The sphere inside will radiate the same power it absorbs. The power radiated by the ...
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2answers
61 views
+100

How stable are Benard Cells when the thermal gradient begins decreasing?

I've found lots of information on the formation of Benard Cells and convection currents but very little information about what happens to the self-organized structures when the energy gradient being ...
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0answers
18 views

Work done by a turbine in a reversible adiabatic process

Why is the change in enthalpy equal to work done in this case. I believe the change in internal energy should be the answer.
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3answers
58 views

Can Thermal Energy be converted into usable energy?

I'm going to go out on a limb and ask a question that will probably be a bit humiliating: Can Thermal Energy be converted into usable energy? I've been reading about conservation of energy, and I know ...
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1answer
32 views

Freezing or melting Brine

In the freezing of brine, is it possible to do it at a rater or in such a way that the water only freezes or the ice is somehow isolated? Or conversely is this possible in the melting of brine-ice?
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0answers
46 views

Euler Equation Arbitrary Quantities

I have a question about the Euler equation. For some state I can write down: $$ U = TS - pV + \mu N$$ In this equation $T$, $p$, $V$, and $N$ are directly measurable so they have fixed values. ...
3
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1answer
68 views

How do door-less refrigerators keep cool air inside?

I got curious when I saw a door-less refrigerator for the first time. It was in a shop and I couldn't control myself so I began to look around to see how they work but I couldn't figure it out. They ...
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1answer
41 views

Question Regarding Nucleation in Phase Change

When liquid turns into solid a nucleus is required. Is the same requirement present for solid-liquid (melting)? Do condensation also require nucleation sites like in freezing? When I look at ...
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0answers
20 views

SI Units for Isothermal Incompressibility and Specific heat capacity at constant volume

I am looking for the SI units of Isothermal Incompressibility and Specific heat capacity at constant volume. What I found was that Isothermal compression was measured in $\frac{m^2}{N}$ so I'm ...
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1answer
19 views

How can I compute heat retention of a heated ceramic floor?

The U.S. department of energy suggests electric radiant floor heat can be cost effective if the floor is preheated during off peak electricity hours if installed within an appropriately massive ...
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1answer
69 views

Why wouldn't a room temperature superconductor violate the laws of physics?

I understand that the laws of physics (as we know them) cannot be violated, but what I would like to know is what I'm missing, because I know there is some explanation that I'm not accounting for. ...
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1answer
88 views

Conservation of energy when focusing black body radiation on another black body

This question about whether or not it is possible to focus black-body radiation to make something hotter than the radiation's source was answered mostly negative: the second law of thermodynamics ...
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0answers
11 views

dynamic structure factor for nucleation

I know that using the peak position/moment of structure factor or may be by first zero or minimum of pair correlation function we can estimate the characteristic length scale in a phase separating ...
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0answers
14 views

Calculating entropy [duplicate]

Suppose 30 gram of metal Gallium melts at 36 °C. The specific heat of fusion is 80.3 kJ/kg and the melting temperature is 29.9 °C. Explain if the melting of the Gallium in this process is ...
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1answer
56 views

'Mechanical equilibrium does not necessarily mean that the pressure is same at all points of the system' - why?

If there are variations in pressure or elastic stress within the system,parts of the system may move, or expand or contract. Eventually these motions,expansions or contractions will cease, and when ...
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1answer
37 views

Derivation of temperature-volume relationship for a reversible adiabatic expansion of an ideal gas

We start with $\delta q = 0$ and $dU = C_{V}(T)dT = \delta w$. Why can we take the heat capacity at constant volume, when this process is an expansion so volume increases?
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3answers
3k views

How does water help extinguish fire?

How does water extinguish fire? Heat energy from the fire is transferred to the water, isn't that how it works? How does water deprive oxygen and stop combustion? How is the specific heat of water ...
2
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1answer
26 views

How can Wien's Displacement Law be 'changed' to a version for frequency?

Wien's Displacement Law stated that for a blackbody emitting radiation, $$\lambda_{max}=\dfrac{1}{T}$$ where $T$ is the temperature of the body and $\lambda_{max}$ is the maximum wavelength of ...
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1answer
70 views

Difference between heat and work

According to the Kinetic Theory of Matter, temperature is nothing but a measure of the kinetic energy of matter. My textbook says that the change in internal energy of a system is the heat gained plus ...
22
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5answers
2k views

Is it possible to focus the radiation from a black body to make something hotter than that black body?

My previous question wasn't specific enough. I'll try to be more specific. Let's imagine we have a hot body let's say 6000K hot that emits lots of thermal radiation. Let's assume 1kW of radiative ...
3
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4answers
213 views

Why does filling a compressed air cylinder produces heat?

And the opposite follow-up question: why does opening the air cylinder makes the air cooler? What I know is that I can't find these answers using the ideal gas law, because that is an equation of ...
49
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3answers
5k views

Is it possible to start fire using moonlight?

You can start fire by focusing the sunlight using the magnifying glass. I searched the web whether you can do the same using moonlight. And found this and this - the first two in Google search ...
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0answers
44 views

Thermodynamics: Caclulate final temperature with hot metal in gas [closed]

I've been working on this problem for a couple of hours now, and I think that I simply don't understand the basic principles that tie into the problem: (note that I attempted this problem and it was ...
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0answers
41 views

Chemical potentials at triple point

How to prove that chemical potential of each phase at triple point are equal? My attempt: At phase transition, $P$ and $T$ are constant and using $dG=0$(Gibbs free energy at minimum) at equilibrium ...
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0answers
26 views

What makes fluorescent lamps lower in heat than high pressure lamps coated with phosphorus?

Why does a fluorescent lamp give off more pure visible light (without heat) than a high-pressure lamp? Is the heat given off by high-pressure lamps caused by the intensity of light being emitted from ...
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1answer
67 views

Understanding the phase separation of a chemically reactive mixture

Let's consider a mixture of solvent and components $A$ and $B$ such that $A$ can transform into $B$ with a rate $k_1$, and the reverse reaction with a rate $k_2$. $A \rightleftharpoons^{k_1}_{k_2} B ...
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0answers
43 views

I have got some weird data after a homemade battery experiment

Okay, so i've been searching for some time, for perpetual motion machines of the second kind. So things got into my mind, such as this homemade solar cell. And i thought in my mind:" It's not the ...
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2answers
79 views

Are critical exponents below and above the critical point always same?

The scaling relations don't distinguish the the critical exponents below and above the critical value. In the mean field level, I understand these critical exponents are same whatever one approaches ...
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1answer
56 views

Density as a derivative of the Helmholtz energy

The equation of state for nitrogen has been formulated using the Helmholtz energy as the fundamental property with independent variables of density and temperature. The equation of state in ...
1
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1answer
25 views

Is every imaginable quasi-static locus (with non-decreasing entropy) physically realisable?

If I have an isolated composite system which is dependent on several extensive properties (say $U_i, V_i$ and $N_i$ for each subsystem $i$) then a quasi-static locus is just a continuous curve through ...
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1answer
46 views

Shock-waves, Bangs and the Speed of Sound

I was watching this video of an erupting volcano. Some Guys in the comments tried to estimate how far away the volcano is by using the delay until the "shock-wave" hits the camera and the speed of ...
10
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0answers
174 views

List of known universality classes

I am working with RG and have a pretty good idea of how it works. However I have noticed that even though the idea of universality class is very general and makes it possible to classify critical ...
3
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3answers
62 views

Why does the air pressure at the surface of the earth exactly equal the weight of the entire air column above it

Why does the air pressure at the surface of the earth (resulting from collisions of molecules on the surface of the earth which has to do with the velocity of the particles) exactly equal the weight ...
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1answer
44 views

Is the Gibbs-Duhem equation always valid?

The derivation of the Gibbs-Duhem equation from Wikpedia uses: As shown in the Gibbs free energy article, the chemical potential is just another name for the partial molar (or just partial, ...
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1answer
61 views

Thermodynamic Internal Energy Reference States

The internal energy, $U$, of a given thermodynamic state, $\boldsymbol{R}$, as introduced in thermodynamics textbooks like Callen seems to be defined as: $U (\boldsymbol{R}) = \Delta U ...
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1answer
43 views

Why does Air Pressure not Matter in Phase Equilibrium?

So long as we are in the ideal gas range, the presence of air does not affect the liquid/solid-vapour equilibrium in any way (such as equilibrium vapour pressure, melting point etc.). Is there a ...
1
vote
1answer
106 views

Work Done in a Thermodynamic System (Cylinder with Piston) [closed]

Heated Piston A cylinder of cross-sectional area 0.0314m^2, filled with argon (a monatomic ideal gas), is sealed with a piston of mass M=25kg that is free to move up and down. The piston is ...
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1answer
47 views

Expectation value of $a_i^\dagger a_i$ for thermal density matrix

Suppose we have some heat bath with Hamiltonian, $$H=\sum_n \left(a^{\dagger}_na_n+\frac{1}{2}\right)\hbar\omega_n$$ and a density matrix $\rho=Z\exp(-\beta H)$ for some normalisation $Z$. ...
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2answers
46 views

Does turning a light off save electricity in a room heated by an electric space heater?

Suppose I've got a basement that kept heated to 20 degrees Celsius using an electric space heater with thermostat. There are no windows in this basement (light is not escaping the room in any ...
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2answers
150 views

Violation of the second law of thermodynamics? [closed]

Here is a machine which seems to violate the second law of thermodynamics: $A$ and $B$ are point black bodies of the same temperature (initially). everything is rotationally symmetric around the ...
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0answers
20 views

What is the best way to heat air? [closed]

I want to know which way we could create hot air most efficient, how can we convert all energy input into air temperature and decrease air density as much as possible? Is the plasma (such as fire) is ...
0
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0answers
32 views

gibbs free energy and maximum work

I'm getting confused between two important results.... Gibbs free energy G = H-TS where h is the enthalpy and S is the entropy. This relation indicates only when the temperature and pressure are ...
0
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0answers
166 views

What is the minimum volume in which, for a given temperature difference $\Delta T$, natural convection would occur?

I'm trying to figure out if convection will occur in a plastic bottle slice and what length of slice would be optimal to achieve high R value transparent insulation on the cheap. The bottle slice is ...
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1answer
75 views

Confusion in the pressure-volume work in an irreversible process

Let there be a certain amount of gaseous substance kept in a cylinder fitted with a weightless & frictionless piston. The initial volume and pressure of the gas are $V_1$ & $P_1$ respectively. ...
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3answers
39 views

Why radiant heat only travels as infra-red waves and not as other types of em waves like UV rays

Heat is a form of energy and em waves carry energy as momentum of photons. So, I think, radiant heat can travel at any frequency. Please correct me.
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2answers
87 views

Contradiction on Entropy of the “universe” decreasing

I have hit what seems to be a contradiction concerning entropy. In my class, we measured the entropy change of photosynthesis which is obviously a non-spontaneous process. We used the formula, ...
2
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0answers
41 views

Evaluate functional integral in partition function of thermal $\phi^4$ theory (Kapusta/Gale p.34-35)

my question is about some steps in the book "Finite Temperature Field Theory" by Kapusta and Gale. To explain the setting, I how to evaluate the functional integrals in the expression $$ \ln ...
2
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2answers
161 views

Glass tends to be cold?

In winter, our glass window serves as a separator of coldness outside and warmth inside our room. We know that the window feels cold when we touch it. Since the air temperature is different in ...
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0answers
72 views

Isentropic and Isothermal Compressibility

Why isentropic compressibility is greater than isothermal compressibility ? While deriving $C_p$ and $C_v$ relation, I got $\frac{Cp}{Cv}$ as $\frac{ke}{Kt}$. Which tells us that $Ke > Kt$. But ...