# 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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### Heat loss from water surface heated in a room

I have one question regarding total heat losses from water surface. Water is heated with immersion heater in open condition in a room. Heat losses from the top of the pan to air is to be calculated. ...
55 views

### What is the gas entropy as a functional of a one-particle distribution function?

There are some discrepancies on how to introduce entropy in classical kinetic theory. In what follows $f(r,p,t)$ is the usual one-particle distribution function of a monatomic gas, normalised to the ...
60 views

### Effective Area of Isotropic Antenna: Explanation?

I'm reading some introduction to antenna theory and I've often puzzled on the equation: $$A_{eff} = \frac{\lambda^2}{4\pi}$$ which relates the effective area by which an antenna captures radiation ...
988 views

### I don't understand why a cavity with a hole is a black body [duplicate]

It makes no sense to me that a cavity with a hole is identical to a black body. Sure, the cavity will probably be a near perfect absorber, but I don’t see how it will be a perfect emitter as well. ...
47 views

### High pitched noise as hot object hits water?

https://www.youtube.com/watch?v=9qSEfcIfYbw Just wondering what this phenomenon was called, and its cause? Would have googled, but I don't even know what it's called.
114 views

### rocket propulsion [closed]

As I understand it, in rocket propulsion a converging/diverging nozzle is used to convert the random velocity vectors of the combustion chamber gases into a mostly unidirectional velocity field at the ...
534 views

### Why doesn't fire flow back INTO the cylinder?

Let us take into a consideration a (pressurized) kerosene stove. The pressure drives fuel out of the nozzle, like or in a much simpler arrangement as in Question- Why doesn't fire flow back ...
71 views

### Gibbs free energy for an ideal gas (problem)

so I'm trying to find the gibbs free energy for an ideal gas $$g = u -Ts + pv$$ Hence, $$dg = du -Tds -sdT +pdv +vdp = -sdt + vdp$$ The entropy is $$s = c_{p} \ln T -R \ln P + s_{o}$$ Which ...
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### Landau's derivation of the law of entropy increase - clarification

In Landau&Lifshitz V: Statistical Physics the following derivation of the law of increase of entropy is given. I need help understanding several crucial steps; I'll briefly summarize the notations ...
188 views

### What is the theoretical instantaneous temperature of a gas?

When we measure the temperature of a gas we typically integrate the molecular collisions and wind up with an 'average' temperature due to the sensor comprising a relatively large thermal mass. And ...
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### There is some attempt to build a magnetic flywheel (reservoir of motion)

We know how flywheel works! There is some attempt to substitute the flywheel-friction mechanism for some magnetic torque ? Exist some mechanism that uses thermal cycle of gases for generate magnetic ...
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### Partition function: Number of states? Doesn't add up for ising

While trying to really understanding the partition function in statistical mechanics, I tried looking at it for a 2D ising model, as that's been helpful for me for all kinds of thermodynamic values. ...
152 views

### Why is the change in entropy for a given energy input dependent on the temperature?

In my thermodinamics class, we saw that $$dS=\frac{dQ}{T}$$ and $$\Delta S=\int_{a}^{b}\frac{dQ}{T}$$ My question is: why for the same energy input $dQ$ the entropy increases more in lower ...
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### How quickly does a gas reach a uniform temperature in an enclosure?

I'm just looking for ballpark figures and reasoning. Suppose a gas (air) at STP was introduced into a 10cm diameter sphere of infinite heat capacity and conductivity at one degree above the gas. How ...
116 views

### Chemical reaction A+B$\leftrightarrow$C. Equilibrium VS Non Equilibrium

Could you please confirm or say why I am wrong? Let us consider the steady state of the chemical reaction $A+B \leftrightarrow^{k_+}_{k_-} C$, with $k_+$ and $k_-$ the forward and backward rates. ...
255 views

### Find Solution to the Heat Equation with Bessel Functions and Initial Values

I'm trying to find the time dependent solution to the heat equation in a very long cylinder (the problem only depends on radius, r, and time, t) with the initial value $T(r,0)=T_1$ and the Dirichlet ...
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### If I accelerate a mass quickly enough through a short distance (i.e. unit step function), would I create wind, sound, and heat?

As far as I know, the movement of air molecules can result in what we call wind, sound, or heat (and maybe some other things I'm not thinking of), depending on the assumptions about its motion. Wind ...
498 views

### 100°C = 100 K =?

I'm in first year. Our class is in lesson " Heat and Thermodynamics". While solving a numerical problem of a reversible engine he told us that 100 degree Celsius is equal to 100 kelvin. I inquired but ...
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### Thermodynamics of expanding nitrogen bubble as it rises in water

Problem An air bubble is released from a nitrogen tank 20 meters under water. The bubble has a radius of 1 mm. What will be the radius of the bubble when it reaches the surface? My Questions Is it ...
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### Given the Pair Correlation Functions for a Multi-component Fluid, is There a Simple Way to Obtain the Helmholtz Free Energy?

I think it may not be possible. Given all the pair correlation functions (e.g. calculated from Ornstein Zernike theory), it seems possible to obtain the internal energy (assuming different species ...
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### What is a “fundamental thermodynamic relation”?

My professor handed me a quiz in which he said something like (I don't remember exactly) "write down the four fundamental thermodynamic relations for $A,G,H,U$". I showed the differential elements of ...
452 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 ...
13k views

### Does placing a lid on a pot affect the rate of boiling?

On the subject of cooking, someone once told me "If you have too much water in your pot after cooking a dish, just turn up the heat and let it boil without a lid for a few minutes". Now I've thought ...
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### why doesn't liquid metal vaporize in a vacuum?

I am wondering why molten metal in a vacuum of electron beam and machines never turns to gas like liquid water does when exposed to a vacuum.
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### Is Boltzmann constant $k_B$ constant?

I heard in a lecture that Boltzmann constant $k_B$ is not constant in some special cases. Do you know the title of the article which contains this one? Do you think this idea is true?
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### Does ambient day/night temperature change have significant energy generation potential

Is there significant energy to capture in daily day/night temperature change? For instance, I see from posts such as the following that there are materials with varying coefficients of linear thermal ...
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### Why does the rate of melting of an ice block vary in different methods?

I saw that an ice block melts faster when it is placed in a microwave rather than when it is exposed to flame. Could it be because the flame creates a layer of water vapor between the flame and ice? ...
791 views

### In counting degrees of freedom of a linear molecule, why is rotation about the axis not counted?

I was reading about the equipartition theorem and I got the following quotations from my books: A diatomic molecule like oxygen can rotate about two different axes. But rotation about the axis ...
520 views

### Carnot vs. Otto

Otto cycle is reversible and according to Carnot theorem should have the same efficiency as Carnot $\eta_{Carnot}=1-T_C/T_H$. The efficiency of Otto is $\eta_{Otto}=1-\frac{T_4-T_1}{T_3-T_2}$ (see ...
373 views

### Are we feeling the sun's heat?

If I may summarize something I told a friend in a pub... "We don't get heat from the sun at all. Heat requires matter to move through and there's too much vacuum between the sun and us for any heat ...
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### Does a vacuum, suddenly opened, become hotter than its surroundings?

Suppose you have an insulated container that is equipped with a valve to let air in. Initially the container is evacuated. You then quickly open the valve, allowing air to rush in. What is the ...
106 views

### Third law of thermodynamics

Does Third Law of thermodynamics violates Quantum Mechanics, I mean Why third law of Thermodynamics holds if we think about it in terms of Quantum Mechanics ?
828 views

### Mass, energy, and entropy.

I have a seemingly simple question about the relation between these three that for some reason doesn't make sense to me. If entropy is the disorder of a system, then a low entropy state is one of ...
821 views

### Thermodynamics : Work done on gas inside container by the force due to pressure exerted by external atmosphere

Suppose a container is fitted with a massless and frictionless piston lying on a table such that pressure due to gas is greater than pressure due to external atmosphere. Hence piston will move. Now ...
215 views

### Average surface temperature of Earth [duplicate]

I had a question in my school exam. Will the average surface temperature of the Earth be lower or higher, if there was no atmosphere? Now, the answer expected is "The avg temp will be lower, because ...
281 views

### What does this extraneous solution to an elementary thermodynamics problem represent?

I recently had the following homework problem (Knight 3e, exercise 16.72, page 467): The cylinder in Figure CP16.72 has a moveable piston attached to a spring. The cylinder's cross-section area ...
341 views

### Existence of gusts of wind, an anomaly?

Enthusiast + Student, not a pro, so pardon my ignorance. How can wind possibly flow in gusts? The way I understand it, a gust is a pocket of air which hits you at slightly higher speed. But how ...
50 views

### How water anomalies affects the Mpemba effect?

I understand basic explanation of Mpemba effect like this: Distant molecules are easier to realign into new formations. But if we compared two bottles of water like in Mpemba effect and one of them ...
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### How can heaters using the same type of fuel be more or less efficient than each other? [duplicate]

I can understand there being a cost difference if you used gas vs electricity to heat your home. But for heaters using the same energy source, how can they be more or less efficient than each other? ...
48 views

### Activation energy as a function of defects

Electrical conductivity has a form of Arrhenius equation. It is assumed that activation energy decreases with defect density in the insulating state and becomes zero in the conducting state. Can one ...
288 views

### Is equilibrum condition consequence of second law of thermodynamics?

Equilibrum state is generally stated to be most stable state. Is equilibrium state consequence of second law of thermodynamics at microscopic level. i.e a system tends to reach the equilibrium state ...
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### Is it possible to tell time within a steady state?

(Inspired from this question (A) which unfortunately the link in the selected answer is now 404 broken (which might have contained my answer), thus cannot read any deeper than that) From the above ...
133 views

### Energy conservation of permanent magnets engine

How to explain the fact that magnet can attract an object (apply a work W) without losing a (significant) part of its internal energy? How to apply the energy conservation principle? Please think to ...
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### Why does this derivation of the dependence of free energy on pressure not work?

It is well known that the Gibbs Free Energy of a gas depends on the pressure via the following formula: $$G_m(p) = G^\circ_m + RT\ln{\frac{p}{p^{\circ}}}$$ Where $G_m$ is the molar gibbs free energy ...
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### Energy under constant pressure and temp

Conceptual problem: let A + 2B -> C + 2D be a reaction under constant pressure P and temp T, that goes all the way towards the products. A,B,C,D are ideal gases. let $\Delta H$ be the molar enthalpy ...
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### Fall in temperature due to expansion

The following lines are from my book. The gas is heated and allowed to expand at such a rate that the fall in temperature due to expansion is less than the rise in temperature due to the heat ...
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### Determination of “Heat transfer coefficient” between block of aluminium and air

I need to build a temperature control system for a hot plate of aluminium, which I intend to heat with an electric cartridge, and the temperature I would be measuring with a PT100 to close the loop. ...
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### What is an expression/physical law that relates high frequency thermal fluctuations to gas pressure?

When a gas is compressed the 'ideal gas law' can predict what the increase in gas temperature will be. But that's just a mean temperature, right? At a quantum level the frequency of molecular ...