# 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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### Thermodynamics Work Done At constant pressure and charles law

If we fix a piston in the mouth of a cylindrical container in such a way that the piston settles when the pressure from outside the piston is equal to that of the gas within the container, and ...
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### How do we know that the Law of Cooling is true?

According to James Stewart's Calculus book (exercise 14, page 609), Newton’s Law of Cooling states that the rate of cooling of an object is proportional to the temperature difference between the ...
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### Maximal work from cooling a portion of water

Let's consider a portion of water of mass $m$. We want to find out how much work (electric energy) we can acquire by cooling it from temperature $T_1$ to temperature $T_0$. The most efficient way is ...
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### Drying clothes with the sun's heat, without any air

Will my wet clothes dry if I hang them under the sun, and if there is no air around the clothes? In other words, do I need both air and heat to dry wet clothes, or is heat alone (in the imagined ...
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### Apparent Contradiction in Helmholtz Free Energy

I am looking at some mathematics of HelmHoltz free energy. Naturally, wikipedia is one of the (hopefully) more reliable sources of information. In its derivation section, the last equation states that ...
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### Blackbody radiation and emissive power

According to blackbody radiation theory, and thanks to Planck, we now know that there is a energy density, $u(\lambda,T)$ [$J/m^3$], associated with a certain wavelength at a particular temperature. ...
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### Is the Boltzmann brain problem really clearly established as a problem?

Starting from the assumption of a cosmology in which Boltzmann brains dominate over evolved ones, it is not immediately obvious to me that there is a real problem, since only Boltzmann brains ...
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### Why is pale coloured skin said to absorb more UV?

Many resources state that light skin/pale skin absorbs more UV than dark-colour skin. Doesn't black absorb maximum radiation? For an example, see this article: Natural selection therefore ...
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### Is liquid helium expensive because of its rarity or because of its low liquidification temperature?

I once argued with my roommate about this problem. Of course it is rare. But presumably the low liquidification temperature makes the cost high too. So which is the primary reason?
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### Change in entropy of the Universe for charging/discharging a capacitor

I just need to know what exactly happens when charging/discharging a capacitor. Is there any heat transfer between the capacitor and the surroundings? Is there a heat transfer between other parts of ...
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### How to calculate liquid pressure regardless of the gravity?

Assume we have a balloon in a tank filled with a liquid , without any gravitational forces: We just know the Temperature, $V_0$ and $V_1$ I want to calculate the pressure exerted from the liquid on ...
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### What would it be like “inside” a star?

This question was triggered by a discussion regarding the computer game Elite: Dangerous, where spaceships routinely operate in close proximity to stars (two or three light seconds away), at which ...
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### Why is the Specific Heat of Helium 36 times greater than Xenon?

Given that these are single atoms, why should the specific heat difference be so great? And more specifically, why does it take 36 times as much energy to raise the temperature of a given mass of ...
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### Which is a more effective cooling method?

Assume I have a refrigerator at 0C and I put a can of soda at the center of the refrigerator. The temperature inside the refrigerator will be increased to 5C after I put the soda in, but will go back ...
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### When to use h = Cp∆T or u = Cv∆T

I'm getting myself confused on when to use h = cp∆T or u = cv∆T where cp is the specific ...
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### 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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### 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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### Is it possible to efficiently extract phonons from a lattice?

The population inversion generated in lasers occurs in multi-level atomic systems. As an example, the Nd:YAG energy diagram is shown below. The pump pushes the Nd ions up to the upper levels, and ...
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### Convection and radiation heat transfer

It's been some years since my last physics exam and I really need a refreshment in this. I'm stuck in getting a rough estimation of heat exchange between 2 objects separeted by air: let's say object 1 ...
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### 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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### Do some of the laws of thermodynamics break down in quantum mechanics?

I do not know if this is a stupid question as I am not an expert in thermodynamics and certainly no expert in QM. So, we know that the laws of thermodynamics are laws based on statistics. They ...
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### Spin correlation function identity

The correlation function G between two spins is usually defined as $$G=\langle \sigma_a \sigma_b\rangle - \langle \sigma_a\rangle \langle\sigma_b\rangle$$ The $\sigma$ are the value of the spins (+...
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### How fast do I have to dry myself for a hot shower to heat my body?

I am not a physicist. I would like to know how fast do I have to dry myself after taking a hot shower to get more heat from the shower, than lost because the water on my skin increasing heat exchange ...
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### What is meant by a “stiff” or “soft” equation of state (wrt neutron stars)?

I am currently trying to understand the history of the development of the equations of state and structure of neutron stars. In my textbook, I frequently encounter phrases such as "The ...
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### Rough/ballpark thermodynamics and black body temperature question

This is probobly pretty basic but I got into a debate with someone and the temperature of Titan came up, and I did some quick and dirty calculations - as follows. Titan is about 9.5 times as far from ...
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### Can bulk modulus ever be negative? Can somebody illustrate with an example!

I was particularly confused when my teacher proved that $C_p > C_v$ for substances other than ideal gas for which the difference between the two is R . However the proof involved assuming that the ...