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5 votes

Why is it so much harder to keep the same room cool than to keep it warm?

I am asking why the cool room warms up faster (loses the cold after you turn off the AC) then the warm room loosing cool down (lose the heat after you turn off the heater). That's not a universal ...
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0 votes

What does $C^{XY}_{\ell}$ mean when we weasure $a_{\ell m}$ in the sky?

It seems to be a mix between the 2 observales but are the 𝑎𝑋ℓ𝑚 and (𝑎𝑌ℓ𝑚)∗ are measured like in the first case in eq(1) with eq(2) ? Yes, for example say $X$ was temperature, then $a_{\ell m}^X$...
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Slope of constant pressure line on $T$-$S$ plot

$$U=T\,\mathrm{d}S-P\,\mathrm{d}V+\mu\,\mathrm{d}N$$ We want $\frac{\partial T}{\partial S}$. We know $T=\frac{\partial U}{\partial S}$, so $\frac{\partial T}{\partial S}$ is the second derivative of ...
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1 vote
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Equilibrium temperature and phase changes

The simplest way to handle this, or even more complex combinations, is to proceed in two steps: Pick a temperature and fraction solid/liquid for the final composition. This choice will almost ...
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5 votes

The entropy given by stefan Boltzmann's law looks remarkably similar to the volume of the sphere; $S(T)=\frac{4}{3}\sigma T^3$

It's a coincidence, as the lack of $\pi$ indicates. The entropy per surface of a blackbody in $D$-dimensional space is $\frac{D+1}{D}\sigma T^D$. (You can deduce it e.g. by generalizing this.) By ...
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Radial dependence of temperature for adiabatic expansion of ideal gas

I don't know if it could be of use for your problem, but the ideal gas with a homogeneous gravity field is well known classical problem of statistical physics. It induces a non-homogeneous ...
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  • 515
0 votes

What exactly is the specific heat of a gas?

specific heat is the quantity of heat required to raise the temperature of one gram of a substance by one Celsius degree and there is a pressure dependence. Please see Does specific heat change with ...
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0 votes

How long can you survive 1 million degrees?

An environment of 1 million Kelvin would irradiate the human body (which is about $2\,\, m^2$) with around $10^{17}$ Watts per the black body law: $$Q=\sigma_{SB}AT^4$$ If we estimate the human body ...
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1 vote

Can low temperature plasma exist?

If the density is extremely small, the recombination rate of free electrons with ionized atoms (or just protons) can exactly balance the thermal ionization rate even at reasonably low temperatures. It ...
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  • 3,221
7 votes

How can the universe be hot or dense in the first moments after the big bang when it has no matter?

Fundamentally, "matter" is manifestation of energy excitations of a universal, all-encompasing substrate (the vacuum or "aether" or spacetime - properly understood, these all refer ...
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30 votes
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How can the universe be hot or dense in the first moments after the big bang when it has no matter?

In Physics, hot is an adjective meaning at high temperature. Heat is a different concept. In the case of ordinary matter, the temperature can be associated with atomic speeds. However, it is possible ...
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4 votes

How can the universe be hot or dense in the first moments after the big bang when it has no matter?

Remember that when we talk about the big bang, all we can actually do is take the current universe and extrapolate the clock backwards in time. We do this using General Relativity ($\Lambda \mathrm{...
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0 votes

Adiabatic free expansion of real (Van der Waal's model) gas below/at/above inversion temperature

this is more a comment on your answer: in any free expansion the pressures in each side are not kept constant: the pressure decreases in one side and increases in the other side. The correct ...
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1 vote

Does the vacuum have a heat capacity ratio? Since there is nothing to 'warm' up

The correct answer is $γ=NaN$. That is to say there's not exactly one correct value. Imagine you take a gas and then expand its volume to an astronomical degree. What you've created is effectively an ...
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-1 votes

Can I use combustion gases for heating?

Found this on the net. The typical furnace outlet temperature of flue gases is usually around 1200 °C which will decreases gradually along the pathway of heat transfer, while the temperature of the ...
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2 votes

Can I use combustion gases for heating?

Of course. the device that does this is called a wood-burning stove.
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0 votes

Green function dependence on temperature

In the Heisenberg picture, the operators evolve according to the Heisenberg equation $$i\hbar \frac{d}{dt} A = [A, H]_-$$ This governs the time evolution of the annihilation operator \begin{align*} i\...
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2 votes

Conductivity of doped semiconductors at absolute zero

I'll need to revisit the theory about this topic in particular to ensure I'm not saying anything wrong, but my first instinct/thaught would be that there is still a gap between the band and the dopant ...
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0 votes

Is thermal vibration a property of a particle moving back and forth by itself, or of a system of particles colliding?

It depends on how you model the particles in the gas. In classical mechanics, simple point like particles should follow Newton’s laws: an object traveling at some velocity will continue to travel like ...
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1 vote

How to calculate heat dissipation of metal box?

This is effectively impossible to calculate from first principles. The problem is that the cooling will be dominated by convection i.e. air in the room around the box will flow over the surfaces and ...
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1 vote

What is the difference between absolute zero Kelvin and almost absolute zero?

In some thermodynamic contexts, the inverse temperature $\beta=1/T$ is a more meaningful quantity. This makes it more clear that 0 Kelvin, or $\beta = \infty$ is approachable but unattainable. And ...
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  • 2,337
2 votes
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What is the difference between absolute zero Kelvin and almost absolute zero?

The difference is that $0$ is not the same as $10^{-9}$. The latter is nearly zero, but it's not zero. Why does it matter? Consider for example Charles's Law, which says that for gases, $V/T$ is a ...
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2 votes
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Heating things with a lens. Theoretical limit vs. energy conservation

The size of the lens only determines the intensity of the radiation that shines on the target object, whereas temperature is determined by Planck's law (thermal equilibrium) and hence, also by the ...
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  • 7,105
0 votes
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Ideal Fermi gas close to $T=0 \rm K$

Here, the substitution and what he talks about is what is more commonly known as the "Sommerfeld expansion". As the temperature $T \rightarrow 0$, one can see that $\beta(\epsilon_{k} - \mu) ...
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