Questions tagged [thermodynamics]
Covers the study of (primarily homogeneous) macroscopic systems from a heat/energy/entropy point of view. Consider also using the tag: [statistical-mechanics].
1,919
questions
4
votes
1
answer
3k
views
Do conventional ovens heat by thermal conduction or radiation?
I am curious which energy transfer mechanism dominates in a conventional oven, thermal conduction or thermal radiation?
A naive guess would be that early on, when the food is still cold, the heat ...
4
votes
2
answers
3k
views
What is enthalpy? [closed]
After an intense research reading documents and Q&A, I am still very confused about the concept Enthalpy.
Etymology says: "to warm in". [ἐνθάλπω (enthálpō, “to warm in”)]
I interpret: "to put ...
4
votes
3
answers
2k
views
Is there an upper limit to temperature in thermodynamics or statistical mechanics
In many presentations of statistical mechanics where we have a system of particles having mass, such as the molecules of an ideal gas, the temperature is often equated to the average relative velocity ...
3
votes
3
answers
2k
views
Entropy and reversible paths
I'm a little bit confused about calculating entropy changes along irreversible paths by integrating over a reversible path. When using the central equation I can understand the argument, entropy and ...
3
votes
1
answer
488
views
Is there a mathematical relationship between time and entropy? [closed]
If there is a relation between time and entropy, what is it?
Are there limitations for this equation?
Or if there is no relationship between them, what is the current state of research?
3
votes
3
answers
2k
views
The temperature in space is about 3K, however there are almost no atoms in space. How can there be residual heat?
My understanding is that heat is essentially atomic vibrations. If there are almost no atoms, how can there be residual heat?
Also, as I understand in space there is no heat transfer via convection ...
3
votes
4
answers
640
views
Thermodynamics second law variational statement query
In thermodynamics as I understand entropy is a state function.
A state function is a property whose value does not depend on the path taken to reach that specific value. In contrast, functions that ...
3
votes
1
answer
260
views
Would a giant satellite laser work as a weapon?
If someone built a satellite to float about in space, which used mirrors to focus and magnify energy from the sun directly into one direct spot (like in the James Bond film Die Another Day) would it ...
3
votes
1
answer
664
views
Why internal energy $U(S, V, N)$ is a homogeneous function of $S$, $V$, $N$?
Question. Why internal energy $U(S, V, N)$ is a homogeneous function of $S$, $V$, $N$? How to prove that? That is why does
$\lambda\cdot U(S,V,n)=U(\lambda\cdot S, \lambda\cdot V, \lambda\cdot N)$?
...
3
votes
1
answer
41k
views
Degrees of freedom in a diatomic molecule [duplicate]
We know that a monatomic compound can only have 3 degrees of freedom as we can consider it to be a point mass. However now that we consider a diatomic molecule, there are 3 degrees of freedom in ...
3
votes
3
answers
3k
views
Mathematical proof of non-negative change of entropy $\Delta S\geq0$
I understand that we can prove that for any process that occurs in an isolated and closed system it must hold that
$$\Delta S\geq0$$
via Clausius' theorem. My question is, how can I prove this in a ...
3
votes
1
answer
76
views
Find work given an equation of state
I've seen textbooks saying that for a given equation of state, for example
$$ M = \frac{C_cH}{T}. $$
Where $M$ is the magnetic moment of a paramagnetic material and $H$ is the magnetic intensity.
In ...
2
votes
3
answers
42k
views
Why specific heat at constant pressure is greater than specific heat at constant volume?
I know the relation between specific heat at constant volume and pressure and I also know how to calculate it.
Thing is, I don't understand its concept
I want to know why at constant pressure, ...
2
votes
2
answers
719
views
How does the work-energy theorem relate to the first law of thermodynamics?
The work energy theorem states that the net work on a particle is equal to the change in the kinetic energy of the particle:
$$W_{net}=\Delta K $$
My first question is whether this formula (the work-...
2
votes
1
answer
526
views
Where are particles dissappearing in thermal equilibrium?
I am incredibly puzzled over dark matter relic abundance plots like this (taken randomly from google), where it shows number density of some particle species in a comoving volume
In thermal ...
2
votes
1
answer
544
views
The definition of entropy
As history of thermodynamics say, it was a mystery that what is the required condition for a given energy conversion to take place? Like there are two possible events each conserving energy but only ...
2
votes
3
answers
377
views
Discrete Cavity Radiation vs Continuous Blackbody radiation, violation of the 2nd law of thermodynamics?
Blackbodies emit a continuous spectrum of radiation, whereas a cavity with reflective walls at thermal equilibrium contains a discrete spectrum.
According to Kirchoff, "smoothing out" the ...
2
votes
1
answer
6k
views
How much oxygen would be consumed on a 1 cm squared surface which is on fire?
I'm trying to figure out how much oxygen the Human Torch produces when he is on fire. I figure if I knew how much oxygen on average (per second?) is consumed by a 1 cm squared surface which is ...
2
votes
2
answers
496
views
How much energy does lowering an object into a black hole generate?
An object of mass m is slowly lowered into a black hole of mass 1000 m. Is the amount of braking energy larger than $0.6 mc^2$?
Now what if, after lowering the mass close to the event horizon, we ...
2
votes
2
answers
3k
views
Is thermodynamic equilibrium static or dynamic?
Suppose a system has two parts, $A$ & $B$. They were initially at different temperatures and hadn't achieved mechanical equilibrium. After attainting thermodynamic equilibrium, do they continue to ...
2
votes
1
answer
2k
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
...
2
votes
2
answers
151
views
Closure conditions in the form of equation of state
While reading the book "Riemann solvers and numerical methods for fluid dynamics" By E. Toro, the very first paragraph is:
"In this chapter, we present the governing equations for the ...
2
votes
1
answer
338
views
Use of the solid angle in deriving the Maxwell-Boltzmann distribution
This is from a course in statistical mechanics, specifically the derivation of the Maxwell-Boltzmann distribution. We have an integral over the momentum space
$$Z = \frac{V}{(2\pi \hbar )^3}\int d^3p ...
2
votes
1
answer
205
views
Evolution of Temperature in Time and Space in an Infinite Bar
An infinitely long, uniform bar is perfectly insulated and one end
is kept at a constant temperature ($T_s$). How does its temperature
evolve in time and space?
Because the bar's cross-section is ...
2
votes
1
answer
4k
views
Heat of vaporization of water - dependence on relative humidity?
Does the heat of vaporization of water depend strongly on the relative humidity of the gas into which it evaporates?
Some context: If we want to calculate the dew point of water, we find the ...
1
vote
2
answers
387
views
Is entropy $S$ a fundamental quantity like Temperature?
What I'm trying to say is that $$S=\int\limits_{T_1}^{T_2}\frac{\mathrm dQ}{T}\tag{1}$$ depends only on the initial and final states. Why is that so? Is it like a "law" (like Newton's law of gravity ...
1
vote
7
answers
1k
views
If heat is merely molecular motion, what is the difference between a hot, stationary baseball and a cool, rapidly moving one?
This is from the Exercises for the Feynman Lectures on Physics, exercise Exercise 1.1. I believe that a hot stationary ball has more thermal energy due to the inter-molecular motion of the baseball, ...
1
vote
1
answer
167
views
Equation-of-state for a photon gas
What is the equation-of-state for the photon gas ?
Remarks:
Here Photon gas = Black body radiation. For more background see How does radiation become black-body radiation?, Does thermal energy ...
1
vote
2
answers
235
views
How can entropy of a quantum subsystem increase?
Suppose that Hamiltonian of a global system is time-independent. Two subsystems encompass this global system. Adopt Heisenberg picture, and density matrix and state vector are constant. von Neumann ...
1
vote
1
answer
4k
views
What makes metal change its color during heating?
What happens on atomic and molecular level?
Is this true also for non-metals and metalloids?
1
vote
2
answers
21k
views
How to calculate temperature of an incandescent bulb filament?
Suppose we have a light bulb, for which we know its power rating, like voltage of $12\mathrm V$, and power consumption of $10\mathrm W$. We also know it's a halogen bulb with a tungsten filament ...
1
vote
2
answers
527
views
Lowest temperature possible in the universe?
The third law of thermodynamics states that nothing can reach to absolute zero temperature. What is the lowest possible temperature that can be in the universe? Has any experiment reached to a ...
1
vote
1
answer
246
views
Why are $S = -k_B\sum_i P_i \ln P_i$ and $S = k_B \ln\Omega$ equivalent?
This might be a silly question, but I don't see the equivalence relation between these two equations. Could somebody explain to me how to derive one from the other? Thanks in advance!
1
vote
3
answers
11k
views
Degree celsius vs. Celsius degree
While reading my copy of "Resnick and Halliday", I noticed that the book has made a distinction b/w the terms " degree celsius" and "celsius degree". When I googled this, I was able to find only one ...
1
vote
3
answers
2k
views
Pressure and altitude
I am going to ask a simple question, for sure.
The pressure with respect to the altitude is given by this formula
Where
sea level standard atmospheric pressure p0 = 101325 Pa
sea level standard ...
1
vote
1
answer
337
views
Heat preserving performance of container relative to content
This question has been addressed in the case of a thermos bottle:
Performance of a thermos bottle relative to contents
I am asking the question again without the hypothesis that it is a
thermos ...
1
vote
2
answers
1k
views
Electric power transmission
If we want to transmit electic current for a long distance, we must minimize a heat that releases because of the resistanse. We cannot make a cable wide because it is expensive and it will be massive. ...
0
votes
1
answer
106
views
Does the Clausius statement of the second law apply to microscopic phenomena? [duplicate]
If we have two interacting gases of different temperatures, then it may be possible that a packet of particles(*) which move at high speed go from the hot gas into the cold one (by chance) and raise ...
0
votes
2
answers
127
views
Is $TdS = dQ$ true for processes involving mass transfer?
In his famous thermodynamics textbook, Callen writes
The identification of $-PdV$ as the mechanical work and of $TdS$ as the heat transfer is valid only for quasi-static processes.
My question -- is ...
0
votes
2
answers
398
views
Clarification on Clausius Inequality
Consider a heat reservoir which gains heat $Q$ irreversibly at temperature $T$ from the surroundings which is at temperature $T_0$. The entropy change of reservoir is then given by $\frac{Q}{T}$, ...
-1
votes
3
answers
31k
views
Why is a degree Celsius exactly the same as a Kelvin?
How on earth is it possible that the difference between two temperatures in Celsius and Kelvin is exactly the same. Given the historical definition of Celsius, I find it hard to believe that this is ...
-2
votes
2
answers
290
views
Can anyone prove this overstated-but-almost-never-justified fact from thermodynamics?
Clausius inequality states that $\oint {\delta Q\over T}$ equals zero for a system undergoing a reversible cycle, whereas it can’t be greater than zero for an irreversible cycle.
But everywhere, I ...
149
votes
10
answers
55k
views
Why is my hand not burned by the air in an oven at 200 °C?
I have this problem from University Physics with Modern Physics (13th Edition):
The inside of an oven is at a temperature of 200 °C (392 °F). You can put your hand in the oven without injury as ...
143
votes
7
answers
42k
views
Why doesn't water boil in the oven?
I put a pot of water in the oven at $\mathrm{500^\circ F}$ ($\mathrm{260^\circ C}$ , $\mathrm{533 K}$). Over time most of the water evaporated away but it never boiled. Why doesn't it boil?
132
votes
2
answers
13k
views
Why does ice cream get harder when colder?
What would seem to be a silly question actually does have some depth to it. I was trying to scoop out some of my favorite soft name-brand ice cream when I noticed it was frozen solid, rather than its ...
131
votes
4
answers
58k
views
Why can I touch aluminum foil in the oven and not get burned?
I cook frequently with aluminum foil as a cover in the oven. When it's time to remove the foil and cook uncovered, I find I can handle it with my bare hands, and it's barely warm.
What are the ...
105
votes
6
answers
11k
views
Explain it to me like I'm a physics grad: Greenhouse Effect
What is the mechanism by which increasing $\rm CO_2$ (or other greenhouse gases) ends up increasing the temperature at (near) the surface of the Earth?
Mostly what I'm looking for is a big-picture ...
81
votes
8
answers
19k
views
Why does fire make very little sound?
Sound is air particles vibrating (thus hitting each other to make longitudinal waves) and heat is the vibration of air molecules. Because we can only assume that heat made from fire is a higher ...
76
votes
8
answers
23k
views
If I stood next to a piece of metal heated to a million degrees, but in a perfect vacuum, would I feel hot?
A friend of mine told me that if you were to stand beside plate of metal that is millions of degrees hot, inside a 100% vacuum, you would not feel its heat. Is this true? I understand the reasoning ...
60
votes
2
answers
6k
views
Why does pressure in a thermos increase after shaking up hot water and soap?
Whenever I wash my thermos, I put hot water and then some soap in; then I seal the one end with my hand or use the lid. After shaking it up, if I slowly remove the lid or my hand, it expels a little ...