All Questions
Tagged with enthalpy or thermodynamics
15,737 questions
6
votes
1
answer
732
views
Which Friedmann equation is redundant?
For flat FLRW cosmology, we can write down two Friedman equations and one matter equation. Namely,
\begin{align}
H^2 & =\frac{8 \pi G}{3} \rho, \tag{1} \\
\frac{\ddot{a}}{a} &= -\frac{4 \pi G}{...
- 852
16
votes
2
answers
24k
views
Does tea stay hotter with the milk in it?
A little thought experiment, similar to this one: Imagine you are making a cup of tea when the door bell rings. You've poured the boiling water into a cup with a teabag in it. As you're just about to ...
- 261
15
votes
2
answers
801
views
The entropic cost of tying knots in polymers
Imagine I take a polymer like polyethylene, of length $L$ with some number of Kuhn lengths $N$, and I tie into into a trefoil knot. What is the difference in entropy between this knotted polymer and ...
- 945
8
votes
1
answer
8k
views
Why are lasers inefficient?
Why are lasers inefficient? Is it because of the heat lost during lasing? Why couldn't there be thermocouples or turbines in parts of the cooling circuits to extract something out of that heat?
- 197
5
votes
2
answers
592
views
Proof that Statistical Mechanics is a model of Themodynamics
The laws of thermodynamics are essentially four axioms of a mathematical theory.
The expectation values of a statistical ensemble are supposed to satisfy the axioms of thermodynamics (under the ...
3
votes
2
answers
484
views
What devices could be taken round dwellings to measure the thermal conductivities of buildings in situ?
Your lateral thinking and knowledge of lab kit could help us solve a tricky measuring problem in building-physics.
One of the problems we get in modelling the heat demands of buildings, is getting an ...
- 4,154
2
votes
1
answer
592
views
Using heat energy to increase temperature
It's been a long time since I studied thermodynamics in college years ago, but I was just wondering this:
I know that a large bathtub of 10 C water has more heat energy than a small cup of 20 C water,...
- 921
0
votes
1
answer
2k
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Heat & thermodynamics question based on heat loss [closed]
A Sphere A is placed on a smooth table.Another sphere B is suspended as shown in the figure.Both the spheres are identical in all respects.Equal quantity of heat is supplied to both spheres.All kinds ...
- 1,035
2
votes
1
answer
6k
views
Simple formula for liquid heat transfer
I'm trying to do a simple simulation of a solar panel coupled through some piping to a boiler, with the aid of a pump. My input constants could be something like: volume of liquid inside the panel, ...
- 171
2
votes
1
answer
2k
views
heater in a perfectly insulated box
Imagine a perfectly insulated box, placed inside the box is an electric heater. The heater is switched on and the box is left to reach equilibrium with its surroundings. What is the final temperature ...
- 301
6
votes
1
answer
7k
views
Entropy of two expanding and mixing ideal gases
I am currently stuck with the following problem:
...
- 1,613
84
votes
8
answers
126k
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Will a hole cut into a metal disk expand or shrink when the disc is heated?
Take a metal disc and cut a small, circular hole in the center.
When you heat the whole thing, will the hole's diameter increase or decrease? and why?
What will happen to the diameter of disc?
- 857
5
votes
2
answers
726
views
Why can't Schwarzschild wormholes exist?
So, I've recently been reading up on Schwarzschild wormholes and I've learned that they cannot exist becuase they violate the 2nd Law of Thermodynamics.
What I'm asking is: Why do they violate the ...
- 53
17
votes
2
answers
5k
views
What's the difference between different speeds of sound?
In astrophysics, I often come across the speed of sound. I understand that, broadly, it represents the speed at which perturbations travel through a medium. But there's more than one speed of sound. ...
- 9,785
8
votes
3
answers
3k
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Why are the virial coefficients functions of temperature only?
I have read in multiple places that the virial coefficients in the virial equation of state, $Z=1+{B \over v}+{C \over v^2}...$, are functions of temperature only and are independent of pressure (or ...
- 698
13
votes
5
answers
4k
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Is there a relativistic generalization of the Maxwell-Boltzmann velocity distribution?
The Maxwell-Boltzmann velocity distribution in 3D space is
$$
f(v)dv = 4\pi \left(\frac{m}{2\pi k_B T}\right)^{3/2} v^2 \exp\left(-\frac{m v^2}{2k_B T}\right)dv$$
It gives the probability for a ...
- 10k
16
votes
2
answers
19k
views
Minimal temperature achievable by vanilla Peltier element?
I wonder, are there any fundamental issues leading to reduced performance of Peltier elements at cryogenic temperatures (-100C and lower)?
What is theoretical/practical minimum temperature ...
- 2,462
3
votes
1
answer
355
views
Physicality of claims regarding AC ice energy storage units
Many of us may remember the problems from thermodynamics where we had to determine if a pretend air conditioning unit supplier's claims violated the 2nd law. I want to pose a similar question ...
- 21.3k
4
votes
2
answers
245
views
Solar chamber trapped light
Was wondering if trapping light in a reflective chamber could heat rods sufficiently enough to heat water? Trying to think of new ways in which we can utilise solar energy.
- 247
8
votes
1
answer
1k
views
How is information defined from a thermodynamics point of view?
How is information defined from a thermodynamics point of view ? I came across some definitions using the concept of free energy of a system. If I have information stored in a finite volume of space ...
- 1,772
1
vote
1
answer
167
views
Is there a timeline in urban area night temperature?
Fact: We all know that during the day concrete absorbs heat and releases it during the night, making urban areas hotter than rural areas.
I observed that after sunset the ambient temperature is going ...
- 13
4
votes
2
answers
753
views
What are the easiest to get/make LN2 superconductors?
I am starting to build multistage Peltier cooler at the moment, and it should be able to reach -100C at least (but if I fail I can always get boring LN2).
Doing some experiments with superconductors ...
- 2,462
1
vote
2
answers
307
views
Relationship of the abundance of an isotope and the vapor pressure
Say I have a sample of some alkali metal - Rubidium for example. A sample of Rubidium consists of approximately 27% $^{87}$Rb and 73% $^{85}$Rb. Does the vapor pressure of each isotope individual ...
- 695
3
votes
1
answer
2k
views
What is wrong with this reasoning on chemical potential?
One definition of the chemical potential is the change of internal energy of the system with respect to a particle of the added substance with the system entropy, molar volume, and all other species ...
- 698
1
vote
2
answers
498
views
Restrictions on defining microstates (Entropy)
If we have an isolated system $Sb$ with thermodynamic entropy $Eb=X$
(and growing by the 2nd law of thermodynamics),
we could define an abstract system $Sa$ (containing the system $Sb$) but define the ...
- 2,927
5
votes
4
answers
792
views
Understanding Heat
Heat or thermal energy as understood is some kind of vibrations of molecules / atoms of the matter. If the molecules are tightly bound in case of solids, it is to-and-fro motion what we call vibration,...
- 977
2
votes
2
answers
110
views
In a gas of particles, how is the displacement vector related to the number density?
Suppose I have a gas of particles that is initially uniformly distributed so that the number density is $n_0$ (number of particles per unit volume), and then I displace the particles by the vector ...
- 852
5
votes
3
answers
19k
views
Why it is said that Universe is an isolated system?
I have read in many places
"Entropy of an isolated systems never
decreases"
And as a corollary:
"As Universe is an isolated system(I) then its entropy is constantly increasing(II)"
I) If ...
- 2,927
2
votes
1
answer
23k
views
How to calculate heat exchange/deltaT in a moving volume of fluid?
I am building a preheater for a maple syrup evaporator and am going to use the steam generated by the heating process to pre-heat the incoming sap from, say 5 degrees C to (hopefully) something on the ...
- 149
6
votes
2
answers
3k
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how does human brain compare to a modern CPU in energy per bit?
Can someone compare the energy efficiency of human brain as a computer ? What is the energy in joules / flop ? may be some reasonable assumptions on the computational load of common tasks such as ...
- 1,772
10
votes
6
answers
3k
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How is thermodynamic entropy defined? What is its relationship to information entropy?
I read that thermodynamic entropy is a measure of the number of microenergy states. What is the derivation for $S=k\log N$, where $k$ is Boltzmann constant, $N$ number of microenergy states.
How is ...
- 1,772
13
votes
3
answers
2k
views
Hydrogen as a fuel in Sun
The source of Sun's incessant energy is hydrogen; which is continuously converting to helium through nuclear fusion reaction releasing energy. Why does not all hydrogen convert into helium in one big ...
- 977
2
votes
2
answers
36k
views
In summer are the upper storey flats more hot or the lower storey flats?
I have often heard neighbours talking things like in a multi storied apartment, the upper flats are more hot in summer then lower flats (or vice versa?) and similarly for some comparison in winter? ...
- 655
4
votes
2
answers
3k
views
Evaporation from a capillary tube
Consider a capillary tube (say from a liquid / capillary thermometer), that means a tube of small internal diameter which holds liquid by capillary action . The tube is fulfilled with water and closed ...
- 2,717
-1
votes
2
answers
2k
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A fan in a hot room at what point does it put in more energy that it dissipates
If a fan that is using 50 watts is moving 1 m³/min of air. Lets say the walls are the same temperature as the air so there’s no heat dissipation there. How do I know if the fan is putting in more ...
- 734
8
votes
2
answers
22k
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Why did my liquid soda freeze once I pulled it out of the fridge?
Can someone explain in both layman's terms and also technically why when I pulled my glass filled with liquid soda from the freezer, the liquid soda quickly froze?
Doesn't this violate the 2nd law of ...
- 183
2
votes
2
answers
1k
views
the form of a kettle
What is the best form for a kettle, that is, to have the water boil the fastest? I am particularly interested in the following case: for one given kettle (whose volume is constant) containing ...
- 2,930
20
votes
5
answers
3k
views
Does the Moon's core still contain significant heat?
On earth, using earth-sheltering techniques can significantly reduce the temperature fluctuations on a structure. Would the same statement be true as well on the Moon? Does the Moon's core still ...
Walter
2
votes
2
answers
2k
views
Limit on geothermal energy that could be extracted before the earth's magnetic field collapsed?
This is more of a theoretical thought-experiment question.
Basically, how much geothermal energy can we extract before the loss of the magnetic field makes it a terribly bad idea?
Will the ...
- 367
12
votes
3
answers
27k
views
Why does a thermometer in wind not show a lower temperature than one shielded from it?
I'm a little familiar with the physics and thermodynamics of the wind chill effect, but this question seems to come up from time to time:
Why, given two temperature sensors or thermometers in the ...
- 1,335
3
votes
5
answers
6k
views
Can heat be transfered via magnetic field in a vacuum?
Say you want to store hot coffee in a container surrounded by a vacuum. To remove all sources of conductive energy loss the container is suspended in the vacuum by a magnetic field and does not have a ...
59
votes
7
answers
24k
views
Is there any proof for the 2nd law of thermodynamics?
Are there any analytical proofs for the 2nd law of thermodynamics?
Or is it based entirely on empirical evidence?
- 1,394
2
votes
1
answer
578
views
Vapor pressure higher than equilibrium vapor pressure
Is there any mechanism by which the vapor pressure of a liquid substance (lets say in a vacuum) is measured to be greater than the expected saturated vapor pressure at that temperature?
Specifically, ...
- 695
13
votes
3
answers
5k
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LED Thermal Modeling (How to solve heat equation with constant heat source)
I have a mechanical design with LEDs that generate heat. I want to estimate the temperature at the LED junction vs. time, but especially at steady state.
Knowing the LED voltage drop and current, I ...
- 231
13
votes
3
answers
7k
views
Inflating a balloon (expansion resistance)
I am doing a quick calculation on how to calculate the pressure needed to inflate a perfectly spherical balloon to a certain volume, however I have difficulties with the fact that the balloon (rubber) ...
- 333
1
vote
1
answer
220
views
thermal energy while calculating Langevin Forces
I have a quick question from thermodynamics.
I remember that we take kT/2 as the kinetic energy per degree of freedom in kinetic theory of gases. But when we do langevin forces (for example in ...
- 1,772