Questions tagged [equilibrium]

Equilibrium is the condition of a system in which all competing influences are balanced. These can range from forces (static/dynamic equilibrium), to chemical processes (chemical equilibrium) and temperature gradients (thermal equilibrium).

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Why is the Earth so fat?

I made a naive calculation of the height of Earth's equatorial bulge and found that it should be about 10km. The true height is about 20km. My question is: why is there this discrepancy? The ...
Mark Eichenlaub's user avatar
8 votes
3 answers
3k views

Why are thermodynamic potentials minimised?

Why is it that, at equilibrium, certain potentials are minimised? That is, for a system at constant temperature and pressure, the Gibbs free energy is minimised, and for fixed volume and temperature, ...
Toby Peterken's user avatar
43 votes
8 answers
6k views

Prove that negative absolute temperatures are actually hotter than positive absolute temperatures

Could someone provide me with a mathematical proof of why, a system with an absolute negative Kelvin temperature (such that of a spin system) is hotter than any system with a positive temperature (in ...
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8 votes
2 answers
772 views

Assumptions of thermodynamic & chemical equilibrium in fluid-dynamics

I am reading about the Euler equations of fluid dynamics from Leveque's Numerical Methods for Conservation Laws. After introducing the mass, momentum and energy equations, some thermodynamic concepts ...
smilingbuddha's user avatar
23 votes
6 answers
4k views

Is thermodynamics only applicable to systems in equilibrium?

So I was going through callen's thermodynamics book and their he says that thermodynamics is only applicable to systems which are in equilibrium and that naturally raised a few questions in my mind ...
Metric's user avatar
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7 votes
2 answers
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How are the Lagrange points determined?

According to Hyper Physics, there are 5 equilibrium, or Lagrange points of the Earth-Moon system and only 2 of them are said to represent stable equilibrium points. This made me think if there is an ...
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25 votes
4 answers
31k views

Why does the nature always prefer low energy and maximum entropy?

Why does the nature always prefer low energy and maximum entropy? I've just learned electrostatics and I still have no idea why like charges repel each other. http://in.answers.yahoo.com/question/...
Know Nothing's user avatar
16 votes
14 answers
16k views

Why do rain drops fall with a constant velocity?

While reading my physics book. I came across a line that says that: Rain drop falls with a constant velocity because the weight(which is the force of gravity acting on body) of the drop is balanced ...
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2 votes
1 answer
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Estimate the reaction force on each leg of a 4-legged table

Assume you have a table with four legs with a weight placed somewhere inside the boundary created by the legs. The task is to determine the reaction force at each leg. Here are the problem ...
Kyle's user avatar
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13 votes
5 answers
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Adiabatic piston: why is Callen's argument flawed?

This question is related to this one. I would ask you to read that question and my answer to the question itself before answering this one. The problem is the following. In his book Thermodynamics, ...
valerio's user avatar
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9 votes
4 answers
2k views

What maximizes entropy? [duplicate]

Liouville theorem states that the phase space distribution function of a system remains unchanged with the system evolution, $$ \frac{\text{d}\rho}{\text{d}t}=\frac{\partial \rho}{\partial t}+\sum_{i=...
Roger V.'s user avatar
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7 votes
1 answer
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What's the point with equilibrium in thermodynamics?

All the thermodynamics books I saw until now state that in thermodynamics we are mainly concerned with equilibrium states (I know there's "non-equilibrium thermodynamics", but I'm interested ...
Gold's user avatar
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7 votes
5 answers
628 views

Slow thermal equilibrium

I have a question which is inspired by considering the light field coming off an incandescent lightbulb. As a blackbody radiation field, the light is in thermal equilibrium at temperature $T$, which ...
Emilio Pisanty's user avatar
5 votes
1 answer
3k views

Definitions in thermodynamics: temperature, thermal equilibrium, heat

I'm currently reading Fermi's "Thermodynamics" and I'm trying to grasp the (possibly different) right definitions for temperature, thermal equilibrium, heat. To clarify, I'm looking for definitions ...
pppqqq's user avatar
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-1 votes
1 answer
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Has the zeroth law of thermodynamics ever been proven?

Has the zeroth law of thermodynamics ever been proven? I know the zeroth law of thermodynamics probably follows from the second law of thermodynamics but according to The statistical nature of the 2nd ...
Timothy's user avatar
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20 votes
4 answers
718 views

Do unstable equilibria lead to a violation of Liouville's theorem?

Liouville's theorem says that the flow in phase space is like an incompressible fluid. One implication of this is that if two systems start at different points in phase space their phase-space ...
UtilityMaximiser's user avatar
6 votes
1 answer
3k views

Calculating Frequency of Oscillations About a Stable Equilibrium Point

Assume I have a particle $m$ moving in one dimension where function $U(x) = -Ax + Bx^2$ describes the potential energy. I am trying to figure out how I can calculate the frequency of small ...
A. Radek Martinez's user avatar
20 votes
2 answers
3k views

Why is (von Neumann) entropy maximized for an ensemble in thermal equilibrium?

Consider a quantum system in thermal equilibrium with a heat bath. In determining the density operator of the system, the usual procedure is to maximize the von Neumann entropy subject to the ...
joshphysics's user avatar
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19 votes
4 answers
20k views

Why can't charge be in a stable equilibrium in electrostatic field?

I am reading Feynman's Lectures volume_II where he discussed the impossibility of the presence of stable equilibrium in an electrostatic field. There are no points of stable equilibrium in any ...
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14 votes
1 answer
34k views

Difference between steady state and equilibrium?

In semiconductor physics, what is the difference between steady state and equilibrium. How analysis of devices varies in these processes?
johndaniel's user avatar
12 votes
3 answers
3k views

How is quantum mechanics consistent with statistical mechanics?

Let's say we have an harmonic oscillator (at Temperature $T$) in a superposition of state 1 and 2: $$\Psi = \frac{\phi_1+\phi_2}{\sqrt{2}}$$ where each $\phi_i$ has energy $E_i \, .$ The probability ...
Ivan's user avatar
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10 votes
6 answers
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Why a system should be at its lowest energy state for its stability?

Every possible reaction in chemistry is to attain stability. In physics, the alignment of an electric dipole in an external electric field and in all other physical systems (at least those I study in ...
Rajath Radhakrishnan's user avatar
6 votes
3 answers
47k views

When a ball is tossed straight up, does it experience momentary equilibrium at top of its path?

This question has been asked many times all over the Internet and answers can be found on places such as yahoo and ask.com, but I'm not satisfied with those answers and I don't trust the validity of ...
B.K.'s user avatar
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4 votes
2 answers
412 views

What are the conditions for a heap of identical hard spheres to be in a static equilibrium?

It is common to see a heap of conical shape formed by a large number of similar size hard spherical objects, for example, a heap of pebbles, sand etc. Suppose we want to model this system as a ...
Maxim Umansky's user avatar
3 votes
1 answer
462 views

Reaction forces in pyramid stacking of steel coils

I was tasked with solving this problem at work as we are out of coil holders and we bought 140 coils and need to stack them asap. Companies that make the storage holders are on back order for several ...
Robert's user avatar
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3 votes
1 answer
1k views

Is there always an equilibrium point in space for gravity?

For instance, for a two-body system, there will always be a point in-between the two in which the forces of gravity completely cancel, and if I were there then I would experience no net force. I ...
QCD_IS_GOOD's user avatar
  • 6,735
2 votes
5 answers
197 views

Free body diagram on a rack of wooden blocks [closed]

Considering a rack of blocks, and that I am able to hold it up in a horizontal line without it collapsing. I am trying to draw the free body diagram of one of the blocks in the middle portion, block A....
esdoublelef's user avatar
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 ...
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2 votes
1 answer
825 views

Numerically solving the TOV equations: how to find the max radius and mass?

I'm solving the TOV (Tolman-Oppenheimer-Volkoff) equations using Mathematica 13.2, and I'm having some issues at finding the outer radius $r_{\text{max}}$ and mass $M_{\text{max}}$ of the spherical ...
Cham's user avatar
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71 votes
5 answers
11k views

Why does a system try to minimize its total energy?

Why does a system like to minimize its total energy? For example, the total energy of a $H_2$ molecule is smaller than the that of two two isolated hydrogen atoms and that is why two $H$ atoms try to ...
SRS's user avatar
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15 votes
2 answers
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Spikes on ferrofluid?

What exactly are the spikes, or peaks and valleys, caused by in pictures such as these Wikipedia states that "From the point of view of magnetic energy, peaks and valleys are energetically favorable",...
QCD_IS_GOOD's user avatar
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14 votes
4 answers
428 views

How quickly should a fluid come to hydrostatic equilibrium?

Let's say I'm holding a one-liter water bottle, full of water, which I then drop. Before dropping the water bottle, the equilibrium is for there to be a pressure gradient in the water canceling the ...
Mark Eichenlaub's user avatar
12 votes
2 answers
19k views

How do simple bird waterers work?

I made a simple bird waterer made from two 1.25L PET bottles. Let's call these two bottles the reservoir and the tray. Setup The tray is a bottle with the top cut off to leave only the bottom third. ...
fiat's user avatar
  • 223
12 votes
4 answers
1k views

Is "equilibrium state" equivalent to "well-defined state variables"?

Follow up to Intuitively, why is a reversible process one in which the system is always at equilibrium? and How slow is a reversible adiabatic expansion of an ideal gas? Suppose you have a piston ...
Mark Eichenlaub's user avatar
11 votes
2 answers
5k views

Do airlocks in space decompress violently as they do in movies?

The standard scene follows: The good guys have finally captured the enemies and have them in an airlock chamber on a spaceship. A button is pressed and out they go, violently propelled by the blast of ...
kettlecrab's user avatar
11 votes
1 answer
21k views

Question on the hydrostatic paradox

The hydrostatic paradox, states that the height of the water in any container is independent of the shape of the container. This does make sense since it is only the height of the fluid ...
stochastic13's user avatar
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10 votes
3 answers
15k views

What is the state of the equilibrium for a second derivative equal to zero?

Considering a potential energy of $U$, and a displacement of $x$, the force is given by $F=-\frac{\partial U}{\partial x}$. Since equilibrium is defined as the point at which $F=0$, we can express ...
Noah P's user avatar
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8 votes
1 answer
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Local thermal vs local thermodynamic equilbrium?

I know that their is a difference between thermal equilibrium and thermodynamic equilibrium. The former only requires no heat transfer whilst the latter also requires mechanical equilibrium, dynamical ...
Quantum spaghettification's user avatar
6 votes
2 answers
9k views

What determines the (minimum) angle at which a domino falls over?

Dominoes, when placed upright, remain that way. Sometimes, even if you tip them a little bit, they will go back to their upright position. However, if you tip them too far, they will fall over. ...
Justin L.'s user avatar
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5 votes
6 answers
678 views

Horizontal Rack of blocks revisited a.k.a. if it were on top of lava, which block would you choose to stand on?

This question is a follow up to the following question asked by esdoublelef Free body diagram on a rack of wooden blocks To repeat, consider a rack of blocks (each block is a cube), that are held up ...
silverrahul's user avatar
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5 votes
2 answers
342 views

Do chemical potentials stay constant in chemical equilibrium?

I am uncertain whether chemical potentials stay constant during chemical equilibrium. Consider a closed container divided into two parts 1 and 2 filled with ideal gas particles. The barrier between ...
Ray Siplao's user avatar
5 votes
0 answers
107 views

Empirical equivalence of shifted chemical potential $\mu_i$

It is often said that, in classical thermodynamics, entropy $S$ and energy $U$ are defined only up to an additive constant proportional to the total amount of substance $N=\sum_i N_i$ (where the sum ...
Jens's user avatar
  • 51
4 votes
3 answers
567 views

How a thermodynamical system relaxes to equilibrium?

For a system in contact with a reservoir with well-defined constant temperature $T$, its change in Helmholtz energy satisfies the following inequality: $$\Delta F \le -W_{by}$$ where $W_{by}$ denotes ...
Jimmy Yang's user avatar
4 votes
1 answer
373 views

Is this formula for the energy of a configuration of 3 fluids physically reasonable?

I have studied for a couple of months now a mathematical model of the energy of a configuration of immiscible fluids situated in a fixed container such that the fluids fill the container. In other ...
Beni Bogosel's user avatar
3 votes
4 answers
1k views

Could a gas be in thermodynamical equilibrium even with a gravitational field?

Let us consider a cylinder filled with ideal gas, and let us assume that the gravitational field is parallel to one of its sides, well in this case, how would the system be in equilibrium? Suppose ...
Yassin Rany's user avatar
3 votes
0 answers
510 views

Three-log problem [closed]

Three uniform round rigid cylindrical logs of the same size and weight are placed on a horizontal plane. The two at the bottom are touching each other, the third one is placed on the top as shown in ...
Maxim Umansky's user avatar
2 votes
2 answers
110 views

Is bouncing ball (100% collision) an oscillatory motion/SHM or both or none?

My teacher told me bouncing ball (100% elastic) is oscillatory motion that does not have a stable equilibrium position and restoring force. It is just to and fro motion and thus called oscillatory ...
Krishna Sharma's user avatar
2 votes
4 answers
8k views

Why are excess charges in a conductor at the surface?

I’ve been told that coulomb repulsion pushes excess electrons to the surface of a conductor (i.e. sphere) electrostatic equilibrium, and this symmetry causes the net electric field inside to be zero. ...
lightweaver's user avatar
  • 1,479
2 votes
2 answers
492 views

How many temperatures has a plasma?

In nonthermal plasma, not all particles move in the same way. The electrons are different from other particles. Both can be described as having a temperature separately. But that would mean, one piece ...
Volker Siegel's user avatar
2 votes
1 answer
3k views

Situation of Stable, Neutral and Unstable Equilibrium

Recently, I was reading about stability of equilibrium. I came across the definitions for different types of equilibrium. Neutral Equilibrium: The kind of equilibrium of a body so placed that when ...
Prasanna Venkatesan's user avatar