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Questions tagged [equilibrium]

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How does the sea level reach a fixed height?

Consider a flat body of water on the Earth. On all tiny elements of water in that body, the buoyant force is equal to the force of gravity due to Earth. Now if I introduce the moon to this body, (or ...
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0answers
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Minimisation of grand free potential at equilibrium?

I have a couple of questions regarding the grand potential at equilibrium. I am well aware that in the grand canonical ensemble, particles and energy can be exchanged. I also understand why the grand ...
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4answers
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Normal Force on an object on the earth’s surface

If a body is on the earth’s surface at equilibrium, the forces acting on it are gravitational (by the earth’s centre) and normal (by the surface). These balance each other and allow the body to ...
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2answers
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Confused about forces acting in this simple statics problem

I'm trying to solve this statics problem to find the internal tension that results, in each member: It shows 3 idealised weightless rigid members hinged in a vertical plane, in an isoceles triangle. ...
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“No stable equilibrium halfway between two equal point charges”

From my instructors notes. Obviously if we displace the negative charge(from the halfway point) in either horizontal direction then it will not return to its initial position. If we displace a ...
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0answers
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Unstable equilibrium due to an arbitrary electrostatic configuration

Suppose n charges are put in an arbitrary electrostatic configuration and a small test is placed at a null point (i.e., where $\vec{E}=0$ ) of the configuration. The task is to show that the ...
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2answers
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Why does the $\phi$-cubed theory have no ground state?

In the book of Sredinicki's, he claimed that the $\phi^3$ theory has no ground state, hence this is not a physical theory. My question is that I can't see why this system has no ground state. And I ...
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0answers
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What does the “equilibrium” mean in DFT?

In DFT, we usually call the "equilibrium position" as the ion positions that corresponds to the global minimum of the (ground-state) Born-Oppenheimer potential energy surface (PES). Let's consider a ...
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1answer
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Entropy of the big bang

At the moment of big bang, all the matter was in perfect order, that is entropy 0 so what force or disturbance would occur to begin the chaos and the entropy start to increase?
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2answers
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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 ...
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1answer
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Equilibrium and non-equilibrium thermodynamic states?

I have the following two states. I have classified them as equilibrium and non-equilibrium and I am not sure if that is the right category my problem is I need to understand what parameters to look ...
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6answers
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Why do atoms tend towards electrical stability?

If an oxygen atom has six electrons, then it has an unfilled orbital and the oxygen atom may share electrons from two hydrogen atoms (and form water) in order to become more stable. But why does ...
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1answer
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Equilibrium between Steam and Ice

Problem Statement: $15 \ g$ of steam at $373 \ K$ ($100^{\circ} \ C$) is mixed with $25 \ g$ of ice at $273 \ K$ ($0^{\circ} \ C$). What is the final temperature of the mixture. My Question: The ...
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2answers
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How much of an object's mass behaves as if it is located at the center of gravity? [closed]

We recently finished up a physics lab where a 200g weight was balanced at the end of a ruler on a fulcrum (measuring torque). In the analysis portion of the lab, we are asked: "How much of the mass ...
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1answer
38 views

Can spontaneous fluctuations cause instantaneous non-equilibrium?

The fluctuation-dissipation theorem says that the linear response of a given system to an external perturbation is expressed as the fluctuation properties of the system in thermal equilibrium. Does ...
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1answer
54 views

Does entropy increase with heat flow?

In an exam, I had a scenario where 2 bodies with different temperatures were put together and over time their temperatures mixed and eventually became uniform. My intuition tells me that as a result, ...
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0answers
31 views

What exactly constitutes equilibrium processes?

It is almost always implied that the fields of statistical physics/thermodynamics mostly consider systems in equilibrium. Simple things as defining the entropy become very hard once we are in the non-...
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0answers
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How does thermalisation make incoherent waves into coherent ones?

Thermalisation is the process by which out-of-equilibrium systems reach equilibrium. Coherence refers to the phases of waves being a constant difference apart. While reading a paper on axion stars, I ...
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1answer
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Does the First law of thermodynamics hold if the final state is NOT in equilibrium?

So we know the first low of thermodynamics holds for isolated systems $$\Delta E = W+Q\tag{1}$$ Now assuming we are in a thermally isolated system such that $(Q=0)$ we obtain $$\Delta E= W\tag{2}$$...
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Is there a difference between false (metastable) and local equilibria?

Or is it permissible to use the terms interchangeably? If there is a difference, what is it?
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4answers
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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 ...
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1answer
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Affect of temperature on carbonation loss

When opening a bottle of soda (any carbonated drink) some of the 'fizz' rushes out. When you replace the cap, CO2 from the soda migrates to fill the empty space in the bottle and eventually an ...
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2answers
52 views

Why quasistatic doesn't imply reversible process? [duplicate]

Quasistatic process are almost always in equilibrium. We know that equilibrium implies zero entropy change. And zero entropy change implies that the process is reversible. So why quasistatic doesn't ...
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2answers
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Clarify Torque and static friction [duplicate]

I came to this problem and solved it but I don't fully understand how this works. The problem is: A 200lb man climbs to the top of a 20ft ladder that is leaning against a smooth wall at an angle ...
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1answer
57 views

Grand free energy as force towards equilibrium

My book on thermal physics shows that, for a system in thermal and diffusive equilibrium (with a reservoir that can supply both energy and particles), the grand free energy, $\Phi$, tends to decrease. ...
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0answers
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How to get electromagnetic field tensor of photon gas?

Momentum spectrum of ideal photon gas depends on its temperature.But sometimes I need these thermal photons to act like 'electromagnetic field'.I want to take them into charged particles' motion ...
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0answers
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Rotor finds equilibrium in TWO positions, why and how?

I have a Mendocino motor, a toy with magnets, solar panels, and coils. Such a thing requires careful balancing (eg. with tiny bits of tape) to achieve sufficient balance that it will start rotating ...
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0answers
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difference between translational and dynamic translation equilibrium

What is the difference between translation equilibrium and dynamic translational equilibrium ? Also what is the difference between rotational and dynamic rotational equilibrium ? definitions : ...
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1answer
122 views

Thermodynamic Stability - Convexity - Concavity of Thermodynamical potential

The thermodynamic stability principle requires convexity of the internal energy upon all of its independent variables. When we pass through the Legendre transforms to build all the other ...
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2answers
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(Two fixed charges add in a third), why does it matter which side I put the third charge if I change signs accordingly?

So the question is "Two charges, 1.0 micro-Coulombs and -3.0 micro-Coulombs, are 10 cm apart. Where can a third charge be located so that no net electrostatic force acts on it?". Now the instructor ...
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3answers
355 views

Maximum entropy at equilibrium for closed system: Local maximum or global maximum?

For a closed system at equilibrium the entropy is maximum. Is this a local maximum or is it a global maximum? I am an undergraduate physics student and it seems that the possibility of entropy ...
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3answers
60 views

Electrostatics fundamentals

Suppose a charge is kept at the centre of a square frame and the vertices of the frame have four more charges fixed to the frame which are identical to each other but not necessary that these four are ...
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1answer
56 views

Definition of equilibrium for thermodynamics and statistical mechanics

Is the definition of equilibrium for thermodynamics and statistical mechanics the same? From my understanding, a system is in thermodynamic equilibrium if its macroscopic variables are not changing. ...
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1answer
36 views

Local equilibrium of slow time varying thermal system

I'm trying to differentiate a thermal system in local equilibrium (and slow time varying) v/s a non-equilibrium system. For a thermal system which is slowly time varying, how does one define local ...
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1answer
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Proving that $c_V$ is positive

On my professor's lecture notes he claims that $c_V>0$. Intuitively, this would have been my guess as well but he mentions how you can prove this by constructing a setup: "Construct an isolated ...
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1answer
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Can body of one temperature ever cause higher temperature in another body?

If you have one body that radiates heat at a constant rate and, forgive my simple assumption, thus constant temperature, and a body that may absorb this heat, may the temperature of second body ever ...
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1answer
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2 body problem question

I am taking a classical mechanics course and i am struggling to determine the equilinrium points for this potential $$Q= -(k/r) + ar + (L^2/mr^2).$$ The problem is that i have to solve a cubic ...
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4answers
67 views

Thermodynamic process

"During this process, the system is always in thermal equilibrium with the surroundings." Does this statement means the process is adiabatic? Since the temperature of both systems is the same, there ...
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1answer
63 views

Why particles of thermodynamic systems have a preference for low energy levels?

While reading Ken Dill's Molecular Driving Force, I noticed this paragraph when the author is trying to explain the Boltzmann distribution. Particles don’t have an intrinsic preference for lower ...
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1answer
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Overconstrained equilibrium binary mixture phase seggregation

Assume a binary mixture, with the component $A$ in concentration $c$ and the component $B$ in concentration $1-c$. The total energy $E(c)$ is thus given by a function of one variable $c$ (if we do not ...
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1answer
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Thermodynamics ($dE<0, dH<0, dG<0$ )

I didn't understand the derivation of why Gibbs free energy becomes negative when change is spontaneous. Our chemistry teacher's derivation was like this. $$dE = TdS - PdV \leq TdS - P_{ext} \cdot ...
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1answer
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Equilibria and conserved quantities in a superposition of central potentials

I have a Lagrangian made up of a superposition of $k$ central repulsive potentials centered at $(a_k,b_k)$. Each potential is somewhat strange - they take the form $V(r)=-\ln\left(r\right)$. The whole ...
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1answer
56 views

Why does a system assume ground state at absolute zero temperature?

I am going through Huang, Statistical Mechanics. He says at 0 kelvin, a quantum system assumes ground state so that $S=k_B ln(G)$ holds where $G$ is the degeneracy of the ground state . My question ...
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1answer
48 views

Earnshaw's theorem and stability of charge inside a hollowed cavity of conductor

When a positive charge q is placed in the center of a spherical cavity of a spherical neutral conductor, it induces a negative charge $q$ to line the inner wall of the conductor, and a positive charge ...
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4answers
128 views

Work done but no change in velocity

I’m taking an introductory course in thermodynamics right now and I have learned that in a quasi static process, one way to increase internal energy is to do work on a system or supply heat to it. But ...
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1answer
61 views

Laplace's heat equation equilibrium functions

I simulated the Laplace's heat equation on a grid (200x200) with boundary conditions of 0 (a cold source) on the top and left and 1 (of hot source) on the bottom and right. (see gif) I stopped ...
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0answers
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Why are the electrodes in thermodynamic equilibrium for a working solar cell?

I was reading a paper that performed simulation of a solar cell under illuminated conditions. Obviously, in these conditions, the device is out of thermodynamic equilibrium. For example the ...
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1answer
43 views

Is temperature equilibrium actually reached in a mixer tap, or is it merely a jumbling of hot/cold droplets?

Most kitchen and bathroom sink faucets have a mixer tap which blends the hot and cold inflows into a warm, uniform outflow stream. It's difficult to believe that complete heat transfer and ...
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1answer
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Relationship between binding energy of atoms in a solid and compressibility defined by bulk modulus

Consider an atom which has η nearest neighbours. The equilibrium binding energy $L_0$ of $N$ atoms is: $$ L_0 = \frac{Nηϵ}{2} $$ The bulk modulus $K$ of a solid of volume $V$ compressed under pressure ...
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1answer
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Why does the thermal equilibrium change so quickly?

I performed an experiment recently where I tested the specific heat capacity of different metals in water. Once the metals were removed from a beaker and placed into calorimeter with water, thermal ...