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Questions tagged [equilibrium]

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Tension in a string attached to a spring with deformation at a point

I am trying to calculate the tension in a string in the configuration shown in the diagram below. Here is a verbal description of the setup: The string is fixed at one end with the other end connected ...
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1answer
23 views

Can I calculate tension in a preloaded string by superposition when the string is deformed?

I am trying to calculate the tension in a string with preloaded tension after a force is applied to the string as in the diagram below. The string would be under tension prior to the force $F$ being ...
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1answer
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How can a rope's tension be at a maximum as a body is about to move up a slope in the rope's direction? [closed]

Note: I do NOT need this exercise's answer. I have no problem working the math out and noticing the truth of my title. A box of mass 0.5 kg is placed on a plane which is inclined at an angle of 40 ...
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Why symmetry leads to stability?

In the whole course of physics I observed a very common thing present around us which is symmetry. Symmetry leads to stability everywhere. For example:-Pauli's Exclusion principle tends to make the ...
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1answer
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Is $d U = 0$ at thermodynamic equilibrium?

At thermodynamic equilibrium, there is thermal, mechanical, and diffusive equilibrium. Does this imply: $$d\mu = dT = dV = 0$$ $$dU = TdS - PdV \implies dU = TdS$$ Here, I know entropy is maximum, ...
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An attempt at justifying effects of minima for Gibbs free energy and Helmholtz free energy

I'm trying to justify that at constant volume, thermal equilibrium is achieved at a minimum of the Helmholtz function. For any thermodynamic process, $$dF = dU - TdS - SdT$$ $$\text{if $dF = 0$}$$ ...
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5answers
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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 ...
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2answers
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How to find Lagrange point $L_2$ of the Earth-Moon system?

I was tasked in my programming lesson to find a value of L1 correct to 5 significant figures, but in my discussion section I began to wonder why there was no second solution to the equation I was ...
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1answer
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Trying to understand the derivation of the Gibbs adsorption isotherm

I have looked at several deritions online, and the clearest I have found is that on the Wiki page The problem being considered is that of two phases, with an interface of a given area between them. ...
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1answer
57 views

Earnshaw's Theorem in Chemical Bonds

Earnshaw's theorem says that: A collection of point charges cannot be maintained in a stable stationary equilibrium configuration solely by the electrostatic interaction of the charges. I also know ...
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1answer
27 views

Static equilibrium (possibility)

In static equilibrium for a ladder leaning against the wall it is supposed to rotate about a point. Why we are not assuming it to slide down as in our practical life?
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1answer
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Does this example contradict Earnshaw's theorem in one dimension?

This is basically a continuation of the post here. Consider electrostatics in $1$-dimension (say, the $x$-axis). Now consider a positive charge $+q$ located at $x=0$, and two equal negative charges $...
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Facing a paradox: Earnshaw's theorem in one dimension

Consider a one-dimensional situation on a straight line (say, $x$-axis). Let a charge of magnitude $q$ be located at $x=x_0$, the potential satisfies the Poisson's equation $$\frac{d^2V}{dx^2}=-\frac{\...
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1answer
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Is there an analogue of Earnshaw’s theorem for inverse cube force laws?

Earnshaw’s theorem states that point charges cannot be held in electrostatic equilibrium by an electric field. Is there an analogue of this for inverse cube force laws?
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5answers
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How can I prove that a state of equilibrium is unstable?

In the particular problem I encountered, an electric field was zero at the origin and we were meant to prove that a particle at the origin was in an unstable state of equilibrium. Is it enough to ...
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1answer
163 views

Neutral conductor and charged insulator brought near each other

What happens when a charged insulator is placed near an uncharged neutral metallic conductor? I know it attracts each other because of charging by induction (electrons redistribute). But would the ...
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0answers
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Entropy derivation question

I’m using Atkins thermodynamics book and he has a section I don’t quite understand. The line I’m particuarly question is the one befor (5.2) where he says “by Kelvins formulation of the second law”. I’...
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1answer
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Figure Description of Equilibrium in a Crystal Lattice

I am reading an Introductory Semiconductor Physics by Parker. The textbook states the following: The ionic crystal itself is formed by the ions coming together in an equilibrium configuration. ...
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5answers
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How does the sea level reach a fixed height? [closed]

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
64 views

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
63 views

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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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
71 views

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
65 views

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
40 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
55 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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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
62 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
47 views

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 ...
2
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1answer
68 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
19 views

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
170 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
171 views

(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
417 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
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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
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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. ...