Questions tagged [equilibrium]

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How long does it take for an electron to reach equilibrium with blackbody radiation?

While teaching a course on electrodynamics, I thought of an interesting question that I think deserves some attention. Consider an ensemble of electrons all with momentum $\hbar \mathbf{k}$ ...
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How to solve for stable/unstable equilibrium in this question [on hold]

A solid cube of side b is at rest on curved surface of a cylinder(horizontal axis) of radius R. Assume no slipping. We have to derive condition for stable equilibrium. My approach: Slightly we can ...
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1answer
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Finding saturation pressure of steam in a mixture

Suppose you have a gaseous mixture of nitrogen and pure steam (no liquid water), at temperature $T$ and net pressure $P$. The mole fraction of steam in the mixture is $x$. You are told that the steam ...
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2answers
49 views

Why is the force not shown when the body is split into components?

In the given figure, for ease of determining the reactions, the body has been split. The component figures shows internal forces but not the load P. How is this possible ? (Though the figure does not ...
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0answers
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Potential Energy Graph for Nuclear Force

when you draw PE energy graph wrt to distance between two interacting nucleons, at distance $r$ around 1 fm, graph has the lowest PE and is point of stable equilibrum. For $r >$ around 1fm, nuclear ...
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1answer
27 views

Equator water pipeline flow at sea level?

If a water pipeline was built at sea level eg. Gulf of Mexico and the other end was at sea level in southern Chile, then the pipeline was primed with pumps, would the water then gravity feed and flow ...
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0answers
25 views

Where can I find careful discussions of equilibrium?

I want to know what physical equilibrium is precisely. I'd like to understand dynamical equilibrium vs. static equilibrium, and I'd like to understand approaches to understand non-equilibrium ...
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0answers
45 views

If a string had mass in an Atwood machine, would the objects be at rest?

So I understand that the tension produced by a string in an Atwood Machine is constant throughout the string assuming it is ideal, but what if it wasn't? Wouldn't there be a tension gradient, in which ...
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1answer
44 views

In a truss, where is the reaction force acting on?

A truss, its free body diagrams and also of the pin at C is shown in the figure. Ay and P, in figure II may be equated because they are acting in opposite direction on the same body(the whole truss), ...
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44 views

Mathematical formulation of the concept of temperature

We were taught the following. Consider three systems $A$, $B$ and $C$, which have exactly two independent variables each, $(X,Y)$, $(X’,Y’)$ and $(X’’,Y’’)$. Thermal equilibrium for $A$ and $B$ is ...
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2answers
33 views

Pressure of a system at the thermodynamic equilibrium

I'm reading a book in which it says that a system is at mechanical equilibrium if the pressure is constant in time but not necessarily uniform in space at any fixed point (for example it can be caused ...
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4answers
50 views

Will friction on tyres due to road still act if the vehicle is moving with constant velocity?

A car moves with the help of friction from ground...while analyzing a car moving with constant velocity..should one consider the frictional force acting at the contact point of road and wheel? If ...
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1answer
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Microstates of the system in microcanonical ensemble

Suppose I have a gas enclosed in a thermally insulated box, and so I suppose, this is an example of a system in a micro-canonical ensemble. Now, I want to understand which microstates of the system ...
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1answer
29 views

Proving that motion of an $n$ dimensional oscillator can be written as a linear combination of “sine waves”

Here is a related question which might provide some context: LINK. Let's consider an oscillator with equation of motion in $n$ dimensions: $$ \frac{d^2}{dt^2} \vec{x} = K \vec{x}. $$ Given that $\...
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1answer
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How does my body temperature stay lower than ambient air temperature if it goes over 37°C?

I know it's because of sweat, but why should the sweat over my skin get heat from the body, cooling it under the ambient temperature, instead of getting it from the hotter air all around me? If I ...
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1answer
53 views

Why charges should go to the surface of conductor? [duplicate]

How can we show that charges should go to the surface of a conductor, assuming that system should minimize its energy? (With no additional assumptions and maybe using variation method!)
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1answer
68 views

Why is detailed balance required for thermodynamic consistency?

why does the Wegscheider condition or detailed balance have to be true for chemical reactions and what does it mean to violate it in terms of thermodynamics? here is an attempt to derive a simple ...
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5answers
514 views

Why does a force divide equally on multiple supports/legs?

Imagine a bridge spanning across two pillars/supports with a weight of $80N$. {Any external forces affecting the system are assumed to be negligible.} We know that the force exerted on these 2 ...
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2answers
59 views

Multiple ropes carrying an object

In this problem, the three objects are identical in shape and mass. The suspended horizontal object is uniform and ropes are attached to the object at same distances (the distance between $T_4$ and $...
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3answers
92 views

Is constant temperature and thermal equilibrium same thing?

Can you give some examples which would reveal whether they are same things or not?
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1answer
33 views

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
25 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
36 views

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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0answers
75 views

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
43 views

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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0answers
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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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53 views

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
37 views

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
62 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
29 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
78 views

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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2answers
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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
54 views

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
207 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
35 views

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
28 views

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
85 views

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
71 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
76 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
92 views

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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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
50 views

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
81 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 ...
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1answer
46 views

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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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 ...