Questions tagged [equilibrium]

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oil and water- The Archimides' law

I have a problem as on the topic, see the below. First I added $25\hbox{cm}^3$ water in to a beaker. Then I added $2N$ oil onto it. But the level of the water was still $25\hbox{cm}^3$. My problem ...
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314 views

Brownian motion and equilibrium

I would like to know if when you consider a system in which you have Brownian motion if it is considered a system in equilibrium or far from equilibrium and why. i.e., is Brownian motion considered as ...
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1answer
61 views

Collisions in System in Equilibrium

Are all collisions between gaseous molecules and boundary in a system at Equilibrium, elastic? Why?
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1answer
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If ice water equilibrium exists at 0C at 1atm then why don't oceans stay near freezing all time

The arctic ice is basically a giant floating sheet of ice laid upon the ocean so the ocean and ice are (probably) at equilibrium in the arctic. And since all oceans are connected shouldn't all of them ...
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5answers
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Is there anything in the universe that is completely static? [closed]

Based on my current, admittedly deficient, understanding of physics everything in the universe is in constant change and motion. Is that understanding correct or is there something that is static? ...
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2answers
192 views

Conceptual Question about Suspension Bridge Forces

Two walkways are suspended from vertical rods. In the first scenario, one long rod supports both walkways and is attached to the ceiling. In the second scenario, two shorter rods are attached to each ...
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0answers
135 views

How long does it take for a balloon to deflate?

Since gas flows from an area of high pressure to low pressure, upon opening an aperture on say a balloon one would expect the contents to diffuse out until both the inside and outside reach a pressure ...
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0answers
29 views

Thermodynamics - thermal equillibrium

If I got a heat source (water at $100^oC$), the heat reservoir (infinite lake with the temperature $5^oC$) and my thermodynamic process proceeds at $20^oC$ somewhere above the source. The process is ...
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0answers
69 views

What is the meaning of free energy out of equilibrium?

So in statistical mechanics the free energy $F$ (either canonical or grand canonical) can be found from the partition sum $\mathcal{Z}$ as: $$ \mathcal{Z}=\exp(-\beta F). $$ As far as I can read it ...
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1answer
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Electric potential definition last portion

The amount of work done in carrying a unit positive charge from infinity to a point in the field against field keeping electrostatic equilibrium. What does it mean "keeping electrostatic equilibrium"?
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1answer
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How to determine the increment term of normal and shear stress in infinitesimal cube ?

Consider a solid body, if it is in equilibrium then every infinitesimal part of the body is in equilibrium so i take an infinitesimal cube with side lengths, $dx$, $dy$ and $dz$ and try to write the ...
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3answers
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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 ...
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1answer
110 views

Why are there time averages in the virial theorem if one of its hypotheses is that the system is in equilibrium?

Why are there time averages in the virial theorem if one of its hypotheses is that the system is in equilibrium? According to my textbook, the virial theorem is that for a gravitationally bound system ...
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2answers
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Stationary particle (quantum)

Setting the potential oprator to zero in the Schrodinger Equation gives us the description of a free particle. Will setting the momentum operator (squared) to zero gives us a meaningful description ...
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3answers
493 views

Same Charge or Same Potential?

I was studying about sharing of charge between two charged conductors when I came across this statement - When two conductors are joined together through a conducting wire, charge begins to flow ...
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1answer
250 views

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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1answer
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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 ...
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2answers
318 views

Entropy irreversibilities

How can we say that a system is reversible or irreversible if not stated? I encountered a book example wherein a piston cylinder device encounters an isobaric process and experiences a transfer of ...
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2answers
216 views

Equilibrium in Electrostatics

In case of electrostatics,from Earnshaw's theorem we get that we can't put a charge in a stable equilibrium with mere electrostatic forces, which is pretty logical as for being consistent with Laplace ...
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1answer
105 views

Thermal equilibrium analogy

If a glass of water at 100°C is placed in contact with a glass of water at 0°C, ignoring room temperature, the ideal temperature of water in both glasses would be 50°C, right? But since entropy causes ...
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2answers
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How does an Anti-tilt chair work?

These chairs are designed as per European Educational Standard Part 1 and 2. These are claimed by the manufacturer as Anti tilt, so basically it makes it relatively difficult for a child to fall when ...
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1answer
86 views

$E_C$-$E_F$ in the equation for concentration of electrons at equilibrium

I'm trying to calculate the charge carrier concentration for silicon doped with phosphorous at a level of $10^{16}$ per cubic centimeter. I have the formula: $$n_0=n_ie^{-(E_C-E_F)/kT}$$ I found $...
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2answers
359 views

Expanding function about equilibrium?

This is Example 2.3.4 from Analytical Mechanics 7th Ed by Fowles & Cassiday. It uses the Morse function $V(x)$, given as $$V(x) = V_0[1 - e^{-(x-x_0)/\delta}]^2 - V_0.$$ The question is: "...
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1answer
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Air-drying a dishwasher: door cracked or wide open?

My dishwasher's design is focused on noise, so there is no air vent which would allow noise to escape. When the dishes are done, I always open the dishwasher to fully dry the still-hot dishes. But, ...
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90 views

Equilibrium states during irreversible process

Is it possible for there to be equilibrium states during an irreversible process? What about when $P_{in}=P_{ex}$ during an irreversible gas expansion?
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1answer
141 views

How to deduce the following equilibrium equation?

I have seen the following free-body diagram in an engineering paper, J. Compos. Constr. 9, 388 (2005) (e-print): As it is mentioned in the paper $f_j$ is the stress in the FRP layer (which acts ...
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1answer
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Solving fulcrum / seesaw problem with multiple stacked weights

I came across an article that 6 year olds in the UK will be asked questions like this in maths class: The question is intended to be an algebra problem of balancing the total weight on each side (...
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1answer
314 views

Is this system in stable or unstable equilibrium? [closed]

6 positive, identical charges $+q$ are placed in a hexagon. A negative charge whose value I want to determine is placed at the center to keep the system in equilibrium. Which means $dU/dr = 0$. But ...
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1answer
59 views

How much water can exist as a gas in a closed container?

If i leave water in a closed container at some ambient temperature what proportion of that water will turn into a gas at equilibrium? Maybe it is easier to solving this by first answering what is the ...
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1answer
757 views

Taylor expansion of potential energy

According to the book "Applied quantum Mechanics, Anthony Levi", The Hamiltonian of a monatomic linear chain is given by: The first term on the right side comes from the sum of the kinetic energy of ...
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1answer
72 views

Does $\{\rho(q,p),H(q,p)\}=0$ necessarily imply $\rho(q,p)=\rho(H)$?

In classical statistical mechanics, the Liouville's theorem tells that for a system in equilibrium the Poisson bracket $$\{\rho(q,p),H(q,p)\}=0.\tag{1}$$ 1. Does it necessarily imply $\rho(q,p)=\rho(...
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1answer
75 views

Re-interpretation of the third law of thermodynamics

Consider the first paragraph of this paper: A fundamental problem in thermodynamic and statistical physics is to study the response of a system in thermal equilibrium to an outside perturbation....
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1answer
377 views

Zeroth law of thermodynamics confusion

I quote from Zemansky's "Heat & Thermodynamics"; "Imagine two systems A and B separated from each other by an adiabatic wall but each in contact with a third system C through diathermic walls, ...
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2answers
795 views

Why is that a system with a concave-up entropy-energy graph cannot have a stable thermal equilibrium?

This is from Schroeder's An Introduction to Thermal Physics: Let A and B be a system both with a concave-up entropy-energy graph. The systems will achieve thermal equilibrium when they are at the ...
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1answer
108 views

Minimum number of charges in an oscillatory system

I was just curious actually. Is it possible to release a set of point-sized charges in space, each particle at rest, such that after a period of time, the system returns to its initial ...
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1answer
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Potential energy function term meaning [closed]

I can't seem to understand what this question is asking. I am not sure really what the second and third terms actually represent in the equation. Near the equilibrium position $r_0$, the ...
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1answer
99 views

Why does mechanical equilibrium depend only on potential energy?

As far as I understand, for a system to be considered in equilibrium, the sum of the forces that is applied to it must be $0$: $\vec F = 0$ which is $\partial \frac{E_p}{\partial x}\bigg\rvert _{x=...
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1answer
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Method of joints: The direction of forces

I have the following question: I need to find the load in truss element DF. I calculated the reaction forces at E and D. Fex = 360 KN Fy = 405 KN Then, I used the method of sections at point F ...
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3answers
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Taking moment about a point

Let's say I have the following diagram: Since it's a fixed beam, I know that there will be three components acting on point A: 1) Force acting in x direction 2) Force acting in y direction 3) ...
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1answer
234 views

The direction of forces in statics problem

I was always confused on how we define the directions of the forces in equilibrium problems. Let's say I have a simple structure: In this case, I assumed that Ray and Rcy are pointing up to counter ...
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3answers
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What does “no characteristic length or time scale” mean?

When looking into the topic of "self-organized criticality," (SOC) one often comes across descriptions of SOC as a state where "the system has no characteristic length or time scale." (Examples here ...
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Balanced forces in scales

We have scales-like mechanism with a spring. Rigidity of the spring $C=10\ \mathrm{kg/cm}$, initial length $L_0=60\ \mathrm{mm}$ and diameter $d=\ \mathrm{10mm}$. Without any weight the scales are in ...
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2answers
307 views

Find force $F$ for equilibrium? [closed]

The masses of all the objects are mentioned inside them. Masses of both inclined planes are same and angle of inclination is same too.The inclines are smooth and the bar is Rigid. We have to find ...
2
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1answer
231 views

What is the difference between an object being at the point of toppling and an object being at the point of slipping? [closed]

Does friction take on its maximum value in both cases, since there is relative motion between the two objects? And in general, what does rolling without slipping and slipping without rolling mean? ...
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1answer
383 views

Tension In A Massive Rope

My book says :the horizontal component of the tension in a massive rope is constant. It does not seem intuitive to me as I don't think that the above approximation (I guess) is possible.I don't know ...
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1answer
740 views

Three hinged rods problem

This is an edited version (I added a solution and a physics concept related query) of a closed question that I'd like to see the answer to Three identical rods are connected by hinges to each other, ...
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1answer
279 views

What does “equilibrium under its own gravity” mean for a body of fluid?

A spherical body of radius $R$ consists of a fluid of constant density and is in equilibrium under its own gravity. If $P(r)$ is the pressure at $r(r < R)$, then the correct option(s) is(are)? ...
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Summation of Forces through a Lever Linkage

I'm trying to brush up on some of my force and torque stuff so I can do some modelling in software. My problem is, it's been a while and I'm unsure of how correct my work is. I have a system as ...
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2answers
148 views

How does in an expanding universe the equilibrium number density change with temperature?

In a "slowly" expanding Universe, suppose the number density of a particle species $A$ undergoing a equilibrium decay $A\to B+C$. The equilibrium number density depletes with the expansion and the ...
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How to calculate equilibria of the sum of “similar” Hamiltonians

Determining equilibria of a Hamiltonian system is equivalent to finding the zeroes of a function, so in general it is a hard task- But what if we have some additional information on the Hamiltonians? ...