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3
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3answers
159 views

Average temperature of the universe

Is it possible to define some average temperature of the universe? If yes, what fixes this temperature and how t estimate today's temperature? Is it different from the temperature of the black-body ...
0
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1answer
59 views

simple physical system involving convergence to equilibrium at decreasing speed

I want to make an analogy of an economic model (Solow 1956) with a simple physical system that exists in reality. I am not looking for a mathematical formula to describe the system (I have that ...
-2
votes
2answers
629 views

Why are planets in unstable equilibrium in a stable orbit

It is well known that a planet in stable orbit is in unstable equilibrium. If e.g. the moon was just a few m/s slower in velocity, or a few m closer to the earth than it is, the gravity would ...
1
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4answers
2k views

Equilibrium using Center of Buoyancy and Gravity (Problem)

I don't understand a solution to a homework question. Specifically, I don't understand the logical sequence of each bolded statement in the answer. (b) To ensure that a ship is in stable ...
1
vote
1answer
317 views

Meaning of chemical equilibrium between two phases

Suppose two phases 1 and 2 of water, say ice and water, are kept in a closed container, at a fixed temperature $T$ and fixed pressure $P$? Then I have the following question: Is phase 1 in ...
2
votes
2answers
86 views

How's equilibrium possible here?

Here's the question.......Two point masses $m$ and $2m$ are attached at each end of a light rod. The rod is pivoted at the center and is free to move in a vertical plane. Then find the angle $A$ when ...
6
votes
1answer
141 views

Do the stars in a galaxy have a thermal kinetic energy distribution?

I think, there is practically everything given to that: many point-like masses, able to exchange energy pseudo-randomly, and far long enough time to reach a thermodynamical equilibrium. Of course, ...
1
vote
1answer
151 views

What is the cause for mechanical equilibrium in statistical mechanics?

In classical thermodynamics, mechanical equilibrium is defined as the state of a system in which there is no net flow of volume as there should be no net pressure within the system. Ok. ...
1
vote
0answers
66 views

I need to remove water from a system [closed]

I work in an industry and I need to remove water from a system. The scenario: There is a washer and a blower on a conveyor belt. The item being washed is plastic (not sure what type) and can't be ...
2
votes
2answers
215 views

Stable equilibrium in a gravitational field with 2 masses [closed]

Suppose you have two point masses $M_1, M_2$ in space and you locate the point $A$ on the line between them such that an object placed at $A$ is in equilibrium (the only forces acting on the obect are ...
0
votes
1answer
98 views

Why do forces/pressures/electrons/temperatures/etc equalize

What causes various forces/etc. like to equalize? I can understand water in a tank being affected by gravity but why do high pressure systems want to move to low pressure systems (vaccuums), electrons ...
1
vote
1answer
198 views

Boltzmann equation (Number density)

I'm trying to understand the Boltzmann equations use in the early Universe. The derivation is somewhat tedious, but in the end I end up with: $$a^{-3}\frac{d}{dt}\left(n_1a^3\right) = n_1^{(0)}n_2^{(...
0
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1answer
336 views

What is dynamic equilibrium actually?

Recently I have asked question sort of like that: Is thermodynamic equilibrium static or dynamic ? But then I wanted to realize what actually a dynamic equilibrium is. I read the wiki article, ...
0
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2answers
390 views

Thermodynamic equilibrium of an isolated system

When an arbitrary system is isolated and left to itself, its property will in general change in time. ....after a sufficiently long time,the temperature will become the same at all points and then the ...
2
votes
2answers
154 views

Is Boltzmann distribution contradicting with the fundamental assumption of statistical thermodynamics?

In equilibrium statistical physics the fundamental assumption of statistical thermodynamics states that the occupation of any microstate is equally probable (i.e. $p_i=1/\Omega, S=-k_B\sum p_i\,{\rm ...
0
votes
2answers
3k views

Understanding mechanical equilibrium

In this site, mechanical equilibrium is defined as when there is no net force between the system and the boundary, it is in mechanical equilibrium. Now, what does it mean by net force? What would ...
1
vote
2answers
497 views

Is thermodynamic equilibrium static or dynamic?

Suppose, two parts of a system $A$ & $B$. They were at varying temperatures and hadn't achieved mechanical equilibrium. After the relaxation time, $A$ & $B$ were at thermal and mechanical ...
0
votes
1answer
335 views

Downward force applied at center of see saw [closed]

If we are given the length of a see saw of negligible weight, and the weights and distances from the center of the two people on the ends of the see saw, how can we find the downward force being ...
1
vote
0answers
148 views

Equilibrium Points in Lagrangian Mechanics

Suppose we have a one particle system with generalized coordinates $q_i$. In classical mechanics, the corresponding Lagrangian is $L = T - V$. Assume $V(q)$ is time-independent. What additional ...
3
votes
1answer
1k 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 ...
0
votes
3answers
261 views

Finding minimum force to hold a particle in equilibrium [closed]

A particle of mass $m$ is suspended from a point A on a vertical wall by means of a light inextensible string of length $b$. Find the magnitude and direction of the minumum force $\boldsymbol P$ that ...
0
votes
2answers
2k views

Electric charge in stable equilibrium

Why an electric charge can't be held in a position of stable equilibrium, neither by an electrostatic field nor by a mechanical force ?
0
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2answers
57 views

Physical Explanation of the steady state in RC circuits

Given a simple RC circuit with a resistance $R$ connected to a capacitor $C$, the differential equation that describes the voltage is $$C \dot V + V/R = 0$$ whose solution is $$V= V(t) = V_0 e^{-t/\...
10
votes
1answer
2k views

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 ...
2
votes
1answer
236 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 ...
0
votes
1answer
372 views

Equilibrium Question? [closed]

For the figure shown a 5.0m long board is hinged 1.75m from its right end and allowed to rotate freely. You push with an upward force of 250N on the end of the board away from the pivot. A bucket of ...
1
vote
0answers
44 views

problem about heat in thermodynamics

$$\nabla G=-T\nabla S-T\nabla S_{sur}=-Q+q $$ $$\nabla G=-T\nabla S-T\nabla S_{sur}=-(U-W_{rew})+q $$ $$\nabla G=-T\nabla S-T\nabla S_{sur}=-(q+W-W_{rew})+q $$ $$\nabla G=-T\nabla S-T\nabla ...
0
votes
1answer
148 views

Is state defined for non-equilibrium systems?

Equilibrium thermodynamic states follow the state principle and Gibb's phase rule when it comes to fixing a state when given a minimum of (typically) 2 state properties. For non-equilibrium states, I ...
1
vote
0answers
81 views

Why is it valid to assume that the liquid-vapour interface is always saturated during evaporation?

For steady state evaporation, I read from two textbooks which simply states that the partial pressure of vapour of interest (say water) at the liquid-vapour interface is equal to the saturation ...
4
votes
2answers
3k views

Understanding the Jacobian Matrix

Taking the example of a two dimensional system, desribred by the following ODE's: \begin{align} \frac{dx_1}{dt}&=f_1(x_1,x_2)\\ \frac{dx_2}{dt}&=f_2(x_1,x_2) \end{align} The Jacobian Matrix ...
0
votes
1answer
74 views

Electrical charge equilibrium and the piezoelectric effect

About the piezoelectric effect: if I press a crystal the electrical charges segregate and an electrical voltage appears on the other two faces of the crystal. See this link for example: http://www....
2
votes
0answers
133 views

Detailed Balance for Quantum Master Equations from System Hamiltonians with Degenerate Spectrum

Kossakowski, Andrzej, et al. ("Quantum detailed balance and KMS condition." Communications in Mathematical Physics 57.2 (1977): 97-110) gave a proof that the stationary state of a quantum dynamical ...
0
votes
1answer
253 views

'Mechanical equilibrium does not necessarily mean that the pressure is same at all points of the system' - why?

If there are variations in pressure or elastic stress within the system,parts of the system may move, or expand or contract. Eventually these motions,expansions or contractions will cease, and when ...
2
votes
3answers
462 views

Unstable equilibrium in a pendulum

Consider a pendulum with a bob and a massless, rigid, hinged rod attached to the bob. The bob is at rest at the bottom most position. Neglecting friction, is it possible to impart such a velocity (...
2
votes
1answer
105 views

Can't understand the meaning of a formula describing a stable equilibrium?

I've found this formula in a book and I can't understand its meaning: $$\frac{df}{dx}\biggr|_{x_0} < 0$$ It describes the condition of an object to being in a $\textit{stable}$ equilibrium. At ...
1
vote
1answer
58 views

Do Monte-Carlo updates have a physical significance in stat. mech?

One of the archetypical example to introduce Monte-Carlo methods in stat. mech. is to work out the properties of the 2D square lattice Ising model and compare the obtained results with Onsager's exact ...
1
vote
2answers
1k views

How does the temperature in a bucket of water with ice change over time?

I'm curious what happens to the temperature of the water in a bucket of water with ice in it. Let's start with a bucket with just ice in it, and we add water, in a room which is room temperature. It ...
3
votes
1answer
294 views

What's the point with equilibrium in Thermodynamics?

All the Thermodynamics books I saw until now state that in Thermodynamics we are mainly concearned with equilibrium states (I know there's "non-equilibrium Thermodynamics", but I'm interested on the ...
4
votes
1answer
229 views

H-theorem and Boltzmann equation applied to Boltzmann distribution

Using the Boltzmann equation: $$ \frac{dH}{dt} = \int_0^{\infty} dr \int_0^{\infty} ds W(r,s)[p_r - p_s][\ln{p_r} - \ln{p_s}],$$ and assuming $p_r = e^{-\beta r}$, the equation looks like $$ \frac{...
1
vote
3answers
222 views

Ice and liquid water interacting across a boundary

Imagine we have two thermodynamic systems, one a mass of ice and the other an equal mass of liquid water, with both at 273.16K. Each system is isolated, except that they can interact with each other ...
4
votes
2answers
98 views

Does the entropy of a system vary in the equilibrium?

I am not physicist and this question may seem trivial. But I understand that in the equilibrium the magnitudes such as temperature or volumen do not vary. Is the same for entropy? My logical says that ...
0
votes
1answer
217 views

Finding equilibrium of mechanical system

A system is described as follows: Consider a system consisting of two rotating bars of length $l$ and with uniform mass density and each with total mass $m$. The bars are attached to a common ...
3
votes
1answer
305 views

Why is $\omega = \sqrt{K/m}$ valid for a quantum oscillator?

I'm working in the 3rd edition of Modern Physics by Serway, Moses, and Moyer. In 6.6, it talks about a quantum oscillator. I don't fully understand how the definition of frequency works. Now, we ...
0
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1answer
77 views

Transmission/Equilibrium Question

Suppose there are two wheels linked by some kind of apparatus with a transmission, in a vacuum. Wheel A is spinning, wheel B is not. Is it empirically possible for the transmission to transfer the ...
7
votes
2answers
1k 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 ...
13
votes
4answers
255 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 ...
3
votes
1answer
536 views

Lagrangian formulation of the problem: small oscillations around an equilibrium

I'm having trouble understanding how some conclusions are made in my book. I'm studying from a coursebook based on Goldstein's "Classical Mechanics", here's what's written in my book, with my problems ...
-1
votes
2answers
2k views

Why mountain roads are winded up rather than keeping them straight? [closed]

In my opinion, to prevent the line passing from the center of gravity of a body (e.g bike or car) trying to climb the mountain or hill to fall outside the base of support so to maintain stable ...
1
vote
1answer
1k views

Mechanics question - ladder leaning against a wall

I am struggling with part a) of this question: Here is the solution: In part a) they prove that $F \leq \mu R$ is false for this system. So as $F$ is neither less than nor equal to $\mu R$ it ...
0
votes
1answer
142 views

What is the relevance of potential energy as a concept?

As far as i understand, for an object to have a fixed quantity of potential energy, it must be in current equilibrium and have no net force applied to it (otherwise the potential would convert to ...