What is Equilibrium: Definition and 1000 Discussions

Thermodynamic equilibrium is an axiomatic concept of thermodynamics. It is an internal state of a single thermodynamic system, or a relation between several thermodynamic systems connected by more or less permeable or impermeable walls. In thermodynamic equilibrium there are no net macroscopic flows of matter or of energy, either within a system or between systems.
In a system that is in its own state of internal thermodynamic equilibrium, no macroscopic change occurs.
Systems in mutual thermodynamic equilibrium are simultaneously in mutual thermal, mechanical, chemical, and radiative equilibria. Systems can be in one kind of mutual equilibrium, though not in others. In thermodynamic equilibrium, all kinds of equilibrium hold at once and indefinitely, until disturbed by a thermodynamic operation. In a macroscopic equilibrium, perfectly or almost perfectly balanced microscopic exchanges occur; this is the physical explanation of the notion of macroscopic equilibrium.
A thermodynamic system in a state of internal thermodynamic equilibrium has a spatially uniform temperature. Its intensive properties, other than temperature, may be driven to spatial inhomogeneity by an unchanging long-range force field imposed on it by its surroundings.
In systems that are at a state of non-equilibrium there are, by contrast, net flows of matter or energy. If such changes can be triggered to occur in a system in which they are not already occurring, the system is said to be in a meta-stable equilibrium.
Though not a widely named "law," it is an axiom of thermodynamics that there exist states of thermodynamic equilibrium. The second law of thermodynamics states that when a body of material starts from an equilibrium state, in which, portions of it are held at different states by more or less permeable or impermeable partitions, and a thermodynamic operation removes or makes the partitions more permeable and it is isolated, then it spontaneously reaches its own, new state of internal thermodynamic equilibrium, and this is accompanied by an increase in the sum of the entropies of the portions.

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  1. Kaushik

    Equilibrium in a system containing I2 and I

    Summary:: . Consider a container consisting of I2 and I in equilibrium. I2 <-> 2I (Is there any to write chemical equations here?) Will both of them separately contribute to the total pressure (as partial pressures)? If no, why? If yes, should we use mole fraction or volume composition to...
  2. E

    B Semantic question about equilibrium

    If a body experiences acceleration in one direction and no acceleration in another (orthogonal) direction, is it accurate to state that the body is in static equilibrium in one direction only? Or is static reserved for strictly no acceleration in any direction? Apologies if this seems pedantic!
  3. r12214001

    Chemistry Relationship between chemical equilibrium and rate equilibrium

    Q: Is it true that the value of K depends on the amounts of reactants and products mixed together initially? Explain. A:NO, Equilibrium and rates of reaction (kinetics) are independent of each other. A reaction with a large equilibrium constant value may be a fast reaction or a slow reaction...
  4. Prabs3257

    Chemistry Is the N Factor of Oxalic Acid in Titration Correctly Applied?

    I got a answer by equation the moles i took the n factor of oxalic acid as 2 and the naoh solution came out to be 2M but i got an answer of 4grams but the answer given is 2 grams am i correct ??
  5. C

    A Use of the Stefan Boltzmann Law in a Non Equilibrium system?

    Hi everyone, I am currently trying to work something out in regards to non-equilibrium thermodynamics. If I have a block of metal in vacuum that is being heated by a laser with a constant power P, is it even possible to be able to describe the emission of radiation by the block via the Stefan...
  6. benny1993

    Understanding Static Equilibrium and Beam Forces: A Troubleshooting Guide

    I have been trying to work on this problem, but I keep getting the answer wrong. I would appreciate if someone could help me understand what I did wrong. Below is my answer and solution:
  7. penroseandpaper

    Ladder in static equilibrium

    Hi everyone, I'm studying an introductory physics course at college, but our teacher is off sick. On top of that, a planned strike means we won't be getting a new science tutor until the start of next month - instead, we've been told to read the textbook and answer the questions. I've got up...
  8. G

    To find the equilibrium position of a mass attached to an elastic string

    Hi, can anyone see if I have made an error in answering this simple question, my answer is not the one given in the textbook. Here's my diagram of the system in equilibrium. So the mass has dropped a distance x below the midpoint of PQ: So the forces balance as: T1 = g + T2 So, using...
  9. posto002

    How Can I Improve My Algebra Skills for Solving Equilibrium Problems?

    I got the equation --> d=mL/[2*(√(2M)^2-m^2)] where M is one of the masses at either end (multiplied by 2 because... there's two of them.) and m is the mass in the middle of the system. Something that a TA told me was that L, the horizontal distance of the system changes. I'm confused by this...
  10. A

    Static equilibrium of a bar attached to a hinge

    item Fx Fy torque(z) rod 0 -mg -mg* (3/2) Fred 0 -Mg (-Mg) * (3/2) hinge -hx hy 0 wire F* cos(theta) F* sin(theta) 3*F* sin(theta) total 0 0 0 theta= tan^(-1) (2/3) = 33.7 degree 3*F* sin(theta) = (8*9.8)* (3/2) + (53*9.8) * (3/2) F= 538.7N tension is 538.7N
  11. B

    How to find the equilibrium point between two masses? (Topic 6.2)

    Morning all I've recently come across a problem where I get conceptually but cannot apply mathematically if that makes sense. I understand the position of the third mass must be at the equilibrium point of ##m_1## (##9.0×10^{24}kg##), so ##\Sigma F = 0## right? And not even necessarily zero...
  12. Kaushik

    Problem on the required force to keep the gate of a dam at equilibrium

    ##\int \!dτ = \int \!P.dA.x ## Here, I am taking torque of the part above the axis##(τ_1)## ,which is clockwise, and the torque of the part below the axis ##(τ_2)##, which is anticlockwise, separately. Now, let me consider a thin strip ##dx## at a height ##x## from the axis. ##τ_1 =...
  13. Prabs3257

    Electrostatic force equilibrium

    I was thinking that we can equate the electrostatic potential energy and the spring energy (as the force is similar to that of a spring so energy will also be 1/2kx^2 ) but i am not getting the correct ans but by equating the net force on one charge to kr i am getting the correct ans can...
  14. D

    Chemistry Equilibrium of a Sparingly Soluble Ionic Compound

    Answer given is 6.61g.
  15. minadd

    Find all forces & the coeff. static friction of a pulley at equilibrium

    Given: (block a) m = .500kg (block b) m = .200kg that's all... Fg (block a) = (.5)(9.81) = 4.905N = Fn Fg (block b) = (.2)(9.81) = 1.962N i think acceleration might go like... a = (1.962)/(.5 + .2) = 2.8m/s^2 but isn't it supposed to be at equilibrium? So 0m/s^2. How am I supposed to find...
  16. lachgar

    How to form the stress tensor component from the equilibrium equation?

    Good evening everybody. This is my suggestion for answer. The tensor is diagonal and the compression is a plane stress equilibre equation div(σ)=0 so: So, does that means that = f(y.z) = Ay+Bz and =f(x.z)= Cx+Dz A,B,C and D are constants. Is that what the question meant? Thank you in...
  17. Diracobama2181

    What is the chemical potential of a diatomic gas in equilibrium?

    It is my assumption that I need to find the chemical potential of the atoms $$\mu_A$$ and for the molecules $$\mu_{A_2}$$, then use $$\mu_{A_2}+\mu_{A}=0$$ to arrive at the given identity. For $$\mu_A$$, I found that $$\mu_A=k_BTln(n_A\lambda ^3)$$, where $$n_a=\frac{N_a}{V}$$ and $$\lambda$$...
  18. Mr-Thirty

    Concentrations at equilibrium (strong base weak acid)

    So my professor covered acids and bases recently, and this is how he presented the method (please see attached word document). I believe its the same as the I.C.E method, though I am not sure. Anyways my problem is perhaps conceptual, but I don't understand why the concentrations at equilibrium...
  19. Adesh

    Chemistry Why do we say that the concentration of solids and liquids are constant?

    I was introduced to the concepts like molarity, molality , mole fraction, formality, normality etc. during general chemistry and it was said that these concepts are used to measure the concentration of solutions. But as the thing called Chemical Equilibrium came I was surprised when I read "In...
  20. J

    Thermodynamic equilibrium with fixed energy/entropy

    If you take a system with fixed entropy S0 and let it evolve, it reachs equilibrium. Let Ueq be the energy of the system at equilibrium. Now take the same system with fixed energy U=Ueq (S is not fixed anymore), how do you know that the equilibrium reached is the same as before, that means with...
  21. M

    Engineering Calculate Static Equilibrium: SOH/CAH/TOA?

    When calculating static equilibrium, i know that you have to split for example, F1 into F1x and F1y. when using soh/cah/toa is it always SIN for the Y axis and always Cos for the X axis? I've been following this method and getting it wrong some times it's really stressing me out, Thanks for any...
  22. T

    Other Far from equilibrium statistics

    Hello! I would like your help to study Science graduate level books and articles, in the following subjects: 1. Far from equilibrium statistics. 2. Information theory and entropy. 3. Negentropy. 4. And Maxwell's demon. My main goal is to be able to understand and explore the Maxwell's demon...
  23. M

    A Fluid Equilibrium Interface Shape

    Hi PF! Suppose a 2D rectangular channel with wall-normal perpendicular to the downward gravity force ##F## rests is static equilibrium. What shape would the liquid take? Looking at the equations of motion, I think the following would have to be satisfied: $$\frac{d P}{dy} = -\rho F\\ \Delta P...
  24. A

    Center of mass and equilibrium

    ?
  25. jhongg7

    Equilibrium equations - spherical coordinates

  26. E

    B How does this osmotic system reach equilibrium?

    [Mentor's note - this post has been edited to ask the interesting physics question while not falling afoul of the forum rule prohibiting discussions of perpetual motion] On the left is pure water (blue), and on the right is concentrated salt water (green). The red walls in the middle are...
  27. E

    Question about equilibrium position

    Suppose we have a reversible reaction $$aA + bB <=> cC + dD$$ which is exothermic in the forward direction. If the temperature of the system decreases, the rate of the forward reaction will increase so that the quotient Q increases to the new (higher) value of Kc. This I think I am ok with (I am...
  28. S

    Tension and speed of bowling ball pendulum passing the equilibrium position

    For part a I used conservation of energy. -m*g*cos(θ)*L+1/2*m*0^2=-m*g*L +1/2*m*v^2 => v = sqrt(2*g*L(1-cos(θ )). b) For b I was think that T = mg in the equilibrium point but that doesn't invole θ in the answer. So that's why I tought that T*cos(θ ) = mg. So that the tension is mg/cos(θ). But...
  29. J

    I Does the postulate of equal a priori probability apply only to equilibrium?

    In deducing the zeroth law of thermodynamics in micro-canonical ensemble, there is a frequently-mentioned example. Suppose we put two isolated system, system 1 and 2, in contact and allowing them to exchange heat. The total energy of the combined system is $$E = {E_1} + {E_2}$$ The total...
  30. A

    What mistake did I make when solving for the equilibrium of a rigid object?

    Here's the task: My attempt at a solution (I choose C as an axis): However, the textbook solution says D should be 58.8. What am I doing wrong?
  31. QuarkDecay

    A Equilibrium point of matter and radiation density

    We want to calculate the ao/a(teq) of the equilibrium point between ρm and ρr (ρm= ρr ) My book solves it this way; ρm(t) / ρr(t)= a(t) ⇒ ⇒ (ρm/ ρr)teq =1 = = (ρm/ ρr)o * a(teq)/ ao I don't understand the a(teq)/ ao part. If ρm(t)= ρο/αo3 and ρr(t)= ρο/αo4 then it should be ρm(t)/ ρr(t) =...
  32. J

    Sum of torques, equilibrium problem

    For part a, A force must be applied so that the entire mass can be 'held up'. Therefore the necessary force must be equal to the gravitation force on all the objects: m(rod) * 9.8 + m(LB) * 9.8 + m(RB) * 9.8 = 137 N For part b, (This is where I'm confused) let's set the point at which the...
  33. brotherbobby

    Normal reactions at the bases of two light supports

    For equilibrium, using ##\Sigma \vec F = 0##, we get ##n_1 + n_2 = 300\; \text{N}##. Taking the system as a whole and applying ##\Sigma \vec \tau = 0## about the hinge (pin) at the top from where the load is hung, we get ##n_1 \times (0.8) \times 4 = n_2 \times (0.6) \times 3##, by taking...
  34. Creedence

    I Equilibrium state of sourceless EM field

    Given a box made of perfectly conducting material. At some part of it at t=0 there is a localized magnetic field. It's sourceless and there aren't any dissipation. After t=0 it starts to spread and fill the box. What is the equilibrium state? Thanks for the answer(s), Robert
  35. S

    Spring is stretched, then attached to a mass, what is its speed at x?

    Dear all, I am back with another Spring problem. I have tried to use the insights I gained from your help last time: https://www.physicsforums.com/threads/find-v-x-of-a-mass-suspended-from-a-spring.972942/page-3#post-6190934 I figured I start with calculating the new equilibrium by ## x...
  36. A

    Torque: find the force necessary for a body to be in equilibrium

    The question doesn't specify whether we're talking about translation or rotational equilibrium, so I suppose it's both: In order for the body to have translational equilibrium: 60 N + F2 = 0 F2 = -60N However, in order to have rotational equilibrium: 60 N * 3m + F2 *8 m = 0 60 N * 3m - 60 N...
  37. andrea1313

    Black cube, maximal and minimal value of equilibrium temperature T

    So i had this problem and I want a rigourous solution. The answer should be : Tmin=(I/sigma)^(1/4) and Tmax=(sqrt(3I)/sigma)^1/4
  38. greenrichy

    Find the stretch of a steel wire in a static equilibrium problem.

    If I can determine the weight of that heavy object placed on the plank, I will be able to determine the stretch of that wire. But, when using the second condition for static equilibrium (torques of the system equal to 0), I always end up with two unknowns, no matter what point of rotation I...
  39. Gbox

    Finding inveriance size, equilibrium and fluctuations

    So I answered 1 and 2, got: 1. ##\vec(r)(\theta,\phi)=l(sin \theta cos \phi, sin \theta sin \phi, -cos \theta)## 2. ##L=\frac{ml^2 (\dot{\theta}^2+\dot{\phi}^2 sin^2 \theta)}{2}+mglcos \theta## 3. a ##mlsin \theta -mgsin \theta =l^2 \ddot{\theta}## , b. ##ml^2 \ddot{\phi}=0## 4. I know...
  40. EEristavi

    Rotational Motion / Static Equilibrium - Mechanics

    I have a solution, However Cant understand 1 point.Now, This is the solution: ##N_2 l cos\theta + \frac 1 2 F_g l cos\theta - f_2 l sin\theta = 0## ## N_2(1 - \mu tan\theta) + \frac 1 2 F_g = 0## This is the the point that I don't like - yes it is less that 0, but it's even less that...
  41. N

    Static equilibrium force decomposition problem

    If you see the $\sum \tau_0 = L\cdot N_1 \cdot cos \theta - LF_1 sin \theta - L/2 \cdot G cos \theta$, all the trigonemetric parts are all opposite of what i can understand, given the angle as drawed in the Picture/url. Please help me :)https://pasteboard.co/IiXr8qA.png
  42. Lone Wolf

    Static equilibrium -- interpretation of forces

    I solved this question correctly, however I have a question regarding how I should work with the weight of the firefighter climbing the ladder. When drawing the force diagram for this problem, I should only include forces acting on the ladder, right? Which means I would represent the normal...
  43. D

    Rigid body equilibrium question

    Problem in the picture. I have a question regarding the problem in the picture. Ma=0 -2Kn*1a+By*4a=0 --> By=0,5kN Mb=0 2kN*3a-Ay*4a=0 --> Ay = 1,5kN Ax=0kN Ay is the pin support and By is the roller support. According to the solution By should be 1kN but isn't that the wrong answer? 2kN has...
  44. D

    Rigid body equilibrium problem

    Hello Ihave gotten as far as coming up with an equation for the sum of moments and it goes as follows: bh*1/2b-1/2hb*1/3b=0 the answer for h i get is wrong and i don't know if i am missing something. moment arm on the b is 1/2b and the moment arm on h is 1/3h because of the way water pressure...
  45. cianfa72

    Daniell cell initial equilibrium

    Hello, consider again a simple Daniell cell with salt bridge in place but no external electric connection between electrodes. Reading from some source e.g. link1 it seems upon Zinc and Copper metal rods are dipped into their own ions solution, both will gain a negative charge (-) due to an...
  46. J

    Find the equilibrium temperature & the amount of ice/water

    That's how far I've gotten and now I'm stuck!
  47. M

    Asymmetrical Static Equilibrium

    a) Answer that I found: 56.31 degrees b) Answer that I found: 121.71 Kg For b) not sure if I need to add the 20 N and 5 N to the total of the cargo mass.
  48. D

    Find the tensions of two strings in mechanical equilibrium

    I tried to do the free body diagram of the problem, but I{m confused because the rule is inclined. Should I just use the given angles to calculate the forces on x and y? Also, how should I calculate the torques? Since I don't know the dimensions of the rule. Can you help me? I think that once I...
  49. Alif Yasa

    Find the equilibrium angle of this unusual driven pendulum geometry

    -I tried to draw the forces on the hoop when it is in the equilibrium state. I know there are friction and normal force on the contact point of the shaft and the hoop -I also put the weight force to the M object -But when i used the torque equilibrium, where the pivot is the contact point of the...
  50. D

    A position of stable equilibrium, and the period of small oscillations

    I tried by taking the derivative of the potential to find the critic points and the I took the second derivative to find which of those points are minimum points. I found that the point is ##x=- a##. I don't understand how to calculate the period, since I haven't seen anything about the harmonic...
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