Equilibrium Definition and 1000 Threads

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. L

    Equilibrium concentration question

    It has been a bit since I took chemistry, but I have been wondering something, and can't seem to remember. Let's say that I have the following: Fe3+ + SCN- <-> FeSCN2+ How can I calculate the point where Fe3+ concentrations become effectively zero? That would say, if I know that I have 1M Fe3+...
  2. I

    What is the tension required for the crosspiece in an A-frame structure?

    ∑Homework Statement You are designing the crosspiece for the A-frame structure in the figure below. Beams AB and AC are 5.00 m long and have a mass of 375.0 kg each. How much tension must the crosspiece EF withstand? Assume that the mass of the crosspiece and the friction at points B and C are...
  3. L

    Stable Equilibrium of Two Hemispheres: a<3b/5

    Homework Statement A solid hemisphere with radius b has its flat surface glued to a horizontal table. Another solid hemisphere with radius a rests on top of the hemisphere of radius b so that the curved surfaces in contact. The surfaces of hemispheres are rough, meaning no slipping occurs...
  4. A

    Solve Spring Equilibrium Homework: Find a-g | Physics

    Homework Statement A mass (m = 1.20 kg), originally at rest, is attached to one end of an unstretched spring (k = 790 N/m) and the other end is fixed to a wall. The mass is then pulled with a constant force to stretch the spring. As a result, the system comes to a momentary stop after the mass...
  5. H

    How to calculate the tension needed for a crosspiece in an A-frame structure?

    Homework Statement You are designing the crosspiece for the A-frame structure in the figure below. Beams AB and AC are 3.35 m long and have a mass of 350.0 kg each. How much tension must the crosspiece EF withstand? Assume that the mass of the crosspiece and the friction at points B and C are...
  6. L

    Static equilibrium and rope tension

    Homework Statement A rope of negligible mass is stretched horizontally between two supports. When an object of weight 3160N is hung at the centre of the rope, the rope is observed to sag by 35.0cm. What is the tension in the rope? Homework EquationsThe Attempt at a Solution [/B] Sum of...
  7. C

    Non equilibrium boson distribution function

    In statistical mechanics the boson distribution function has the well known form ##f = \frac{1}{e^{E/T} - 1},## (in the special case of zero chemical potential). As one considers the non-equilibrium variant this generalize to ##f = \frac{1}{e^{\frac{E}{T(1+ \Theta)}} - 1},## for some function...
  8. P

    Exploring Equilibrium Constants & Kinetics: A Closer Look

    Consider the following generic equilibrium: aM + bN ⇌ cO + dP An equilibrium constant, K, can be defined as: $$K = \frac{[O]^c [P]^d}{[M]^a [N]^b}$$ But couldn't we also define another equilibrium constant similarly with coefficients that are in the same ratio as our original equation? For...
  9. J

    Torque equilibrium. Where would the masses need to be placed?

    A triple beam balance, like the scales we use every week in lab, works using rotational equilibrium. If a 253 g mass is placed on the pan, 1 cm from the pivot, and there are three masses, 1 g, 10 g, and 50 g masses on the other side can be slid back and forth between 0 cm and 10 cm from the...
  10. gfd43tg

    Equilibrium conversion of batch vs. flow reactor

    Hello, I went through a problem in my reaction engineering textbook, and solved them correctly by basically mindlessly plugging into the equation, but now I wanted to get some help interpreting the results. So we had a reversible, elementary gas phase reaction that is isothermal and isobaric...
  11. G

    Vibration Massless spring static equilibrium

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  12. L

    Stable equilibrium state question

    If a system is in a stable equilibrium state,its energy is maximum for fixed value of entropy ,the parameters and the amounts of constituents.Im wondering why this is true?Homework EquationsThe Attempt at a Solution
  13. S

    Calculate equilibrium dissolved oxygen at 19.8 Celsius

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  14. T

    Get Help Solving a Cylinder in Equilibrium Problem

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  15. N

    Principle of Virtual Work and static equilibrium

    Homework Statement The system shown in Fig. 2-6 is in static equilibrium. Use the principle of virtual work to find the weights A and B. Neglect the weight of the strings and the friction in the pulleys. Homework Equations Conservation of gravitational potential energy. The Attempt at a...
  16. C

    Why is the chemical potential negligible at equilibrium?

    I am currently reading cosmology and I read the statement "... chemical potential which is nearly always negligible when processes are in equilibrium." So the current understanding I got of chemical potential is through the law: $$dE = - pdV + TdS + \mu dN.$$ Hence for processes which occur at...
  17. B

    Equilibrium position of a charge between other 2 charges

    Homework Statement Place a third charge q between two charges Q1 = -2Q and Q2= 3Q so they stay in electrostatic equilibrium. Knowing that the distance between Q1 and Q2 is equal L Homework Equations (KQ1Q)/(QQ1)2 = (KQ2Q)/(QQ2)2 3. The Attempt at a Solution Let's start by saying that i know...
  18. S

    Mass in Equilibrium with Variable Length Cable

    Homework Statement Consider the diagram below. A 1000 kg block being suspended in equilibrium by cables c and a. Cable c is on a pulley which can vary the length of the cable. Write the constraints of equilibrium. Find the tension in each cable based on varying lengths of cable c between 1.2 m...
  19. Bhupendra

    Solving for Equilibrium Forces on a 5kg Ball

    Homework Statement a spherical ball of mass = 5kg rests between two planes which make angles of 30 degrees and 45 degrees respectively with the horizontal . The system is in equilibrium . find the normal forces exerted on the ball by each of the planes . The planes are smooth . Homework...
  20. B

    What is the Correct Method for Calculating Tension in Equilibrium?

    I don't manage to get this task right. See the pictures. First i take point D and draw a free body diagram. Then i use the equation for equilibrium. I can then calculate the tension in the wires that goes from D to B and from D to C. I then take point B and draw a free body diagram, but all the...
  21. S

    Pressure Equilibrium 2a+b: Why is Velocity x27?

    2a+b⇌product if pressure is increased 3 times of the initial pressure the velocity of the forward reaction will be of___________ the previous velocity? Actually i got 9 times but the answer is given 27 times Can anyone please give the hint about it? Thanks in advance!
  22. S

    STATICS: Applying Force Equilibrium

    The ball of weight W is held in place as shown in the left figure by the inclined surface and the compressed spring. The FBD of the ball is shown in the right figure. Determine the force exerted by the spring on the ball, and the force exerted by the inclined surface on the ball in terms of the...
  23. M

    Thermodynamic equilibrium of ideal gas and mechanical system

    Homework Statement Hi I am new to Physics Forums and desperately need some help! This is my first class back since undergrad and my math skills have gotten a little rusty. I think that's where I am failing. Okay here goes: http://tinypic.com/r/2zqf04k/8 There is a 4 subsystem...
  24. P

    Equilibrium Statistical Physics, Plischke ex. 1.2

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  25. O

    Electrostatic Equilibrium Problem

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  26. J

    Finding the charge when three charges are in equilibrium

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  27. B

    Multiple definitions for equilibrium

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  28. B

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    I've been thinking about the building-design simulation, and am hoping that someone can point me toward more complete treatises/references, than this hobbyist impression. I'm trying to develop a mathematical outline of simulated deformation physics, with the hope of eventually creating a...
  29. B

    Distance between equilibrium and nonequilibrium states

    Recently, the properties of nonequilibrium many-body quantum systems have aroused great curiosity of physicists. Numerous papers have been published about this area. But how do we measure how far the system is from equilibrium states? There are several proposals have been published, for...
  30. F

    Lennard-Jones Potential: Derive an expression for equilibrium constant

    Edit: well this is frustrating, not sure why the itex things aren't working.. Edit2: I've attached a picture of the question at the bottom as well. Homework Statement The cohesive energy of a solid noble gas may be written as (See picture below) Derive an expression for the...
  31. H

    Why Do Gases Need Same Speed for Thermal Equilibrium?

    When two gases are mixed, if they are to be in thermal equilibrium, they must have the same average molecular speed. Why or why not?
  32. J

    Relationship between Free energy change and Equilibrium constant (K)

    1. I have a question regarding the equation ΔG° = -RT lnK. When solving for the equilibrium constant K, what is the relationship between the vapor pressure of each molecular compound in a reaction? ΔG° = Free energy change R = Universal gas constant T = Temperature K = equilibrium...
  33. S

    What is the Partial Pressure of H2S at Equilibrium?

    Homework Statement H2(g) + S(s) = H2S(g) Kc= 6.8x10^-2 If 0.2 moles of H2 and 1.0 mole of S are heated in a 1L vessel upto 90C, what will be the partial pressure of H2S at equilibrium? Can someone help me with this step by step?Homework Equations The Attempt at a Solution Kc = 6.8X10^-2 = x /...
  34. B

    What is the tension in the rope of a pulley system in equilibrium?

    Homework Statement When a 50-kg mass on a pulley is allowed to drop, the friction in the pulley creates a constant 200 N force upward. What is the tension in the rope? Homework Equations F (upward) = F (downward) The Attempt at a Solution I attempted this problem by assigning the...
  35. Useful nucleus

    Equilibrium with respect to matter flow

    Homework Statement A two component gaseous system has a fundamental equation of the form S=AU1/3V1/3N1/3 + (BN1N2)/N N=N1+N2 where A and B are positive constants. V is volume , S is entropy, U is internal energy, N is mole number. A closed cylinder of total volume 2V0 is separated into two...
  36. wirefree

    Analyzing a String System: Calculating M for Equilibrium

    Question: Two pulleys of masses 12 kg and 8 kg are connected by a fine string hanging over a fixed pulley as shown. Over the 8 kg pulley is hung a fine string with masses 4 kg and M. Over the 12 kg pulley is hung another fine string with masses 3 kg and 6 kg. Calculate M so that the string over...
  37. B

    Equilibrium Stat Mech Vs. Kinetics

    Ensembles Figured it out thanks, next post:
  38. M

    Equilibrium Question involving Torque

    Homework Statement The crane shown below has a mass of 4000 kg and a base of 3.4 m. The arm of the crane is 22 m and attaches to the center of the crane. If the arm is placed at an angle of 30°, what is the largest mass that the crane can hold off the ground without tipping? Homework...
  39. D

    Quartz Crystal Microbalance equilibrium

    Hi guys, I am a new user of QCM and I am having trouble reaching equilibrium. I have isolated the chamber from air flows and vibrations, the crystals are clean and all the other removable parts have been cleaned as well. The real problem is that sometimes I have managed to reach equilibrium...
  40. M

    Translational and Rotational Equilibrium

    Hi Everyone, I'm having some trouble with a problem concerning translational and rotational equilibrium. The question involves a balance with various masses suspended from it (see attached image). The question states that the counterweight is moved from 1cm away from the fulcrum to 2cm away from...
  41. L

    Liquid - polymer equilibrium "contact" distance

    Hi everyone, In this paper ( http://pubs.acs.org/doi/abs/10.1021/la962131c ), the authors calculate the surface energy of polymer and the contact angle of various liquids on these polymers using a so-called "liquid - polymer equilbrium contact dsitance". I'm wondering how to calculate such a...
  42. T

    Equilibrium Question - at what rate does the block accelerate?

    Homework Statement A force is applied to a box 60 degrees from the horizontal. The box weights 5kg. If the force is 120N, and the surface upon which the box rests upon is frictionless, at what rate does the block accelerate? Homework Equations Fnet = ma Fg= mg The Attempt...
  43. WannabeNewton

    Surfactant adsorption chemical equilibrium

    Homework Statement A dilute solution of surfactants can be regarded as an ideal three-dimensional gas. As surfactant molecules can reduce their energy by contact with air, a fraction of them migrate to the surface where they can be treated as a two-dimensional ideal gas. Surfactants are...
  44. TheFerruccio

    Find the equilibrium separation between two materials

    Homework Statement Find the equilibrium separation between Diamond and Copper materials. Homework Equations Lennard-Jones force/unit area: p_a(z)=\frac{8w}{3z_0}\left[\left(\frac{z}{z_0}\right)^{-9}-\left(\frac{z}{z_0}\right)^{-3}\right] I need to find z_0 which is the equilibrium...
  45. S

    Classical mechanics - small movements around equilibrium

    Homework Statement Two cylinders, that can rotate around their vertical axis, are connected with a spring as shown in the picture. Moment of inertia ##J## is the same for both and they also have the same radius ##R##. Distance between the axes is ##L##. Spring with constant ##k## is ##d## long...
  46. C

    Can Static Equilibrium Be Maintained Without Friction in This Beam Scenario?

    A uniform vertical beam of mass 40 kg is acted on by a horizontal force of 520 N at its top and is held, in the vertical position, by a cable as shown. a) Draw a free-body diagram for the beam, clearly labeling all of the forces acting on it. b)Calculate the tension in the cable...
  47. S

    What is the Hydroxide Ion Concentration in an Ionic Equilibrium Problem?

    Homework Statement Calculate the hydroxide ion concentration: 16.5 mL of aqueous sulfuric acid at 1.5 M added to 12.7 mL of sodium hydroxide at 5.5M. 2. The attempt at a solution H2SO4 (aq) + 2 NaOH (aq) <--> 2 H2O (l) + Na2SO4 (aq) Hydrogen Concentration: (16.5 mL / 1000) (1.5 M) =...
  48. S

    Horizontal spring and wood block energy and equilibrium question

    Homework Statement A 200 g wood block is firmly attached to a horizontal spring. The block can slide along a table where the coefficient of friction is 0.40. A force of 10 N compresses the spring 18 cm. If the spring is released from this position, how far beyond its equilibrium position will...
  49. Q

    Pump probe spectroscopy as linear process away from equilibrium

    Greetings everyone, I have recently been trying to generate some theoretical pump probe spectroscopy data. The typical way to do this is via third order time dependent perturbation theory and applying phase matching approximations / Rotating wave approx etc etc. There is however, no...
  50. T

    Engineering Statics Equilibrium System Question

    Homework Statement http://i.imgur.com/VFb5rwj.png Homework Equations The Attempt at a Solution First, I drew the FBD for the system, one horizontal force at A, one force perpendicular to the beam at B, and the weight. Writing out the equilibrium equations: The sum of forces in...
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