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. T

    Equilibrium of Coplanar Forces

    Homework Statement In this experiment, a suspended metre-ruler lies in a series of equilibrium under non-parallel forces. Homework Equations The apparatus consists of the following. A metre rule suspended by strings which make an angle with the vertical. The strings run over pulleys which are...
  2. M

    Solve Static Equilibrium: Beam, 520N Force, Cable Tension

    Homework Statement A uniform vertical beam of mass 40kg is acted on by a horizontal force of 520N 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 labelling all the forces acting on it b) Calculate the tension in the cable...
  3. R

    Solve Particle Equilibrium Homework Statement

    Homework Statement If the 4.4-kg block is suspended from the pulley B and the sag of the cord is d = 0.15 m, determine the force in cord ABC. Neglect the size of the pulley. (Figure 1) Express your answer to three significant figures and include the appropriate units.Homework EquationsThe...
  4. A

    Finding the point of equilibrium

    Homework Statement As a stunt coordinator with a background in physics, you have been asked to determine whether Man can make it across the river and whether the rope will hold his weight. Man is rather small, with a mass of only about 48.7 kg, including his equipment. The crossing distance is...
  5. E

    I Proportion in 2D and 3D static equilibrium?

    If you were to look at a simple 2D or 3D static equilibrium diagram, where a mass is suspended by multiple ropes at different angles, my gut feeling is that there is a fairly simple proportion between the distance between the center of mass and the rope (the distance created opposite of the...
  6. X

    Static equilibrium: setting up the equation

    Homework Statement A rigid wire frame is formed in a right triangle, and set in a vertical plane as shown. Two beads, of weight W1 = 1.5 N and W2 = 2.5 N, slide without friction on the wires, and are connected by a cord. When the system is in static equilibrium, what is the tension in the...
  7. F

    How can dynamic equilibrium be best explained to students?

    This concept, though intuitive to some, appears to be counterintuitive for 20-80% of smart-to-brilliant students I teach. How can the idea of an object having non-zero speed while (balanced) forces act on that object, be conveyed without resorting to just a shuffling of physics terms to give a...
  8. J

    Finding Equilibrium Force in 2D Vector Addition

    Homework Statement Homework Equations [/B] xcomponent of vector= magnitude*cosine of theta ycomponent of vector= magnitude*sine of theta Basic trig operations after this. Honestly I think that I am just having trouble with the algebra The Attempt at a Solution Because the magnitude of the...
  9. Y

    Electrodynamics problem - force equilibrium

    Homework Statement Hello! I have a question about the following problem: Two point masses m1 and m2 are attached to isolating wires to point P. They are both positively charges (charge Q1 and Q2) and in the picture you can see the situation at equilibrium. What is the proportion of the...
  10. ramzerimar

    Dynamics problem: determine maximum angle for equilibrium

    Homework Statement A uniform bar with length c can slide a cylindrical surface with a radius of r. Determine the maximum angle theta that guarantees the equilibrium of the bar if the friction coefficient at the points of contact is u. Homework Equations Friction force = u*N (where N is the...
  11. H

    Equilibrium Problem - Balancing Torques

    Homework Statement [/B] A door 1.00 m wide and 2.00 m high weighs 330 N and is supported by two hinges, one 0.50 m from the top and the other 0.50 m from the bottom. Each hinge supports half the total weight of the door. Assuming that the door’s center of gravity is at its center, find the...
  12. W

    Equilibrium constant Kp,eq issue

    Homework Statement question1-sentence https://www.physicsforums.com/attachments/105257 question1-answer https://www.physicsforums.com/attachments/105258 question2-sentence https://www.physicsforums.com/attachments/105259 question2-answer https://www.physicsforums.com/attachments/105260...
  13. Aafia

    Calculate equilibrium constant?

    Homework Statement [/B] The following reaction was allowed to reach the state of equilibrium 2A+B <====> C The initial amounts of reactant present in one litre of solution were 0.5 mole of A and 0.6 mole of B. At equilibrium the amounts were 0.2 mole of A and 0.45 mole of B and 0.15 mole of...
  14. J

    Zero equilibrium 0 x a force?

    Okay - a classic trap . multiplication times zero . I have a 3 foot bar weighing 10 lbs . its attached along its mid point to an axle . how much torque / force - do I need along its axle ( motorized ) to get it to spin 30 rpm or 60 rpm ? its in equilibrium , but of course it needs FORCE...
  15. D

    Density of star from hydrostatic equilibrium and pressure

    Homework Statement Assume that the pressure p in a star with spherical symmetry is related to the density \rho by the (distinctly unrealistic) equation of state p= \tfrac12 k\rho^2, where k is a constant. Use the fluid equilibrium equation obtained in Problem 23 to find a relation between \rho...
  16. T

    Equilibrium of a uniform bar inclined at an angle

    1. A uniform bar AB of length 3 m and weight 40 N rests in equilibrium with the lower end A on a smooth plane inclined at an angle alpha to the horizontal, where tan alpha = 3/4 and a point C of the rod against a smooth peg. If the rod makes an angle theta with the horizontal, where tan theta =...
  17. T

    Uniform right circular cone hanging in equilibrium

    1. A uniform right circular solid cone of weight W is suspended by two vertical strings attached to the ends A and B of a diameter of its base. If the cone hangs in equilibrium with its vertex vertically below A, find the tension in the strings. 2. Centre of mass of cone from the vertex is 3/4...
  18. Soumalya

    Statics: Equilibrium in 3-Dimensions

    Homework Statement The portable reel is used to wind up and store an air hose. The tension in the hose is 100 N and a vertical 200-N force is applied to the handle in order to steady the reel frame. Determine the minimum force P which must be applied perpendicular to the handle DE and the...
  19. Saracen Rue

    Equilibrium Constant of a Heterogeneous Reaction

    Homework Statement Write an expression for Kc for the following at equilibrium; CaCO3(s) + CO2(g) + H2O(l) ⇔ Ca2+(aq) + 2HCO3-(aq) Homework Equations K = [products]^(coefficients)/[products]^(coefficients) The Attempt at a Solution I know that pure solid and liquid substances should be set...
  20. J

    MHB 3 forces, 5N,6N and 7N ac on a particle which remains in a state of equilibrium.

    3 forces, 5N,6N and 7N act on a particle which remains in a state of equilibrium. What is the angle between the 5N and 6N force?
  21. Eureka99

    The Pressure of a liquid in equilibrium?

    The pressure of a liquid in equilibrium, is equal to the pressure of it's vapour, or to the sum of vapour pressure plus atmospheric pressure? My doubt starts from this problem: At 293 K and 1 atm, the vapour pressure of water is 565.8 Pa. Calculate the vapour pressure of water, when the total...
  22. Soumalya

    Statics: Problem about Equilibrium in 3-dimensions

    Homework Statement The two uniform rectangular plates each weighing 800 kg are freely hinged about their common edge and suspended by the central cable and four symmetrical corner cables. Calculate the tension T in each of the corner cables and the tension T0 in the center cable. All...
  23. A

    Static Equilibrium Problem involving Moments

    Equations: Net torque=0=Net force I believe this is a static equilibrium question but can't figure out how to do it
  24. Andreas C

    I Sun and hydrostatic equilibrium

    I've been trying to make an estimate for the pressure of the sun's photosphere using some sort of hydrostatic equilibrium equation, but I can't seem to get one that even comes close to agreeing with observations. Does anyone know what the right formula is and what the right way to approach the...
  25. R

    Systematic treatment of equilibrium

    Homework Statement Determine [Zn2+], [CN–], and [HCN] in a saturated solution of Zn(CN)2 with a fixed pH of 1.060. The Ksp for Zn(CN)2 is 3.0 × 10–16. The Ka for HCN is 6.2 × 10–10. Homework Equations The Attempt at a Solution I don't have a phone or camera right now to take a picture of my...
  26. Uthpala Kaushalya

    How Does Mixing Ideal Gases in a Thermally Insulated Cylinder Affect Pressure?

    Homework Statement Consider a rigid piston cylinder arrangement. Assume that the cylinder and the piston are covered by a perfectly insulating material and the piston cylinder interface is very smooth. Two types of ideal gases are separated in the cylinder by the piston. The number of gas...
  27. T

    Change in concentration in equilibrium caused by addition

    Homework Statement Cu2+ ions react with Fe2+ ions according to the following reaction. Cu2+ + 2Fe2+ --> Cu + 2Fe3+ At equilibrium, the concentration of Cu2+ ions is not changed by the addition of A) Cu2++B) Fe2+C) CuD) Fe3+. Homework EquationsThe Attempt at a Solution I chose A through...
  28. E

    Trouble with Gibbs Free Energy & Equilibrium Constant Calc.

    Hello. A known equation that is useful for calculating equilbrium constants is: ΔG° = -RT * ln(K) This is all well and good. Given a standard gibbs free energy of reaction for some given reaction, the equilibrium constant for the reaction can be found. My trouble is in which ΔG° to use. For...
  29. L

    Can an object be in equilibrium if it is in motion?

    Homework Statement Can an object be in equilibrium if it is in motion? Explain * This is a prelab question to a lab that deals with equilibrium and torque The attempt at a solution My thought process: - Equilibrium means that: 1) Net external forces = 0 2) Net torque = 0 - Net...
  30. GW150914

    I Why do we need "planetary equilibrium temperature"?

    I mean, currently it seems that scientists are using equilibrium temperature of exoplanets (calculated assuming an Earth-like albedo) to determine whether a planet is habitable or not. But aren't there other more accurate ways to determine surface temperatures of exoplanets? I learned Wien's...
  31. S

    Static Equilibrium Pulley Problem

    Homework Statement The attachment above contains the diagram given in the problem M1 = 25kg M2 = 35kg Friction coefficient between M1 and stucture = 0.15 Friction coefficient between M1 and M2 = 0.25 Friction on pulleys and mass on pulleys is negligible. Find the minimum mass M3 for sliding...
  32. A

    Understanding Torque and Equilibrium in Simple Systems

    There is a stick of known length l and known mass m1 with its balance point in the middle. By placing an object of unknown mass m2 at the far end of the stick, the balance point moves towards the same end as the object by a distance of d. Calculate m2. Second, similar exercise: Stick of known...
  33. U

    Examples of systems which cannot receive work adiabatically?

    In Principles of General Thermodynamics, Hatsopoulus and Keenan (p 442) make the following claim: What, however, would be some physical examples of such a system?
  34. washer

    3D Equilibrium of 4-wheel support

    Homework Statement The plate has a dimension of 460mm X 260mm, subjected to a central load of 400N. Determine the reaction A and B, each representing a roller support as a wheel, located at the center of one edge. (opposite blue arrows have the same resultant value.) Are they the same ? If not...
  35. T

    Thermal Equilibrium of Hot Iron & Water

    Homework Statement A hot (800 K) iron nail of mass 150 g is placed into a 1.0 kg of water at 25°C. What is the common temperature of the 2 objects once they have reached thermal equilibrium? Assume no change of state of the water. Heat capacity of iron nail is 450 J/kg/K Heat capacity of...
  36. jfizzix

    Gravitationally induced electric voltage in conductor

    Here's a random thought that I don't as yet have a satisfactory solution: Let's assume we have a copper column resting on an insulated table. Copper is an electric conductor, which means that some electrons within the copper column move freely throughout the column, subject to electromagnetic...
  37. W

    With which speech goes the Mass through the Equilibrium?

    Homework Statement With which speed goes the Mass through the Equilibrium? Homework Equations Es = 1/2*D*s2 Ekin = 1/2*m*v2 Epot = m*g*h Energy in the zero point = below/ under highest Point: Epot = 1kg * 9,81N/kg*0.67m ≈ 6.573Nm Equilibrium : Ekin, Epot, Es Lowest Point: Es= 0 Ekin = 0...
  38. R

    Classifaction of equilibrium points for a Hamiltonian System

    Homework Statement For the system: \frac{dx}{dt}=x\cos{xy} \: \: \frac{dy}{dt}=-y\cos{xy} (a) is Hamiltonian with the function: H(x,y)=\sin{xy} (b) Sketch the level sets of H, and (c) sketch the phase portrait of the system. Include a description of all equilibrium points and any saddle...
  39. evinda

    MHB Find the stable equilibrium points

    Hello! (Wave) Let a particle be forced to move over the sphere $x^2+y^2+z^2=1$, that is subject to gravitational forces while also to an additional "dynamic" $V(x,y,z)=x+y$. Find the stable equilibrium points , if they exist.I have found a similar example: Let $\vec{F}$ be the gravity field...
  40. JasonR2

    How Do You Solve a Truss Equilibrium Problem with Incorrect Force Calculations?

    Question: Three forces of magnitude 45 kN, 24 kN and 12 kN are applied to the truss connection as shown. Knowing that the connection is in equilibrium, determine the magnitude of the forces C and T. Equations: Fx = 0 Fy = 0 My attempt: Fx=0: -24kN + Tcos40 + Csin20 = 0 Fy = 0: 45kN +...
  41. A

    Equilibrium and principle of moments

    Homework Statement Homework Equations Torque = F x D Manticlockwise = Mclockwise The Attempt at a Solution From what I understand, the moment on left hand side of the pivot point needs to be equal to the moment on right hand side, therefore... (Fx.2)+(5x.14) = (7xd) My initial...
  42. K

    Equilibrium velocity of a propeller-driven aircraft

    Hi - I just want to know if I'm approaching this correctly. I'm trying to calculate the equilibrium velocity of a propeller-driven aircraft when supplied a given amount of power. I'm iterating: min_velocity = 0.01 max_velocity= 200 aircraft_velocity = 5 for i in range(steps): force = 0.5 *...
  43. RoboNerd

    Question about static equilibrium

    1. Homework Statement problem is above Homework Equations sum of torques = I * angular acceleration = 0 for rotational equilibrium The Attempt at a Solution OK, here's what I tried to do. I wrote sigma Fy = Fleft pier up + F secondleftpierup - Fg,beam = 0 I assumed that the two piers have...
  44. V

    Stable equilibrium of a 4 mass, 6 spring system

    Homework Statement I am making a physics engine for my computer science class and couldn't figure out how to get the spring/mass system to come to equilibrium into a box-like shape such as the one on the left. I can understand why the one on the right is in equilibrium but I don't know how to...
  45. U

    A simple static equilibrium problem

    Homework Statement Suppose we have a folding ladder, so that when its legs are spread it makes an isosceles triangle. Suppose it has a support that runs parallel to the ground. There is a bucket of paint that rests on top of the ladder, and the mass of the ladder is negligible. Data: The legs...
  46. V

    A Question about Chemical Equilibrium

    say we have a reaction: N2O4 <====> 2NO2 for this reaction,if initial number of moles of N2O4 is 'a' can we write t = 0 a - at equilbrium a-x...
  47. chmergatroyd

    Finding weight of a beam using torque & equilibrium?

    Homework Statement A uniform beam at equilibrium is suspended by a cord at X which is 2m from one end of the beam and and 3m from the other end. There is a mass of 28N attached to the 2m end and a mass of 10N on the 3m end. Determine the weight force of the beam Homework Equations...
  48. TheSodesa

    Precipitation of lead hydroxide in the presence of EDTA

    Homework Statement 1. The question EthyleneDiamineTetra-acetate(##EDTA^{4-}##) is used in chemical analysis as a complex-former. ##EDTA^{4-}## is also used to treat heavy metal poisoning, as it forms water soluble complexes with them which can then be easily removed by the body. The reaction...
  49. F

    Equilibrium of NOCL Change w/ Added NO & Cl2

    Homework Statement 2NOCl(g) <--> 2NO(g) +Cl2(g)[/B] What will be the effect on equilibrium concentration of NOCL when equal moles of NOCL and NO are introduced in the mixture at constant temperature? Homework Equations Kconcentration = [NO]2[Cl2] / [NOCl]2 The Attempt at a Solution In my...
  50. Dietrichw

    Static equilibrium, tension and compression

    Homework Statement The figure below shows a light truss formed from three struts lying in a plane and joined by three smooth hinge pins at their ends. The truss supports a downward force of = 1 330 N applied at the point B. The truss has negligible weight. The piers at A and C are smooth. The...
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