What is Van der waals: Definition and 144 Discussions

The Van der Waals radius, rw, of an atom is the radius of an imaginary hard sphere representing the distance of closest approach for another atom.
It is named after Johannes Diderik van der Waals, winner of the 1910 Nobel Prize in Physics, as he was the first to recognise that atoms were not simply points and to demonstrate the physical consequences of their size through the Van der Waals equation of state.

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  1. chikou24i

    How is the Van der Waals interaction potential calculated?

    Hello! In Van der Waals interaction, how to prove that : H= - (2*e^2*x1*x2) / R^3 ?
  2. C

    Van der waals attraction between 1-D wires

    Homework Statement Consider two infinitesimally thin (i.e 1-D) wires of equal length ##L##, and at mutual distance ##d##. Consider the two configurations shown in attachment Estimate the van der Waals interaction between the wires, for ##d \gg L##, in the two situations where: (i) the wires...
  3. S

    How do I calculate van der Waals constants with 2 unknowns (a and b)?

    How would I go about calculating the units for the van der waals constants if I have 2 unknowns? (a and b) (P + a (n/V)^2) (V - n b) = n R T p=Pressure v=Volume n=moles R=gas constant T=temperature
  4. ZetaOfThree

    Young's modulus for cubic Van der Waals crystal

    Homework Statement Calculate the Young’s moduli for Van der Waals solids with sc, bcc, and fcc structures.Homework Equations Definition of Young's Modulus $$Y=\frac{stress}{strain}$$ Total energy due to Van der Waals interaction: $$U_{tot}=\frac{1}{2}N(4 \epsilon) \left( \sum_j{ \left( \frac{...
  5. I

    Virial Expansion of Van Der Waals Equation

    Homework Statement If the compressability factor is given at a certain temperature as a function of pressure: Z(T) = 1+αP+βP2 find α and β in the form α(a, b, T) and β(a, b, T) where a and b are the van der waals coefficients. Homework Equations nRT=(P+a(n/V)2))(V-nb) Z=PV/nRT The Attempt...
  6. G

    Van der Waals Forces Between Particle and Surface

    Hey, I was hoping to be directed to some literature regarding the van der waals forces between a particle and a surface. I have found lots of models regarding particle-particle forces but no extensions of this. Thanks
  7. N

    Interpretation of the Van der Waals Equation

    There is a silly detail about the interpretation of the Van der Waals (VDW) equation that I cannot fully understand. Say we have the Van der Waals equation for one mole: (P + a / V^2 ) (V - b) = RT The usual interpretation is that if you start from the ideal gas law PV=RT, you have to...
  8. koustav

    Van der waals equation and molar specific heat

    the van der waals equation of state is given by (p+an^2/v^2)(v-nb)=nRT.how to show that for a gas obeying the above equation of state (∂Cv/∂V) (taking temperature constant)=0?
  9. U

    Virial Expansion, Van Der Waals Gas

    Homework Statement Taken from 'Concepts in Thermal Physics': Homework Equations The Attempt at a Solution For the step highlighted in red, why does the '-1' go into the integrand?
  10. U

    Van der Waals Equation, Virial Expansion

    Homework Statement Taken from Concepts in thermal Physics: Homework Equations The Attempt at a Solution Shouldn't the van der waal's equation be: p = \frac{RT}{V_m -b} - \frac{a}{V_m^2} pV_m = \frac{VRT}{V_m -b} - \frac{a}{V_m}
  11. E

    Van der Waals Forces: Air vs Water

    Are van der waals forces between two molecules stronger in air of in water (as a medium)?
  12. M

    Thermal Physics problem: Van der Waals

    [b]1. Find an expression that relates T and V in a process in which the entropy S is constant, by doing the following: a) From thermodynamic identity dS=(1/t)dU + (p/v)dV find the expression for dS as a function of T and V (and dT and dV). b) Integrate the expression found in (a)...
  13. Q

    Conceptual Van der Waals Question

    Homework Statement Why does the Van der Waals equation adjust pressure up and volume down? Homework Equations (P + a)(V-b) = nRT (overly simplified, but it gets the point across). The Attempt at a Solution Okay. Volume refers to total free volume in a container in the context...
  14. Kelsi_Jade

    Van der Waals Equation and specific heat

    The problem reads: Consider a gas with constant specific heat cv and the van der waals equation of state (P+ a/v2)(v-b)=RT , where v=V/n A) Find du and the specific internal energy u=U/n B) Find ds and the specifi entropy s=S/n Here's what I've tried so far: A) I took the initial...
  15. C

    Expansion of Van der waals for small pressure

    Homework Statement In the VDW eqn of state, ##(P + a/v^2)(v-b)=RT## write Pv as a function of {P,T} and by expanding the result in powers of P or otherwise show that the first terms of a Virial expansion in powers of P for a VDW gas are given by $$Pv = RT + \left(b - \frac{a}{RT}\right)P +...
  16. fluidistic

    Thermodynamics, Van der Waals gas problem

    Homework Statement Hi guys, I'm stuck on 2 parts for now of the following problem that appeared in an exam I totally bombed: Given the equation ##s=s_0+R\ln \left [ C(v-b)\left (u + \frac{a}{v} \right )^{5/2} \right ]##, 1)Calculate ##u(v,T)## 2)Calculate f in its natural variables (the...
  17. S

    Van der Waals radius of hydrogen deuteride

    Does anyone have a reference for experimental and/or estimated Van der Waals radius for the molecule hydrogen deuteride ? Symbolically, this would be a proton {(p)e-} + {(np)e-} deuterium. I would like to know how closely the two atoms can approach each other to maintain the stable state of a...
  18. fluidistic

    Free expansion of a Van der Waals gas, physical explanation

    Hello! Today I've learned that when a Van der Waals gas undergoes a free expansion, it cools down a bit. :bugeye: This is in contrast with the ideal gas in which case since it does no work and since the process is adiabatic, the internal energy of the ideal gas remains unchanged by the...
  19. F

    Van der Waals force (Lenard-Jones)

    Hi everyone I'd like to know the Lennard-Jones potential energy is just for symmetric particles or we can use it for asymmetric forces too? If not how can I define the potential energy between two asymmetric particles? Thanks
  20. E

    Calculate the molar volume using the van der Waals equation.

    Homework Statement Calculate the molar volume using the van der Waals equation of a gas at P=3000psia, T=60F. The critical pressure and temperature, Pc and Tc, are Pc=408psia, Tc=504F. Homework Equations The given van der Waals equations(s): (P+a/Vm^2)(Vm-b)=RT ----(1)...
  21. T

    Help Me Solve the Van der Waals Equation

    I'm so close to being done in class, though this has thrown me for a loop could anyone help me please?Does the picture show up on here?
  22. O

    Van der Waals gas is not real gas?

    From van der Waals , (P+a/v^2)(v-b)=RT, At critical temperature, I get (∂P/∂V)at constant temperature =0 and (∂^2P/∂V^2) at constant temperature ,T=0. then critical pressure,P = a/(27b^2)--------1 critical volume,v=3b-----------2 critical temperature=8a/(27Rb)----------3 then simultaneous...
  23. O

    Understanding Van der Waals P-V Graph and the Maxwell Equal Area Rule

    http://en.wikipedia.org/wiki/Van_der_Waals_equation (the graph of Maxwell equal area rule) When the temperature is under the critical temperature, then at certain pressure, there will be three possible point for the volume of the system. What does that mean? Do they mean the volume of the...
  24. O

    Understanding P in Van der Waals Gas Equation

    [P + (n2a/V2)](V - nb) = nRT What is the P represent? Is the pressure of ideal gas or the pressure we can measure from the real gas? Consider now it is the pressure of ideal gas but how do we get the value of P from an experiment of real gas? What if it is the pressure of real gas then why do...
  25. C

    Balancing Van der Waals force with a spring

    Homework Statement An AFM tip has a spring constant k, a colloidal bead with radius r is glues onto the cantilever tip. Derive an expression and plot the deformation of the cantilever spring, Δx, as a function of distance the tip of the cantilever is moved towards another bead of the same...
  26. B

    P-Chem: Van der Waals Equation data

    Homework Statement Based on values for the Van der Waals equation tabulated in Table 1.6 in appendix A, determine an approximate molecular volume for CCl4 and CH4. Rationalize these values based on reasonable bond lengths and other parameters. The Lennard-Jones potential energy function is...
  27. Q

    Find (∂V/∂T) of a Van der Waals Gas using variables?

    Find (∂V/∂T) of a Van der Waals Gas using variables? Evaluating a Derivative of the van der Waals Equation using the Cyclic Rule For the van der Waals equation of state: V-b(P+ a/v^2)=RT the derivative (∂V/∂T)p is difficult to obtain directly because finding an equation for V in terms...
  28. M

    Solving for b with van der Waals

    solving for "b" with van der Waals 1. Homework Statement The problem gives me "a", the temperature, volume, number of moles, and pressure. It asks me to solve for "b". I have set up the equation to solve for b, and done it. The answer comes out with the right units but it is negative...
  29. A

    Thermal: taylor series van der waals equation

    Homework Statement Show that at constant volume V and temperature T but decreasing number N=n*N_{A} of particles the Van der Waals equation of state approaches the equation of state of an ideal gas. Hint: Rearrange the equation of state into the explicit functional form P=P(v,T) and use x=1/v...
  30. W

    Van der Waals forces of hydrogen

    Why does the Van der Waals forces of hydrogen weakest in liquid state?
  31. K

    Reversible adiabatic expansion using Van der Waals' equation

    Homework Statement A real gas obeys Van der Waals‟ equation, which for one mole of gas is (p + A/V2)(V-B) = RT and its internal energy is given by U = CvT - A/V where the molar heat capacity at constant volume, Cv , is independent of the temperature and pressure. Show that the relation...
  32. L

    Physical meaning of the extremums of the Van Der Waals equation

    Hi everyone, Homework Statement When I draw an isotherm into a p-V diagram I have to extrem values. If I do an experiment I won't find any extremum, because the van der waals equation isotherm is just theoretical (it's just a line in an experiment). However, I want to know the 'physical'...
  33. G

    Van der Waals, temperature decrease

    Homework Statement I need to find a p-V region where the temperature is decreasing ## (p + \frac{a}{V^2} ) (V-b) = RT ## The Attempt at a Solution So I thought that a place to start would be ## dT = \frac{\partial T}{\partial P} dP + \frac{\partial T}{\partial V} dV < 0 ##...
  34. B

    Understanding Van der Waals Equation for Ideal and Actual Gases | Homework Help

    Homework Statement (P+ an^2/V^2)(V-nb) = nRT I'm having some problems in understanding the above equation. firstly, PV=nRT holds for ideal P and V , so after correction we are writing ideal P and V in terms of measured P and V right? secondly V-nb is the actual volume not the ideal...
  35. J

    Van der Waals Interaction

    I need to report about the Van der Waals interaction in our Quantum Mechanics class. Can someone please help me sort things out about Van der Waals interaction? How do go think should I go about it? In our book, the only thing discussed is the Van der Waals interaction between two hydrogen atom...
  36. H

    Calculating Van Der Waals Forces: Gecko Climbing Walls

    Hi Is there an eqauation to work out the Van Der Waals Force between solids? I am enquiring this in relation to how a gecko is able to walk up a wall. I have done some research and it says that a gecko can walk up walls because of Van Der Waals forces, I want to know is there an equation...
  37. I

    Working out the cp cv difference for van der waals equation

    Homework Statement I've put all the information on the picture as i already created on there in an attempt to help brain storm some ideas that didn't work ... http://dl.dropbox.com/u/48169762/Capture.PNG Homework Equations All on picture :) http://dl.dropbox.com/u/48169762/Capture.PNG The...
  38. S

    Entropy change for van der waals gas Problem

    Homework Statement Consider Entropy change for van der waal gas Prove that ΔH = {bRT[1/(V2-b)-1/(V1-b)]}-2a/V2+2a/V1Homework Equations Ideal gas : PV = nRT , Enthalpy : ΔH = U + PV , Real gas : nRT = (P + a/(V2))(V-b) The Attempt at a Solution I prove from PV = nRT in real gas it's...
  39. S

    Partial Derivative of Van der Waals Equation

    Given that the Van Der Waals equation is (p + (an^2)/v^2)(v-nb)=nRT where n,a,R and b are constants... How to we find the derivative of p wrt v ? How to find the derivative of p wrt T without further differentiation ?? Can anyone teach me how to do this question ? Sincerly thanks~
  40. K

    Van Der Waals EOS Shortcomings & Corrections

    what are the main drawbacks of the van der waals equation of state for real gases and how are they corrected?
  41. R

    Taking the derivative of the van der waals equation

    Homework Statement I'm trying to find dT/dVm for the van der waals equation. I was looking at http://courses.washington.edu/bhrchem/c456/vdw_jtc.pdf" page, but I'm confused about how they reached their result: The Attempt at a Solution I honestly have no idea what they did. dT/dVm of...
  42. M

    Explaining Intermolecular Forces at the Quantum Level

    Can someone enlighten me about the quantum level explanations for the following: 1. Adhesion between two substances 2. Cohesion within a substance 3. Strength of a substance (as in tensile strength, compressive strength and shear strength) 4. Stiffness of a substance (as in Youngs modulus...
  43. R

    Van der Waals equation adjustments

    Homework Statement Why in the van der waals equation, is the pressure correction term added on to the measured pressure but the volume correction term subtracted from the measured volume? Homework Equations (p+n^2a/V^2)(V-nb)=nRT The Attempt at a Solution I thought that the...
  44. R

    Is the Van der Waals Gas Equation More Accurate Than the Ideal Gas Equation?

    Just wondering, for van der waals equation, the real pressure is smaller than the ideal pressure and the real volume is smaller than the ideal volume?
  45. G

    Does the Compressibility Factor of a Gas Relate to Van der Waals Constants?

    Does the compressibility factor of a gas say anything about Van der Waals constants?
  46. W

    Energy Change in Reversible Adiabatic Expansion of a van der Waals gas

    Homework Statement Suppose the equation of state of a gas is Beta p=rho/(1-b rho) - beta a rho^2 where beta=1/kb T, b is a constant and rho is the molecular density, N/V. The internal energy of this gas is given by U=(5/2)N kb T - N a rho Determine the final internal energy of the gas...
  47. Telemachus

    Idel van der Waals fluids and Helmholtz potential

    Homework Statement Hi there. I have this problem which I'm trying to solve. Its from callen, it says: Two ideal van der Walls fluids are contained in a cylinder, sparated by an internal moveable piston. There is one mole of each fluid, and the two fluids have the same values of the van der...
  48. G

    Van der Waals Force: Questions Answered

    hi. first of all i am a little bit confused about the difference between casimir and van der waals forces? isn't it so, that both forces are consequences of Lifgarbagez's theory? so how can you distinguish between them? another question I asked myself is, how do Casimir and Van der Waals force...
  49. D

    Factor a in Van der Waals equation of state off by a factor of 0.1?

    Hello So I have a lab exam in Joule-Thomson effect and I have given values for a and b for VdW equation of state for nitrogen and carbon dioxide. When I take the critical temp. and pressure and put it in the equations: a=\frac{27(T_c R)^2}{64 p_c} and b=\frac{T_c}{8p_c} b gives...
  50. M

    Van der Waals and electrostatic force between polystyrene spheres

    Homework Statement Compare gravitational, electrostatic and van der Waals forces between two 100nm polystyrene spheres which are separated by a) one micrometer b) one millimetre and c) one centimetre Homework Equations Gravitation and electrostatic equations are standard. The...
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