What is Seperation of variables: Definition and 36 Discussions

In mathematics, separation of variables (also known as the Fourier method) is any of several methods for solving ordinary and partial differential equations, in which algebra allows one to rewrite an equation so that each of two variables occurs on a different side of the equation.

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  1. P

    Solving a wave equation with seperation of variables.

    Homework Statement I am trying to solve the given wave equation using separation of variables, u_{tt} - 4u_{xx} = 4 for 0 < x < 2 and t > 0 (BC) u(0,t) = 0 , u(2,t) = -2, for t>0 (IC) u(x,0)=x-x^2 , u_t(x,0)=0 for 0\leq x \leq2 Homework Equations We are told we will need to use, x =...
  2. M

    E&M separation of variables and Fourier

    Homework Statement Boundary conditions are i) V=0 when y=0 ii) V=0 when y=a iii) V=V0(y) when x=0 iv) V=0 when x app infinity. I understand and follow this problem (separating vars and eliminated constants) until the potential is found to be V(x,y) = Ce^(-kx)*sin(ky) Condition ii...
  3. F

    Stationary temperature field inside a house

    Homework Statement House: a room (see figure) has perfectly isolated walls, except the two windows where a convective heat exchange takes place (with the same transfer coefficient). Outside temperature in front of a sun-faced wall-sized panoramic window is T1, while at the back it is...
  4. E

    Potential of dielectric cylinder with constant polarization

    Homework Statement The dielectric cylinder is radius R and thickness d. Origin is at the center of the cylinder, which is oriented along the z-axis. It has polarization P=pz∧I need to calculate the potential V(0,0,h) at h>d/2. Homework Equations σb=P⋅n∧...
  5. E

    Potential around charged infinite cylinder in E field

    Homework Statement The cylinder has a radius a and is perpendicular to the electric field, E(r)=E(x_hat). It also carries charge Q. The potential is of the form V(r,φ)=A0+A0'ln(r)+∑(n=1 to ∞)((Ancos(nφ)+Bnsin(nφ))rn+(An'cos(nφ)+Bn'sin(nφ))r-n) Homework Equations V=-∫E⋅dl The Attempt at a...
  6. Dor

    Solve Laplace equation on rectangle domain

    Homework Statement I'm having issues with a Laplace problem. actually, I have two different boundary problems which I don't know how to solve analytically. I couldn't find anything on this situations and if anybody could point me in the right direction it would be fantastic. It's just Laplace's...
  7. kini.Amith

    Separation of variables to solve Schrodinger equation

    How do we know that separable solutions of Schrodinger equation (in 3d) form a complete basis? I understand that the SE is a linear PDE and therefore every linear combination of the separable solutions will also be a solution , but how do we know that the converse, i.e 'every solution can be...
  8. S

    Diffusion equation by seperation of variables

    Homework Statement A uniform rod of length l has an initial (at time t = 0) temperature distribution given by u(x, 0) = sin(\frac{πx}{l}), 0 \leq x \leq l. The temperature u(x, t) satisfies the classical one-dimensional diffusion equation, ut = kuxx The ends of the rod are...
  9. J

    Diff.Eq. Seperation of variables.

    Homework Statement Solve the given differential equation subject to the indicated initial condition. (e^-y + 1)sinxdx=(1+cosx)d, y(0)=0Homework Equations Basically we have to use separation of varaibles to solve before using initial value condition.The Attempt at a Solution After separation...
  10. T

    Abstract questions about PDEs with respect to Seperation of Variables

    I have two more loosely based questions about PDEs and the separation of variables technique: In the intro of this chapter the author imposed that we "assume" the the solution to a set of special PDEs is: U(x,t) = X(x)T(t) where X and T are the eigenfunctions. My question is how did...
  11. G

    Help - Seperation of variables problem, multiple solutions.

    Help -- Seperation of variables problem, multiple solutions. Homework Statement Suppose that dy/dx = √y and y(0) = 0. What is y(x)? There is more than one answer to this problem. You must list five correct solutions. Homework Equations Seperation of Variables/ integration The...
  12. M

    Schrodinger Equation and seperation of variables

    Hi, In Griffiths QM, it is claimed that to solve the Schrodinger Equation, we take the solution wavefunction \Psi(x,t) to be of a seperable form \psi(x)\phi(t). He then says that a superposition of these seperable forms can always give us the general solution. Can someone help me prove that...
  13. C

    The doubt of Seperation of Variables

    The method of separation of variables is used to solve the problem of partial diffrential equation. For example, when the partial differential equation is: \frac{\partial u}{\partial t}-\alpha\frac{\partial^{2}u}{\partial x^{2}}=0 We could suppose that u(x,t) is a solution concerning...
  14. X

    Seperation constant giving a harmonic dependence. (Seperation of variables)

    Homework Statement http://img18.imageshack.us/img18/8970/bose.png The Attempt at a Solution I'm on part b) where it asks which separation constation gives a harmonic time dependence. From part a) I deduced the equation \frac{d^{2}T}{dt^{2}}\frac{1}{T} = a constant. I'm choosing the constant...
  15. R

    Seperation of Variables to find exact solution

    Can anyone help me please or point me in the right direction, I am needing to find an exact solution for this equation by using separation of variables and compare them to answers i have calculated for Euler's method & Euler-Cauchy method. The equation is dx/dt=x^2/(t+1) when x(0)=1 and t=time...
  16. D

    Solve using seperation of variables

    Hello everyone how yall durrin! Solve the following DEs by Seperation of Variables eliminate natural logarithms and leave your final answer in implicit form (3x+8)(y^2 +4)dx - 4y(x^2+5x+6)dy=0 by separation of variables i get (3x+8)dx/(x^2+5x+6)=4ydy/(y^2 +4) now I am having trouble...
  17. D

    Stuck with integration of differential equation by seperation of variables

    Homework Statement dy/dx=-ylny/x initial conditions y(x=1)=1 express answer in the form f(x,y)=0 Homework Equations The Attempt at a Solution i let u=ln y du/dy=1/y y*du=dy subbing into equation gives int(y/yu du)=-int dx which is equal to int(1/u du)=-int(1/x) dx...
  18. C

    Seperation of variables - Product solutions for unsteady heat conduction

    Hey guys, I was wondering about problem 12C.1 in Transport phenomena by Bird, Stewart and lightfoot. The problem states that a block of material initially at uniform T0 is suddenely exposed to T1 at all surfaces. Assume a solution of T=X(x,t)Y(y,t)Z(z,t) any help with...
  19. S

    Seperation of variables, infinite cubic well

    Suppose one is to find the stationary states of a particle in an infinite cubic well. Inside the box the time independent SE is: - \frac{\hbar}{2m} \big( \frac{\partial ^2 \psi}{\partial x ^2 } + \frac{\partial ^2 \psi}{\partial z ^2 } + \frac{\partial ^2 \psi}{\partial z ^2 } \big)= E\psi...
  20. P

    When can seperation of variables be applied?

    I have some confusion about when separation variables can be applied to a PDE. Can it be applied on any PDE that can separated for any domain? If so, is the use of more "powerful" techniques simply used to save effort? (As you might have to use superposition several times!)
  21. B

    Newton's Law of Cooling (diff eq. - seperation of variables)

    Newton's Law of Cooling (diff eq. -- separation of variables) Homework Statement Fresh coffee sitting in a room cooling...you know the routine. Anyhow T(0) = 90degreesCelcius. Room temp=25degrees Celcius find k. Then he asks us to use Euler's method to estimate coffee temp after five...
  22. S

    Seperation of variables in the Schrodinger equation

    hey guys first post so sorry if this has already been asked :S what exactly is meant by the separation of variables in the schrodinger equation? also what co-ordinate system would i use to solve for an electron in a hydrogen-like ion? thanks
  23. V

    Seperation of variables / Alternative method to solve a DE

    Hej, This question is in the context of General Relativity problem. I'm attemping to compute the Killing Vectors for a Torus. After some juggling around I ended up with the following differential equation \frac{d}{d \theta} \left( \frac{ (a+b \cos \theta) \sin \theta }{b} F(\phi) + g(\theta)...
  24. N

    A PDE I can't solve by seperation of variables

    Homework Statement Homework Equations After simplification, the PDE is (b^2/a^2)(d^2 v/ d x^2) + (d^2 v/ d y^2) = -1 The Attempt at a Solution Obviously, it can't be solved by separation of variables. And I also failed in similarity solution.
  25. T

    Solving an ODE: The Pwer Series and Seperation of Variables

    Homework Statement Solve the following equation by a power series and also by separation of variables. Check that the two agree. Homework Equations N/A The Attempt at a Solution Power Series: (1+x) \frac{dy}{dx} = y (1+x) \frac{1}{dx} = y \frac{1}{dy} The power...
  26. T

    A seperation of variables intergration

    Homework Statement \frac{dV}{dt} = 20 - kV By solving this show that V = A + Be^{-kt} Homework Equations Well I am guessing there is a ln coming into play somewhere during the intergration if the diff the bottom = the top then you get a ln(bottom) The Attempt at a Solution...
  27. S

    Solve a 3-D parabolic equation with seperation of variables.

    Homework Statement Create a 3D parabolic equation and solve it with the method of separation of variables. Homework Equations None The Attempt at a Solution \frac{\partial T}{\partial t}=\alpha [\frac{\partial^{2} T}{\partial x^{2}}+\frac{\partial^{2} T}{\partial...
  28. T

    Seperation of variables (easy)

    Homework Statement show the solution to dy/dx = [1+(1/y)]^1/2 is given by ∫[y^1/2]/[1+y]^1/2 dy = x + c The Attempt at a Solution I know this will be solved by separation of variables... If i take the whole rhs over to the lhs I get... dy/[1+(1/y)]^1/2 = dx RHS no problems when...
  29. G

    How Do You Solve the Differential Equation (x-1)y'=6y?

    Homework Statement (x-1)y'=6y Could someone please help explain to me how to do this problem. Here's my attempt... (x-1)y'=6y (x-1)dy=6y (x-1)=6y/dy \int (x-1)dx=\int 6y/dy ((x^2,2)-x)= 3y^2+C Then I try to get it to equal y and it comes out nothing like the answer. Am I even doing these...
  30. C

    Seperation of variables in the 2 dimensional wave equation

    [SOLVED] Seperation of variables in the 2 dimensional wave equation I'd like to apologize right away for the terrible formatting. I was trying to make it pretty and easy to read but I guess I'm just not used the system yet and I had one problem after another. As you'll see at one point the...
  31. N

    Seperation of variables - first order PDE

    [SOLVED] Seperation of variables - first order PDE Homework Statement I have the expression X'(x)/X(x) = cx. How do I separate the variables? It's the fraction on the left side that annoys me. I know that X'(x) = d(X(x))/dx, but I can't use this here? EDIT: Sorry for the mis-spelled title...
  32. C

    Is Separation of Variables Valid for Solving Partial Differential Equations?

    when is the separation of variables technique for partial differential equations valid? it seems to give a particular general solution (such as a general Fourier series, or series of legendre polynomials) to a problem depending which coordinate system that you are in?
  33. F

    Seperation of Variables (double check please

    Seperation of Variables (double check please :) I have a final coming up, and I want to make sure I have this method down. Q: For the second-order wave equation u_{tt}=u_{xx}, the substitution of u=A(x)B(t) will give second-order equations for A nd B when the x and t variables are...
  34. F

    Can Separation of Variables Solve u_t = u_x for u=A(x)B(t)?

    This is the the first time I've encountered separation with partial differential equations. There are no worked examples, so I need some help to work through this problem. The question seems to be somewhat hand holding, since it seems to be THE introduction. Q: Apply separation of variables...
  35. JasonJo

    Help with DE problem and seperation of variables

    i found the DE for a problem, and it was y'(t) = .06 - y/1040 and then the problem gave me the following hint: Find the amount of sugar after t minutes. Note: When you solve by separation of variables, keep the coefficient of y on the right side and bring over to the denominator on the...
  36. M

    Solving by seperation of variables.

    I've got a few 1st order ODEs which I have problems solving. I am new to the subject and self-taught so I may have a little difficulty absorbing. The question is... 1. \frac {dy}{dx} = \frac {y^3}{x^2} for 1. I put it in the form, x^2 dy = y^3 dx \frac {dy}{y^3} = \frac...