Delta Definition and 1000 Threads

  1. H

    How to Compute an Integral Involving a Delta Function and Sine?

    Homework Statement Compute ∫^{∞}_{-∞}dx (x2+a2)-1δ(sin(2x)), without calculating the resulting sum. Homework Equations This is a very specific integral which ,has a delta function δ operating on sin function The Attempt at a Solution Does anyone know this integral ? I...
  2. M

    Understanding the Delta Function: Exploring the Role of k in Equations

    in this equation why does k take on both positive and negative values? isn't k a fixed constant that can only be positive or negative
  3. TheFerruccio

    Question regarding Kronecker Delta

    Question regarding Kronecker Delta and index notation I am reading a book which covers the Kronecker delta and shows some examples of how it works. One of the examples confuses me, because it seems to be impossible. This book uses the notation that a repeated index is a summation over the...
  4. P

    The Double Dirac Delta Function Potential wave functions

    Homework Statement Consider the double Dirac delta function V(x) = -α(δ(x+a) + δ(x-a)). Using this potential, find the (normalized) wave functions, sketch them, and determine the # of bound states. Homework Equations Time-Independent Schrodinger's Equation: Eψ = (-h^2)/2m (∂^2/∂x^2)ψ +...
  5. P

    A question about Dirac Delta Potential Well solution

    In Griffith's Introduction to Quantum Mechanics, on page 56, he says that for scattering states (E > 0), the general solution for the Dirac delta potential function V(x) = -aδ(x) (once plugged into the Schrodinger Equation), is the following: ψ(x) = Ae^(ikx) + Be^(-ikx), where k = (√2mE)/h...
  6. Y

    Question about Dirac Delta function

    In page 555, Appendix B of Intro to electrodynamics by D Griffiths: \nabla\cdot \vec F=-\nabla^2U=-\frac{1}{4\pi}\int D\nabla^2\left(\frac{1}{\vec{\vartheta}}\right)d\tau'=\int D(\vec r')\delta^3(\vec r-\vec r')d\tau'=D(\vec r) where ##\;\vec{\vartheta}=\vec r-\vec r'##. Is it supposed to be...
  7. Y

    How Do I Prove Symmetry and Scaling Properties of the Dirac Delta Function?

    I want to proof (1)##\delta(x)=\delta(-x)## and (2) ## \delta(kx)=\frac{1}{|k|}\delta(x)## (1) let ##u=-x\Rightarrow\;du=-dx## \int_{-\infty}^{\infty}f(x)\delta(x)dx=(0) but \int_{-\infty}^{\infty}f(x)\delta(-x)dx=-\int_{-\infty}^{\infty}f(-u)\delta(u)du=-f(0) I cannot proof (1) is equal as I...
  8. Y

    Question on Dirac Delta function in Griffiths

    My question is in Griffiths Introduction to Electrodynamics 3rd edition p48. It said Two expressions involving delta function ( say ##D_1(x)\; and \;D_2(x)##) are considered equal if: \int_{-\infty}^{\infty}f(x)D_1(x)dx=\int_{-\infty}^{\infty}f(x)D_2(x)dx\;6 for all( ordinary) functions f(x)...
  9. T

    Why Don't Phases Short Out in Three Phase Delta Transformers?

    Dumb question - if the phases are tied together in a three phase delta transformer why don't the phases short out against each other in the same way the transmission lines would fault phase to phase if the touched each other in the overhead? I think I understand the wye winding situation in...
  10. Seydlitz

    Variation of Epsilon Delta Proof

    Homework Statement Prove that if ##\left |x-x_{0} \right | < \frac{\varepsilon }{2}## and ##\left |y-y_{0} \right | < \frac{\varepsilon }{2}## then ##|(x+y)-(x_0+y_0)| < \varepsilon ## and ##|(x-y)-(x_0-y_0)| < \varepsilon ##Homework Equations Postulate and proof with real numbers as well...
  11. ryanuser

    What is the Difference Between Delta x and Just x?

    Why would we use delta sign such as Delta x but we can just call the missing part x rather than delta x, I mean what's the difference between using delta x and just x?
  12. A

    How Do You Determine the Wave Function Across a Delta Potential Barrier?

    Homework Statement V(x) = \begin{cases} \infty & , & x<0 \\ -g*\delta (x-a) & , & x>0 \end{cases} \text{ }Write the wave function in the left side of x=a and right side of x=a (BOUND STATE) The Attempt at a SolutionIn the right side o x=a i would say that it is \psi...
  13. R

    Differentiating delta function composed with a function

    Dear all, I just wondered whether there was any standard identity to help me solve this equation: $$ \int \delta(f(x))^{\prime\prime}g(x) dx $$ Thanks in advance for your help.
  14. D

    Dirac Delta Function: Explanation & Usage

    I know this probably belongs in one of the math sections, but I did not quite know where to put it, so I put it in here since I am studying Electrodynamics from Griffiths, and in the first chapter he talks about Dirac Delta function. From what I've gathered, Dirac Delta function is 0 for...
  15. A

    Delta V as a Function of Altitude

    Greetings everyone! I have a question on how the delta-V required to reach different orbits is determined. I refer to lift-off delta-V. I'm curious to find the relationship between altitude and delta V required to get to the height. From what I have found out, the lift-off delta v to a...
  16. C

    Quick question about Kronecker delta function

    So in the infinite square well, the eigenfunctions are ## \psi_n (x) = \sqrt{\dfrac{2}{a}} \sin \left( \dfrac{n \pi}{a} x \right) ## Each state is orthogonal to each other, and so ## \displaystyle \int \psi_m (x) ^* \psi_n (x) dx = \delta_{mn} ## Does this also hold if they were cosines?
  17. F

    Formal definition of Dirac's delta and charge densities

    Hi, I'm new to this forum but I've been aware of its existence for a while and it's pretty cool. I finally came up with a question to post so here i am :) I've read a few nice posts on this forum about this topic, but I couldn't find the answer to what I'm looking for. I'm familiar with the...
  18. S

    Real analysis epsilon delta problem

    I've been reading through Spivak's calculus, and the problem is the answer key i have a hold of is for a different edition so it often doesn't answer the correct questions. Anyways, here they are: Chapter 5 problem 10 b. Prove that lim x-> 0 f(x) = lim x-> a f(x-a) c. Prove that lim...
  19. Goddar

    Can I Solve This Complicated Equation with a Dirac Delta Function?

    Hi there, my version of Mathematica may be too old and I'm not finding this one by hand so any help would be appreciated: ψ''(z)=[k2/4 –M2 –kδ(z)]ψ(z), where δ(z) is the Dirac delta, k and M constants. i can solve the same equation without the M^2 term by exp(k|z|/2), but this one proves to...
  20. M

    Solve the Mystery of Y or Delta 3 Phase

    Hello, For our final lab we will be given a box with three wires, the neuter is missing. In the box there is a Y or delta configuration and we can use everything to find out if it is a Y or a delta configuration. So, voltmeters or watt meters, or even something like tin foil. Can someone...
  21. T

    Defiunition of kroneker delta as a tensor

    hi, the delta symbol as a tensor (in the minkovski space, in case one has to be specific), what is it exactly? is it \delta^a_b = \frac{\partial{x^a}}{\partial{x^b}} is it \delta^a_b = g^{ac} g_{cb}or is there some other definition?thanks
  22. N

    Vectors delta velocity calculations

    Hello, Two waves originated from point A and point B, waves are 'velocity-direction-dependent', can be treated as 'vectors', it took t2 time for the wave from point B to reach A, and it took time t2 for the wave from point B to reach A. Given the fact the two vectors have the same...
  23. S

    Archived Calculus for delta function based on wave function

    Homework Statement Two-electron Wavefunction: ψ(r1,r2,r12) = exp(-Ar1-Br2-Cr12), r12 = |r1-r2| A, B, and C are coefficients Calculate <ψ|δ(r12)|ψ> and <ψ|δ(r1)|ψ> Homework Equations NO The Attempt at a Solution <ψ|δ(r12)|ψ> = ∫∫dv1dv2ψ2(r1,r2,r12)δ(r12)...
  24. R

    How can boundary conditions be written for a DEQ with Dirac delta?

    Hi All, so I'm trying to tackle this DEQ: f''[x] = f[x] DiracDelta[x - a] - b, with robin boundary conditions f'[0] == f[0], f'[c] == f[c] where a,b, and c are constants. If you're curious, I'm getting this because I'm trying to treat steady state in a 1D diffusion system where...
  25. V

    Solving Delta Connected Load: Find Phase Current in PU and SI Units

    A balanced 3 phase load is rated at S=10KVA and 500V. The device is operating at 90% nominal voltage and 100 % line current. The question is: if the load is delta connected, find the phase current in per unit and SI units. I calculated correctly that the per unit line current value is...
  26. psparky

    Why do we sometimes choose delta over wye for transformers and generators?

    Speaking of delta and wye transformers and generators, why do we sometimes use one over the other? The WYE's seem easier to ground...but why pick a delta over a wye...or a wye over a delta in certain situations?
  27. B

    Double Delta function potential well

    Consider a one-dimensional system described by a particle of mass m in the presence of a pair of delta function wells of strength Wo > 0 located at x = L, i.e. V(x) = -Wo (x + L) - Wo(x - L) This is a rough but illuminating toy model of an electron in the presence of two positive. charges...
  28. H

    Divergence of inverse square field and Dirac delta

    \nabla \cdot \frac{\mathbf{r}}{|r|^3}=4 \pi \delta ^3(\mathbf{r}) What's the proof for this, and what's wrong with the following analysis? The vector field \frac{\mathbf{r}}{|r|^3}=\frac{1}{r^2}\hat{r} can also be written \mathbf{F}=\frac{x}{\sqrt{x^2+y^2+z^2}^3}\hat{x}+...
  29. F

    How Can Mathematical Modeling Describe Oscillations in a Weighted String System?

    A string of length L are connected in x = 0 and x = L. The point x = a, 0<a<L, are a point formed weight with mass m0 attached. Formulate the mathematical problem for small transversal oscillations . I got it to be: ∴ m0*utt(a,t) = s(a,t)(ux(a+,t)-ux(a-,t)) u(x,0) = g(x) ut(x,0) = h(x)...
  30. F

    Confusion regarding delta definition of limit

    I don't quite get the significance of the delta limit definition, if n>N and |sn−s|< ϵ , why does the limit converges does this simply means that there exist a number ε such that if n is great enough it will be greater than s by ε? But this doesn't make sense, because s is the value...
  31. A

    Exploring the Properties of the Dirac Delta Function

    Prove that. \int_a^b f(x)g' (x)\, dx = -f(0) This is supposed to be a delta Dirac function property. But i can not prove it. I thought using integration by parts. \int_a^b f(x)g' (x)\, dx = f(x)g(x) - \int_a^b f(x)'g (x)\, dx But what now? Some properties: \delta...
  32. ShayanJ

    Is Dirac Delta a Function or a Distribution?

    In texts about dirac delta,you often can find sentences like "The delta function is sometimes thought of as an infinitely high, infinitely thin spike at the origin". If we take into account the important property of dirac delta: \int_\mathbb{R} \delta(x) dx=1 and the fact that it is zero...
  33. J

    Integral of delta function multiplied by a Heaviside step function

    Homework Statement consider two functions:ψ(x) which is eqaul to zero at a,that is ψ(a)=0 and f(x)=H(x-a)*β(x)+(1-H(x-a))*γ(x) where H(x-a) is the heaviside step function and β(x),γ(x) is the continuous function. it seems that the derivative of f(x) is not exist. the question is whether...
  34. Avatrin

    Epsilon delta continuity of 1/x at x=1

    This isn't really homework; It's just something that has been bothering me ever since I first learned calculus because I suck at epsilon-delta proofs. Homework Statement Show that 1/x is continuous at x=1 Homework Equations If |x-a|<δ Then |f(x)-f(a)|<ε The Attempt at a Solution...
  35. fluidistic

    Writing up the charge distribution with Dirac's delta

    Homework Statement In electrostatics it's useful to have ##\rho (\vec x )## written with Dirac's delta so that we can know the total charge by integrating the charge distribution over a region of space. Many problems/situations deal with point charges. In Cartesian coordinates for example...
  36. B

    Quick question about Dirac delta functions

    What does the square of a Dirac delta function look like? Is the approximate graph the same as that of the delta function?
  37. F

    Taylor series expansion of Dirac delta

    I'm trying to understand how the algebraic properties of the Dirac delta function might be passed onto the argument of the delta function. One way to go from a function to its argument is to derive a Taylor series expansion of the function in terms of its argument. Then you are dealing with...
  38. T

    Is that star load will make unbalance load, delta will not?

    is that star load will make unbalanced , delta will not? THX
  39. L

    Calculate scattering amplitude by delta function potential

    Homework Statement I need to give scattering amplitude f(θ) in Born approximation to the first order in the case of delta function scattering potential δ(r). The problem is in spherical coordinate and I'll give major equation concerned.Homework Equations The equation for scattering amplitude is...
  40. L

    Limit of integral lead to proof of convergence to dirac delta

    Hi, I try to prove, that function f_n = \frac{\sin{nx}}{\pi x} converges to dirac delta distribution (in the meaning of distributions sure). On our course we postulated lemma, that guarantee us this if f_n satisfy some conditions. So I need to show, that \lim_{n\rightarrow...
  41. J

    A question about Dirac delta function

    Hello, Is this correct: \int [f_j(x)\delta (x-x_i) f_k(x)\delta (x-x_i)]dx = f_j(x_i)f_k(x_i) If it is not, what must the left hand side look like in order to obtain the right handside, where the right hand side multiplies two constants? Thanks!
  42. J

    Simple equations in Dirac Delta function terms

    Hi there, I'm trying to comprehend Dirac Delta functions. Here's something to help me understand them; let's say I want to formulate Newton's second law F=MA (for point masses) in DDF form. Is this correct: F_i = \int [m_i\delta (x-x_i) a_i\delta (x-x_i)]dx Or is it this: F_i = [\int...
  43. J

    MHB How to Expand and Simplify the Expression of Kronecker Delta?

    Hi, I'm working on a problem stated as: Expand the following expression and simplify where possible $$ \delta_{ij}\delta_{ij} $$ I'm pretty sure this is correct, but not sure that I am satisfying the expand question. I'm not up to speed in linear algebra (taking a continuum mechanics course) -...
  44. B

    Integrating the Dirac Delta function

    Homework Statement I am trying to integrate the function \int _{-\infty }^{\infty }(t-1)\delta\left[\frac{2}{3}t-\frac{3}{2}\right]dt Homework Equations The Attempt at a Solution I think the answer should be \frac{5}{4} because \frac{2}{3}t-\frac{3}{2}=0 when t=9/4. then (9/4-1) = 5/4...
  45. A

    The meaning of the delta dirac function

    Homework Statement For a function ρ(x,y,z) = cδ(x-a), give the meaning of the situation and describe each variable. Homework Equations As far as units go, I know that: ρ(x,y,z) = charge density = C/ m^3 δ(x-a) = 1/m and if those two are correct, then b must have units of (C/m^2)...
  46. elfmotat

    Dirac Delta Function: Does it Have a Residue?

    Does the Dirac delta fuction have a residue? Given the close parallels between the sifting property and Cauchy's integral formula + residue theory, I feel like it should. Unfortunately, I have no idea how they tie together (if they do at all).
  47. G

    Dirac Delta Integral: Compute \int_ {-\infty}^\infty e^{ikx}δ(k^2x^2-1)dx

    Homework Statement compute the integral: \int_ {-\infty}^\infty \mathrm{e}^{ikx}\delta(k^2x^2-1)\,\mathrm{d}xHomework Equations none that I have The Attempt at a Solution I don't actually have any work by hand done for this because this is more complex than any dirac delta integral I have...
  48. G

    Dirac delta function how did they prove this?

    Hi all, I'm familiar with the fact that the dirac delta function (when defined within an integral is even) Meaning delta(x)= delta(-x) on the interval -a to b when integral signs are present I want to prove this this relationship but I don't know how to do it other than with a limit...
  49. Z

    Is Dirac's Delta to the fourth power equal to 0.5?

    hello, Please attached snapshot. Does the integral 7.9 equal to 0.5 \inthμσ dzμ/dτ dzσ/dτdτ ? I'm confused as to the fourth power of Dirac's Delta. Then where does the derivative on x go ? For more on this, see MTW pg180 thanks
  50. R

    Delta Transformers: How 2 Hot Phases Keep From Blowing Up

    How is that two hot phases can be put into a transformers and it not blow up? i have asked this question many different times to different engineers at work and I'm tired of hearing "just because". please explain in detail
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