A Green's function for problems involving linear isotropic media

AI Thread Summary
The discussion focuses on solving the problem of a sphere of isotropic dielectric media in a uniform electric field, as presented in Griffiths' textbook. The approach involves modeling the permittivity with a piecewise function and deriving Poisson's equation. The use of Green's functions and the image-charge method is emphasized to find the potential, particularly distinguishing cases for charges inside and outside the sphere. A multipole expansion is suggested for cases with an asymptotically constant electric field, noting that only dipole terms are necessary due to symmetry. A reference to a manuscript in German is provided for further reading on the discussed methods.
spin_100
Messages
15
Reaction score
1
TL;DR Summary
Is there a way to tackle problems involving linear isotropic dielectric media with permittivity separated by a boundary directly using an appropriate green's function? I am studying electrodynamics from Jackson's electrodynamics and after learning about the power of using green's function to solve boundary value problems, I was wondering if there is something similar to this for dielectric media. I have shown my approach here but I am stuck at a point.
I am considering a simple problem of a sphere of isotropic dielectric media (permittivity ## \epsilon ## and Radius ##R##) placed in a uniform electric field ## E_0 ## (z-direction). The problem is from Griffiths Chapter 4, example 7.
Since, it is a linear dielectric material, ## D = \epsilon E ## Since there is a discontinuity in ## \epsilon ##
We can model ## \epsilon (r) = \epsilon \theta (R-r) + \epsilon_0 \theta (r-R)##
Taking the divergence of D and since there are no free charges. (external charges) we get $$ 0 = \epsilon(r) \nabla \cdot E + \delta (R-r) ( \epsilon_0 - \epsilon ) E \cdot r$$

Then, we get the possion's equation $$ \nabla \cdot E = - \delta (R-r) ( \epsilon_0 - \epsilon ) E \cdot r $$ After that I don't know how to proceed further. I solved for the potential using an appropriate green's function but the result I am getting is wrong.
 
Last edited:
Physics news on Phys.org
You can get the Green's function using the image-charge concept. The Green's ##G(\vec{x},\vec{x}')## function is given by the solution of the problem having a "unit charge" at ##\vec{x}'##. Then you need an image charge (distinguishing the cases when ##\vec{x}'## is inside or outside the sphere). The case of a homogeneous electric field is given by taking two charges ##\pm Q## to infinity and moving them to ##\pm \infty \vec{e}_z## (for the asymptocally constant electric field in the ##z## direction).

You can also solve the problem with the asymptotically constant electric field by making a multipole-expansion ansatz. From symmetry it's a priori clear that you only need to go to the dipole order.
 
vanhees71 said:
You can get the Green's function using the image-charge concept. The Green's ##G(\vec{x},\vec{x}')## function is given by the solution of the problem having a "unit charge" at ##\vec{x}'##. Then you need an image charge (distinguishing the cases when ##\vec{x}'## is inside or outside the sphere). The case of a homogeneous electric field is given by taking two charges ##\pm Q## to infinity and moving them to ##\pm \infty \vec{e}_z## (for the asymptocally constant electric field in the ##z## direction).

You can also solve the problem with the asymptotically constant electric field by making a multipole-expansion ansatz. From symmetry it's a priori clear that you only need to go to the dipole order.
Please provide a reference where such a method is discussed.
 
This is from Griffiths' Electrodynamics, 3rd edition, page 352. I am trying to calculate the divergence of the Maxwell stress tensor. The tensor is given as ##T_{ij} =\epsilon_0 (E_iE_j-\frac 1 2 \delta_{ij} E^2)+\frac 1 {\mu_0}(B_iB_j-\frac 1 2 \delta_{ij} B^2)##. To make things easier, I just want to focus on the part with the electrical field, i.e. I want to find the divergence of ##E_{ij}=E_iE_j-\frac 1 2 \delta_{ij}E^2##. In matrix form, this tensor should look like this...
Thread 'Applying the Gauss (1835) formula for force between 2 parallel DC currents'
Please can anyone either:- (1) point me to a derivation of the perpendicular force (Fy) between two very long parallel wires carrying steady currents utilising the formula of Gauss for the force F along the line r between 2 charges? Or alternatively (2) point out where I have gone wrong in my method? I am having problems with calculating the direction and magnitude of the force as expected from modern (Biot-Savart-Maxwell-Lorentz) formula. Here is my method and results so far:- This...
Back
Top