International Journal of Engineering Trends and Technology (IJETT) - Volume4 Issue7- July 2013
Analytical Technique For Electromagnetic Field
Using Finite Element Method
Nilesh Yadav#1, Neeraj Kumar*2
#
Faculty of Engineering & Department of Electrical Engineering & Shri Satya
Sai Institute of Science & Technology SH-18 Bhopal Indore Road
Opp. Oil Fed Plant Sehore [M.P.]
Abstract— In this paper, we will introduce the methodical
procedure to effect the solution of the type of problem that is
governed by Poisson's or Laplace's equation in higher
dimensions. The technique that will be employed to obtain this
solution is finite difference method , finite element method and
method of moments. It is pedagogically convenient to introduce
the technique with an example and then carefully work through
the details.
Combining equation (1) with equation (4) and obtain
2V  
Keywords— 2 dimension , 3 dimension , FDM , MOM
I. INTRODUCTION
We study that a static electric field E would be created in a
vacuum from a volume charge distribution ρv. This physical
phenomenon was expressed through a partial differential
equation. We have been able to write this partial differential
equation using a general vector notation as
•E 
v
0
(1)
Which is Gauss’s law in a differential form. Here we have
applied a shorthand notation that is common for the vector
derivatives by using a vector operator ∇ called the “del
operator”, which in Cartesian coordinates is




ux  uy  uz
x
y
z
(2)
The static electric field is a conservative field which implies
that
 E  0
(3)
This means that the electric field could be represented as the
gradient of a scalar electric potential V
E  V
Recall the vector identity
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(4)
(5)
  V  0
v
0
(6)
where we have used the relation that ∇2 = ∇ • ∇2.Equation (5)
is called Poisson's equation[1]. If the charge density ρv in the
region of interest were equal to zero, then Poisson's equation
is written as
2V  0
(7)
These equations forms the basis for solving the boundary
value problems and harmonics associated with it.
II. FINITE DIFFRENCE METHOD USING MATLAB
There are different methods for the numerical solution of the
two dimensional Laplace’s and Poisson’s equation. Some of
the techniques are based on a differential formulation that was
introduced earlier[2]. The
Finite difference method is
considered here and the Finite element methodis discussed in
the next paper. Other techniques are based on the integral
formulation of the boundary value problems such as the
Method of momentsis described later. The boundary value
problems become more complicated in the presence of
dielectric interfaces which are also considered in this paper.
The finite difference method (FDM) considered
here is an extension of the method already applied to a onedimensional problem. This method allows MATLAB to be
more directly involved inthe solution of the boundary value
problems. We will discuss this method here using a problem
that is similar to that presented in eqution. We will describe
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the technique to obtain and to solve a suitable set of coupled
equations that can be interpreted as a matrix equation.
The algorithm that we use is based on the
approximation for the second derivatives in Cartesian
coordinates. In this case, we assume a square grid with a step
size h in both directions for a two-dimensional calculation
 2 V(x, y) 
1
 V  x  h, y   V  x  h, y 
h2 
 V  x, y  h   V  x, y  h   4V  x, y   (8)
This leads to the following “star shap” representation for a
two-dimensional Laplace’s equation as shown in Figure 1(a)
V0 
V1  V2  V3  V4
4
(b)
Figure 1. (a) The general five-point scheme, (b) The three-point scheme at the
corner.
(9)
The voltage at the center is approximated as being the
averageof the voltages at the four tips of the star. [4]For the
three-dimensional case, the square is replaced with a cube and
a seven-point scheme is applied. In this case, the coefficient
1/4 in equation (9) is simply replaced with 1/6.
However, in Poisson's equation, it will modify the charge
density that is to be evaluated at the central point (0). For the
special case of a corner point, this five point scheme has to be
modified to a three point one as shown in Figure 1(b). In this
case the principle of the average value simply gives[4]
V0 
(a)
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V1  V2
2
(10)
This principle can be applied iteratively for the computation
of the potential in the points of the square grid as shown in
Figure 2. This method is also called a relaxation method. After
computing the first iteration, we determine the potential at the
other points withinthe nine-point mesh. This will involve two
more iterations as shown in Figure 3. In the second iteration,
all of the potentials at the locations indicated by a solid circle
in Figure 3(a) are now known.[6] The values indicated by a
square are to be computed in this iteration using eqution (9).
In the third iteration, the values of the potential indicated by
the solid circles and squares are known from the previous
two iterations or as initial values in the calculation. Again
employing equation(9), the values of the potential at the
locations indicated by the diamonds can be computed. In this
mesh, it is assumed that the potentials at the boundaries are
alreadygiven in the statement of the problem, hence the
potentials at the locations indicated by the hollow circles are
also known as shown in Figure 3(b).
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This iterative procedure can continue until the computed
values at all of the points in the decreasing meshesbecome
closer to each other. The accuracyof the calculation can be
insured by repeating the calculation with a different initial
mesh size. A mesh with a shape and orientation that is
different than the one used here could also beemployed in a
numerical calculation.[8] This is particularly useful in
calculations involving unusual shapes. It is also possible to
scale the various dimensions in order to use this particular
mesh. A critical restrictionis also found on the square mesh
size in that the first point must be in the center of the square.
This point will be evaluated from the four boundaries of the
square. This will restrict the number of internal points N of the
square to contain the following number of points
Figure 2. The square grid in two dimensions in Cartesian coordinates.
1 2; 32; 72, 152; 312’ 632;… [2 N-1]2
(11)
This is called the array size.
III. METHOD OF MOMENT USING MATLAB
In the previous chapter, we found that the electric potential V
could be computed from a known charge distribution. This
was accomplished using integral
V  x, y,z  
(a)
(b)
Figure 3. The second and third iterations. (a) The values of the potential
indicated by the solid circles are known. The values at the locations of the
solid squares are computed in the second iteration. (b) The potentials at the
boundaries indicated by the hollow circles are assumed to be known. The
potentials at the locations indicated by the diamonds are computed in the
third iteration.
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1
4  0

V
 V  x ', y ',z ' 
R
dx 'dy 'dz '
(12)
where R is the distance between the charge located at the
point (x’,y’,z’) and the point of observation at the point (x,y,z).
If the charge distribution is known, then the potential can be
easily computed. We note that equation (12) can be converted
into a summation and hence the integral can be evaluated
numerically There are cases, however, where the potential
may actually be known and the charge distribution may be
unknown. Static fields abound with such problems. An
example would be the determination of an unknown surface
charge distribution on a conductor if the potential of the
conductor was specified. The technique that will be
introduced is called the “Method of Moments” and it will be
identified as “MoM”in the following discussion. This
technique will be very powerful in calculating the capacitance
of various metallic objects. It is also useful in calculating the
capacitance of a transmission line that will be encountered
later.[9] Finally, it is use determining the shapes of various
objects such as planes and rockets that may be impinging
upon a nation by correctly interpreting the reflected high
frequency signals from the objects by the observer.
Consider the configuration shown in Figure 3.
Four charges are located in space. A Cartesian coordinate
system isalso introduced and the location of the centers of the
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four charges are specified with reference to this coordinate
system. The potential at two of the charges (Q1 and Q2) is
specified to be V1 = V2 = -1 and the potential atthe other two
(Q3and Q4) is specified to be V3 = V 4 = +1. The value of the
individual charges is unknown. In order to obtain a unique
solution for the values, of these 4 charges, we must be able to
writedown 4 equations that will describe the potential at the 4
defined locations. We assume that the region is a vacuum and
we can use superposition. We write four linear equations for
the potentials at the four points
The four equations in (13) can also be written in matrix
notation. Remember that MATLAB was originally created in
order to solve problems of the type
[P][Q] = [V]
where [V] is the column vector of the known potentials, [Q] is
the column vector of the unknown charges and [P] is the
square matrix of coefficients





1 
P 
40 





Figure 3. Four charges distributed in space.The potential at the indicated
points are assumed to be
V = - 1 and V = + 1.
1 
1  Q1
Q2
Q3
Q4




40  r1  r1
r1  r2
r1  r3
r1  r4



Q3
1  Q1
Q2
Q4 
1 





40  r3  r1
r3  r2
r3  r3
r3  r4 
Q3
1  Q1
Q2
Q4 
1 





40  r4  r1
r4  r2
r4  r3
r4  r4 
1 
Q3
1  Q1
Q2
Q4




40  r2  r1
r2  r2
r2  r3
r2  r4



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4

j1
1
.Q j
ri  rj
1
r1  r1
1
r1  r2
1
r1  r3
1
r2  r1
1
r2  r2
1
r2  r3
1
r3  r1
1
r3  r2
1
r3  r3
1
r4  r1
1
r4  r2
1
r4  r3
1 
r1  r4 

1 

r2  r4 
1 

r3  r4 
1 

r4  r4 
(16)
This matrix is symmetric because the potential between the
charge and the point of observation depends upon the
magnitudeof the distance R between the two points.[11]
The diagonal terms of this matrix (i = j) appear to
give us problems since they become very large. These terms
are called singular.We remove this singularity with an
approximation.The approximation makes the assumption that
the potential at these singular points isevaluated at the edgeof
the spherical charge that has a radius a and not at the center. It
maintains that potential throughout the interior of the spherical
charge.[13] The diagonal elements of a matrix [P] are
(13)
Pi, j 
1
40 a
(17)
This P matrix is helpful in determining the boundary
conditions prevailing in electromagnetics.
IV. CONCLUSION
This can be written using the summation sign as
1
Vi 
40
(15)
(14)
Solving boundary value problems for potentials has led us to
certain general conclusions concerning the methodical
procedure. First, nature has given us certain physical
phenomena that can be described by partial or ordinary
differential equations. In many cases,these equations can be
solved analytically.Other cases may require numerical
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International Journal of Engineering Trends and Technology (IJETT) - Volume4 Issue7- July 2013
solutions. The analytical solutions contain constants of
integration. Nature also providesus with enough information
that will allow us to evaluate these constants and thus obtain
the solution for the problem.Assuming that neither
mathematical nor numerical mistakes have been made, we can
rest assured that this is the solution.
V. REFERENCES
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[4]
[5]
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[8]
[9]
[10]
[11]
[12]
[13]
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