CHAPTER 2
BASIC IN ELECTROMAGNETIC AND
NUMERICAL METHODS
The engineers or the sciences need to know the physic of electromagnetic field
that it permeates through all space. Electromagnetic filed comprises of electric field
and magnetic field. The electric field is produced by non-moving charges and the
magnetic field is produced by moving charges(currents) and depicts as the source of
the field. The way in which charges and currents interact with the electromagnetic
field is described by Maxwell’s equations and Lorentz Force Law [2].
The principle of electromagnetic and magnetostatics are used for designing the
permanent magnet synchronous motor in this thesis. Finite element methods are used
for the numerical computation of Maxwell’s equations.
2.1 BIOT – SAVART’S LAW
Following the Biot – Savart’s law, the mathematical relation between the
magnetic field intensity and the current which produces the filed is as follows [3]
2
12
1 1 12
2 4 R
dH I dL a π
= × Eq. 2-1
where Figure 2-1 identifies each term in the equation. Subscripts are included in
this introduction to the Biot – Savart law to clarify the location of each element.
Equation 2-1 is analogous to the Coulomb’s law equation for the electric field
resulting from a differential charge,
2
12
1 12
2 4 R
a dE dQ= πε Eq. 2-2
To get the total field resulting from a current can be found by integrating the
contributions from each segment by,
= ∫ × 4 R2
H IdL aR
π
Eq. 2-3
4
I1
dH2
P2
R12 = R12 a12
dL1
FIGURE 2-1 Illustration of the law of Biot-Savart showing magnetic field arising
from a differential segment of current
Progression from Equation 2-1 to 2-3 is possible because, just like electric
fields, they can be added by superposition.
2.2 AMPERE’S CIRCUITAL LAW
In electrostatic problems that feature considerable symmetry, it is easier to
apply Gauss’s law to solve for the electric field intensity than using the Coulomb’s
law. Likewise, in magnetostatic problems with sufficient symmetry, Ampère’s
circuital law can be applied more easily than the law of Biot-Savart.
D ds Q (Gauss's law integral form)
s
∫∫ ⋅ = − Eq. 2-4
Ampère’s circuital law says that the integration of H around any closed path is
equal to the net current enclosed by that path. This is state in equation form as
∫H⋅dL = Ienc (Ampère's law−integral form) Eq. 2-5
The line integral of H around a closed path is termed the circulation of H. The
path of the circulation does not matter in solving for the current enclosed, but in
practical application, a symmetrical current distribution is given and you want to
solve for H, so it is important to make a careful selection of an Amperian path
(analogous to a Gaussian surface) that is everywhere either tangential or normal to H,
and over which H is constant.
The direction of the circulation is chosen such that the right-hand rule is
satisfied. That is, with the thumb in the direction of the current, the fingers will curl in
the direction of the circulation.
Example : Infinite-length line current
Given a infinite-length line current I lying along the z -axis, use Ampère’s law
to determine the magnetic field by integrating the magnetic field around a circular
path of radius ρ lying in the x−y plane.
5
I
L
ρ
y
x
z
FIGURE 2-2 Amperian path around an infinite length of current
From Ampère’s law,
∫ ⋅ = ∫ ⋅ =
L L
H dl Hϕ dl I Eq. 2-6
by symmetry, the magnetic field is uniform on the given path so that
∫ = =
L
Hϕ dl Hϕ2πρ I Eq. 2-7
or
ϕ πρ 2
H = I Eq. 2-8
2.3 Maxwell’s equations
All four Maxwell’s equations for static fields have been defined in both integral
form and differential form. Maxwell’s equations for time varying fields contain
additional terms which form a compete set of coupled equations (all four equations
must be satisfied simultaneously). Four Maxwell’s equations are de-coupled into two
sets of two equations : two for electrostatic field and two for magnetostatic fields.
Maxwell’s equations for static fields are:
Integral form Differential form
∫∫ ⋅ = ∫∫∫ =
s v
D ds ρv dv Qenclosed ∇⋅D = ρ v Eq. 2-9
∫ ⋅ =
L
E dl 0 ∇×E = 0 Eq. 2-10
∫∫ ⋅ =
s
B ds 0 ∇⋅B = 0 Eq. 2-11
∫ ⋅ = ∫∫ ⋅ =
S
enclosed
L
H dl J ds I ∇×H = J Eq. 2-12
6
Equation 2-9 is called Gauss’s law (electric fields).
Equation 2-10 is called Faraday’s law.
Equation 2-11 is called Gauss’s law (magnetic fields).
Equation 2-12 is called Ampère’s law.
The differential, or point, form of Maxwell’s equations are easily derived by
applying the divergence theorem and Stoke’s theorem to the integral form of the
equations.
2.3.1 Differential operators in electromagnetic
The differential form of governing equations in electromagnetic (Maxwell’s
equations and related equations) are defined in terms of four different differential
operators: the gradient operator, the divergence operator, the Laplacian operator. All
of these operators can be defined in terms of the gradient (nabla ∇ ) operator.
Operators involving ∇ :
Note that the two operators that operate on vectors (divergence and curl) are the two
operators found in the differential form of Maxwell’s equations. Certain
characteristics of the vector fields in Maxwell’s equations can be determined based on
the divergence and curl results for these fields.
Characteristics of F based on ∇⋅F
Vectors with nonzero divergence (∇⋅F ≠ 0) vary in the direction of the field.
Vectors with zero divergence (∇⋅F =0) do not vary in the direction of the field.
The divergence of a vector F at a point P can be visualized by enclosing the
point by an infinitesimally small differential volume and examining flux vector in and
out of the volume. If there is a net flux out of the volume (more flux out of the volume
than into the volume), the divergence of F is positive at the point P. If there is a net
flux into the volume (more flux into the volume than out the volume), the divergence
of F is negative at the point P.
(∇⋅ ) > 0 p F (∇⋅ ) < 0 p F
P F
P F
Operator Example Operand Result
Gradient ∇V = − E scalar Vector
Divergence v ∇⋅D = ρ vector Scalar
Laplacian
ε
ρv
V
−
∇2 = scalar Scalar
Curl ∇×H = J vector Vector
7
If the net flux into the differential volume is zero (the flux into the volume
equals the flux out of the volume), the divergence of F is zero at the point P.
(∇⋅ ) = 0 P F
According to Gauss’s law for electric fields in differential form,
∇⋅D = ρ v Eq. 2-13
the divergence of the electric flux density is zero in a charge free region (ρv = 0) and
non zero in a region where charge is present. Thus, the divergence of the electric flux
density locates the source of the electrostatic field (net positive charge = net flux out
and net negative charge = net flux in).
According to Gauss’s law for magnetic fields in differential form,
∇⋅B = 0 Eq. 2-14
the divergence of the magnetic flux density is always zero since there is no magnetic
charge (net flux = 0).
Characteristics of F based on ∇×F
Vectors with nonzero curl (∇×F≠0 ) vary in a direction perpendicular to the direction
of the field.
Vectors with zero curl (∇×F = 0 ) do not vary in a direction perpendicular to the
direction of the field.
The curl of vector F at a point P can be visualized by inserting a small paddle
wheel into the field (interpreting the vector F as a force field) and see if the paddle
wheel rotates or not. If there is an imbalance of the force on the sides of the paddle
wheel, the wheel will rotate and the curl of F is in the direction of the wheel axis
(according to the right hand rule). If the forces on both sides are equal, there is no
rotation, and the curl is zero. The magnitude of the rotation velocity represents the
magnitude of the curl of F at P. The curl of the vector field F is therefore a measure
of the circulation of F around point P.
CHAPTER 2
BASIC IN ELECTROMAGNETIC AND
NUMERICAL METHODS
The engineers or the sciences need to know the physic of electromagnetic field
that it permeates through all space. Electromagnetic filed comprises of electric field
and magnetic field. The electric field is produced by non-moving charges and the
magnetic field is produced by moving charges(currents) and depicts as the source of
the field. The way in which charges and currents interact with the electromagnetic
field is described by Maxwell’s equations and Lorentz Force Law [2].
The principle of electromagnetic and magnetostatics are used for designing the
permanent magnet synchronous motor in this thesis. Finite element methods are used
for the numerical computation of Maxwell’s equations.
2.1 BIOT – SAVART’S LAW
Following the Biot – Savart’s law, the mathematical relation between the
magnetic field intensity and the current which produces the filed is as follows [3]
2
12
1 1 12
2 4 R
dH I dL a π
= × Eq. 2-1
where Figure 2-1 identifies each term in the equation. Subscripts are included in
this introduction to the Biot – Savart law to clarify the location of each element.
Equation 2-1 is analogous to the Coulomb’s law equation for the electric field
resulting from a differential charge,
2
12
1 12
2 4 R
a dE dQ= πε Eq. 2-2
To get the total field resulting from a current can be found by integrating the
contributions from each segment by,
= ∫ × 4 R2
H IdL aR
π
Eq. 2-3
4
I1
dH2
P2
R12 = R12 a12
dL1
FIGURE 2-1 Illustration of the law of Biot-Savart showing magnetic field arising
from a differential segment of current
Progression from Equation 2-1 to 2-3 is possible because, just like electric
fields, they can be added by superposition.
2.2 AMPERE’S CIRCUITAL LAW
In electrostatic problems that feature considerable symmetry, it is easier to
apply Gauss’s law to solve for the electric field intensity than using the Coulomb’s
law. Likewise, in magnetostatic problems with sufficient symmetry, Ampère’s
circuital law can be applied more easily than the law of Biot-Savart.
D ds Q (Gauss's law integral form)
s
∫∫ ⋅ = − Eq. 2-4
Ampère’s circuital law says that the integration of H around any closed path is
equal to the net current enclosed by that path. This is state in equation form as
∫H⋅dL = Ienc (Ampère's law−integral form) Eq. 2-5
The line integral of H around a closed path is termed the circulation of H. The
path of the circulation does not matter in solving for the current enclosed, but in
practical application, a symmetrical current distribution is given and you want to
solve for H, so it is important to make a careful selection of an Amperian path
(analogous to a Gaussian surface) that is everywhere either tangential or normal to H,
and over which H is constant.
The direction of the circulation is chosen such that the right-hand rule is
satisfied. That is, with the thumb in the direction of the current, the fingers will curl in
the direction of the circulation.
Example : Infinite-length line current
Given a infinite-length line current I lying along the z -axis, use Ampère’s law
to determine the magnetic field by integrating the magnetic field around a circular
path of radius ρ lying in the x−y plane.
5
I
L
ρ
y
x
z
FIGURE 2-2 Amperian path around an infinite length of current
From Ampère’s law,
∫ ⋅ = ∫ ⋅ =
L L
H dl Hϕ dl I Eq. 2-6
by symmetry, the magnetic field is uniform on the given path so that
∫ = =
L
Hϕ dl Hϕ2πρ I Eq. 2-7
or
ϕ πρ 2
H = I Eq. 2-8
2.3 Maxwell’s equations
All four Maxwell’s equations for static fields have been defined in both integral
form and differential form. Maxwell’s equations for time varying fields contain
additional terms which form a compete set of coupled equations (all four equations
must be satisfied simultaneously). Four Maxwell’s equations are de-coupled into two
sets of two equations : two for electrostatic field and two for magnetostatic fields.
Maxwell’s equations for static fields are:
Integral form Differential form
∫∫ ⋅ = ∫∫∫ =
s v
D ds ρv dv Qenclosed ∇⋅D = ρ v Eq. 2-9
∫ ⋅ =
L
E dl 0 ∇×E = 0 Eq. 2-10
∫∫ ⋅ =
s
B ds 0 ∇⋅B = 0 Eq. 2-11
∫ ⋅ = ∫∫ ⋅ =
S
enclosed
L
H dl J ds I ∇×H = J Eq. 2-12
6
Equation 2-9 is called Gauss’s law (electric fields).
Equation 2-10 is called Faraday’s law.
Equation 2-11 is called Gauss’s law (magnetic fields).
Equation 2-12 is called Ampère’s law.
The differential, or point, form of Maxwell’s equations are easily derived by
applying the divergence theorem and Stoke’s theorem to the integral form of the
equations.
2.3.1 Differential operators in electromagnetic
The differential form of governing equations in electromagnetic (Maxwell’s
equations and related equations) are defined in terms of four different differential
operators: the gradient operator, the divergence operator, the Laplacian operator. All
of these operators can be defined in terms of the gradient (nabla ∇ ) operator.
Operators involving ∇ :
Note that the two operators that operate on vectors (divergence and curl) are the two
operators found in the differential form of Maxwell’s equations. Certain
characteristics of the vector fields in Maxwell’s equations can be determined based on
the divergence and curl results for these fields.
Characteristics of F based on ∇⋅F
Vectors with nonzero divergence (∇⋅F ≠ 0) vary in the direction of the field.
Vectors with zero divergence (∇⋅F =0) do not vary in the direction of the field.
The divergence of a vector F at a point P can be visualized by enclosing the
point by an infinitesimally small differential volume and examining flux vector in and
out of the volume. If there is a net flux out of the volume (more flux out of the volume
than into the volume), the divergence of F is positive at the point P. If there is a net
flux into the volume (more flux into the volume than out the volume), the divergence
of F is negative at the point P.
(∇⋅ ) > 0 p F (∇⋅ ) < 0 p F
P F
P F
Operator Example Operand Result
Gradient ∇V = − E scalar Vector
Divergence v ∇⋅D = ρ vector Scalar
Laplacian
ε
ρv
V
−
∇2 = scalar Scalar
Curl ∇×H = J vector Vector
7
If the net flux into the differential volume is zero (the flux into the volume
equals the flux out of the volume), the divergence of F is zero at the point P.
(∇⋅ ) = 0 P F
According to Gauss’s law for electric fields in differential form,
∇⋅D = ρ v Eq. 2-13
the divergence of the electric flux density is zero in a charge free region (ρv = 0) and
non zero in a region where charge is present. Thus, the divergence of the electric flux
density locates the source of the electrostatic field (net positive charge = net flux out
and net negative charge = net flux in).
According to Gauss’s law for magnetic fields in differential form,
∇⋅B = 0 Eq. 2-14
the divergence of the magnetic flux density is always zero since there is no magnetic
charge (net flux = 0).
Characteristics of F based on ∇×F
Vectors with nonzero curl (∇×F≠0 ) vary in a direction perpendicular to the direction
of the field.
Vectors with zero curl (∇×F = 0 ) do not vary in a direction perpendicular to the
direction of the field.
The curl of vector F at a point P can be visualized by inserting a small paddle
wheel into the field (interpreting the vector F as a force field) and see if the paddle
wheel rotates or not. If there is an imbalance of the force on the sides of the paddle
wheel, the wheel will rotate and the curl of F is in the direction of the wheel axis
(according to the right hand rule). If the forces on both sides are equal, there is no
rotation, and the curl is zero. The magnitude of the rotation velocity represents the
magnitude of the curl of F at P. The curl of the vector field F is therefore a measure
of the circulation of F around point P.
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