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ECE 2317 Applied Electricity and Magnetism. Prof. D. Wilton ECE Dept. Notes 25. Notes prepared by the EM group, University of Houston. Magnetic Field. v.  r. N. S. q. This experimental law defines the magnetic flux density vector B. Lorentz Force Law:.

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slide1

ECE 2317

Applied Electricity and Magnetism

Prof. D. Wilton

ECE Dept.

Notes 25

Notes prepared by the EM group, University of Houston.

slide2

Magnetic Field

v

r

N

S

q

This experimental law defines the magnetic flux density vector B.

Lorentz Force Law:

In general, (with both E and B present):

The units of B are Webers/m2 or Tesla [T].

slide3

Magnetic Gauss Law

z

S (closed surface)

N

N

y

x

S

B

Magnetic pole (not possible) !

S

No net flux out !

slide4

Magnetic Gauss Law: Differential Form

Apply the divergence theorem:

Since this applies for any volume,

B field flux lines either close on themselves or at infinity!

slide5

Ampere’s Law

I

y

x

Iron filings

(exact value)

slide6

Ampere’s Law (cont.)

Note: the definition of one Amp is as follows:

1 [A] current produces:

So

slide7

Ampere’s Law (cont.)

Define:

H is called the “magnetic field”

The units of H are [A/m].

(for single infinite wire)

slide8

Ampere’s Law (cont.)

y

C

I

x

The current is inside a closed path.

slide9

y

C

I

x

Ampere’s Law (cont.)

The current is outside a closed path.

slide10

C

Ampere’s Law (cont.)

Hence

Ampere’s Law

Iencl

Although this law was “derived” for an infinite wire of current, the assumption is made that it holds for any shape current.

This is an experimental law.

“Right-Hand Rule”

slide11

Amperes’ Law: Differential Form

C

S

(from Stokes’ theorem)

S is a small planar surface

Since the unit normal is arbitrary,

slide12

Maxwell’s Equations (Statics)

electric Gauss law

magnetic Gauss law

Faraday’s law

Ampere’s law

In contrast to electric fields, the sources for magnetic fields produce a circulation and are vectors! !

slide13

Maxwell’s Equations (Dynamics)

electric Gauss law

magnetic Gauss law

Faraday’s law

Ampere’s law

slide14

Displacement Current

Ampere’s law:

A

“displacement current”

insulator

h

z

+ + + + + + + + + + +

(This term was added by Maxwell.)

Q

I

The current density vector that exists inside the lower plate.

slide15

Ampere’s Law: Finding H

  • 1) The “Amperian path” C must be a closed path.
  • 2) The sign of Iencl is from the RH rule.
  • 3) Pick C in the direction of H (as much as possible).
  • 4) Wherever there is a component of H along path C , it
  • must be constant. => Symmetry!
slide16

z

I

y

C

r

x

Example

Calculate H

First solve for H .

“Amperian path”

An infinite line current along the z axis.

slide17

z

I

y

C

r

x

Example (cont.)

slide18

Example (cont.)

Hd cancels

2)Hz = 0

Hzdz=0

I

h

C

 = 

3)H = 0

I

Magnetic Gauss law:

h

S

slide19

z

I

y

r

x

Example (cont.)

slide20

Example

y

coaxial cable

b

I

a

c

a

I

r

x

C

b

z

c

The outer jacket of the coax has a thickness of t = c-b

 < a

Note: the permittivity of the material inside the coax does not matter here.

b <  < c

slide21

y

b

a

r

x

C

c

Example (cont.)

The other components are zero, as in the wire example.

This formula holds for any radius, as long as we get Ienclcorrect.

slide22

y

b

a

r

x

C

c

Example (cont.)

 < a

a < < b

b < < c

 > c

Note: There is no magnetic field outside of the coax (“shielding property”).

slide23

y

b

a

r

x

C

c

Example (cont.)

 < a

a < < b

b < < c

 > c

Note: There is no magnetic field outside of the coax (“shielding property”).

slide24

Example

Solenoid

n = N/L= # turns/meter

a

Calculate H

z

L

I

Find Hz

 < a

h

C

slide25

 > a

Hz

C

h

Example (cont.)

slide26

Example (cont.)

The other components of the magnetic field are zero:

1)H = 0 since

C

2)H = 0from

S

slide27

n = N/L = # turns/meter

a

z

L

I

Example (cont.)

Summary:

slide28

Example

z

Calculate H

y

-

x

x

+

Infinite sheet of current

top view

y

slide29

Hx-

x

Hx+

y

Example (cont.)

(superposition of line currents)

(magnetic Gauss Law)

By symmetry:

slide30

Example (cont.)

w

-

2y

x

+

C

y

Note: No contribution from the left and right edges (the edges are perpendicular to the field).

slide31

Example (cont.)

Note: The magnetic field does not depend on y.

slide32

y

I

h

I

x

w

z

w >> h

Example

Find H everywhere

slide33

Example (cont.)

y

h

x

Two parallel sheets of opposite surface current

slide34

“bot”

“top”

y

I

h

-I

x

magnetic field due to top sheet

magnetic field due due to bottom sheet

Example (cont.)

Infinite current sheet approximation of finite width current sheets

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