1. Magnetic and Electromagnetic
DR. MOHD IRFAN HATIM MOHAMED DZAHIR

2. What you should know at the end of this chapter
Magnetic field
Magnetic Flux
Flux Density
Permeability
Different magnetic materials
Reluctance
Ohms Law for Magnetic Circuits
Magnetizing Force
Hysteresis
Ampere’s Circuital Law
Airgap
Faradays Law
Lenz’s Law

3. Application of magnetic effects

4. Speaker
The shape of pulsating waveform of the input current is determined by the sound to be reproduced by the speaker.
The higher the pitch of the sound pattern, the higher the oscillating frequency between the peaks and valleys resulting higher frequency of the vibration of the cone.

5. Coaxial High-Fidelity Loudspeaker
(b) Basic operation
(a) Overall view
(a) Cross-sectional view

6. Hall Effect Sensor
(b) Effect on electron flow
(a) Orientation of controlling
Hall effect sensor is a semiconductor device that generates an output voltage when exposed to the magnetic field.
The difference in potential is due to separation of charge established by the Lorentz force.
The direction of force can be determined by left-hand rule.

7. Bicycle Speed Indicator
Use as sensor for alarm systems

8. Magnetic Field Magnetic field Magnetic flux lines (Φ)
exists in the region surrounding a permanent magnet
can be represented by magnetic flux lines
Magnetic flux lines (Φ)
Representation of magnetic field.
do not have origins or terminating points
exist in continuous loops
radiate from the north pole to the south pole
returning to the north pole through the metallic bar

9. Magnetic Field
The strength of a magnetic field in a particular region is directly related to the density of flux lines in that region
Magnetic field strength at point a is twice that at point b since twice as many magnetic flux lines are associated with the perpendicular plane at point a than at point b.

10. If like poles are brought together, the magnets repel
Magnetic Field
Continuous magnetic flux line will strive to occupy as small an area as possible.
This results in magnetic flux lines of minimum length between the unlike poles
If unlike poles of two permanent magnets are brought together, the magnets attract
If like poles are brought together, the magnets repel

11. Magnetic Field
If a nonmagnetic material, such as glass or copper, is placed in the flux paths surrounding a permanent magnet, an almost unnoticeable change occurs in the flux distribution
if a magnetic material, such as soft iron, is placed in the flux path, the flux lines pass through the soft iron rather than the surrounding air because flux lines pass with greater ease through magnetic materials than through air.

12. Magnetic Field
If a nonmagnetic material, such as glass or copper, is placed in the flux paths surrounding a permanent magnet, an almost unnoticeable change occurs in the flux distribution
if a magnetic material, such as soft iron, is placed in the flux path, the flux lines pass through the soft iron rather than the surrounding air because flux lines pass with greater ease through magnetic materials than through air.

13. Magnetic Field
The previously stated principle is used in shielding sensitive electrical elements and instruments that can be affected by stray magnetic fields

14. Magnetic Field
A magnetic field is present around every wire that carries an electric current
Right-hand rule can be used to determine the direction of magnetic flux line

15. Magnetic Field
If the conductor is wound in a single-turn coil the resulting flux flows in a common direction through the center of the coil.
A coil of more than one turn produces a magnetic field that exists in a continuous path through and around the coil

16. Magnetic Field
The field strength of the coil can be effectively increased by placing certain materials, such as iron, steel, or cobalt, within the coil to increase the flux density within the coil
The whole concept  electromagnetic

17. Magnetic Field 2 type of magnets Electromagnet Permanent magnet
A material such as steel or iron that will remain magnetized for long periods of time without the aid of external means.
Electromagnet
Electromagnet
Magnetic effects introduce by the flow of charge or current.
Flux distribution is quite similar to permanent magnet
Have north and south pole
Concentration of flux line is less than that of permanent magnet
Field strength may be increase by placing a core made of magnetic materials (iron, steel, cobalt)
Parameters affecting field strength
Currents
Number of turn
Material of the core
Without core
With core

18. Magnetic Field Right Hand Rule Case 1 Case 2
Thumb : Direction of current flow
Other fingers : Direction of magnetic flux
Case 2
Thumb : Direction of magnetic flux
Other fingers : Direction of current flow

19. Magnetic Flux Representation of magnetic field.
Group of force lines going from the north pole to the south pole
In the SI system of units, magnetic flux is measured in webers (Wb) and is represented using the symbol phi (𝚽).
1 Weber = 108 lines
Similar to current in electric circuit

20. Flux Density The number of flux lines per unit area Use symbol ‘B’
Measured in Tesla (T)
Magnitude of flux density
If 1 weber of magnetic flux passes through an area of 1 square meter, the flux density is 1 tesla.

21. Example 1
Find the flux and the flux density in the two magnetic cores shown in following figure. The diagram represents the cross section of a magnetized material. Assume that each dot represents 100 lines or 1 µWb.

22. Example 1 For figure a Flux is simply the number of lines
Finding flux density

23. Example 1 For figure b Finding flux density
Note : the core with the largest flux does not necessarily have the highest flux density.

24. Example 2
If the flux density in a certain magnetic material is 0.23 T and the area of the material is 0.38 in2 , what is the flux through the material? Convert the area to m2
1 m = inch

25. Magnetomotive force
External force or 'Pressure' required to set up the magnetic flux lines within the magnetic material.
The cause of a magnetic field
Similar to the applied voltage in electric circuit
Measured in ampere-turns (At)
The magnetomotive force (mmf),  is proportional to the product of the number of turns around the core (in which the flux is to be established) and the current through the turns of wire

26. Permeability Definition
the measure of the ability of a material to support the formation of a magnetic field within itself
degree of magnetization that a material obtains in response to an applied magnetic field.
Measure of the ease in which magnetic flux lines can be established in the material
Ability of magnetic material to conduct flux

27. Permeability Permeability of air (free space) Relative permeability
The ratio of the permeability of a material to that of free space

28. Nonmagnetic materials
Permeability
Material
Description
Example µr
Nonmagnetic materials
Permeability same as that of free space
copper, aluminium, glass, air and wood
µr = 1
Diamagnetic
Permeability slightly less than that of free space.
Bismuth, pyrolitic carbon
µr < 1
Paramagnetic
Permeability slightly more than that of free space.
magnesium, molybdenum, lithium, and tantalum
1 < µr < 100
Ferromagnetic
materials have a very high level permeability
Iron, nickel, steel and alloys of these materials
µr  100

29. Reluctance
The reluctance of a material to the setting up of magnetic flux lines in the material
Unit : Ampere-turns / Weber
Compare this to the resistance in electric circuit

30. Ohm’s Law for Magnetic Circuit
Recall
Effect = Flux
Cause = Magnetomotive force
Opposition = Reluctance
For Magnetic Circuit

31. Example 3
Calculate the reluctance of a torus (a doughnut-shaped core) made of low-carbon steel. The inner radius of the torus is 1.75 cm and the outer radius of the torus is 2.25 cm. Assume the permeability of low- carbon steel is 2X10-4 Wb/ At m Solution:
The length is equal to the circumference of the torus measured at the average radius a b c

32. Example 4
Mild steel has a relative permeability of 800. Calculate the reluctance of a mild steel core that has a length of 10 cm and has a cross section of 1.0 cm X 1.2 cm. Solution:

33. Magnetizing Force Magnetomotive force per unit length
Also called magnetic field intensity
Symbol : H
Independent of the type of core material
determined solely by the number of turns, the current, and the length of the core.

34. B-H Relationship
Flux density (B) and magnetizing force are related by the equation
However, we know that So

35. Hysterisis
Hysteresis is a characteristic of a magnetic material whereby a change in magnetization lags the application of the magnetic field intensity.
The magnetic field intensity (H) can be readily increased or decreased by varying the current through the coil of wire, and it can be reversed by reversing the voltage polarity across the coil.
In other word, hysteresis is the lagging effect between the flux density, B of a material and the magnetizing force, H applied.

36. Hysterisis
Series magnetic circuit used to define the hysteresis curve.

37. Hysterisis
The entire curve (shaded) is called the hysteresis curve.

38. Hysterisis
The flux density B lagged behind the magnetizing force H during the entire plotting of the curve. When H was zero at c, B was not zero but had only begun to decline. Long after H had passed through zero and had equaled to –Hd did the flux density B finally become equal to zero

39. Hysteresis
If the entire cycle is repeated, the curve obtained for the same core will be determined by the maximum H applied

40. Normal magnetization curve for three ferromagnetic materials.

41. Magnetic Equivalent Circuit
Magnetic circuit
Electric circuit

42. Ampere’s Circuital Law
The algebraic sum of the rises and drops of the mmf around a closed loop of a magnetic circuit is equal to zero. Or
The sum of the rises in mmf equals the sum of the drops in mmf around a closed loop.
Similar to KVL in electric circuit

43. Ampere’s Circuital Law
Steel
Cobalt
Iron

44. Flux 
The sum of the fluxes entering a junction is equal to the sum of the fluxes leaving a junction
Similar to KCL in electric circuit

45. Series Magnetic Circuit
2 types of problem:
 is given, and the impressed mmf, NI must be computed – design of motors, generators and transformers
NI is given, and the flux  of the magnetic circuit must be found – design of magnetic amplifiers
B-H curve is used
to find H if B is given
to find B if H is given

46. Example 5: Series Magnetic Circuit

47. Example 5: Series Magnetic Circuit
Part a: Finding I

48. Example 5 Use B-H curve to find H Part a: Finding I For cast steel
When B=0.2  H=170 At/m
170

49. Example 5
Part a: Finding I
Use Ampere’s circuital law

50. Example 5
Part b: Finding µ and µr

51. Example 6

52. Example 6
Length of each material
Area

53. Example 6
Finding H for sheet steel

54. Example 6
Finding H for cast iron

55. Example 6
Use Ampere’s circuital law

56. Example 7: NI is given, find flux 

57. Air Gaps Fringing Only ideal case will be covered in this course
The spreading of the flux lines outside the common area of the core for the air gap.
Only ideal case will be covered in this course

58. Air Gaps For ideal case Mmf drop across the air gap
Permeability of air is assumed to be equal to permeability of free space
so magnetizing force of air gap can be determined by:

59. Example 8 : Air Gap

60. Example 8 : Air Gap

61. Example 8 : Air Gap

62. Example 9 : Air Gap

63. Example 9 : Air Gap

64. Example 9 : Air Gap

65. Application of magnetic effects

66. Faraday’s law of electromagnetic induction
Michael Faraday discovered the principle of electromagnetic induction in 1831.
Basically he found that moving a magnet through a coil of wire induced a voltage across the coil,
Two observation:
The amount of voltage induced in a coil is directly proportional to the rate of change of the magnetic field with respect to the coil (d /dt).
The amount of voltage induced in a coil is directly proportional to the number of turns of wire in the coil (N).

67. Faraday’s law of electromagnetic induction
First observation S N
Magnet is moved at certain rate and certain voltage is produced
Magnet is moved at faster rate and creating a greater induced voltage.

68. Faraday’s law of electromagnetic induction
Second observation
Magnet is moved through a coil and certain voltage is produced
Magnet is moved at same speed through coil that has greater number of turn and greater voltage is induced

69. Faraday’s law of electromagnetic induction
Faraday’s Law is stated as follows:
The voltage induced across a coil of wire equals the number of turns in the coil times the rate of change of the magnetic flux.
Faraday's law is expressed in equation form as

70. Example 10 : Faraday’s Law
Apply Faraday's law to find the induced voltage across a coil with 500 turns that is located in a magnetic field that is changing at a rate of 8000 µWb/s.

71. Lenz’s Law Defines the polarity or direction of the induced voltage.
“an induced effect is always such as to oppose the cause that produced it.”
“When the current through a coil changes, an induced voltage is created as a result of the changing electromagnetic field and the polarity of the induced voltage is such that it always opposes the change in current.”

72. Lenz’s Law
The magnetic flux linking the coil of N turns with a current I has the distribution shown in Fig
If the current through the coil increases in magnitude, the flux linking the coil also increases.
We just learned through Faraday’s law, however, that a coil in the vicinity of a changing magnetic flux will have a voltage induced across it.
The result is that a voltage is induced across the coil in Fig due to the change in current through the coil.
It is very important to note in Fig that the polarity of the induced voltage across the coil is such that it opposes the increasing level of current in the coil.