1. 1 Introduction to Electromagnetic Theory Charges, Electric and Magnetic Fields Assoc. Prof. Dr. Wanchai Pijitrojana Room 412-7 Research and Lab Bldg. pwanchai@engr.tu.ac.th

2. Lecture 2 2 Objectives To provide an introductory course on classical electromagnetics, Maxwell’s equations, and electromagnetic fields in materials. To provide an introductory course on classical electromagnetics, Maxwell’s equations, and electromagnetic fields in materials. To prepare knowledge for students who want to continue in the fields of electrical engineering and electrophysics. To prepare knowledge for students who want to continue in the fields of electrical engineering and electrophysics.

3. Lecture 2 Textbooks 3 by Richard E. Dubroff (Author), S. V. Marshall (Author), G. G. Skitek (Author)

4. Lecture 2 James Clerk Maxwell 4

5. Lecture 2 James Clerk Maxwell James Clerk Maxwell, one of the world's greatest physicists, was Professor of Natural Philosophy at King's from 1860 to 1865, perhaps the most fertile years of his career. It was during this period that he demonstrated that magnetism, electricity and light were different manifestations of the same fundamental laws, and described all these, as well as radio waves, radar, and radiant heat, through his unique and elegant system of equations. These calculations were crucial to Albert Einstein in his production of the theory of relativity 40 years later, and led Einstein to comment that 'One scientific epoch ended and another began with James Clerk Maxwell'. James Clerk Maxwell, one of the world's greatest physicists, was Professor of Natural Philosophy at King's from 1860 to 1865, perhaps the most fertile years of his career. It was during this period that he demonstrated that magnetism, electricity and light were different manifestations of the same fundamental laws, and described all these, as well as radio waves, radar, and radiant heat, through his unique and elegant system of equations. These calculations were crucial to Albert Einstein in his production of the theory of relativity 40 years later, and led Einstein to comment that 'One scientific epoch ended and another began with James Clerk Maxwell'. Maxwell also made a major contribution to the understanding of colour vision and the first demonstration of colour photography. He produced notable work on the theory of rolling curves, on the composition of Saturn's rings, on the nature of gases and on electrical measurements. Maxwell also made a major contribution to the understanding of colour vision and the first demonstration of colour photography. He produced notable work on the theory of rolling curves, on the composition of Saturn's rings, on the nature of gases and on electrical measurements. At King's the James Clerk Maxwell Building at the Waterloo Campus commemorates him, as does a chair in Physics named after him and a society for undergraduate physicists. At King's the James Clerk Maxwell Building at the Waterloo Campus commemorates him, as does a chair in Physics named after him and a society for undergraduate physicists.

6. Lecture 2 6 Electromagnetic Fields Electromagnetics is the study of the effect of charges at rest and charges in motion. Electromagnetics is the study of the effect of charges at rest and charges in motion. Some special cases of electromagnetics: Some special cases of electromagnetics: Electrostatics: charges at rest Electrostatics: charges at rest Magnetostatics: charges in steady motion (DC) Magnetostatics: charges in steady motion (DC) Electromagnetic waves: waves excited by charges in time-varying motion Electromagnetic waves: waves excited by charges in time-varying motion

7. Lecture 2 7 Electromagnetic Fields Maxwell’s equations Fundamental laws of classical electromagnetics Special cases Electro- statics Magneto- statics Electro- magnetic waves Kirchoff’s Laws Statics: Geometric Optics Transmission Line Theory Circuit Theory Input from other disciplines

8. Lecture 2 8 Electromagnetic Fields transmitter and receiver are connected by a “field.”

9. Lecture 2 9 Electromagnetic Fields When an event in one place has an effect on something at a different location, we talk about the events as being connected by a “field”. When an event in one place has an effect on something at a different location, we talk about the events as being connected by a “field”. A field is a spatial distribution of a quantity; in general, it can be either scalar or vector in nature. A field is a spatial distribution of a quantity; in general, it can be either scalar or vector in nature.

10. Lecture 2 10 Electromagnetic Fields Electric and magnetic fields: Electric and magnetic fields: Are vector fields with three spatial components. Are vector fields with three spatial components. Vary as a function of position in 3D space as well as time. Vary as a function of position in 3D space as well as time. Are governed by partial differential equations derived from Maxwell’s equations. Are governed by partial differential equations derived from Maxwell’s equations.

11. Lecture 2 11 Electromagnetic Fields A scalar is a quantity having only an amplitude (and possibly phase). A scalar is a quantity having only an amplitude (and possibly phase). A vector is a quantity having direction in addition to amplitude (and possibly phase). A vector is a quantity having direction in addition to amplitude (and possibly phase). Examples: voltage, current, charge, energy, temperature Examples: velocity, acceleration, force

12. Lecture 2 12 Electromagnetic Fields Fundamental vector field quantities in electromagnetics: Fundamental vector field quantities in electromagnetics: Electric field intensity Electric field intensity Electric flux density (electric displacement) Electric flux density (electric displacement) Magnetic field intensity Magnetic field intensity Magnetic flux density Magnetic flux density units = volts per meter (V/m = kg m/A/s 3 ) units = coulombs per square meter (C/m 2 = A s /m 2 ) units = amps per meter (A/m) units = teslas = webers per square meter (T = Wb/ m 2 = kg/A/s 3 )

13. Lecture 2 13 Electromagnetic Fields Universal constants in electromagnetics: Universal constants in electromagnetics: Velocity of an electromagnetic wave (e.g., light) in free space (perfect vacuum) Velocity of an electromagnetic wave (e.g., light) in free space (perfect vacuum) Permeability of free space Permeability of free space Permittivity of free space: Permittivity of free space: Intrinsic impedance of free space: Intrinsic impedance of free space:

14. Lecture 2 14 Electromagnetic Fields In free space: Relationships involving the universal constants: Relationships involving the universal constants:

15. Lecture 2 15 Electromagnetic Fields sources J i, K i Obtained by assumption from solution to IE fields E, H Solution to Maxwell’s equations Observable quantities

16. Lecture 2 16 Maxwell’s Equations Maxwell’s equations in integral form are the fundamental postulates of classical electromagnetics - all classical electromagnetic phenomena are explained by these equations. Maxwell’s equations in integral form are the fundamental postulates of classical electromagnetics - all classical electromagnetic phenomena are explained by these equations. Electromagnetic phenomena include electrostatics, magnetostatics, electromagnetostatics and electromagnetic wave propagation. Electromagnetic phenomena include electrostatics, magnetostatics, electromagnetostatics and electromagnetic wave propagation. The differential equations and boundary conditions that we use to formulate and solve EM problems are all derived from Maxwell’s equations in integral form. The differential equations and boundary conditions that we use to formulate and solve EM problems are all derived from Maxwell’s equations in integral form.

17. Lecture 2 17 Maxwell’s Equations Various equivalence principles consistent with Maxwell’s equations allow us to replace more complicated electric current and charge distributions with equivalent magnetic sources. Various equivalence principles consistent with Maxwell’s equations allow us to replace more complicated electric current and charge distributions with equivalent magnetic sources. These equivalent magnetic sources can be treated by a generalization of Maxwell’s equations. These equivalent magnetic sources can be treated by a generalization of Maxwell’s equations.

18. Lecture 2 18 Maxwell’s Equations in Integral Form (Generalized to Include Equivalent Magnetic Sources) Adding the fictitious magnetic source terms is equivalent to living in a universe where magnetic monopoles (charges) exist.

19. Lecture 2 19 Continuity Equation in Integral Form (Generalized to Include Equivalent Magnetic Sources) The continuity equations are implicit in Maxwell’s equations.

20. Lecture 2 20 Contour, Surface and Volume Conventions CS dSdS open surface S bounded by closed contour C dS in direction given by RH rule V S dSdS volume V bounded by closed surface S dS in direction outward from V

21. Lecture 2 21 Electric Current and Charge Densities J c = (electric) conduction current density (A/m 2 ) J c = (electric) conduction current density (A/m 2 ) J i = (electric) impressed current density (A/m 2 ) J i = (electric) impressed current density (A/m 2 ) q ev = (electric) charge density (C/m 3 ) q ev = (electric) charge density (C/m 3 )

22. Lecture 2 22 Magnetic Current and Charge Densities K c = magnetic conduction current density (V/m 2 ) K c = magnetic conduction current density (V/m 2 ) K i = magnetic impressed current density (V/m 2 ) K i = magnetic impressed current density (V/m 2 ) q mv = magnetic charge density (Wb/m 3 ) q mv = magnetic charge density (Wb/m 3 )

23. Lecture 2 23 Maxwell’s Equations - Sources and Responses Sources of EM field: Sources of EM field: K i, J i, q ev, q mv K i, J i, q ev, q mv Responses to EM field: Responses to EM field: E, H, D, B, J c, K c E, H, D, B, J c, K c

24. Lecture 2 24 Maxwell’s Equations in Differential Form (Generalized to Include Equivalent Magnetic Sources)

25. Lecture 2 25 Continuity Equation in Differential Form (Generalized to Include Equivalent Magnetic Sources) The continuity equations are implicit in Maxwell’s equations.

26. Lecture 2 26 Electromagnetic Boundary Conditions Region 2 Region 1

27. Lecture 2 27 Electromagnetic Boundary Conditions

28. Lecture 2 28 Surface Current and Charge Densities Can be either sources of or responses to EM field. Can be either sources of or responses to EM field. Units: Units: K s - V/m K s - V/m J s - A/m J s - A/m q es - C/m 2 q es - C/m 2 q ms - W/m 2 q ms - W/m 2

29. Lecture 2 29 Electromagnetic Fields in Materials In time-varying electromagnetics, we consider E and H to be the “primary” responses, and attempt to write the “secondary” responses D, B, J c, and K c in terms of E and H. In time-varying electromagnetics, we consider E and H to be the “primary” responses, and attempt to write the “secondary” responses D, B, J c, and K c in terms of E and H. The relationships between the “primary” and “secondary” responses depends on the medium in which the field exists. The relationships between the “primary” and “secondary” responses depends on the medium in which the field exists. The relationships between the “primary” and “secondary” responses are called constitutive relationships. The relationships between the “primary” and “secondary” responses are called constitutive relationships.

30. Lecture 2 30 Electromagnetic Fields in Materials Most general constitutive relationships : Most general constitutive relationships :

31. Lecture 2 31 Electromagnetic Fields in Materials In free space, we have: In free space, we have:

32. Lecture 2 32 Electromagnetic Fields in Materials In a simple medium, we have: In a simple medium, we have: (independent of field strength) linear (independent of field strength) (independent of position within the medium) isotropic (independent of position within the medium) (independent of direction) homogeneous (independent of direction) (independent of time) time-invariant (independent of time) non-dispersive (independent of frequency)

33. Lecture 2 33 Electromagnetic Fields in Materials  = permittivity =  r  0 (F/m)  = permittivity =  r  0 (F/m)  = permeability =  r  0 (H/m)  = permeability =  r  0 (H/m)  = electric conductivity =  r  0 (S/m)  = electric conductivity =  r  0 (S/m)  m = magnetic conductivity =  r  0 (  /m)  m = magnetic conductivity =  r  0 (  /m)

34. Lecture 2 34 Phasor Representation of a Time- Harmonic Field A phasor is a complex number representing the amplitude and phase of a sinusoid of known frequency. A phasor is a complex number representing the amplitude and phase of a sinusoid of known frequency. time domain frequency domain phasor

35. Lecture 2 35 Phasor Representation of a Time- Harmonic Field Phasors are an extremely important concept in the study of classical electromagnetics, circuit theory, and communications systems. Phasors are an extremely important concept in the study of classical electromagnetics, circuit theory, and communications systems. Maxwell’s equations in simple media, circuits comprising linear devices, and many components of communications systems can all be represented as linear time-invariant ( LTI ) systems. (Formal definition of these later in the course …) Maxwell’s equations in simple media, circuits comprising linear devices, and many components of communications systems can all be represented as linear time-invariant ( LTI ) systems. (Formal definition of these later in the course …) The eigenfunctions of any LTI system are the complex exponentials of the form: The eigenfunctions of any LTI system are the complex exponentials of the form:

36. Lecture 2 36 Phasor Representation of a Time- Harmonic Field If the input to an LTI system is a sinusoid of frequency , then the output is also a sinusoid of frequency  (with different amplitude and phase). If the input to an LTI system is a sinusoid of frequency , then the output is also a sinusoid of frequency  (with different amplitude and phase). A complex constant (for fixed  ); as a function of  gives the frequency response of the LTI system.

37. Lecture 2 37 Phasor Representation of a Time- Harmonic Field The amplitude and phase of a sinusoidal function can also depend on position, and the sinusoid can also be a vector function: The amplitude and phase of a sinusoidal function can also depend on position, and the sinusoid can also be a vector function:

38. Lecture 2 38 Phasor Representation of a Time- Harmonic Field Given the phasor (frequency-domain) representation of a time-harmonic vector field, the time-domain representation of the vector field is obtained using the recipe: Given the phasor (frequency-domain) representation of a time-harmonic vector field, the time-domain representation of the vector field is obtained using the recipe:

39. Lecture 2 39 Phasor Representation of a Time- Harmonic Field Phasors can be used provided all of the media in the problem are linear  no frequency conversion. Phasors can be used provided all of the media in the problem are linear  no frequency conversion. When phasors are used, integro-differential operators in time become algebraic operations in frequency, e.g.: When phasors are used, integro-differential operators in time become algebraic operations in frequency, e.g.:

40. Lecture 2 40 Time-Harmonic Maxwell’s Equations If the sources are time-harmonic (sinusoidal), and all media are linear, then the electromagnetic fields are sinusoids of the same frequency as the sources. If the sources are time-harmonic (sinusoidal), and all media are linear, then the electromagnetic fields are sinusoids of the same frequency as the sources. In this case, we can simplify matters by using Maxwell’s equations in the frequency-domain. In this case, we can simplify matters by using Maxwell’s equations in the frequency-domain. Maxwell’s equations in the frequency-domain are relationships between the phasor representations of the fields. Maxwell’s equations in the frequency-domain are relationships between the phasor representations of the fields.

41. Lecture 2 41 Maxwell’s Equations in Differential Form for Time-Harmonic Fields

42. Lecture 2 42 Maxwell’s Equations in Differential Form for Time-Harmonic Fields in Simple Medium

43. Lecture 2 43 Electrostatics as a Special Case of Electromagnetics Maxwell’s equations Fundamental laws of classical electromagnetics Special cases Electro- statics Magneto- statics Electro- magnetic waves Kirchoff’s Laws Statics: Geometric Optics Transmission Line Theory Circuit Theory Input from other disciplines

44. Lecture 2 44 Electrostatics Electrostatics is the branch of electromagnetics dealing with the effects of electric charges at rest. Electrostatics is the branch of electromagnetics dealing with the effects of electric charges at rest. The fundamental law of electrostatics is Coulomb’s law. The fundamental law of electrostatics is Coulomb’s law.

45. Lecture 2 45 Electric Charge Electrical phenomena caused by friction are part of our everyday lives, and can be understood in terms of electrical charge. Electrical phenomena caused by friction are part of our everyday lives, and can be understood in terms of electrical charge. The effects of electrical charge can be observed in the attraction/repulsion of various objects when “charged.” The effects of electrical charge can be observed in the attraction/repulsion of various objects when “charged.” Charge comes in two varieties called “positive” and “negative.” Charge comes in two varieties called “positive” and “negative.”

46. Lecture 2 46 Electric Charge Objects carrying a net positive charge attract those carrying a net negative charge and repel those carrying a net positive charge. Objects carrying a net positive charge attract those carrying a net negative charge and repel those carrying a net positive charge. Objects carrying a net negative charge attract those carrying a net positive charge and repel those carrying a net negative charge. Objects carrying a net negative charge attract those carrying a net positive charge and repel those carrying a net negative charge. On an atomic scale, electrons are negatively charged and nuclei are positively charged. On an atomic scale, electrons are negatively charged and nuclei are positively charged.

47. Lecture 2 47 Electric Charge Electric charge is inherently quantized such that the charge on any object is an integer multiple of the smallest unit of charge which is the magnitude of the electron charge e = 1.602  10 -19 C. Electric charge is inherently quantized such that the charge on any object is an integer multiple of the smallest unit of charge which is the magnitude of the electron charge e = 1.602  10 -19 C. On the macroscopic level, we can assume that charge is “continuous.” On the macroscopic level, we can assume that charge is “continuous.”

48. Lecture 2 48 Coulomb’s Law Coulomb’s law is the “law of action” between charged bodies. Coulomb’s law is the “law of action” between charged bodies. Coulomb’s law gives the electric force between two point charges in an otherwise empty universe. Coulomb’s law gives the electric force between two point charges in an otherwise empty universe. A point charge is a charge that occupies a region of space which is negligibly small compared to the distance between the point charge and any other object. A point charge is a charge that occupies a region of space which is negligibly small compared to the distance between the point charge and any other object.

49. Lecture 2 49 Coulomb’s Law Q1Q1 Q2Q2 Force due to Q 1 acting on Q 2 Unit vector in direction of R 12

50. Lecture 2 50 Coulomb’s Law The force on Q 1 due to Q 2 is equal in magnitude but opposite in direction to the force on Q 2 due to Q 1. The force on Q 1 due to Q 2 is equal in magnitude but opposite in direction to the force on Q 2 due to Q 1.

51. Lecture 2 51 Electric Field Consider a point charge Q placed at the origin of a coordinate system in an otherwise empty universe. Consider a point charge Q placed at the origin of a coordinate system in an otherwise empty universe. A test charge Q t brought near Q experiences a force: A test charge Q t brought near Q experiences a force: Q QtQt

52. Lecture 2 52 Electric Field The existence of the force on Q t can be attributed to an electric field produced by Q. The existence of the force on Q t can be attributed to an electric field produced by Q. The electric field produced by Q at a point in space can be defined as the force per unit charge acting on a test charge Q t placed at that point. The electric field produced by Q at a point in space can be defined as the force per unit charge acting on a test charge Q t placed at that point.

53. Lecture 2 53 Electric Field The electric field describes the effect of a stationary charge on other charges and is an abstract “action-at-a-distance” concept, very similar to the concept of a gravity field. The electric field describes the effect of a stationary charge on other charges and is an abstract “action-at-a-distance” concept, very similar to the concept of a gravity field. The basic units of electric field are newtons per coulomb. The basic units of electric field are newtons per coulomb. In practice, we usually use volts per meter. In practice, we usually use volts per meter.

54. Lecture 2 54 Electric Field For a point charge at the origin, the electric field at any point is given by For a point charge at the origin, the electric field at any point is given by

55. Lecture 2 55 Electric Field For a point charge located at a point P’ described by a position vector For a point charge located at a point P’ described by a position vector the electric field at P is given by the electric field at P is given by Q P O

56. Lecture 2 56 Electric Field In electromagnetics, it is very popular to describe the source in terms of primed coordinates, and the observation point in terms of unprimed coordinates. In electromagnetics, it is very popular to describe the source in terms of primed coordinates, and the observation point in terms of unprimed coordinates. As we shall see, for continuous source distributions we shall need to integrate over the source coordinates. As we shall see, for continuous source distributions we shall need to integrate over the source coordinates.

57. Lecture 2 57 Electric Field Using the principal of superposition, the electric field at a point arising from multiple point charges may be evaluated as Using the principal of superposition, the electric field at a point arising from multiple point charges may be evaluated as

58. Lecture 2 58 Continuous Distributions of Charge Charge can occur as Charge can occur as point charges (C) point charges (C) volume charges (C/m 3 ) volume charges (C/m 3 ) surface charges (C/m 2 ) surface charges (C/m 2 ) line charges (C/m) line charges (C/m)  most general

59. Lecture 2 59 Continuous Distributions of Charge Volume charge density Volume charge density Q encl  V’

60. Lecture 2 60 Continuous Distributions of Charge Electric field due to volume charge density Electric field due to volume charge density Q encl dV’ V’ P

61. Lecture 2 61 Electric Field Due to Volume Charge Density

62. Lecture 2 62 Continuous Distributions of Charge Surface charge density Surface charge density Q encl  S’

63. Lecture 2 63 Continuous Distributions of Charge Electric field due to surface charge density Electric field due to surface charge density Q encl dS’ S’ P

64. Lecture 2 64 Electric Field Due to Surface Charge Density

65. Lecture 2 65 Continuous Distributions of Charge Line charge density Line charge density Q encl  L’

66. Lecture 2 66 Continuous Distributions of Charge Electric field due to line charge density Electric field due to line charge density Q encl  L’ P

67. Lecture 2 67 Electric Field Due to Line Charge Density

68. Lecture 2 68 Electrostatic Potential An electric field is a force field. An electric field is a force field. If a body being acted on by a force is moved from one point to another, then work is done. If a body being acted on by a force is moved from one point to another, then work is done. The concept of scalar electric potential provides a measure of the work done in moving charged bodies in an electrostatic field. The concept of scalar electric potential provides a measure of the work done in moving charged bodies in an electrostatic field.

69. Lecture 2 69 Electrostatic Potential The work done in moving a test charge from one point to another in a region of electric field: The work done in moving a test charge from one point to another in a region of electric field: a b q

70. Lecture 2 70 Electrostatic Potential In evaluating line integrals, it is customary to take the dl in the direction of increasing coordinate value so that the manner in which the path of integration is traversed is unambiguously determined by the limits of integration. In evaluating line integrals, it is customary to take the dl in the direction of increasing coordinate value so that the manner in which the path of integration is traversed is unambiguously determined by the limits of integration. x 35 ba

71. Lecture 2 71 Electrostatic Potential The electrostatic field is conservative : The electrostatic field is conservative : The value of the line integral depends only on the end points and is independent of the path taken. The value of the line integral depends only on the end points and is independent of the path taken. The value of the line integral around any closed path is zero. The value of the line integral around any closed path is zero. C

72. Lecture 2 72 Electrostatic Potential The work done per unit charge in moving a test charge from point a to point b is the electrostatic potential difference between the two points: The work done per unit charge in moving a test charge from point a to point b is the electrostatic potential difference between the two points: electrostatic potential difference Units are volts.

73. Lecture 2 73 Electrostatic Potential Since the electrostatic field is conservative we can write Since the electrostatic field is conservative we can write

74. Lecture 2 74 Electrostatic Potential Thus the electrostatic potential V is a scalar field that is defined at every point in space. Thus the electrostatic potential V is a scalar field that is defined at every point in space. In particular the value of the electrostatic potential at any point P is given by In particular the value of the electrostatic potential at any point P is given by reference point

75. Lecture 2 75 Electrostatic Potential The reference point ( P 0 ) is where the potential is zero (analogous to ground in a circuit). The reference point ( P 0 ) is where the potential is zero (analogous to ground in a circuit). Often the reference is taken to be at infinity so that the potential of a point in space is defined as Often the reference is taken to be at infinity so that the potential of a point in space is defined as

76. Lecture 2 76 Electrostatic Potential and Electric Field The work done in moving a point charge from point a to point b can be written as The work done in moving a point charge from point a to point b can be written as

77. Lecture 2 77 Electrostatic Potential and Electric Field Along a short path of length  l we have Along a short path of length  l we have

78. Lecture 2 78 Electrostatic Potential and Electric Field Along an incremental path of length dl we have Along an incremental path of length dl we have Recall from the definition of directional derivative : Recall from the definition of directional derivative :

79. Lecture 2 79 Electrostatic Potential and Electric Field Thus: Thus: the “del” or “nabla” operator