Classical Electromagnetism
Richard Fitzpatrick
Professor of Physics
The University of Texas at Austin Contents
1 Maxwell’s Equations 7 1.1 Introduction ...... 7 1.2 Maxwell’sEquations...... 7 1.3 ScalarandVectorPotentials...... 8 1.4 DiracDeltaFunction...... 9 1.5 Three-DimensionalDiracDeltaFunction...... 9 1.6 Solution of Inhomogeneous Wave Equation ...... 10 1.7 RetardedPotentials...... 16 1.8 RetardedFields...... 17 1.9 ElectromagneticEnergyConservation...... 19 1.10 ElectromagneticMomentumConservation...... 20 1.11 Exercises...... 22
2 Electrostatic Fields 25 2.1 Introduction ...... 25 2.2 Laplace’s Equation ...... 25 2.3 Poisson’sEquation...... 26 2.4 Coulomb’sLaw...... 27 2.5 ElectricScalarPotential...... 28 2.6 ElectrostaticEnergy...... 29 2.7 ElectricDipoles...... 33 2.8 ChargeSheetsandDipoleSheets...... 34 2.9 Green’sTheorem...... 37 2.10 Boundary Value Problems ...... 40 2.11 DirichletGreen’sFunctionforSphericalSurface...... 43 2.12 Exercises...... 46
3 Potential Theory 49 3.1 Introduction ...... 49 2 CLASSICAL ELECTROMAGNETISM
3.2 AssociatedLegendreFunctions...... 49 3.3 SphericalHarmonics...... 50 3.4 Laplace’s Equation in Spherical Coordinates ...... 51 3.5 Poisson’sEquationinSphericalCoordinates...... 52 3.6 MultipoleExpansion...... 53 3.7 AxisymmetricChargeDistributions...... 56 3.8 DirichletProbleminSphericalCoordinates...... 57 3.9 NewmannProbleminSphericalCoordinates...... 58 3.10 Laplace’s Equation in Cylindrical Coordinates ...... 59 3.11 Poisson’sEquationinCylindricalCoordinates...... 64 3.12 Exercises...... 69
4 Electrostatics in Dielectric Media 73 4.1 Polarization...... 73 4.2 Boundary Conditions for E and D ...... 75 4.3 Boundary Value Problems with Dielectrics ...... 75 4.4 EnergyDensityWithinDielectricMedium...... 81 4.5 ForceDensityWithinDielectricMedium...... 82 4.6 Clausius-MossottiRelation...... 85 4.7 DielectricLiquidsinElectrostaticFields...... 86 4.8 Exercises...... 89
5 Magnetostatic Fields 93 5.1 Introduction ...... 93 5.2 Biot-SavartLaw...... 93 5.3 Continuous Current Distribution ...... 93 5.4 CircularCurrentLoop...... 95 5.5 LocalizedCurrentDistribution...... 98 5.6 Exercises...... 100
6 Magnetostatics in Magnetic Media 103 6.1 Magnetization...... 103 6.2 Magnetic Susceptibility and Permeability ...... 104 6.3 Ferromagnetism...... 105 6.4 Boundary Conditions for B and H ...... 107 6.5 PermanentFerromagnets...... 108 6.6 UniformlyMagnetizedSphere...... 110 6.7 SoftIronSphereinUniformMagneticField...... 113 6.8 MagneticShielding...... 114 6.9 MagneticEnergy...... 116 6.10 Exercises...... 117
7 Wave Propagation in Uniform Dielectric Media 119 CONTENTS 3
7.1 Introduction ...... 119 7.2 FormofDielectricConstant...... 120 7.3 AnomalousDispersionandResonantAbsorption...... 122 7.4 Wave Propagation in Conducting Media ...... 124 7.5 HighFrequencyLimit...... 126 7.6 PolarizationofElectromagneticWaves...... 127 7.7 FaradayRotation...... 128 7.8 WavePropagationinMagnetizedPlasmas...... 131 7.9 WavePropagationinDispersiveMedia...... 132 7.10 Wave-FrontPropagation...... 136 7.11 SommerfeldPrecursor...... 139 7.12 MethodofStationaryPhase...... 144 7.13 GroupVelocity...... 145 7.14 BrillouinPrecursor...... 147 7.15 SignalArrival...... 149 7.16 Exercises...... 150
8 Wave Propagation in Inhomogeneous Dielectric Media 153 8.1 Introduction ...... 153 8.2 LawsofGeometricOptics...... 153 8.3 FresnelRelations...... 155 8.4 TotalInternalReflection...... 161 8.5 Reflection by Conducting Surfaces ...... 166 8.6 Ionospheric Radio Wave Propagation ...... 167 8.7 WKBApproximation...... 168 8.8 Reflection Coefficient...... 172 8.9 ExtensiontoObliqueIncidence...... 173 8.10 Ionospheric Pulse Propagation ...... 177 8.11 Measurement of Ionospheric Electron Density Profile ...... 179 8.12 Ionospheric Ray Tracing ...... 180 8.13 AsymptoticSeries...... 183 8.14 WKBSolutionasAsymptoticSeries...... 188 8.15 StokesConstants...... 189 8.16 WKB Reflection Coefficient...... 194 8.17 JeffriesConnectionFormula...... 197 8.18 Exercises...... 198
9 Radiation and Scattering 201 9.1 Introduction ...... 201 9.2 BasicAntennaTheory...... 201 9.3 Antenna Directivity and EffectiveArea...... 204 9.4 AntennaArrays...... 208 9.5 ThomsonScattering...... 210 4 CLASSICAL ELECTROMAGNETISM
9.6 RayleighScattering...... 212 9.7 Exercises...... 213
10 Resonant Cavities and Waveguides 215 10.1 Introduction ...... 215 10.2 Boundary Conditions ...... 215 10.3 Cavities with Rectangular Boundaries ...... 217 10.4 Quality Factor of a Resonant Cavity ...... 218 10.5 Axially Symmetric Cavities ...... 219 10.6 Cylindrical Cavities ...... 222 10.7 Waveguides...... 223 10.8 DielectricWaveguides...... 225 10.9 Exercises...... 230
11 Multipole Expansion 231 11.1 Introduction ...... 231 11.2 MultipoleExpansionofScalarWaveEquation...... 231 11.3 Angular Momentum Operators ...... 233 11.4 MultipoleExpansionofVectorWaveEquation...... 234 11.5 PropertiesofMultipoleFields...... 237 11.6 Solution of Inhomogeneous Helmholtz Equation ...... 240 11.7 SourcesofMultipoleRadiation...... 242 11.8 RadiationfromLinearCentre-FedAntenna...... 245 11.9 SphericalWaveExpansionofVectorPlaneWave...... 248 11.10MieScattering...... 250 11.11Exercises...... 254
12 Relativity and Electromagnetism 255 12.1 Introduction ...... 255 12.2 RelativityPrinciple...... 255 12.3 LorentzTransformation...... 256 12.4 Transformation of Velocities ...... 260 12.5 Tensors...... 261 12.6 PhysicalSignificanceofTensors...... 265 12.7 Space-Time ...... 266 12.8 ProperTime...... 270 12.9 4-Velocity and 4-Acceleration ...... 271 12.10CurrentDensity4-Vector...... 272 12.11Potential4-Vector...... 273 12.12GaugeInvariance...... 274 12.13RetardedPotentials...... 274 12.14 Tensors and Pseudo-Tensors ...... 276 12.15ElectromagneticFieldTensor...... 278 CONTENTS 5
12.16DualElectromagneticFieldTensor...... 281 12.17TransformationofFields...... 283 12.18PotentialDuetoaMovingCharge...... 284 12.19FieldDuetoaMovingCharge...... 285 12.20RelativisticParticleDynamics...... 286 12.21ForceonaMovingCharge...... 288 12.22ElectromagneticEnergyTensor...... 289 12.23 Accelerated Charges ...... 292 12.24LarmorFormula...... 295 12.25RadiationLosses...... 299 12.26 Angular Distribution of Radiation ...... 300 12.27SynchrotronRadiation...... 301 12.28Exercises...... 304 6 CLASSICAL ELECTROMAGNETISM Maxwell’s Equations 7 1 Maxwell’s Equations
1.1 Introduction
This chapter gives a general overview of Maxwell’s equations.
1.2 Maxwell’s Equations
All classical (i.e., non-quantum) electromagnetic phenomena are governed by Maxwell’s equa- tions, which take the form ρ ∇·E = , (1.1) 0 ∇·B = 0, (1.2) ∂B ∇×E = − , (1.3) ∂t ∂E ∇×B = µ j + µ . (1.4) 0 0 0 ∂t Here, E(r, t), B(r, t), ρ(r, t), and j(r, t)representtheelectric field-strength,themagnetic field- strength,theelectric charge density,andtheelectric current density, respectively. Moreover,
−12 2 −1 −2 0 = 8.8542 × 10 C N m (1.5) is the electric permittivity of free space, whereas
−7 −2 µ0 = 4π × 10 NA (1.6) is the magnetic permeability of free space. As is well known, Equation (1.1) is equivalent to Coulomb’s law (for the electric fields generated by point charges), Equation (1.2) is equivalent to the statement that magnetic monopoles do not exist (which implies that magnetic field-lines can never begin or end), Equation (1.3) is equivalent to Faraday’s law of electromagnetic induction, and Equation (1.4) is equivalent to the Biot-Savart law (for the magnetic fields generated by line currents) augmented by the induction of magnetic fields by changing electric fields. Maxwell’s equations are linear in nature. In other words, if ρ → αρand j → α j,whereα is an arbitrary (spatial and temporal) constant, then it is clear from Equations (1.1)–(1.4) that E → α E and B → α B. The linearity of Maxwell’s equations accounts for the well-known fact that the electric fields generated by point charges, as well as the magnetic fields generated by line currents, are superposable. Taking the divergence of Equation (1.4), and combining the resulting expression with Equa- tion (1.1), we obtain ∂ρ + ∇·j = 0. (1.7) ∂t 8 CLASSICAL ELECTROMAGNETISM
In integral form, making use of the divergence theorem, this equation becomes d ρ dV + j · dS = 0, (1.8) dt V S where V is a fixed volume bounded by a surface S . The volume integral represents the net electric charge contained within the volume, whereas the surface integral represents the outward flux of charge across the bounding surface. The previous equation, which states that the net rate of change of the charge contained within the volume V is equal to minus the net flux of charge across the bounding surface S , is clearly a statement of the conservation of electric charge. Thus, Equa- tion (1.7) is the differential form of this conservation equation. As is well known, a point electric charge q moving with velocity v in the presence of an electric field E and a magnetic field B experiences a force
F = q (E + v × B). (1.9)
Likewise, a distributed charge distribution of charge density ρ and current density j experiences a force density f = ρ E + j × B. (1.10)
1.3 Scalar and Vector Potentials
We can automatically satisfy Equation (1.2) by writing
B = ∇×A, (1.11)
where A(r, t)istermedthevector potential. Furthermore, we can automatically satisfy Equa- tion (1.3) by writing ∂A E = −∇φ − , (1.12) ∂t where φ(r, t)istermedthescalar potential. The previous prescription for expressing electric and magnetic fields in terms of the scalar and vector potentials does not uniquely define the potentials. Indeed, it can be seen that if A → A −∇ψ and φ → φ + ∂ψ/∂t,whereψ(r, t) is an arbitrary scalar field, then the associated electric and magnetic fields are unaffected. The root of the problem lies in the fact that Equation (1.11) specifies the curl of the vector potential, but leaves the divergence of this vector field completely unspecified. We can make our prescription unique by adopting a convention that specifies the divergence of the vector potential—such a convention is usually called a gauge condition. It turns out that Maxwell’s equations are Lorentz invariant. (See Chapter 12.) In other words, they take the same form in all inertial frames. Thus, it makes sense to adopt a gauge condition that is also Lorentz invariant. This leads us to the so-called Lorenz gauge condition (see Section 12.12),
∂φ µ + ∇·A = 0. (1.13) 0 0 ∂t Maxwell’s Equations 9
Equations (1.11)–(1.13) can be combined with Equations (1.1) and (1.4) to give 1 ∂ 2φ ρ −∇2φ = , 2 2 (1.14) c ∂t 0 1 ∂ 2A −∇2A = µ j, (1.15) c 2 ∂t 2 0 where 1 c = √ = 2.988 × 108 ms−1 (1.16) 0 µ0 is the velocity of light in vacuum. Thus, Maxwell’s equations essentially boil down to Equa- tions (1.14) and (1.15).
The Dirac delta function, δ(t − t), has the property
δ(t − t) = 0fort t. (1.17) In addition, however, the function is singular at t = t in such a manner that ∞ δ(t − t) dt = 1. (1.18) −∞
It follows that ∞ f (t) δ(t − t) dt = f (t), (1.19) −∞ where f (t) is an arbitrary function that is well behaved at t = t. It is also easy to see that δ(t − t) = δ(t − t). (1.20)
1.5 Three-Dimensional Dirac Delta Function
The three-dimensional Dirac delta function, δ(r − r), has the property
δ(r − r) = 0forr r. (1.21) In addition, however, the function is singular at r = r in such a manner that δ(r − r) dV = 1. (1.22) V Here, V is any volume that contains the point r = r.(Also,dV is an element of V expressed in terms of the components of r, but independent of the components of r.) It follows that f (r) δ(r − r) dV = f (r), (1.23) V 10 CLASSICAL ELECTROMAGNETISM
where f (r) is an arbitrary function that is well behaved at r = r. It is also easy to see that
δ(r − r) = δ(r − r). (1.24)
We can show that 1 ∇ 2 = −4πδ(r − r). (1.25) |r − r| (Here, ∇ 2 is a Laplacian operator expressed in terms of the components of r, but independent of the components of r.) We must first prove that 1 ∇ 2 = 0forr r, (1.26) |r − r| in accordance with Equation (1.21). If R = |r − r| then this is equivalent to showing that 1 d d 1 R 2 = 0 (1.27) R 2 dR dR R for R > 0, which is indeed the case. (Here, R is treated as a radial spherical coordinate.) Next, we must show that ∇ 2 1 = − dV 4π, (1.28) V |r − r | in accordance with Equations (1.22) and (1.25). Suppose that S is a spherical surface, of radius R, centered on r = r. Making use of the definition ∇ 2φ ≡∇·∇φ, as well as the divergence theorem, we can write 1 1 1 ∇ 2 dV = ∇·∇ dV = ∇ · dS |r − r| |r − r| |r − r| V V S d 1 = 4π R 2 = −4π. (1.29) dR R
(Here, ∇ is a gradient operator expressed in terms of the components of r, but independent of the components of r. Likewise, dS is a surface element involving the components of r, but indepen- dent of the components of r.) Finally, if S is deformed into a general surface (without crossing the point r = r) then the value of the volume integral is unchanged, as a consequence of Equa- tion (1.26). Hence, we have demonstrated the validity of Equation (1.25).
1.6 Solution of Inhomogeneous Wave Equation
Equation (1.14), as well as the three Cartesian components of Equation (1.15), are inhomogeneous three-dimensional wave equations of the general form 1 ∂ 2 −∇2 u = v, (1.30) c 2 ∂t 2 Maxwell’s Equations 11
where u(r, t) is an unknown potential, and v(r, t) a known source function. Let us investigate whether it is possible to find a unique solution of this type of equation. Let us assume that the source function v(r, t) can be expressed as a Fourier integral,