MASSACHUSETTS INSTITUTE OF TECHNOLOGY Physics Department Physics 8.07: Electromagnetism II December 18, 2012 Prof. Alan Guth

FORMULA SHEET FOR FINAL EXAM Exam Date: December 19, 2012 ∗∗∗

Some sections below are marked with asterisks, as this section is. The asterisks indicate that you won’t need this material for the quiz, and need not understand it. It is included, however, for completeness, and because some people might want to make use of it to solve problems by methods other than the intended ones. Index Notation: ·B  = Ai Bi , A

×B  i = "ijk Aj Bk , A "ijk "pqk = δip δjq − δiq δjp det A = "i1 i2 ···in A1,i1 A2,i2 · · · An,in

Rotation of a Vector: Ai = Rij Aj ,

Orthogonality: Rij Rik = δjk j=1

Rotation about z-axis by φ: Rz (φ)ij Rotation about axis n ˆ by φ:∗∗∗

j=2

 i=1  cos φ − sin φ   = i=2    sin φ cos φ i=3 0 0

(RT T = I) j=3

0 0 1

      

R(ˆ n, φ)ij = δij cos φ + n ˆin ˆ j (1 − cos φ) − "ijk n ˆ k sin φ . Vector Calculus: ∂i ≡

Gradient:

 ϕ)i = ∂i ϕ , (∇

Divergence:

 ·A  ≡ ∂i A i ∇

Curl:

 × A)  i = "ijk ∂j Ak (∇

Laplacian:

 · (∇  ϕ) = ∇2 ϕ = ∇

∂ ∂xi

∂ 2ϕ ∂xi ∂xi

Fundamental Theorems of Vector Calculus: 

b

Gradient:

 ϕ · d, = ϕ(b) − ϕ(a) ∇

 a



 Divergence:

V

 Curl: S

 · da A

 ·A  d3 x = ∇ S

where S is the boundary of V    · d, (∇ × A) · da = A P

where P is the boundary of S

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p. 2

Delta Functions:     ϕ(r )δ 3 (r − r  ) d3 x = ϕ(r  ) ϕ(x)δ(x − x ) dx = ϕ(x ) ,   d dϕ   ϕ(x) δ(x − x ) dx = − dx dx x=x


δ(x − xi ) , g(xi ) = 0 δ(g(x)) = |g  (xi )| i    1  r −  r  · = 4πδ 3 (r − r  ) ∇ = −∇2  3 |r − r | |r − r  |     x  rˆj 1 δij − 3ˆ ri rˆj 4π j δij δ 3 (r) ≡ ∂i 3 = −∂i ∂j + = ∂i 2 3 r r 3 r r  r − d 8π   3  · 3(d · rˆ)ˆ ∇ = − (d · ∇)δ (r ) 3 r3  r − d 4π  δ 3 (r )  × 3(d · rˆ)ˆ ∇ = − d × ∇ 3 3 r Electrostatics:  , where F = qE  1 1 (r − r  ) qi (r − r  )  = E(r ) = r  ) d3 x 3 ρ(  4π"0 i |r − r  |3 4π"0 |r − r | "0 =permittivity of free space = 8.854 × 10−12 C2 /(N·m2 ) 1 = 8.988 × 109 N·m2 /C2 4π"0   r 1 ρ(r  ) 3      E(r ) · d, = d x V (r ) = V (r 0 ) − 4π"0 |r − r  |  r0  ·E  = ρ ,  ×E  = 0,  = −∇V  ∇ E ∇ "0 ρ (Poisson’s Eq.) , ρ = 0 =⇒ ∇2 V = 0 (Laplace’s Eq.) ∇2 V = − "0 Laplacian Mean Value Theorem (no generally accepted name): If ∇2 V = 0, then the average value of V on a spherical surface equals its value at the center. Energy:

 1 1 qi qj 1 1 ρ(r )ρ(r  ) W = = d3 x d3 x 2 4π"0 rij 2 4π"0 |r − r  |

1 W = 2



ij

i=j 

1 d xρ(r )V (r ) = "0 2 3



 2 3 E  d x

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p. 3

Conductors:  = σn Just outside, E ˆ "0 Pressure on surface:

1  2 σ |E|outside

Two-conductor system with charges Q and −Q: Q = CV , W = 12 CV 2 N isolated conductors: Vi =



Pij Qj ,

Pij = elastance matrix, or reciprocal capacitance matrix

Cij Vj ,

Cij = capacitance matrix

j

Qi =

j

a a2 Image charge in sphere of radius a: Image of Q at R is q = − Q, r = R R Separation of Variables for Laplace’s Equation in Cartesian Coordinates:  V =

cos αx sin αx



cos βy sin βy



cosh γz sinh γz

 where γ 2 = α2 + β 2

Separation of Variables for Laplace’s Equation in Spherical Coordinates: Traceless Symmetric Tensor expansion:   1 ∂ 1 2 ∂ϕ r + 2 ∇2θ ϕ = 0 , ∇ ϕ(r, θ, φ) = 2 r ∂r r ∂r where the angular part is given by   1 ∂ ∂ϕ 1 ∂ 2ϕ 2 sin θ + ∇θ ϕ ≡ sin θ ∂θ ∂θ sin2 θ ∂φ2 2

()

()

∇2θ Ci1 i2 ...i n ˆ i1 n ˆ i2 . . . n ˆ i = −,(, + 1)Ci1 i2 ...i n ˆ i1 n ˆ i2 . . . n ˆ i , ()

where Ci1 i2 ...i is a symmetric traceless tensor and n ˆ = sin θ cos φ eˆ1 + sin θ sin φ eˆ2 + cos θ eˆ3 . General solution to Laplace’s equation:   () ∞

C () i2 ...i rˆi1 rˆi2 . . . rˆi , V (r ) = Ci1 i2 ...i r  + i1+1 r =0

where r = rrˆ

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Azimuthal Symmetry:  ∞ 

B  A r + +1 { zˆi1 . . . zˆi } rˆi1 . . . rˆi V (r ) = r =0 where { . . . } denotes the traceless symmetric part of . . . . Special cases: {1} = 1 { zˆi } = zˆi { zˆi zˆj } = zˆi zˆj − 13 δij



 zˆi δjk + zˆj δik + zˆk δij  { zˆi zˆj zˆk zˆm } = zˆi zˆj zˆk zˆm − 71 zˆi zˆj δkm + zˆi zˆk δmj + zˆi zˆm δjk + zˆj zˆk δim    1 δij δkm + δik δjm + δim δjk + zˆj zˆm δik + zˆk zˆm δij + 35 { zˆi zˆj zˆk } = zˆi zˆj zˆk −

1 5

Legendre Polynomial / Spherical Harmonic expansion: General solution to Laplace’s equation:    ∞

Bm  V (r ) = Am r + +1 Ym (θ, φ) r =0 m=−





2π

Orthonormality:

π

dφ 0

0

sin θ dθ Y∗
m
(θ, φ) Ym (θ, φ) = δ
 δm
m

Azimuthal Symmetry:  ∞ 

B  V (r ) = A r + +1 P (cos θ) r =0

Electric Multipole Expansion: First several terms:  1  Q p  · rˆ 1 rˆi rˆj Q + · · · , where V (r ) = + 2 + ij r 2 r3 4π"0 r    3 3 Q = d x ρ(r ) , pi = d x ρ(r ) xi Qij = d3 x ρ(r )(3xi xj −δij |r |2 ) ,  dip (r ) = − 1 ∇  E 4π"0  ×E  dip (r ) = 0 , ∇



p · rˆ r2

 =

1 3(p · rˆ)ˆ r − p 1 − pi δ 3 (r ) 3 4π"0 r 3"0

 ·E  dip (r ) = 1 ρdip (r ) = − 1 p · ∇δ  3 (r ) ∇ "0 "0

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Traceless Symmetric Tensor version: V (r ) =

∞ 1 1 () Ci1 ...i rˆi1 . . . rˆi , +1 r 4π"0 =0

where () Ci1 ...i

(2, − 1)!! = ,!

 ρ(r ) { xi1 . . . xi } d3 x

(r ≡ rrˆ ≡ xi eˆi )

∞

(2, − 1)!! r  1 = { rˆi1 . . . rˆi } rˆi1 . . . rˆi , ,! r +1 |r − r  |

for r  < r

=0

(2, − 1)!! ≡ (2, − 1)(2, − 3)(2, − 5) . . . 1 =

(2,)! , with (−1)!! ≡ 1 . 2 ,!

Reminder: { . . . } denotes the traceless symmetric part of . . . . Griffiths version:  ∞ 1 1  V (r ) = r  ρ(r  )P (cos θ  ) d3 x +1 r 4π"0 =0



where θ = angle between r and r  . ∞

r 1 < = P (cos θ  ) , +1  |r − r  | r>

∞

√

=0

1 P (x) =  2 ,!



d dx

1 = λ P (x) 2 1 − 2λx + λ =0

 (x2 − 1) ,

(Rodrigues’ formula) 

P (1) = 1

1



P (−x) = (−1) P (x)

−1

dx P
(x)P (x) =

2 δ
 2, + 1

Spherical Harmonic version:∗∗∗ ∞  1 4π qm V (r ) = Ym (θ, φ) 4π"0 2, + 1 r +1 =0 m=−



where qm =

∗  Ym r ρ(r  ) d3 x

∞



1 4π r  ∗   = Y (θ , φ )Ym (θ, φ) , 2, + 1 r +1 m |r − r  | =0 m=−

for r  < r

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Electric Fields in Matter: Electric Dipoles:  p = d3 x ρ(r ) r  r δ 3 (r − r d ) , where r d = position of dipole ρdip (r ) = −p · ∇  = (p · ∇  )E  =∇  (p · E)  F (force on a dipole)   = p × E (torque on a dipole)  U = −p · E Electrically Polarizable Materials:  (r ) = polarization = electric dipole moment per unit volume P  ·n ρbound = −∇ · P , σbound = P ˆ  ≡ "0 E  +P  , D

 ·D  = ρfree , ∇

 ×E  = 0 (for statics) ∇

Boundary conditions: ⊥ ⊥ Eab ove − Ebelow =

σ "0

  E above − Ebelow = 0

⊥ ⊥ Dab ove − Dbelow = σfree

    D above − Dbelow = Pabove − Pbelow

Linear Dielectrics:  = "0 χe E,  P

χe = electric susceptibility  = "E  " ≡ "0 (1 + χe ) = permittivity, D " = 1 + χe = relative permittivity, or dielectric constant "r = "0

N α/"0 , where N = number density of atoms 1 − Nα 30  or (nonpolar) molecules, α = atomic/molecular polarizability (P = αE)  1  ·E  d3 x (linear materials only) Energy: W = D 2  W (Even if one or more potential differences are Force on a dielectric: F = −∇ held fixed, the force can be found by computing the gradient with the total charge on each conductor fixed.) Clausius-Mossotti equation: χe =

Magnetostatics: Magnetic Force:  = q (E  + v × B)  = dp , F dt

where p = γm0v ,

1 γ=  1−

v2 c2

8.07 FORMULA SHEET FOR FINAL EXAM, FALL 2012

  = F

p. 7

  = Id, × B

 d3 x J × B

Current Density:

 J · da

Current through a surface S: IS = S

Charge conservation:

∂ρ  · J = −∇ ∂t

Moving density of charge: J = ρv Biot-Savart Law:      d, × (r − r  ) K(r ) × (r − r  )  µ0 µ0  B (r ) = I = da 4π |r − r  |3 4π |r − r  |3    µ0 J(r ) × (r − r  ) 3 = d x 4π |r − r  |3 where µ0 = permeability of free space ≡ 4π × 10−7 N/A2 Examples:  = µ0 I φˆ Infinitely long straight wire: B 2πr  = µ0 nI0 zˆ , where n = turns per Infinitely long tightly wound solenoid: B unit length  0, z) = Loop of current on axis: B(0,

µ0 IR2 zˆ 2(z 2 + R2 )3/2

 r ) = 1 µ0 K  ×n Infinite current sheet: B( ˆ, n ˆ = unit normal toward r 2 Vector Potential:   )coul = µ0 A(r 4π



J(r  ) 3  d x , |r − r  |

 =∇  ×A , B

 ·A  coul = 0 ∇

 ·B  = 0 (Subject to modification if magnetic monopoles are discovered) ∇   (r ) = A(  r ) + ∇Λ(  r ) for any Λ(r ). B  =∇  ×A  is Gauge Transformations: A unchanged. Amp`ere’s Law:

  · d, = µ0 Ienc B

 ×B  = µ0 J , or equivalently ∇ P

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p. 8

Magnetic Multipole Expansion: Traceless Symmetric Tensor version: ∞ µ0 () { rˆi1 . . . rˆi } Mj;i1 i2 ...i r +1 4π =0  (2, − 1)!! () d3 xJj (r ){ xi1 . . . xi } where Mj ;i1 i2 ...i = ,!  Current conservation restriction: d3 x Sym(xi1 . . . xi−1 Ji ) = 0

Aj (r ) =

i1 ...i

where

Sym i1 ...i

means to symmetrize — i.e. average over all

orderings — in the indices i1 . . . i Special cases:  , = 1:



d3 x Ji = 0 d3 x (Ji xj + Jj xi ) = 0

, = 2:

 × rˆ  r ) = µ0 m Leading term (dipole): A( , 4π r 2 where 1 (1) mi = − "ijk Mj;k 2   1 1  m  = I r × d, = d3 x r × J = Ia , 2 2 P  where a = da for any surface S spanning P S

 × rˆ µ0 3(m  · rˆ)ˆ r−m  2µ0  dip (r ) = µ0 ∇  ×m = m  δ 3 (r ) B + 2 3 4π r 3 4π r  ·B  dip (r ) = 0 ,  ×B  dip (r ) = µ0 Jdip (r ) = −µ0 m  δ 3 (r )  ×∇ ∇ ∇ Griffiths version:

∞ µ0 I 1  A(r ) = (r  ) P (cos θ  )d, 4π r +1 =0

Magnetic Fields in Matter: Magnetic Dipoles:   1 1  d3 x r × J = Ia m  = I r × d, = 2 P 2

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p. 9

 r δ 3 (r − r d ), where r d = position of dipole  ×∇ Jdip (r ) = −m  =∇  (m  F  · B) (force on a dipole)   =m  ×B  U = −m  ·B

(torque on a dipole)

Magnetically Polarizable Materials:  (r ) = magnetization = magnetic dipole moment per unit volume M  ×M  ,  bound = M  ×n K ˆ Jbound = ∇  = Jfree ,  ,  ×H  ·B  =0  ≡ 1B  −M ∇ ∇ H µ0 Boundary conditions: ⊥ ⊥ ⊥ ⊥ ⊥ ⊥ Hab Bab ove − Bbelow = 0 ove − Hbelow = −(Mabove − Mbelow )       ˆ) H ˆ B above − Bbelow = µ0 (K × n above − Hbelow = Kfree × n Linear Magnetic Materials:  = χm H,  χm = magnetic susceptibility M  = µH  B µ = µ0 (1 + χm ) = permeability, Magnetic Monopoles:  r ) = µ0 qm rˆ ;  B( Force on a static monopole: F = qm B 4π r 2  = µ0 qe qm rˆ , where rˆ points Angular momentum of monopole/charge system: L 4π from qe to qm µ0 qe qm 1 = h ¯ × integer Dirac quantization condition: 4π 2 Connection Between Traceless Symmetric Tensors and Legendre Polynomials or Spherical Harmonics: (2,)! { zˆi1 . . . zˆi } n ˆ i1 . . . n ˆ i P (cos θ) =  2 (,!)2 For m ≥ 0, (,m)

ˆ i1 . . . n ˆ i , Ym (θ, φ) = Ci1 ...i n (,m)

ˆ+ ˆ+ ˆim+1 . . . zˆi } , where Ci1 i2 ...i = dm { u i1 . . . u im z  2m (2, + 1) (−1)m (2,)! , with dm = 2 ,! 4π (, + m)! (, − m)! 1 ex + ieˆy ) and u ˆ+ = √ (ˆ 2 ∗ Form m < 0, Y,−m (θ, φ) = (−1)m Ym (θ, φ)

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p. 10

More Information about Spherical Harmonics:∗∗∗  2 + 1 ( − m)! m P (cos θ)eimφ Y£m (θ, φ) = 4π ( + m)! £ where P£ m (cos θ) is the associated Legendre function, which can be defined by P£m (x) =

£+m

(−1)m 2 m/2 d (1 − x ) (x2 − 1)£

£ £+m 2 ! dx

Legendre Polynomials:

SPHERICAL HARMONICS Ylm(θ , φ) l=0

1

Y00 =

4π 3 sin θeiφ 8π

Y11 = l=1

3 cos θ 4π

Y10 =

Y22 =

l=2

1 4

15 sin2 θe2iφ 2π 15 sin θ cosθeiφ 8π

Y21 = -

Y20 =

5 ( 32 cos2θ 4π

1 ) 2

35 sin3 θe3iφ 4π

Y33 = -

1 4

Y32 =

1 4

105 sin2 θ cos θe2iφ 2π

Y31 = -

1 4

21 sinθ (5cos2θ -1)eiφ 4π

l=3

Y30 =

7 ( 5 cos3θ 4π 2

3 2

cos θ)

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p. 11

Maxwell’s Equations:   ×E  = − ∂B , (iii)∇ ∂t

 ·E  = 1ρ (i) ∇ "0

  ×B  = µ0 J + 1 ∂E (iv)∇ c2 ∂t

 ·B  =0 (ii) ∇

1 c2  + v × B)  Lorentz force law: F = q(E where µ0 "0 =

∂ρ  · J = −∇ ∂t Maxwell’s Equations in Matter: Charge conservation:

: Polarization P and magnetization M  · P , ρb = −∇

 ×M  , Jb = ∇

ρ = ρf + ρb ,

J = Jf + Jb

Auxiliary Fields:   ≡ B −M  , H µ0

 ≡ "0 E  + P D

Maxwell’s Equations:  ·D  = ρf (i) ∇  ·B  =0 (ii) ∇

  ×E  = − ∂B , (iii)∇ ∂t   ×H  = Jf + ∂D (iv)∇ ∂t

For linear media:  = "E  , D

 = 1B  H µ

where " = dielectric constant, µ = relative permeability  ∂D = displacement current Jd ≡ ∂t Maxwell’s Equations with Magnetic Charge:   = −µ0 Jm − ∂B ,  ×E (iii)∇ ∂t   = µ0 Je + 1 ∂E  ·B  = µ0 ρm  ×B (iv)∇ (ii) ∇ c2 ∂t   1    Magnetic Lorentz force law: F = qm B − 2 v × E c

 ·E  = 1 ρe (i) ∇ "0

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p. 12

Current, Resistance, and Ohm’s Law:  + v × B)  , where σ = conductivity. ρ = 1/σ = resistivity J = σ(E Resistors: V = IR ,

P = IV = I 2 R = V 2 /R

, Resistance in a wire: R = ρ , where , = length, A = cross-sectional area, and ρ = A resistivity   V0 −t/RC Charging an RC circuit: I = , Q = CV0 1 − e−t/RC e R  + v × B)  · d, , where v is either the velocity EMF (Electromotive force): E ≡ (E of the wire or the velocity of the charge carriers (the difference points along the wire, and gives no contribution) Inductance: Universal flux rule: Whenever the flux through a loop changes, whether due to a  or motion of the loop, E = − dΦB , where ΦB is the magnetic flux changing B dt through the loop Mutual inductance: Φ2 = M21 I1 , M21 = mutual inductance µ0 d,1 · d,2 (Franz) Neumann’s formula: M21 = M12 = 4π P1 P2 |r 1 − r 2 | Self inductance: Φ = LI ,

E = −L

dI ; dt

L = inductance

Self inductance of a solenoid: L = n2 µ0 V , where n = number of turns per length, V = volume  V0  R t L 1−e Rising current in an RL circuit: I = R Boundary Conditions: D1⊥ − D2⊥ = σf 1 E1⊥ − E2⊥ = σ "0

 − E  = 0 E 1 2  −D  = P − P  D

B1⊥ − B2⊥ = 0 H1⊥ − H2⊥ = M2⊥ − M1⊥

 −H  = −n f H ˆ×K 1 2  = −µ0 n   −B ˆ×K B 1 2

1

2

1

2

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p. 13

Conservation Laws:

  1 1  2 2  |B| Energy density: uEM = "0 |E | + 2 µ0 = 1 E  ×B  Poynting vector (flow of energy): S µ0 Conservation of energy:  d  · da Integral form: [UEM + Umech ] = − S dt ∂u  , where u = uEM + umech  ·S Differential form: = −∇ ∂t 1  1 ; 2 Si is the density of momentum in the i’th Momentum density: ℘EM = 2 S c c direction     1 1 1 2 2   Maxwell stress tensor: Tij = "0 Ei Ej − δij |E| + Bi Bj − δij |B| 2 µ0 2 where −Tij = −Tji = flow in j’th direction of momentum in the i’th direction Conservation of momentum:    d 1 3 Si d x = Tij daj , for a volume V Integral form: Pmech,i + 2 dt c V S bounded by a surface S ∂ Differential form: (℘mech,i + ℘EM,i ) = ∂j Tji ∂t Angular momentum:  × B)]  Angular momentum density (about the origin): ,EM = r ×℘EM = "0 [r ×(E

Wave Equation in 1 Dimension: 1 ∂ 2f ∂2f − = 0 , where v is the wave velocity v 2 ∂t2 ∂z 2 Sinusoidal waves: f (z, t) = A cos [k(z − vt) + δ] = A cos [kz − ωt + δ] where ω = angular frequency = 2πν ν = frequency ω v = = phase velocity δ = phase (or phase constant) k k = wave number λ = 2π/k = wavelength T = 2π/ω = period A = amplitude Euler identity: eiθ = cos θ + i sin θ ˜ i(kz−ωt) ] , where A˜ = Aeiδ ; “Re” is usually Complex notation: f (z, t) = Re[Ae dropped. ω dω = group velocity Wave velocities: v = = phase velocity; vgroup = k dk

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p. 14

Electromagnetic Waves:  1 ∂ 2E =0, c2 ∂t2 Linearly Polarized Plane Waves:

− Wave Equations: ∇2 E

− ∇2 B

 1 ∂ 2B =0 c2 ∂t2

 r −ωt) ˜0 is a complex amplitude, n  (r , t) = E ˜0 ei(k· n ˆ , where E ˆ is a unit vector, E and ω/|k| = vphase = c. n ˆ · k = 0 (transverse wave)

 = 1k ˆ×E  B c Energy and Momentum: u = "0 E02 cos2 (kz − ωt + δ) , (k = k zˆ)   averages to 1/2 ! " 1 = 1E  ×B  = uc zˆ ,  = "0 E02 S I (intensity) = |S| µ0 2 1  u ℘EM = 2 S = zˆ c c Electromagnetic Waves in Matter: # µ" n≡ = index of refraction µ0 "0 c v = phase velocity = n   1  |2 + 1 |B|  2 u= "|E 2 µ  = nk ˆ×E  B c  = 1E  ×B  = uc zˆ S µ n Reflection and Transmission at Normal Incidence: Boundary conditions: "1 E1⊥ = "2 E2⊥ B1⊥

=

B2⊥

i(k1 z−ωt)

ET

El

V2

V1 Bl

Incident wave (z < 0): ˜0,I e  I (z, t) = E E

X

 ,  = E E 1 2 1  1  B1 = B . µ1 µ2 2

BT Z

eˆx

 I (z, t) = 1 E ˜0,I ei(k1 z−ωt) eˆy . B v1

ER

BR

V1

Interface Y

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p. 15

Transmitted wave (z > 0):  T (z, t) = E ˜0,T ei(k2 z−ωt) eˆx E ˜0,T ei(k2 z−ωt) eˆy .  T (z, t) = 1 E B v2 Reflected wave (z < 0):  R (z, t) = E ˜0,R ei(−k1 z−ωt) eˆx E  R (z, t) = − 1 E ˜0,R ei(−k1 z−ωt) eˆy . B v1 ω must be the same on both sides, so ω c ω c = v1 = , = v2 = n1 k2 n2 k1 Applying boundary conditions and solving, approximating µ1 = µ2 = µ0 ,   n1 − n2 ˜ 2n1 ˜ ˜0,I E0,R = E0,I E E0,T = n1 + n2 n1 + n2 Electromagnetic Potentials:  =∇  ×A , The fields: B

  = −∇  V − ∂A E ∂t

+∇ Λ ,  = A Gauge transformations: A  ·A =0 Coulomb gauge: ∇

=⇒

 ·A  = − 1 ∂V Lorentz gauge: ∇ c2 ∂t 2

2

V =−

1 ρ, "0

2

V =V −

∇2 V = −

1 ρ "0

∂Λ ∂t  is complicated) (but A

=⇒

 = −µ0 J , A

where

2

≡ ∇2 −

1 ∂2 c2 ∂t2

= D’Alembertian

Retarded time solutions (Lorentz gauge):    r  , tr ) r  , tr ) 1 1 3  ρ( 3  J(  V (r , t) = , A( r , t) = d x d x 4π"0 |r − r  | 4π"0 |r − r  | where tr = t −

|r − r  | c

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p. 16

Li´enard-Wiechert Potentials (potentials of a point charge): V (r , t) =

1 q  4π"0 |r − r p | 1 −

qvp  (r , t) = µ0 A  4π |r − r p | 1 −

 vp c

 vp c

·ˆ

·ˆ



=

vp V (r , t) c2

where r p and vp are the position and velocity of the particle at the retarded time tr , and  = r − r p ,

= |r − r p | ,

ˆ =

r − r p |r − r p |

Fields of a point charge (from the Li´enard-Wiechert potentials):  r , t) = E(

 2 q |r − r p | 2 ( ) + ( ) v  u  r −  r ) × (  u ×  a − c p p p 4π"0 (u · (r − r p ))3

 r , t) = 1 ˆ × E(  r , t) B( c where u = c ˆ − vp Radiation: Radiation from an oscillating electric dipole along the z axis: p(t) = p0 cos(ωt) ,

p0 = q0 d

Approximations: d  λ  r,   cos θ p0 ω V (r, θ, t) = − sin[ω(t − r/c)] 4π"0 c r  r , t) = − µ0 p0 ω sin[ω(t − r/c)] zˆ A( 4πr   2 µ p ω sin θ 0 0  r , t) = 1 rˆ × E(  r , t)  =− E cos[ω (t − r/c)] θˆ , B( c 4π r 2    1  µ0 p0 ω 2 sin θ   Poynting vector: S = (E × B ) = rˆ cos[ω(t − r/c)] µ0 c 4π r ! "  µ p2 ω 4  sin2 θ $ 2% 1 0 0  = cos = Intensity: I = S r ˆ , using r2 2 32π 2 c  ! " 2 4  · da = µ0 p0 ω Total power: P  = S 12πc

8.07 FORMULA SHEET FOR FINAL EXAM, FALL 2012

p. 17

Magnetic Dipole Radiation: Dipole moment: m  (t) = m0 cos(ωt) zˆ , at the origin   µ0 m0 ω 2 sin θ   r , t) = 1 rˆ × E(  r , t) E=− cos[ω(t − r/c)] φˆ , B( 4πc r c m0 , θˆ → −φˆ Compared to the electric dipole radiation, p0 → c General Electric Dipole Radiation: ¨  r , t) = 1 rˆ × E(  r , t) = − µ0 [ˆ  (r , t) = µ0 [(ˆ E r · p¨ )ˆ r − p¨ ] , B( r ×p] 4πr c 4πrc Multipole Expansion for Radiation: The electric dipole radiation formula is really the first term in a doubly infinite series. There is electric dipole, quadrupole, . . . radiation, and also magnetic dipole, quadrupole, . . . radiation. Radiation from a Point Particle: When the particle is at rest at the retarded time, q  rad = E [ ˆ × ( ˆ × ap )] 2 4π"0 c |r − r  | 2 2 rad = 1 |E  rad |2 ˆ = µ0 q a Poynting vector: S 16π 2 c µ0 c where θ is the angle between ap and ˆ .

Total power (Larmor formula): P =



sin2 θ 2

 ˆ

µ0 q 2 a 2 6πc

(valid for vp = 0 or |vp |  c) Li´enard’s Generalization if vp
= 0:  2     µ0 q 2 γ 6  v ×  a  P = a2 −  = 6πc c 

µ0 q 2 dpµ dpµ 6πm20 c dτ dτ   For relativists only

Radiation Reaction: Abraham-Lorentz formula: 2 rad = µ0 q a˙ F 6πc The Abraham-Lorentz formula is guaranteed to give the correct average energy loss for periodic or nearly periodic motion, but one would like a formula that works under general circumstances. The Abraham-Lorentz formula leads to runaway solutions which are clearly unphysical. The problem of radiation reaction for point particles in classical electrodynamics apparently remains unsolved.

8.07 FORMULA SHEET FOR FINAL EXAM, FALL 2012

p. 18

Vector Identities:

Triple Products

A . (B x C) = B . (C x A) = C . (A x B) A x (B x C) = B(A . C) - C(A . B) Products Rules

∆

(f g) = f ( g) + g ( f)

∆

(A . B) = A x ( x B) + B x ( x A) + (A . )B + (B . )A

∆

(f A) = f ( . A) + A . ( f)

∆

(A x B) = B . ( x A) - A . ( x B)

∆

x (f A) = f ( x A) - A x ( f)

∆

(A x B) = (B . )A - (A . )B + A ( . B) - B( . A)

∆

∆

∆

∆

∆

∆

∆

∆

∆

∆

∆

∆

∆

∆

∆

∆

Second Derivatives

∆

. ( x A) = 0

∆

x ( f) = 0

∆

x ( x A) =

∆

∆

( . A) - 2A ∆

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∆ ∆

∆

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8.07 Electromagnetism II

)DOO

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