Electromagnetism: Example Sheet 3 Professor David Tong, February 2015 1. A steady current I flows along a cylindrical conductor of constant circular crosssection and uniform conductivity σ. Show, using the relevant equations for E and J, that the current is distributed uniformly across the cross-section of the cylinder, and calculate the electric and magnetic fields just outside the surface of the cylinder. Verify that the integral of the Poynting vector over unit length of the surface is equal to the rate per unit length of dissipation of electrical energy as heat. 2. A monochromatic wave with fields ˆ ei(kz−ωt) Einc = E0 x , Binc = E0 i(kz−ωt) ˆe y c propagates in empty space z < 0. A perfect conductor fills the region z ≥ 0. Show that if the reflected fields are given by ˆ ei(−kz−ωt) Eref = −E0 x , Bref = E0 i(−kz−ωt) ˆe y c then the total fields E = Einc + Eref and B = Binc + Bref satisfy the Maxwell equations and the relevant boundary conditions at z = 0. What surface current flows in the plane z = 0? Compute the Poynting vector in the region z < 0 and compute its value averaged over a period T = 2π/ω. Use the result of Q6, Sheet 2 to show that the average force per unit area on the conductor is f¯ = 0 E02 . 3. Perfectly conducting planes are positioned at y = 0 and y = a. Show that a monochromatic plane wave can propagate between the plates in the y direction only if the frequency is given by ω = nπc/a with n ∈ Z. 4. Perfectly conducting planes are positioned at y = 0 and y = a. Show that a monochromatic wave may propagate between the plates in the direction z if the field components are nπy Ex = ωA sin sin(kz − ωt) a nπy By = kA sin sin(kz − ωt) a nπy nπA Bz = cos cos(kz − ωt) a a 1 with A a constant and n ∈ Z. Show that the wavelength λ is given by 1 1 n2 = − λ2 λ2∞ 4a2 where λ∞ is the wavelength of waves of the same frequency in the absence of conducting plates. 5. Consider a plane polarized electromagnetic wave described by the vector and scalar potentials, A(r, t) = A0 ei(k·r−ωt) and φ(r, t) = φ0 ei(k·r−ωt) with arbitrary A0 . Use Maxwell’s equations to find a relationship between A0 and φ. Find a gauge transformation such that the new vector potential is “transversely polarised”, i.e. A0 · k = 0. What is the scalar potential φ in this gauge? 6. For constant electric and magnetic fields, E and B, show that if E · B = 0 and E2 − c2 B2 6= 0 then there exist frames of reference where either E or B are zero, but not both. [Hint: it suffices to take just Ey and Bz non zero and consider Lorentz transformations along the x-direction with speed v < c.] 7. In d + 1 space-time dimensions, the equations of electromagnetism are given by ∂µ F µν = µ0 J ν with Fµν = ∂µ Aν − ∂ν Aµ where µ, ν = 0, 1, . . . d How many components does the electric field? How many components does the magnetic field have? What is the potential energy between two charges q1 and q2 ? How many linearly independent polarisation states does an electromagnetic wave have? [Note: Pay particular attention to the cases d = 1 and d = 2, partly because they’re special and partly because they can actually be realised in experiment. For d ≥ 4, you may denote the area of a (d − 1)-dimensional sphere as Sd−1 .] 8. An electromagnetic wave is reflected by a perfect conductor at x = 0. The electric field has the form b [f (t− ) − f (t+ )] E(t, x) = y where f is an arbitrary function and ct± = ct ± x. Show that this satisfies the relevant boundary condition at the conductor. Find the corresponding magnetic field B. 2 Show that under a Lorentz transformation to a frame moving with speed v in the x-direction the electric field is transformed to r 0 t+ 1 c−v 0 0 0 0 b ρf (ρt − ) − f where ρ = E (t , x ) = y ρ ρ c+v bF (t− ), find the wave that is reflected after it Hence for an incident wave E(t, x) = y hits a perfectly conducting mirror moving with speed v in the x-direction. 9. A particle of rest mass m and charge q moves in a constant uniform electric field E = (E, 0, 0). It starts from the origin with initial momentum p = (0, p0 , 0). Show that the particle traces out a path in the (x, y) plane given by qEy E0 x= cosh qE p0 c p where E0 = p20 c2 + m2 c4 is the initial kinematic energy of the particle. 10? . For a general 4-velocity, written as uµ = γ(c, v), show that ! −E · v/c Fµν uν = γ E+v×B In the rest-frame of a conducting medium, Ohm’s law states that J = σE where σ is the conductivity and J is the 3-current. Assuming that σ is a Lorentz scalar, show that Ohm’s law can be written covariantly as Jµ + 1 ν (J uν )uµ = σF µν uν 2 c where J µ is the 4-current and uµ is the (uniform) 4-velocity of the medium. If the medium moves with 3-velocity v in some inertial frame, show that the current in that frame is 1 J = ρv + σγ E + v × B − 2 (v · E)v c where ρ is the charge density. Simplify this formula, given that the charge density vanishes in the rest-frame of the medium. 3

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