Question: Consider an ideal stationary magnetic dipole m
Consider an ideal stationary magnetic dipole m in a static electric ï¬eld E. Show that the ï¬elds carry momentum
/
[Hint: There are several ways to do this. The simplest method is to start with
![Consider an ideal stationary magnetic dipole m in a static electric field E. Show that the fields carry momentum
/
[Hint: There are several ways to do this. The simplest method is to start with
and use integration by parts to show that
So far, this is valid for any localized static configuration. For a current confined to an infinitesimal neighborhood of the origin we can approximate V (r) ≈ V (0) − E(0) · r. Treat the dipole as a current loop, and use Eqs. 5.82 and 1.108.]21](../images/consider-an-ideal-stationary-magnetic-dipole-m-in-a-static-electric-eld-259605-1.jpg)
and use integration by parts to show that
![Consider an ideal stationary magnetic dipole m in a static electric field E. Show that the fields carry momentum
/
[Hint: There are several ways to do this. The simplest method is to start with
and use integration by parts to show that
So far, this is valid for any localized static configuration. For a current confined to an infinitesimal neighborhood of the origin we can approximate V (r) ≈ V (0) − E(0) · r. Treat the dipole as a current loop, and use Eqs. 5.82 and 1.108.]21](../images/consider-an-ideal-stationary-magnetic-dipole-m-in-a-static-electric-eld-259605-2.jpg)
So far, this is valid for any localized static conï¬guration. For a current conï¬ned to an inï¬nitesimal neighborhood of the origin we can approximate V (r) ≈ V (0) − E(0) · r. Treat the dipole as a current loop, and use Eqs. 5.82 and 1.108.]21
> According to Snell’s law, when light passes from an optically dense medium into a less dense one (n1 > n2) the propagation vector k bends away from the normal (Fig. 9.28). In particular, if the light is incident at the critical angle
> Light from an aquarium (Fig. 9.27) goes from water / through a plane of glass / into air (n = 1). Assuming it’s a monochromatic plane wave and that it strikes the glass at normal incidence, find the minimum and maximum
> A microwave antenna radiating at 10 GHz is to be protected from the environment by a plastic shield of dielectric constant 2.5. What is the minimum thickness of this shielding that will allow perfect transmission (assuming normal incidence)? [Hint: Use E
> Light of (angular) frequency ω passes from medium 1, through a slab (thickness d) of medium 2, and into medium 3 (for instance, from water through glass into air, as in Fig. 9.27). Show that the transmission coefficient for normal incidence is given by /
> Suppose / (This is, incidentally, the simplest possible spherical wave. For notational convenience, let (kr − ωt) ≡ u in your calculations.) (a) Show that E obeys all four of Maxwell’s equations, in vacuum, and find the associated magnetic field. (b) Calc
> A plane wave traveling through vacuum in the z direction encounters a perfect conductor occupying the region z(( 0, and reflects back: / (a) Find the accompanying magnetic field (in the region z < 0). (b) Assuming B=0 inside the conductor, find the current
> The “inversion theorem” for Fourier transforms states that Use this to determine A˜(k), in Eq. 9.20, in terms of f (z, 0) and f˙(z, 0).
> (a) Show directly that Eqs. 9.197 satisfy Maxwell’s equations (Eq. 9.177) and the boundary conditions (Eq. 9.175). (b) Find the charge density, λ(z, t), and the current, I (z, t), on the inner conductor.
> Work out the theory of TM modes for a rectangular wave guide. In particular, find the longitudinal electric field, the cutoff frequencies, and the wave and group velocities. Find the ratio of the lowest TM cutoff frequency to the lowest TE cutoff frequency
> (a) Find the divergence of the function v = s(2 + sin2 φ) sˆ + s sin φ cos φ φˆ+ 3z zˆ. (b) Test the divergence theorem for this function, using the quarter-cylinder (radius 2,
> Confirm that the energy in the TEmn mode travels at the group velocity. [Hint: Find the time averaged Poynting vector S and the energy density u (use Prob. 9.12 if you wish). Integrate over the cross section of the wave guide to get the energy per unit ti
> Use Eq. 9.19 to determine A3 and δ3 in terms of A1, A2, δ1, and δ2.
> Consider a rectangular wave guide with dimensions 2.28 cm*1.01 cm. What TE modes will propagate in this wave guide, if the driving frequency is 1.70* 1010 Hz? Suppose you wanted to excite only one TE mode; what range of frequencies could you use? What ar
> Show that the mode TE00 cannot occur in a rectangular wave guide. [Hint: In this case ω/c=k, so Eqs. 9.180 are indeterminate, and you must go back to Eq. 9.179. Show that Bz is a constant, and hence—applying Faraday’s law in integral form to a cross sect
> (a) Derive Eqs. 9.179, and from these obtain Eqs. 9.180. (b) Put Eq. 9.180 into Maxwell’s equations (i) and (ii) to obtain Eq. 9.181. Check that you get the same results using (i) and (iv) of Eq. 9.179.
> Starting with Eq. 9.170, calculate the group velocity, assuming there is only one resonance, at ω0. Use a computer to graph y ≡ vg/c as a function of x ≡ (ω/ω0)2, from x = 0 to 2, (a) for γ = 0, and (b) for γ = (0.1)ω0. Let (Nq2)/(2mε0ω02) = 0.003. Note
> Find the width of the anomalous dispersion region for the case of a single resonance at frequency ω0. Assume γ
> If you take the model in Ex. 4.1 at face value, what natural frequency do you get? Put in the actual numbers. Where, in the electromagnetic spectrum, does this lie, assuming the radius of the atom is 0.5 Å? Find the coefficients of refraction and dispersi
> (a) Shallow water is nondispersive; waves travel at a speed that is proportional to the square root of the depth. In deep water, however, the waves can’t “feel” all the way down to the bottomâ&#
> Calculate the reflection coefficient for light at an air-to-silver inter- face (μ1 = μ2 = μ0, ε1 = ε0,σ = 6 × 107(Ω · m)−1), at optical frequencies (ω = 4 × 1015/s).
> Express the cylindrical unit vectors sˆ, φˆ, zˆ in terms of xˆ, yˆ, zˆ (that is, derive Eq. 1.75). “Invert” your formulas to get xˆ, yˆ, zˆ in terms of sˆ, φˆ, zˆ (and φ).
> (a) Calculate the (time-averaged) energy density of an electromagnetic plane wave in a conducting medium (Eq. 9.138). Show that the magnetic contribution always dominates. / (b) Show that the intensity is /
> (a) Show that the skin depth in a poor conductor / (independent of frequency). Find the skin depth (in meters) for (pure) water. (Use the static values of ε, μ, and σ ; your answers will be valid, then, only at relatively low frequencies.) (b) Show that
> Show that the standing wave f (z, t) A sin(kz) cos(kvt ) satisfies the wave equation, and express it as the sum of a wave traveling to the left and a wave traveling to the right (Eq. 9.6).
> (a) Suppose you imbedded some free charge in a piece of glass. About how long would it take for the charge to flow to the surface? (b) Silver is an excellent conductor, but it’s expensive. Suppose you were designing a microwave experiment to operate at a
> The index of refraction of diamond is 2.42. Construct the graph analogous to Fig. 9.16 for the air/diamond interface. (Assume μ1= μ2 =μ0.) In particular, calculate (a) the amplitudes at normal incidence, (b) Brewster’s angle, and (c) the “crossover” a
> Analyze the case of polarization perpendicular to the plane of incidence (i.e. electric fields in the y direction, in Fig. 9.15). Impose the boundary conditions (Eq. 9.101), and obtain the Fresnel equations for / Sketch / as functions of θI , for the ca
> Suppose Aeiax + Beibx = Ceicx , for some nonzero constants A, B, C , a, b, c, and for all x . Prove that a = b = c and A + B = C .
> In writing Eqs. 9.76 and 9.77, I tacitly assumed that the reflected and transmitted waves have the same polarization as the incident wave—along the x direction. Prove that this must be so. [Hint: Let the polarization vect
> Calculate the exact reflection and transmission coefficients, without assuming μ1 = μ2 = μ0. Confirm that R + T = 1
> Find all elements of the Maxwell stress tensor for a monochromatic plane wave traveling in the z direction and linearly polarized in the x direction (Eq. 9.48). Does your answer make sense? (Remember that /represents the momentum flux density.) Ho
> Compute the gradient and Laplacian of the function T=r (cos θ sin θ cos φ). Check the Laplacian by converting T to Cartesian coordinates and using Eq. 1.42. Test the gradient theorem for this function, using the path
> In the complex notation there is a clever device for finding the time average of a product. Suppose f (r, t) = A cos (k · r − ωt + δa) and g(r, t) = B cos (k r ωt + Î
> Consider a particle of charge q and mass m, free to move in the xy plane in response to an electromagnetic wave propagating in the z direction (Eq. 9.48—might as well set δ = 0). (a) Ignoring the magnetic force, find the velocity of the particle, as a fun
> The intensity of sunlight hitting the earth is about 1300 W/m2. If sunlight strikes a perfect absorber, what pressure does it exert? How about a perfect reflector? What fraction of atmospheric pressure does this amount to?
> By explicit differentiation, check that the functions f1, f2, and f3 in the text satisfy the wave equation. Show that f4 and f5 do not.
> Two concentric spherical shells carry uniformly distributed charges +Q (at radius a) and −Q (at radius b > a). They are immersed in a uniform magnetic field B = B0 zˆ. (a) Find the angular momentum of the fields (with respect to the center). (b) Now the ma
> In Ex. 8.4, suppose that instead of turning off the magnetic field (by reducing I ) we turn off the electric field, by connecting a weakly10 conducting radial spoke between the cylinders. (We’ll have to cut a slot in the solenoid, so the cylinders can stil
> Consider an infinite parallel-plate capacitor, with the lower plate (at z = −d/2) carrying surface charge density −σ , and the upper plate (at z = +d/2) carrying charge density +σ .
> A charged parallel-plate capacitor (with uniform electric field E = E zˆ) is placed in a uniform magnetic field B = B xˆ, as shown in Fig. 8.6. (a) Find the electromagnetic momentum in the space between the plates. (b) Now a resisti
> Imagine two parallel infinite sheets, carrying uniform surface charge σ (on the sheet at z =d) and -σ (at z=0). They are moving in the y direction at constant speed v (as in Problem 5.17). (a) What is the electromagnetic momentum in a region of area A? (
> (a) Consider two equal point charges q, separated by a distance 2a. Construct the plane equidistant from the two charges. By integrating Maxwell’s stress tensor over this plane, determine the force of one charge on the other. (b) Do the same for charges
> Compute the divergence of the function v = (r cos θ) rˆ + (r sin θ) θˆ + (r sin θ cos φ) φˆ. Check the divergence theorem for this function, using
> Use the cross product to find the components of the unit vector n perpendicular to the shaded plane in Fig. 1.11.
> Calculate the force of magnetic attraction between the northern and southern hemispheres of a uniformly charged spinning spherical shell, with radius R, angular velocity ω, and surface charge density σ . [This is the same as Prob. 5.44, but this time use
> A circular disk of radius R and mass M carries n point charges (q), attached at regular intervals around its rim. At time t=0 the disk lies in the xy plane, with its center at the origin, and is rotating about the z axis with angular velocity ω0, when it
> (a) Carry through the argument in Sect. 8.1.2, starting with Eq. 8.6, but using J f in place of J. Show that the Poynting vector becomes S = E × H, (8.46) and the rate of change of the energy density in the fields is For linea
> A point charge q is a distance a > R from the axis of an infinite solenoid (radius R, n turns per unit length, current I ). Find the linear momentum and the angular momentum (with respect to the origin) in the fields. (Pu
> Because the cylinders in Ex. 8.4 are left rotating (at angular velocities ωa and ωb, say), there is actually a residual magnetic field, and hence angular momentum in the fields, even after the current in the solenoid has been extinguished. If the cylinders
> Consider the charging capacitor in Prob. 7.34. (a) Find the electric and magnetic fields in the gap, as functions of the distance s from the axis and the time t . (Assume the charge is zero at t = 0.) (b) Find the energy density uem and the Poynting vecto
> Suppose you had an electric charge qe and a magnetic monopole qm . The field of the electric charge is / (of course), and the field of the magnetic monopole is Find the total angular momentum stored in the fi
> Work out the formulas for u, S, g, and /in the presence of magnetic charge. [Hint: Start with the generalized Maxwell equations (7.44) and Lorentz force law (Eq. 8.44), and follow the derivations in Sections 8.1.2, 8.2.2, and 8.2.3.]
> Picture the electron as a uniformly charged spherical shell, with charge e and radius R, spinning at angular velocity ω. (a) Calculate the total energy contained in the electromagnetic fields. (b) Calculate the total angular momentum contained in the field
> (a) Check the divergence theorem for the function / using as your volume the sphere of radius R, centered at the origin. (b) Do the same for / (If the answer surprises you, look back at Prob. 1.16.)
> A sphere of radius R carries a uniform polarization P and a uniform magnetization M (not necessarily in the same direction). Find the electromagnetic momentum of this configuration. [Answer: (4/9)πμ0 R3(M × P)]
> A point charge q is located at the center of a toroidal coil of rectangular cross section, inner radius a, outer radius a+w, and height h, which carries a total of N tightly-wound turns and current I . (a) Find the electromagnetic momentum p of this confi
> An infinitely long cylindrical tube, of radius a, moves at constant speed v along its axis. It carries a net charge per unit length λ, uniformly distributed over its surface. Surrounding it, at radius b, is another cylinder, moving with the same velocity
> A very long solenoid of radius a, with n turns per unit length, carries a current Is . Coaxial with the solenoid, at radius b ((a, is a circular ring of wire, with resistance R. When the current in the solenoid is (gradually) decreased, a current Ir is i
> Derive Eq. 8.43. [Hint: Use the method of Section 7.2.4, building the two currents up from zero to their final values.]
> Derive Eq. 8.39. [Hint: Treat the lower loop as a magnetic dipole.]
> Imagine an iron sphere of radius R that carries a charge Q and a uniform magnetization M = M zˆ. The sphere is initially at rest. (a) Compute the angular momentum stored in the electromagnetic fields. (b) Suppose the sphere is g
> Calculate the power (energy per unit time) transported down the cables of Ex. 7.13 and Prob. 7.62, assuming the two conductors are held at potential difference V, and carry current I (down one and back up the other).
> An infinite number of different surfaces can be fit to a given boundary line, and yet, in defining the magnetic flux through a loop, / I never specified the particular surface to be used. Justify this apparent oversight.
> A square loop of wire (side a) lies on a table, a distance s from a very long straight wire, which carries a current I , as shown in Fig. 7.18. (a) Find the flux of B through the loop. (b) If someone now pulls the loop directly away from t
> Express the unit vectors rˆ, θˆ, φˆ in terms of xˆ, yˆ, zˆ (that is, derive Eq. 1.64). Check your answers several ways (rˆ · rˆ =? 1, θˆ · φˆ =? 0, rˆ × θˆ =? φˆ,.. .). Also work out the inverse formulas, giving xˆ, yˆ, zˆ in terms of rˆ, θˆ,
> A metal bar of mass m slides frictionlessly on two parallel conducting rails a distance l apart (Fig. 7.17). A resistor R is connected across the rails, and a uniform magnetic field B, pointing into the page, fills the entir
> (a) Show that Maxwell’s equations with magnetic charge (Eq. 7.44) are invariant under the duality transformation where / and α is an arbitrary rotation angle in “E/B-space.” Charge and c
> Prove Alfven’s theorem: In a perfectly conducting fluid (say, a gas of free electrons), the magnetic flux through any closed loop moving with the fluid is constant in time. (The magnetic &ium
> A certain transmission line is constructed from two thin metal “rib- bons,” of width w, a very small distance h
> The magnetic field of an infinite straight wire carrying a steady cur- rent I can be obtained from the displacement current term in the Ampère/Maxwell law, as follows: Picture the current as consisting of a uniform line charge λ moving along the z axis at
> Suppose J(r) is constant in time but ρ(r, t) is not—conditions that might prevail, for instance, during the charging of a capacitor. (a) Show that the charge density at any particular point is a linear function of time: Ï
> A rectangular loop of wire is situated so that one end (height h) is between the plates of a parallel-plate capacitor (Fig. 7.9), oriented parallel to the field E. The other end is way outside, where the field is essentially
> An infinite wire runs along the z axis; it carries a current I (z) that is a function of z (but not of t ), and a charge density λ(t) that is a function of t (but not of z). (a) By examining the charge flowing into a segment dz in a time dt , show that dλ/
> A transformer (Prob. 7.57) takes an input AC voltage of amplitude V1, and delivers an output voltage of amplitude V2, which is determined by the turns ratio (V2/ V1 N2/N1). If N2 > N1, the output voltage is greater than the input voltage. Why doesn’t thi
> Two coils are wrapped around a cylindrical form in such a way that the same flux passes through every turn of both coils. (In practice this is achieved by inserting an iron core through the cylinder; this has the effect of concentrating th
> Find formulas for r,θ,φ in terms of x, y, z (the inverse, in other words, of Eq. 1.62).
> (a) Use the Neumann formula (Eq. 7.23) to calculate the mutual inductance of the configuration in Fig. 7.37, assuming a is very small /Compare your answer to Prob. 7.22. (b) For the general case (not assuming a is small), show that where
> In the discussion of motional emf (Sect. 7.1.3) I assumed that the wire loop (Fig. 7.10) has a resistance R; the current generated is then I= v Bh/R. But what if the wire is made out of perfectly conducting material, so that R is zero? In that case, the
> A circular wire loop (radius r , resistance R) encloses a region of uniform magnetic field, B, perpendicular to its plane. The field (occupying the shaded region in Fig. 7.56) increases linearly with time (B= α
> The current in a long solenoid is increasing linearly with time, so the flux is proportional to t : Ф =αt . Two voltmeters are connected to diametrically opposite points ( A and B), together with resistors (R1 and
> An atomic electron (charge q) circles about the nucleus (charge Q) in an orbit of radius r ; the centripetal acceleration is provided, of course, by the Coulomb attraction of opposite charges. Now a small magnetic field dB is slowly turned on, perpendicu
> An infinite wire carrying a constant current / direction is moving in the y direction at a constant speed v. Find the electric field, in the quasistatic approximation, at the instant the wire coincides with the z axis (Fig. 7.54). /
> Electrons undergoing cyclotron motion can be sped up by increasing the magnetic field; the accompanying electric field will impart tangential acceleration. This is the principle of the betatron. One would like to keep the radius of the orbit constant dur
> A battery of emf ε and internal resistance r is hooked up to a variable “load” resistance, R. If you want to deliver the maximum possible power to the load, what resistance R should you choose? (You can’t change E and r , of course.)
> (a) Referring to Prob. 5.52(a) and Eq. 7.18, show that for Faraday-induced electric fields. Check this result by taking the divergence and curl of both sides. (b) A spherical shell of radius R carries a uniform surface charge σ . It spins abou
> Refer to Prob. 7.11 (and use the result of Prob. 5.42): How long does is take a falling circular ring (radius a, mass m, resistance R) to cross the bottom of the magnetic field B, at its (changing) terminal velocity?
> (a) Show that / (b) Show that
> A perfectly conducting spherical shell of radius a rotates about the z axis with angular velocity ω, in a uniform magnetic field Calculate the emf developed between the “north pole” and the e
> If a magnetic dipole levitating above an infinite superconducting plane (Prob. 7.45) is free to rotate, what orientation will it adopt, and how high above the surface will it float?
> A familiar demonstration of superconductivity (Prob. 7.44) is the levitation of a magnet over a piece of superconducting material. This phenomenon can be analyzed using the method of images.31 Treat the magnet as a perfect dipole m, a height z above the
> In a perfect conductor, the conductivity is infinite, so E=0 (Eq. 7.3), and any net charge resides on the surface (just as it does for an imperfect conductor, in electrostatics). (a) Show that the magnetic field is constant (∂B/∂t=0), inside a perfect cond
> The magnetic field outside a long straight wire carrying a steady current I is The electric field inside the wire is uniform: where ρ is the resistivity and a is the radius (see Exs. 7.1 and 7.3). Question: What i
> A rare case in which the electrostatic field E for a circuit can actually be calculated is the following:28 Imagine an infinitely long cylindrical sheet, of uniform resistivity and radius a. A slot (corresponding to the batt
> Two long, straight copper pipes, each of radius a, are held a distance 2d apart (see Fig. 7.50). One is at potential V0, the other at V0. The space surrounding the pipes is filled with weakly conducting material of conductivity σ
> Sea water at frequency / Hz has permittivity / permeability μ=μ0, and resistivity ρ=0.23 Ω m. What is the ratio of conduction current to displacement current? [Hint: Consider a parallel-plate capacitor immersed in sea water and driven by a voltage V0
> Suppose the conductivity of the material separating the cylinders in Ex. 7.2 is not uniform; specifically, σ(s)=k/s, for some constant k. Find the resistance between the cylinders. [Hint: Because σ is a function of position, Eq. 7.5 does not hold, the cha
> Suppose a magnetic monopole qm passes through a resistanceless loop of wire with self-inductance L. What current is induced in the loop?27
> Check Corollary 1 by using the same function and boundary line as in Ex. 1.11, but integrating over the five faces of the cube in Fig. 1.35. The back of the cube is open.
> Assuming that “Coulomb’s law” for magnetic charges (qm ) reads work out the force law for a monopole qm moving with velocity v through electric and magnetic fields E and B.26
> Suppose / (The theta function is defined in Prob. 1.46b). Show that these fields satisfy all of Maxwell’s equations, and determine ρ and J. Describe the physical situation that gives rise to these fields.
