Chapter 5: Q27P (page 248)
Find the vector potential above and below the plane surface current in Ex. 5.8.
Short Answer
The vector potential above and below the plane surface current is .
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Chapter 5: Q27P (page 248)
Find the vector potential above and below the plane surface current in Ex. 5.8.
The vector potential above and below the plane surface current is .
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Two long coaxial solenoids each carry current I , but in opposite directions, as shown in Fig. 5.42. The inner solenoid (radius a) has turns per unit length, and the outer one (radius b) has .Find B in each of the three regions: (i) inside the inner solenoid, (ii) between them, and (iii) outside both.

Find the magnetic vector potential of a finite segment of straight wire carrying a current .[Put the wire on the zaxis, from to , and use Eq. 5.66.]
Check that your answer is consistent with Eq. 5.37.
I worked out the multipole expansion for the vector potential of a line current because that's the most common type, and in some respects the easiest to handle. For a volume current :
(a) Write down the multipole expansion, analogous to Eq. 5.80.
(b) Write down the monopole potential, and prove that it vanishes.
(c) Using Eqs. 1.107 and 5.86, show that the dipole moment can be written
A thin glass rod of radius and length carries a uniform surface charge . It is set spinning about its axis, at an angular velocity. Find the magnetic field at a distances from the axis, in the plane (Fig. 5.66). [Hint: treat it as a stack of magnetic dipoles.]
A thin uniform donut, carrying charge and mass , rotates about its axis as shown in Fig. 5.64.
(a) Find the ratio of its magnetic dipole moment to its angular momentum. This is called the gyromagnetic ratio (or magnetomechanical ratio).
(b) What is the gyromagnetic ratio for a uniform spinning sphere? [This requires no new calculation; simply decompose the sphere into infinitesimal rings, and apply the result of part (a).]
(c) According to quantum mechanics, the angular momentum of a spinning
electron is , where is Planck's constant. What, then, is the electron's magnetic dipole moment, in ? [This semi classical value is actually off by a factor of almost exactly 2. Dirac's relativistic electron theory got the 2 right, and Feynman, Schwinger, and Tomonaga later calculated tiny further corrections. The determination of the electron's magnetic dipole moment remains the finest achievement of quantum electrodynamics, and exhibits perhaps the most stunningly precise agreement between theory and experiment in all of physics.
Incidentally, the quantity (e ), where is the charge of the electron and is its mass, is called the Bohr magneton.]
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