Problem 1: Field angular momentum A pure magnetic dipole of dipole moment ñ is placed R + at the center of a spherical shell of radius R carry- ing total charge Q uniformly distributed across its + †ñ + + + + + surface. The dipole and the spherical shell are both initially at rest. (a) Compute the total linear momentum (density j = €0Ë × B) and angular momentum (density l = i x g) stored in the electromagnetic fields in terms of m, Q, and R. (b) The magnetic dipole moment m is slowly changed. Derive the induced electric field Ena (7) = - 40 m x A (1) (HINT: One elegant solution makes use of the dipole vector potential Ãalip (F) = ) (c) Compute the torque on the charged sphere in terms of m, Q, and R. If the dipole moment is turned off completely, how much angular momentum is the sphere left with? (Assume the sphere is heavy enough that its rotation generates negligible magnetic field.) (d) Instead, the magnetic dipole moment m is held fixed and the charged sphere is allowed to gradually expand (driven by electrostatic pressure). Will it begin to rotate? If so, where does the torque come from? +

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Problem 1: Field angular momentum
A pure magnetic dipole of dipole moment ñ is placed
+
+
+
R
at the center of a spherical shell of radius R carry-
ing total charge Q uniformly distributed across its
+ tm £
surface. The dipole and the spherical shell are both
initially at rest.
+
+
(a) Compute the total linear momentum (density g =
E0Ē x B) and angular momentum (density l = i x g) stored in the electromagnetic fields
in terms of m, Q, and R.
(b) The magnetic dipole moment ñ is slowly changed. Derive the induced electric field
Ena(7) = - 4o m x f
4 r2
(1)
(HINT: One elegant solution makes use of the dipole vector potential Ādip(7) = .)
(c) Compute the torque on the charged sphere in terms of m, Q, and R. If the dipole
moment is turned off completely, how much angular momentum is the sphere left with?
(Assume the sphere is heavy enough that its rotation generates negligible magnetic field.)
(d) Instead, the magnetic dipole moment ñ is held fixed and the charged sphere is allowed
to gradually expand (driven by electrostatic pressure). Will it begin to rotate? If so, where
does the torque come from?
Transcribed Image Text:Problem 1: Field angular momentum A pure magnetic dipole of dipole moment ñ is placed + + + R at the center of a spherical shell of radius R carry- ing total charge Q uniformly distributed across its + tm £ surface. The dipole and the spherical shell are both initially at rest. + + (a) Compute the total linear momentum (density g = E0Ē x B) and angular momentum (density l = i x g) stored in the electromagnetic fields in terms of m, Q, and R. (b) The magnetic dipole moment ñ is slowly changed. Derive the induced electric field Ena(7) = - 4o m x f 4 r2 (1) (HINT: One elegant solution makes use of the dipole vector potential Ādip(7) = .) (c) Compute the torque on the charged sphere in terms of m, Q, and R. If the dipole moment is turned off completely, how much angular momentum is the sphere left with? (Assume the sphere is heavy enough that its rotation generates negligible magnetic field.) (d) Instead, the magnetic dipole moment ñ is held fixed and the charged sphere is allowed to gradually expand (driven by electrostatic pressure). Will it begin to rotate? If so, where does the torque come from?
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