Suppose that the magnetic dipole moment of Earth is 6.2 × 1025 J/T. (a) If the origin of this magnetism were a magnetized iron sphere at the center of Earth, what would be its radius? (b) What fraction of the volume of Earth would such a sphere occupy? The radius of Earth is 6.37 × 106 m. Assume complete alignment of the dipoles. The density of Earth's inner core is 13 g/cm3. The magnetic dipole moment of an iron atom is 2.1 × 10-23 J/T. Iron has a molar mass of 55.9 g/mol. (Note: Earth's inner core is in fact thought to be in both liquid and solid forms and partly iron, but a permanent magnet as the source of Earth's magnetism has been ruled out by several considerations. For one, the temperature is certainly above the Curie point.)

Suppose that the magnetic dipole moment of Earth is 6.2 × 1025 J/T. (a) If the origin of this magnetism were a magnetized iron sphere at the center of Earth, what would be its radius? (b) What fraction of the volume of Earth would such a sphere occupy? The radius of Earth is 6.37 × 106 m. Assume complete alignment of the dipoles. The density of Earth's inner core is 13 g/cm3. The magnetic dipole moment of an iron atom is 2.1 × 10-23 J/T. Iron has a molar mass of 55.9 g/mol. (Note: Earth's inner core is in fact thought to be in both liquid and solid forms and partly iron, but a permanent magnet as the source of Earth's magnetism has been ruled out by several considerations. For one, the temperature is certainly above the Curie point.)