ALTERNATING-CURRENT CIRCUITS — Edward Purcell & David Morin
Physics — E&M · magnetic-energy-galaxy
Problem
Magnetic energy in the galaxy * A magnetic field exists in most of the interstellar space in our galaxy. There is evidence that its strength in most regions is between 10−6 and 10−5 gauss. Adopting 3⋅10−6 gauss as a typical value, find, in order of magnitude, the total energy stored in the magnetic field of the galaxy. For this purpose you may assume the galaxy is a disk roughly 1021 m in diameter and 1019 m thick. To see whether the magnetic energy amounts to much, on that scale, you might consider the fact that all the stars in the galaxy are radiating about 1037 joules/second. How many years of starlight is the magnetic energy worth?