(a) The magnetic field inside an air filled solenoid 36cm long and 2cm in diameter is 0.8T. Approximately how much energy is stored in this field? (answer 29J)
(b) Assuming the earth's magnetic field averages about 0.5 x 10-4 T near the surface of the earth, estimate the total enery stored in this field in the first 10km above the earth's surface. (answer 5.1 x 10+15 J)