Problem 30986.

Solenoid length l = 1 m, diameter D = 10 cm. A wire with diameter d = 1 mm is tightly wound around the solenoid. How much current must be passed through the solenoid so that the energy of its magnetic field is equal to the energy of the Earth's magnetic field contained in a room measuring 8*4*2.5 m^3? Find the ratio of the volumetric energy densities of the magnetic field of the solenoid and the Earth. Take the Earth's magnetic field strength to be 40 A/m.