An iron core shaped as a doughnut with round cross-section of radius `a = 3.0 cm` carreis a winding of `N = 1000` turns through which a current `I = 1.0 A` flows. The mean radius of the doughnut is `b = 32 cm`. Using the polt in Fig. Find the magnetic energy strored up in the core. A field strength `H` is suposed to be the same throughout the cross-section and equal to its magnitude in the centre of the cross-section.