Solving a problem in physics section 17 paragraph 80 el

80. The iron core having a ring shape with a square cross section, bears a coil of N = 1000 turns. Inner ring radius a = 0,200 m, outdoor m b = 0,250. Determine the energy W, stored in the core in the case where the current flows through the winding I = 1,26 A. Determination produce approximately by assuming the field intensity along the whole section of the core the same and equal to the value of H in the center section.