420. A current I = 20 A flows along a circular loop with a radius of R = 5 cm. The loop is located in a uniform magnetic field (B = 40 mT) so that the normal to the plane of the loop makes an angle υ = π / 6 with the vector B. Determine the change in the potential the energy of the contour when it is rotated by an angle φ = π / 2 in the direction of increasing the angle υ.