Consider the circuit in the figure $($b$).$ The curved segments are arcs of circles of angle $35$o and radii a $=20$ cm and b $=10$ cm$.$ The straight segments are along radii. Find the magnetic field B at point P $($ the center of curvature$),$ assuming a current of $20$ A clockwise in the circuit. $($Positive out of page and negative into page$)$