$($c$29$p$5)$ Consider the circuit in the figure $($b$).$ The curved segments are arcs of circles of angle $25$ and radii $a=16cm$ and $b=8cm.$ The straight segments are along radii. Find the magnetic field B at point P $($the center of curvature$),$ assuming a current of $12$ A clockwise in the circuit. $($Positive out of page and negative into page$)$

$($b$)$