The magnetic field lines of a current-carrying circular loop form concentric circles around the wire. At the center of the loop, the lines are nearly straight and perpendicular to the plane of the loop. The direction of the field can be determined using the right-hand thumb rule. 
Two identical circular loops \(P\) and \(Q\) each of radius \(r\) are lying in parallel planes such that they have common axis. The current through \(P\) and \(Q\) are \(I\) and \(4I\) respectively in clockwise direction as seen from \(O\). The net magnetic field at \(O\) is: 
Find magnetic field at midpoint O. Rings have radius $R$ and direction of current is in opposite sense. 