Two long conducting wires carrying equal currents are placed to the $Z-axis$. A charged particle is made to move in a circular path of radius ‘a’ with centre of the path at point $(0,2a)$ in clockwise direction. During the course of motion, it passes through four  points $P, Q, R and S$ having coordinates $(0, a), (-a, 2a), (0, 3a) and (a, 2a)$ respectively. The magnitude of force exerted on the particle by the magnetic field created by the wires is maximum when it passes through points.