(a) A very long straight wire PQ of negligible diameter carries a steady current I1 . A rigid square coil ABCD of side l and n turns is set up with sides AB and DC parallel to the coplanar with PQ as shown in Fig. 13.16. The side of the coil AB is at distance d from the wire PQ. Derive an expression for the resultant force on the coil when a steady current I2 flows through it. What is the direction of the force? (b) Calculate the magnitude of the force when I1 = 1 A, I2 = 4 A, d = 0.10 m, n = 10, and l = 0.05 m.