(a) Derive an expression for the magnetic field a distance r from an infinitely-long straight wire carrying current i. [I want a complete derivation.] (b) Two infinitely-long, straight wires are located 0.5 m apart and carry current into the paper. The wire on the left carries a 5 A current while the wire on the right carries a 2 A current. What is the magnitude and direction of the magnetic field at a point half-way between the wires?