(a) A current I flows in a straight wire of length L . Show that the magnitude of the magnetic field at a perpendicular distance x from the midpoint of the wire is given by |B|= n0I/4pi L/xsqrt((L/2)^2+x^2) where n0 is the permeability of free space. What is the direction of the B field? (b) A loop of wire of length l carries a current I . Compare the magnetic fields at the centre of the loop when it is bent into (a) a square and (b) an equilateral triangle.