$1.$ a$)$ Consider a section of an infinitely long, straight wire that

carries a current i$.$ Prove that the magnetic field at a point P$,$

located a distance r from the wire is $0$i$/(2$ r$).$

b$)$ A long straight wire in the horizontal plane carries a current of

$45$ A in north to south direction, what is the magnitude and

direction of field B at a point $3$ m east of the wire?