A long, current carrying wire has a radius R and current I. What is the magnetic field inside, and outside the wire? Now, if it asked what is the field a distance r from the wire, I could simply use the Law of Biot & Savart (mu.I/2.pi.r). I havn't dome electricity in a while and I am stuck on this one. I am sure its simple, I just need a nudge. Thanks guys.