# Help with ampere's law problem

urgent help with ampere's law problem

The problem reads as: Consider a hollow wire of inner radius a and outer radius b carrying a current I. Use Ampere's law to find the magnitude of the magnetic field B(r) as a function of distance r from the center of the wire.

A) What is the current density in the wire?

B) What is B(r) in the center hollow of the wire (r < a)?

C) What is B(r) inside the metal part of the wire (a < r < b)?

D) What is B(r) outside the wire (r > b)?

HEre is the diagram:http://www.mustangmods.com/data/16002/untitled1.jpg

Here is my work, i don't beleive i am doing it right for what it is asking.
http://www.mustangmods.com/data/16002/image15.jpg

can anyone help?

I wish I could help but magnetism isnt a strong point of mine. What I can tell you is this helped me alot last semester when I was taking the course.

A) Current density is current per unit area, your unit area here is $\pi (b^2-a^2)$

B) In a solenoid there is a magnetic field between, but I'm not sure for a cylindrical wire.

C) If there was a magnetic field inside the wire, it would affect the flow of current drastically. I doubt there is one.

D) I'm quite confident there is a magnetic field outside.

Again, don't bet on my answers, but I hope it gives you some pointers until someone else helps/