# Wire with Current and electric field

1. Oct 22, 2007

### emtilt

Wire with Current and associated fields - Urgent help needed

1. The problem statement, all variables and given/known data
A long wire of circular cross section with radius a carries a steady current i. Let the coordinate z denote position along the wire, $$\phi$$ denot orientation around the wire, and r denote perpendicular distance to the wire.
a)Assume a constant current density on the wire. Find an expression for the magnetic field inside and outside the wire.

b)A long 18-guage copper wire has a diameter of 0.0403in. If the current in the wire is i=1A, what is the electric field required to drive the current in this wire?

c) What is the largest magnetic field within the wire? What is the ratio of the magnetic force to the electric force on an electron moving with the current at the position of strongest magnetic field?

I got part A easily enough. But, I have no idea what to do on part b. I couldn't get anywhere even after looking in my textbook, so I'm even wondering if maybe there was an error in the problem and the teacher left something out. Is there a way to do this that I am missing? For part c, I believe the magnetic field is strongest at the surface, but I have no idea how to get that ratio. The only thing I can think of is to write $$F_B=qv\mu_0 i/2\pi a$$ and $$F_E=q\rho j$$

Last edited: Oct 22, 2007
2. Oct 22, 2007

### emtilt

Can anyone help with this? I forgot to mention that I need help somewhat urgently, before tomorrow. I've been staring at this for quite some time and have made no progress.