# Magnetic field caused by long wire

1. Oct 24, 2011

### syhpui2

1. The problem statement, all variables and given/known data

A long, thin wire carrying constant current I = 2 A into the page is surrounded by a concentric cylindrical hollow wire of inner radius a = 0.12 m, and outer radius b = 0.26 m, carrying total current I = 4 A directed out of the page, as shown. Assume the current in the cylindrical hollow wire is distributed uniformly over its cross-sectional area.

Which one of the following statements is valid?
(a) For radii r > b, the magnetic field is the same as the field a distance r from the center of a long wire carrying total current of 6 A directed out of the page.

(b) Ampere's law is valid only in highly symmetric situations, such as the situation depicted in the diagram
.
(c) The magnetic field at the point labeled C is directed out of the page.

(d) The magnitude of the magnetic field at r = a is less than the magnitude of the magnetic field at point C (assume that the distance from the center to point C is 3a).

(e) None of the statements above is valid.

http://i.imgur.com/QNOTY.png

2. Relevant equations

int[B dot dl]= miu I

3. The attempt at a solution

I can understand first A,D is wrong, but what does option B even means? Is that means Biot-Savart Law is always true? What is relationship between Biot-Savart Law and Ampere's Law? C is wrong because the out of screen current is larger which is the enclosed current direction at C?

Thx