A very long wire of circular cross-section (with radius a) is placed so that its
ID: 1493807 • Letter: A
Question
A very long wire of circular cross-section (with radius a) is placed so that its center runs along the r-axis. This wire carries a net current l in the upward (+z) direction; the current is uniformly distributed across the wire. Meanwhile, a very long thin wire is placed along the line (xy) = (+4a,0), and carries the same current l but in the downward (-z) direction. Both wires have length L (which is so much larger than the dimension a that it may be considered infinite). Compare the magnitudes of the magnetic field at the point A (located at the origin), point B located at (x, y, z) = (2a,0,0) (i.e., halfway between the wire centers), and at point C located at (x, y, z) = (-2a,0,0): A. |B_A| > |B_B| = |B-c| B. |B_A| = |B_B| = |B_C| C. |B_B| = |B_C| > |B_A| D. |B_A| > |B_b| > |B_C|Explanation / Answer
Outside teh wire the B field behaviour is the same as a thin wire, according to ampere's law.
This means that B(B)=0 since teh current intensity are teh same but with opposite directions.
Given again accoridng to Ampere's law taht B(c) is not null, teh only viable answer is E