The figure below represents a section of a circular conductor of nonuniform diameter carrying a current of 4.50 A. The radius of cross-section A1 is 0.300 cm.

Question

(a)What is the magnitude of the current density across A1?
A: 1.59155e2 using J=I/((r^2)pi)
(b) If the current density across A2 is one-fourth the value across A1, what is the radius of the conductor at A2?
I cannot figure out part b.

Answer 1

b. I/(pi*r^2) = 1.59e2/4. r = ?.