Physics

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.
(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.

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

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