Chapter 26, Problem 9P

The magnitude J(r) of the current density in a certain cylindrical wire is given as a function of radial distance from the center of the wire’s cross section as J(r) = Br, where r is in meters, J is in amperes per square meter, and B = 2.00 × 10⁵ A/m³. This function applies out to the wire’s radius of 2.00 mm. How much current is contained within the width of a thin ring concentric with the wire if the ring has a radial width of 10.0 µm and is at a radial distance of 1.20 mm?