Halliday, Resnick & Walker — Current and Resistance: Problem 54

54 The magnitude \(J\) of the current density in a certain lab wire with a circular cross section of radius \(R = 2.50 \text{ mm}\) is given by \(J = (3.00 \times 10^8)r^2\), with \(J\) in amperes per square meter and radial distance \(r\) in meters. What is the current through the outer section bounded by \(r = 0.900R\) and \(r = R\)?