Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (4th Edition)

(a) $I = 0.80~A$ (b) $J = 7.1\times 10^7~A/m^2$
(a) We can find the current $I$: $I = J~A$ $I = J~\pi~r^2$ $I = (4.5\times 10^5~A/m^2)~(\pi)~(0.75\times 10^{-3}~m)^2$ $I = 0.80~A$ (b) We can find the current density in the filament: $J = \frac{I}{A}$ $J = \frac{I}{\pi~r^2}$ $J = \frac{0.80~A}{(\pi)~(0.06\times 10^{-3}~m)^2}$ $J = 7.1\times 10^7~A/m^2$