Question

30 C of charge passes in a 5 second period through a resistive material of cross- sectional area of a circle with radius 1 x 10^-3 m. How much of the current goes through the cross-section in A?

Answer 1

The current is

$$I=\frac QT.$$

The current density is

$$J=\frac{I}{A}=\frac {Q}{tA}=\frac {Q}{t\pi r^2},\\\space\\ J=1.91·10^6\text{ A/m}^2.$$