The elastic potential energy stored in a steel wire of length 20 m stretched through 2 cm is 80 J. The cross sectional area of the wire is $\_\_\_\_$ mm$^2$. (Given, $Y = 2.0 \times 10^{11}$ N m$^{-2}$)
The elastic potential energy stored in a steel wire of length 20 m stretched through 2 cm is 80 J. The cross sectional area of the wire is $\_\_\_\_$ mm$^2$. (Given, $Y = 2.0 \times 10^{11}$ N m$^{-2}$)