a
Young's modulus $Y=\frac{F / A}{l / L} \Rightarrow F=\frac{Y A}{L} l$ Work done
$d W=F \times d l=\int_{o}^{I} \frac{Y A}{L} l d l=\frac{1}{2} Y A \frac{l^{2}}{L}$ Given $, A=1 m m^{2}=10^{-6}$
$l=1 m m=10^{-3} m, Y=2 \times 10^{11} N m^{-2}, L=1 m=$
$W=\frac{1}{2} \times 2 \times 10^{11} \times 10^{-6} \times\left(10^{-3}\right)^{2} \mathrm{W}=0.1 \mathrm{J}$