A copper wire 1.8 m long and 0.90 mm across carries a 45 N load…

Step 1. Area. $\pi(0.45 \times 10^{-3}\ \text{m})^2 = 6.36 \times 10^{-7}\ \text{m}^2$.

Step 2. Stretch. $\dfrac{45\ \text{N}}{6.36 \times 10^{-7}\ \text{m}^2} \times \dfrac{1.8\ \text{m}}{1.1 \times 10^{11}\ \text{Pa}} = 1.157 \times 10^{-3}\ \text{m}$.

Step 3. Energy. $\tfrac12 \times 45\ \text{N} \times 1.157 \times 10^{-3}\ \text{m} = 0.0260\ \text{J}$.

Step 4. Per cubic metre. $\dfrac{0.0260\ \text{J}}{6.36 \times 10^{-7}\ \text{m}^2 \times 1.8\ \text{m}} = 2.27 \times 10^{4}\ \text{J/m}^3$.