A steel tape is calibrated at $20^{\circ} \mathrm{C}$. On a cold day when the temperature is $-15^{\circ} \mathrm{C}$, what will be the percent error in the tape? $\alpha_{\text {steel }}=1.1 \times 10^{-5}{ }^{\circ} \mathrm{C}^{-1}$.
For a temperature change from $20^{\circ} \mathrm{C}$ to $-15^{\circ} \mathrm{C}$, we have $\Delta T=-35^{\circ} \mathrm{C}$. Then,
$$
\frac{\Delta L}{L_{0}}=\alpha \Delta T=\left(1.1 \times 10^{-5}{ }^{\circ} \mathrm{C}^{-1}\right)\left(-35^{\circ} \mathrm{C}\right)=-3.9 \times 10^{-4}=-0.039 \%
$$