If a rod of original length $\ell_{1}$ has its temperature changed from $T_{1}$ to $T_{2},$ determine a formula for its new length $\ell_{2}$ in terms of $T_{1}, T_{2},$ and $\alpha .$ Assume $(a) \quad \alpha=$ constant (b) $\alpha=\alpha(T)$ is some function of temperature, and (c) $\alpha=\alpha_{0}+b T$ where $\alpha_{0}$ and $b$ are constants.