The mean free time between collisions for electrons ina wire is expressed as:

$\overline{){\mathbf{\tau}}{\mathbf{=}}\frac{\mathbf{m}\mathbf{\sigma}}{{\mathbf{n}}_{\mathbf{e}}{\mathbf{e}}^{\mathbf{2}}}}$

For electrons in an iron wire:

$\begin{array}{rcl}\mathbf{\tau}& \mathbf{=}& \frac{\mathbf{(}\mathbf{9}\mathbf{.}\mathbf{11}\mathbf{\times}{\mathbf{10}}^{\mathbf{-}\mathbf{31}}\mathbf{)}\mathbf{(}\mathbf{1}\mathbf{\times}{\mathbf{10}}^{\mathbf{7}}\mathbf{)}}{\mathbf{(}\mathbf{8}\mathbf{.}\mathbf{5}\mathbf{\times}{\mathbf{10}}^{\mathbf{28}}\mathbf{)}\mathbf{\left[}{\mathbf{(}\mathbf{1}\mathbf{.}\mathbf{6}\mathbf{\times}\mathbf{10}\mathbf{-}\mathbf{19}\mathbf{)}}^{\mathbf{2}}\mathbf{\right]}}\end{array}$

What is the mean free time between collisions for electrons in an iron wire and an aluminum wire?

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