....¹

Other (i.e., $\ensuremath{\vec{r}}= (r,\theta,\phi)$, etc.) or lower dimensional (i.e., $\ensuremath{\vec{x}}= (x,y)$, etc.) coordinate systems will be employed when appropriate.

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...-direction.²

For the stress tensor, the two coordinate systems (material and laboratory) are usually chosen to the identical. This is the typical case for tensors in engineering applications, but it is not necessary.

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... conserved.³

Ignoring nuclear decay.

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... tensor,⁴

For a discussion of stress and strain (and the number of their independent components) in this thermodynamic discussion, refer to the addendum to this lecture.

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... Modified⁵

This form reduces to the classical Fick's law under special circumstances as described later .

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