Concept: The divergence of the curl is equal to $\mu_0$ times the current (in magneto-statics) (editing)

While the constituents of a derivative (the $f$ and $x$ in $\frac{df}{dx}$) can have physical interpretations, the derivative itself can have a different physical interpretation. For example, $\frac{dx}{dt}$ is a velocity. FIXME: find a better example