- #1
nonequilibrium
- 1,439
- 2
One can easily prove that [itex]\nabla \cdot f[/itex] is invariant under a rotation of the reference frame, however I would like to prove that the divergence operator itself is invariant (same principle, different approach). In other words I want to prove that [itex]\mathbf \nabla = \mathbf e_x \frac{\partial}{\partial x} + \mathbf e_y \frac{\partial}{\partial y} = \mathbf e_{x'} \frac{\partial}{\partial x'} + \mathbf e_{y'} \frac{\partial}{\partial y'}[/itex] where [itex]\mathbf r' = U \mathbf r[/itex] is a coordinate transformation with U orthogonal.
I think matrix notation will simplify things. Rewrite [itex]\nabla = \mathbf e^T \cdot \partial[/itex] where we define [itex]\mathbf e = \left( \begin{align} \mathbf e_x \\ \mathbf e_y \end{align}\right)[/itex] and [itex]\partial = \left( \begin{align} \partial_x \\ \partial_y \end{align} \right)[/itex].
If I remember correctly, the basis transformation is the inverse of the coordinate transformation, i.e. [itex]\mathbf e = U \mathbf e'[/itex]. Also, one can easily check that [itex]\partial = U^T \partial'[/itex] (e.g. [itex]\partial_{x} = \frac{\partial x'}{\partial x} \partial_{x'} + \frac{\partial y'}{\partial x} \partial_{y'} = U_{11} \partial_{x'} + U_{21} \partial_{y'}[/itex])
This gives that [itex]\nabla = \mathbf e^T \cdot \partial = \left( U \mathbf e' \right)^T \cdot \left( U^T \partial' \right) = \mathbf e'^T \; U^T U^T \; \partial' \neq \mathbf e'^T \cdot \partial'[/itex]
Where did I err?
I think matrix notation will simplify things. Rewrite [itex]\nabla = \mathbf e^T \cdot \partial[/itex] where we define [itex]\mathbf e = \left( \begin{align} \mathbf e_x \\ \mathbf e_y \end{align}\right)[/itex] and [itex]\partial = \left( \begin{align} \partial_x \\ \partial_y \end{align} \right)[/itex].
If I remember correctly, the basis transformation is the inverse of the coordinate transformation, i.e. [itex]\mathbf e = U \mathbf e'[/itex]. Also, one can easily check that [itex]\partial = U^T \partial'[/itex] (e.g. [itex]\partial_{x} = \frac{\partial x'}{\partial x} \partial_{x'} + \frac{\partial y'}{\partial x} \partial_{y'} = U_{11} \partial_{x'} + U_{21} \partial_{y'}[/itex])
This gives that [itex]\nabla = \mathbf e^T \cdot \partial = \left( U \mathbf e' \right)^T \cdot \left( U^T \partial' \right) = \mathbf e'^T \; U^T U^T \; \partial' \neq \mathbf e'^T \cdot \partial'[/itex]
Where did I err?