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Vector identities

Notation: $ {\bf a}$ , $ {\bf b}$ , $ {\bf c}$ , $ {\bf d}$ are general vectors; $ \phi $ , $ \psi$ are general scalar fields; $ {\bf a} = a_x\,{\bf e}_x+a_y\,{\bf e}_y+a_x\,{\bf e}_z= (a_x,\,a_y,\,a_z)$ , and so on, where $ {\bf e}_x$ , $ {\bf e}_y$ , and $ {\bf e}_z$ are right-handed Cartesian basis vectors.

$\displaystyle \vert{\bf a}\vert$ $\displaystyle = \left(a_x^{\,2}+a_y^{\,2}+a_z^{\,2}\right)^{1/2}$ (A.73)
$\displaystyle {\bf a}\cdot{\bf b}$ $\displaystyle = a_x\,b_x+a_y\,b_y+a_z\,b_z$ (A.74)
$\displaystyle {\bf a}\times {\bf b}$ $\displaystyle =\left\vert\begin{array}{ccc}{\bf e}_x,& {\bf e}_y,& {\bf e}_z\\ [0.5ex] a_x,& a_y, &a_z\\ [0.5ex] b_x,& b_y,& b_z \end{array}\right\vert$    
  $\displaystyle = (a_y\,b_z-a_z\,b_y)\,{\bf e}_x+(a_z\,b_x-a_x\,b_z)\,{\bf e}_y+(a_x\,b_y-a_y\,b_x)\,{\bf e}_z$ (A.75)
$\displaystyle {\bf a}\times ({\bf b}\times {\bf c})$ $\displaystyle = ({\bf a}\cdot{\bf c})\,{\bf b} - ({\bf a}\cdot{\bf b})\,{\bf c}$ (A.76)
$\displaystyle ({\bf a}\times {\bf b})\times {\bf c}$ $\displaystyle = ({\bf c}\cdot{\bf a})\,{\bf b}- ({\bf c}\cdot{\bf b})\,{\bf a}$ (A.77)
$\displaystyle ({\bf a}\times {\bf b})\cdot({\bf c}\times {\bf d})$ $\displaystyle = ({\bf a}\cdot{\bf c})\,({\bf b}\cdot{\bf d}) - ({\bf a}\cdot{\bf d})\,({\bf b}\cdot{\bf c})$ (A.78)
$\displaystyle ({\bf a}\times {\bf b})\times ({\bf c}\times {\bf d})$ $\displaystyle = ({\bf a}\times {\bf b}\cdot{\bf d})\,{\bf c} - ({\bf a}\times {\bf b}\cdot{\bf c})\,{\bf d}$ (A.79)
$\displaystyle \nabla\phi$ $\displaystyle = \frac{\partial\phi}{\partial x}\,{\bf e}_x+\frac{\partial\phi}{\partial y}\,{\bf e}_y+\frac{\partial\phi}{\partial z}\,{\bf e}_z$ (A.80)
$\displaystyle \nabla(\phi\,\psi)$ $\displaystyle =\phi\,\nabla\psi+\psi\,\nabla\phi$ (A.81)

Richard Fitzpatrick 2016-03-31