Monday, January 4, 2016

vectors - Expressing the magnitude of a cross product in indicial notation


I am trying to teach myself tensor calculus but have reached a stumbling block - expressing the magnitude of a cross product in indicial notation. I know that one can express a cross product of two vectors $\vec{A}$ and $\vec{B}$ in indicial notation as follows:


$$ \vec{A} \times \vec{B} = \epsilon_{ijk}a_j b_k \hat{e}_i$$


But I am not sure how to express the magnitude of the resulting vector using indicial notation. My guess is


$$ \mid \; \vec{A} \times \vec{B} \mid^2 \; = (\vec{A} \times \vec{B})_m(\vec{A} \times \vec{B})_m = \epsilon_{ijk}a_j b_k \hat{e}_i \; \epsilon_{ijk}a_j b_k \hat{e}_i$$



but I seem to recall reading that having an index occur more than twice is undefined. How would I write the magnitude of the cross product using correct notation?



Answer



The problem is that you used the same indices to sum over the elements of the first and second $A\times B$ in your dot product. The product should actually read


$$ \mid \; \vec{A} \times \vec{B} \mid^2 \; = (\vec{A} \times \vec{B})\cdot(\vec{A} \times \vec{B}) = \epsilon_{ijk}a_j b_k \hat{e}_i \cdot\; \epsilon_{pqr}a_q b_r \hat{e}_p$$


From here on you can use the orthogonality of the $\hat{e}_i$s and then use the following relation to simplify your problem


\begin{align}\varepsilon _{ijk}\varepsilon _{imn}=\delta _{jm}\delta _{kn}-\delta _{jn}\delta _{km}\end{align}


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