How do I cross Del with (scalar*vector)?

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Discussion Overview

The discussion centers on the mathematical manipulation of the curl operator (Del) applied to the product of a scalar field and a vector field, specifically examining the expression ∇ x (αB) and its equivalence to the proposed right-hand side involving the curl of α and B. The scope includes mathematical reasoning and homework-related inquiry.

Discussion Character

  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • One participant expresses uncertainty about how to apply the curl operator to a scalar multiplied by a vector, indicating a need for clarification on the left side of the equation.
  • Another participant suggests that the left side can be simplified to εijk Bk,j α, questioning whether it should be broken into two parts.
  • A third participant emphasizes the importance of returning to definitions to ensure the left-hand side equals the right-hand side, recommending exploration of the components of the expression.
  • There is a reference to the curl being related to the cross product, indicating a connection to vector calculus concepts.
  • Areas of Agreement / Disagreement

    Participants do not appear to reach a consensus on the manipulation of the expression, with multiple interpretations and approaches being discussed without resolution.

    Contextual Notes

    Participants express uncertainty about the application of the curl operator and the simplification of the expressions, indicating potential limitations in understanding the definitions and component forms involved.

Fido
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Homework Statement


Show that for any scalar field α and vector field B:

∇ x (αB) = ∇α x B + α∇ x B

Homework Equations



(∇ x B)i = εijk vk,j
(∇α)i = αi
(u x v)i = eijkujvk

The Attempt at a Solution


Since α is a scalar i wasn't quite sure how to cross it with ∇

So on the left side I have:
εijkBk,j αi

I'm pretty sure I'm supposed to solve out the left side to get to the right because that what I did for the previous problem.

The right side all I could get it to simplify to was:
αi x B + α∇ x B
 
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Nabla cross product with something is the curl of the thing right?
http://www.math.ucla.edu/~ronmiech/Calculus_Problems/32B/chap14/section5/930d31/930_31.html
 
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So does that mean that my left side is just εijk Bk,j α ? I feel like it's supposed to be broken up into two parts
 
Last edited:
It needs to if you are going to get RHS = LHS ... if in doubt go back to the definitions.
Maybe look at ##(\alpha B)_i## also see what the RHS looks like in components. Play around until you understand it.
 

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