Expressing a variable from cross product

Click For Summary

Discussion Overview

The discussion revolves around the manipulation of vectors using the cross product, specifically whether cyclic rotation can be applied to express one vector in terms of the others. The context includes theoretical considerations and applications in electromagnetism.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions whether cyclic rotation in the expression \(\vec{a} = \vec{b} \times \vec{c}\) allows for expressing \(\vec{c}\) and \(\vec{b}\) in terms of \(\vec{a}\) and the other vectors, and whether this is valid only for perpendicular vectors.
  • Another participant points out that if \(\vec{a} = \vec{b} \times \vec{c}\) and \(\vec{b}\) and \(\vec{c}\) are not perpendicular, then it is incorrect to express \(\vec{b}\) or \(\vec{c}\) in terms of \(\vec{a}\) due to the perpendicularity condition of the cross product.
  • A later reply provides an identity related to the cross product that may help explore various possibilities in vector manipulation.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of cyclic rotation in the context of the cross product, indicating that the discussion remains unresolved regarding the conditions under which such manipulations are valid.

Contextual Notes

The discussion highlights limitations related to the assumptions about vector orientations and the implications of the cross product's properties, particularly regarding perpendicularity.

v-v
Messages
4
Reaction score
0
Can I use cyclic rotation in [tex]\vec{a}[/tex] = [tex]\vec{b}[/tex] x [tex]\vec{c}[/tex] and say:
[tex]\vec{c}[/tex] = [tex]\vec{a}[/tex] x [tex]\vec{b}[/tex]
[tex]\vec{b}[/tex] = [tex]\vec{c}[/tex] x [tex]\vec{a}[/tex]

for any vectors [tex]\vec{a}[/tex], [tex]\vec{b}[/tex] and [tex]\vec{c}[/tex] or only if they are perpendicular to each other?

If it's only a special case: is there a way to express [tex]\vec{b}[/tex] and [tex]\vec{c}[/tex] from the previous equation?

(I'm asking because of the equation in electromagnetism that says [tex]\vec{E}[/tex]=c([tex]\vec{B}[/tex]x[tex]\vec{U}[/tex]) where I might need to find any of the 3 vectors from the other two)

I hope writing down the matrices, finding the inverses and solving a matrix equation is not the only way.

Thank you :)
 
Mathematics news on Phys.org
Remember that the cross product of any two vectors is perpendicular to the two vectors. If [itex]\vec{a}= \vec{b}\times\vec{c}[/itex] with [itex]\vec{b}[/itex] and [itex]\vec{c}[/itex] not perpendicular, then we cannot have \vec{b}= \vec{c}\times\vec{a}[/itex] because that says [itex]\vec{b}[/itex] is perpendicular to [itex]\vec{c}[/itex] nor can [itex]\vec{c}= \vec{a}\times\vec{b}[/itex] because then [itex]\vec{c}[/itex] would be perpendicular to [itex]\vec{b}[/itex].
 
Thank you :)
 
You can explore the various possibilities with this handy identity:

[tex]\left ( \textbf{a} \times \textbf{b} \right ) \times \textbf{c} = \left ( \textbf{a} \cdot \textbf{c} \right ) \textbf{b} - \left ( \textbf{b} \cdot \textbf{c} \right ) \textbf{a}[/tex]
 

Similar threads

  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 4 ·
Replies
4
Views
5K
Replies
5
Views
2K
  • · Replies 12 ·
Replies
12
Views
2K
  • · Replies 10 ·
Replies
10
Views
3K
  • · Replies 12 ·
Replies
12
Views
3K
Replies
26
Views
3K
  • · Replies 0 ·
Replies
0
Views
3K
  • · Replies 25 ·
Replies
25
Views
2K
Replies
5
Views
2K