# What is a cross product

1. Jul 23, 2014

### Greg Bernhardt

Definition/Summary

The cross product of two vectors $\mathbf{A}$ and $\mathbf{B}$ is a third vector (strictly, a pseudovector or axial vector) $\mathbf{A}\times\mathbf{B}$ perpendicular to both of the original vectors, with magnitude equal to the product of their magnitudes times the (positive) sine of the angle between them, and in the direction determined by the right-hand rule.

It is anti-commutative: $\mathbf{A}\times\mathbf{B}\,=\,-\,\mathbf{B}\times\mathbf{A}$

If the vectors $\mathbf{A}$ and $\mathbf{B}$ are considered as 1-forms, then the wedge product $\mathbf{A}\wedge\mathbf{B}$ is a 2-form (a directed area), and its dual $\ast(\mathbf{A}\wedge\mathbf{B})$ is a dual 1-form, corresponding to the pseudovector $\mathbf{A}\times\mathbf{B}$

Equations

The magnitude of $\mathbf{A}\times\mathbf{B}$ is the area of the paralleogram with sides $\mathbf{A}$ and $\mathbf{B}$:

$$|\mathbf{A}\times\mathbf{B}| = AB \sin \theta$$

In terms of Cartesian components:

$$(\mathbf{A}\times\mathbf{B})_x = A_y B_z - A_z B_y$$

$$(\mathbf{A}\times\mathbf{B})_y = A_z B_x - A_x B_z$$

$$(\mathbf{A}\times\mathbf{B})_z = A_x B_y - A_y B_x$$

The triple product (scalar product) $\mathbf{A}\cdot(\mathbf{B}\times\mathbf{C})$ is the volume of the parallepiped with sides $\mathbf{A}$ $\mathbf{B}$ and $\mathbf{C}$, and therefore:

$$\mathbf{A}\cdot(\mathbf{B}\times\mathbf{C})\,=\,\mathbf{B}\cdot(\mathbf{C}\times\mathbf{A})\,=\,\mathbf{C}\cdot(\mathbf{A}\times\mathbf{B})$$

The repeated cross product $\mathbf{A}\times(\mathbf{B}\times\mathbf{C})$ is a vector perpendicular to $\mathbf{A}$, and is a linear combination of $\mathbf{B}$ and $\mathbf{C}$:

$$\mathbf{A}\times(\mathbf{B}\times\mathbf{C})\,=\,(\mathbf{A}\cdot\mathbf{C})\mathbf{B}\,-\,(\mathbf{A}\cdot\mathbf{B})\mathbf{C}$$

Extended explanation

For two vectors $\mathbf{a} = \left(a_x,a_y,a_z\right)\;\;\mathbf{b}=\left(b_x,b_y,b_z\right)$ in $\mathbb{R}^3$, the cross poduct can be written as the determinant of a 3x3 matrix:

$$\mathbf{a}\times\mathbf{b} = \left|\begin{array}{ccc} \mathbf{i} & \mathbf{j} & \mathbf{k} \\ a_x & a_y & a_z \\ b_x & b_y & b_z\end{array}\right|$$

Where $\mathbf{i}, \mathbf{j}, \mathbf{k}$ is a right-handed orthonormal basis.

Polar vectors and pseudovectors:

A polar (ordinary) vector is reversed under an inversion of the coordinate axes: $\mathbf{a}\ \mapsto\ -\mathbf{a}$

However, the cross product of two polar vectors is not reversed: $\mathbf{a}\ \times\ \mathbf{b}\ \mapsto\ (-\mathbf{a})\ \times\ (-\mathbf{b})\ =\ \mathbf{a}\ \times\ \mathbf{b}$

In other words, the cross product of two polar (ordinary) vectors is invariant (the same) under an inversion of the coordinate axes: this is called a pseudovector (axial vector).

Directed area:

A directed area is a flat surface together with a magnitude (its area), and a sign ($\pm$) indicating a direction of rotation within the surface (alternatively, indicating a preferred normal direction).

A directed area is an elementary 2-form (a wedge product of two 1-forms).

A general 2-form is a sum of directed areas.

Its dual (in three-dimensional space) is a 1-form in the dual space, corresponding to a pseudovector normal to the surface and with magnitude equal to its area.

Triple product 3-form pseudoscalars and directed volume:

The wedge product of three 1-forms is a 3-form.

In three-dimensional space, all 3-forms are multiples of each other, and so a 3-form is essentially a scalar. To be precise, a 3-form is the dual of a 0-form, which is a scalar.

However, under an inversion of the coordinate axes, a 3-form is multiplied by minus-one, and so a 3-form technically is a psuedoscalar.

The triple product $\mathbf{a}\cdot(\mathbf{b}\,\times\,\mathbf{c})$ of three vectors is the dot product of a vector and a pseudovector, and is the pseudoscalar equal to the wedge product $\mathbf{a}\,\wedge\,\mathbf{b}\,\wedge\,\mathbf{c}$ of the three vectors, in the same order. It is the directed volume of the parallepiped whose sides are those three vectors ("directed" because it includes a sign ($\pm$) indicating a direction of rotation around the common vertex).

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