View source for Curl (mathematics)

{{short description|Differential operator describing the rotation at a point in a 3D vector field}}
{{Other uses|Rotation operator (disambiguation)}}
{{Calculus |Vector}}

In [[vector calculus]], the '''curl''' is a [[vector operator]] that describes the [[Differential (infinitesimal)|infinitesimal]] [[rotation]] of a [[vector field]] in three-dimensional [[Euclidean space]]. At every point in the field, the curl of that point is represented by a [[vector (geometry)|vector]]. The attributes of this vector (length and direction) characterize the rotation at that point.

The direction of the curl is the axis of rotation, as determined by the [[right-hand rule]], and the magnitude of the curl is the magnitude of rotation. If the vector field represents the [[flow velocity]] of a moving [[fluid]], then the curl is the '''[[circulation (fluid dynamics)|circulation]] density''' of the fluid. A vector field whose curl is zero is called [[irrotational]]. The curl is a form of [[derivative|differentiation]] for vector fields. The corresponding form of the [[fundamental theorem of calculus]] is [[Kelvin–Stokes theorem|Stokes' theorem]], which relates the [[surface integral]] of the curl of a vector field to the [[line integral]] of the vector field around the boundary curve.

The alternative terminology ''rotation'' or ''rotational'' and alternative notations {{math|'''rot F'''}} and {{math|∇ × '''F'''}} are often used (the former especially in many European countries, the latter, using the [[del operator|del]] (or nabla) operator and the [[cross product]], is more used in other countries) for {{math|curl '''F'''}}.

Unlike the [[gradient]] and [[divergence]], curl does not generalize as simply to other dimensions; some [[#Generalizations|generalizations]] are possible, but only in three dimensions is the geometrically defined curl of a vector field again a vector field. This is a phenomenon similar to the 3-dimensional [[cross product]], and the connection is reflected in the notation {{math|∇ ×}} for the curl.

The name "curl" was first suggested by [[James Clerk Maxwell]] in 1871<ref>[http://www.clerkmaxwellfoundation.org/MathematicalClassificationofPhysicalQuantities_Maxwell.pdf Proceedings of the London Mathematical Society, March 9th, 1871]</ref> but the concept was apparently first used in the construction of an optical field theory by [[James MacCullagh]] in 1839.<ref>[https://archive.org/details/collectedworks00maccuoft Collected works of James MacCullagh]</ref><ref>[http://jeff560.tripod.com/c.html Earliest Known Uses of Some of the Words of Mathematics] [[tripod.com]]</ref>

==Definition==
[[File:Curl.svg|thumb|300px|The components of {{math|'''F'''}} at position {{math|'''r'''}}, normal and tangent to a closed curve {{math|''C''}} in a plane, enclosing a planar [[vector area]] {{math|'''A''' {{=}} ''A'''''n̂'''}}.]]

<!---DEL CROSS F IS NOT A DEFINITION, IT'S AN ALTERNATIVE NOTATION. IT DOESN'T MEAN ANYTHING SO YOU CAN'T BASE A DEFINITION ON IT----->The curl of a vector field {{math|'''F'''}}, denoted by {{math|curl '''F'''}}, or {{math|∇ × '''F'''}}, or {{math|rot '''F'''}}, at a point is defined in terms of its projection onto various lines through the point. If {{math|'''n̂'''}} is any unit vector, the projection of the curl of {{math|'''F'''}} onto {{math|'''n̂'''}} is defined to be the limiting value of a closed [[line integral]] in a plane orthogonal to {{math|'''n̂'''}} divided by the area enclosed, as the path of integration is contracted around the point.

The curl operator maps continuously differentiable functions {{math|''f'' : ℝ<sup>3</sup> → ℝ<sup>3</sup>}} to continuous functions {{math|''g'' : ℝ<sup>3</sup> → ℝ<sup>3</sup>}}, and more generally, it maps [[Smooth function|{{math|''C<sup>k</sup>''}}]] functions in {{math|ℝ<sup>3</sup>}} to {{math|''C''<sup>''k''−1</sup>}} functions in {{math|ℝ<sup>3</sup>}}. [[Image:Curlorient.svg|200px|right|thumb|Convention for vector orientation of the line integral]]

Implicitly, curl is defined:<ref>Mathematical methods for physics and engineering, K.F. Riley, M.P. Hobson, S.J. Bence, Cambridge University Press, 2010, {{ISBN|978-0-521-86153-3}}</ref><ref>Vector Analysis (2nd Edition), M.R. Spiegel, S. Lipschutz, D. Spellman, Schaum’s Outlines, McGraw Hill (USA), 2009, {{ISBN|978-0-07-161545-7}}</ref>

:<math>(\nabla \times \mathbf{F})(p)\cdot \mathbf{\hat{n}} \ \overset{\underset{\mathrm{def}}{}}{=} \lim_{A \to 0}\left( \frac{1}{|A|}\oint_C \mathbf{F} \cdot d\mathbf{r}\right)</math>

where {{math|∮<sub>''C''</sub> '''F''' ⋅ ''d'''''r'''}} is a [[line integral]] along the [[Boundary (topology)|boundary]] of the [[area]] in question, and {{math|{{abs|''A''}}}} is the magnitude of the area. This equation defines the projection of the curl of {{math|'''F'''}} onto {{math|'''n̂'''}}, where {{math|'''n̂'''}} is the normal vector to the surface bounded by {{math|''C''}}; and {{math|''C''}} is defined via the [[right-hand rule]] (see diagram).[[File:Right hand rule simple.png|thumb|Right hand rule for curve orientation. The thumb points in the direction of {{math|'''n̂'''}}, and the fingers curl along the orientation of {{math|''C''}}.]]

The above formula means that the curl of a vector field is defined as the infinitesimal [[area density]] of the ''circulation'' of that field. To this definition fit naturally 
* the [[Kelvin–Stokes theorem]], as a global formula corresponding to the definition, and 
* the following "easy to memorize" definition of the curl in curvilinear [[orthogonal coordinates]], e.g. in [[Cartesian coordinates]], [[spherical coordinates|spherical]], [[cylindrical coordinates|cylindrical]], or even [[Elliptic coordinate system|elliptical]] or [[parabolic coordinates]]:

::<math>
\begin{align}
& (\operatorname{curl}\mathbf F)_1=\frac{1}{h_2h_3}\left (\frac{\partial (h_3F_3)}{\partial u_2}-\frac{\partial (h_2F_2)}{\partial u_3}\right ), \\[5pt]
& (\operatorname{curl}\mathbf F)_2=\frac{1}{h_3h_1}\left (\frac{\partial (h_1F_1)}{\partial u_3}-\frac{\partial (h_3F_3)}{\partial u_1}\right ), \\[5pt]
& (\operatorname{curl}\mathbf F)_3=\frac{1}{h_1h_2}\left (\frac{\partial (h_2F_2)}{\partial u_1}-\frac{\partial (h_1F_1)}{\partial u_2}\right ).
\end{align}
</math>

The equation for each component {{math|(curl '''F''')<sub>''k''</sub>}} can be obtained by exchanging each occurrence of a subscript 1, 2, 3 in cyclic permutation: 1&nbsp;→&nbsp;2, 2&nbsp;→&nbsp;3, and&nbsp;3&nbsp;→&nbsp;1 (where the subscripts represent the relevant indices).

If {{math|(''x''<sub>1</sub>, ''x''<sub>2</sub>, ''x''<sub>3</sub>)}} are the [[Cartesian coordinate system|Cartesian coordinates]] and {{math|(''u''<sub>1</sub>, ''u''<sub>2</sub>, ''u''<sub>3</sub>)}} are the orthogonal coordinates, then 
:<math>h_i = \sqrt{\left (\frac{\partial x_1}{\partial u_i} \right )^2 + \left (\frac{\partial x_2}{\partial u_i} \right )^2 + \left (\frac{\partial x_3}{\partial u_i} \right )^2}</math> 
is the length of the coordinate vector corresponding to {{math|''u<sub>i</sub>''}}. The remaining two components of curl result from [[cyclic permutation]] of [[index notation|indices]]: 3,1,2&nbsp;→&nbsp;1,2,3&nbsp;→&nbsp;2,3,1.

===Intuitive interpretation===
Suppose the vector field describes the [[velocity field]] of a [[fluid flow]] (such as a large tank of [[liquid]] or [[gas]]) and a small ball is located within the fluid or gas (the centre of the ball being fixed at a certain point). If the ball has a rough surface, the fluid flowing past it will make it rotate. The rotation axis (oriented according to the right hand rule) points in the direction of the curl of the field at the centre of the ball, and the angular speed of the rotation is half the magnitude of the curl at this point.<ref>{{citation|first1=Josiah Willard |last1=Gibbs|authorlink1=Josiah Willard Gibbs|first2=Edwin Bidwell|last2=Wilson|authorlink2=Edwin Bidwell Wilson|title=Vector analysis|url=http://hdl.handle.net/2027/mdp.39015000962285?urlappend=%3Bseq=179|year=1901}}</ref>

The curl of the vector at any point is given by the rotation of an infinitesimal area in the ''xy''-plane (for ''z''-axis component of the curl), ''zx''-plane (for ''y''-axis component of the curl) and ''yz''-plane (for ''x''-axis component of the curl vector). This can be clearly seen in the examples below.

==Inverse==
The inverse [[Curl (mathematics)|curl]] of a three-dimensional vector field can be obtained up to an [[Constant of integration|integration constant]] and an unknown [[irrotational field]] with the [[Biot–Savart law]].

== Usage ==
In practice, the above definition is rarely used because in virtually all cases, the curl [[operator (mathematics)|operator]] can be applied using some set of [[curvilinear coordinates]], for which simpler representations have been derived.

The notation {{math|∇ × '''F'''}} has its origins in the similarities to the 3-dimensional [[cross product]], and it is useful as a [[mnemonic]] in [[Cartesian coordinate system|Cartesian coordinates]] if {{math|∇}} is taken as a vector [[differential operator]] [[del]]. Such notation involving [[operator (physics)|operators]] is common in [[physics]] and [[algebra]].

Expanded in 3-dimensional [[Cartesian coordinate system|Cartesian coordinates]] (see ''[[Del in cylindrical and spherical coordinates]]'' for [[Spherical coordinate system|spherical]] and [[Cylindrical coordinate system|cylindrical]] coordinate representations),{{math|∇ × '''F'''}} is, for {{math|'''F'''}} composed of {{math|[''F<sub>x</sub>'', ''F<sub>y</sub>'', ''F<sub>z</sub>'']}}:

:<math>
\nabla \times \mathbf{F} =
\begin{vmatrix} \boldsymbol{\hat\imath} & \boldsymbol{\hat\jmath} & \boldsymbol{\hat k} \\[5pt]
{\dfrac{\partial}{\partial x}} & {\dfrac{\partial}{\partial y}} & {\dfrac{\partial}{\partial z}} \\[10pt]
F_x & F_y & F_z \end{vmatrix}
</math>

where {{math|'''i'''}}, {{math|'''j'''}}, and {{math|'''k'''}} are the [[unit vector]]s for the {{math|''x''}}-, {{math|''y''}}-, and {{math|''z''}}-axes, respectively. This expands as follows:<ref>Arfken, p. 43.</ref>

:<math>
\nabla \times \mathbf{F} =
\left(\frac{\partial F_z}{\partial y} - \frac{\partial F_y}{\partial z}\right) \boldsymbol{\hat\imath} + \left(\frac{\partial F_x}{\partial z} - \frac{\partial F_z}{\partial x} \right) \boldsymbol{\hat\jmath} + \left(\frac{\partial F_y}{\partial x} - \frac{\partial F_x}{\partial y} \right) \boldsymbol{\hat k} = \begin{bmatrix}\frac{\partial F_z}{\partial y} - \frac{\partial F_y}{\partial z} \\ \frac{\partial F_x}{\partial z} - \frac{\partial F_z}{\partial x} \\ \frac{\partial F_y}{\partial x} - \frac{\partial F_x}{\partial y}\end{bmatrix}</math>

Although expressed in terms of coordinates, the result is invariant under proper rotations of the coordinate axes but the result inverts under reflection.

In a general coordinate system, the curl is given by<ref name="Mathworld">{{MathWorld |title=Curl |urlname=Curl}}</ref>

:<math>(\nabla \times \mathbf{F} )^k = \varepsilon^{k\ell m} \nabla_\ell F_m</math>

where {{mvar|ε}} denotes the [[Levi-Civita symbol#Levi-Civita tensors|Levi-Civita tensor]] and {{math|∇}} the [[covariant derivative]], the [[metric tensor]] is used to [[raising and lowering indices|lower the index]] on {{math|'''F'''}}, and the [[Einstein summation convention]] implies that repeated indices are summed over. In Cartesian coordinate system, the covariant derivative reduces to the partial derivative. Equivalently,

:<math>(\nabla \times \mathbf{F} ) = \mathbf{e}_k\varepsilon^{k\ell m} \nabla_l F_m</math>

where {{math|'''e'''<sub>''k''</sub>}} are the coordinate vector fields. Equivalently, using the [[exterior derivative]], the curl can be expressed as:

:<math> \nabla \times \mathbf{F} = \left( \star \big( {\mathrm d} F^\flat \big) \right)^\sharp </math>

Here {{music|flat}} and {{music|sharp}} are the [[musical isomorphism]]s, and {{math|<small>★</small>}} is the [[Hodge star operator]]. This formula shows how to calculate the curl of {{math|'''F'''}} in any coordinate system, and how to extend the curl to any [[orientation (mathematics)|oriented]] three-dimensional [[Riemannian metric|Riemannian]] manifold. Since this depends on a choice of orientation, curl is a [[Chirality (mathematics)|chiral]] operation. In other words, if the orientation is reversed, then the direction of the curl is also reversed.

==Examples==

===Example 1===
Take the [[vector field]]:

:<math>\mathbf{F}(x,y,z)=y\boldsymbol{\hat{\imath}}-x\boldsymbol{\hat{\jmath}}.</math>

For clarity, this can be decomposed as follows:

:<math>\mathbf{F}_x =y, \mathbf{F}_y = -x, \mathbf{F}_z =0.</math>

Its corresponding plot:

[[Image:Uniform curl.svg|center|250px]]

Upon visual inspection, the field can be described as "rotating". If the vectors of the field were to represent a linear [[force]] acting on objects present at that point, and an object were to be placed inside the field, the object would start to rotate clockwise around itself. This is true regardless of where the object is placed.

Calculating the curl:

:<math>\nabla \times \mathbf{F} =0\boldsymbol{\hat{\imath}}+0\boldsymbol{\hat{\jmath}}+ \left({\frac{\partial}{\partial x}}(-x) -{\frac{\partial}{\partial y}} y\right)\boldsymbol{\hat{k}}=-2\boldsymbol{\hat{k}}
</math>

The resulting vector field describing the curl would be uniformly going in the negative {{Math|''z''}} direction. The results of this equation align with what could have been predicted using the [[Right-hand rule#A rotating body|right-hand rule]] using a [[Cartesian coordinate system#In three dimensions|right-handed coordinate system]]. Being a uniform vector field, the object described before would have the same rotational intensity regardless of where it was placed.

The plot describing the curl of {{math|'''F'''}}:

[[Image:Curl of uniform curl.png|center|250px]]

===Example 2===
Take the vector field:

:<math>\mathbf{F}(x,y,z)=-x^2\boldsymbol{\hat{\jmath}}.</math>

Its corresponding plot:

[[Image:Nonuniform curl.svg|250px|center]]

Upon initial inspection, curl existing in this graph would not be obvious. However, taking the object in the previous example, and placing it anywhere on the line {{Math|''x'' {{=}} 3}}, the force exerted on the right side would be slightly greater than the force exerted on the left, causing it to rotate clockwise. Using the right-hand rule, it can be predicted that the resulting curl would be straight in the negative {{Math|''z''}} direction. Inversely, if placed on {{Math|''x'' {{=}} −3}}, the object would rotate counterclockwise and the right-hand rule would result in a positive {{Math|''z''}} direction.

Calculating the curl:

:<math>{\nabla} \times \mathbf{F} =0\boldsymbol{\hat{\imath}}+0\boldsymbol{\hat{\jmath}}+ {\frac{\partial}{\partial x}}(-x^2) \boldsymbol{\hat{k}}=-2x\boldsymbol{\hat{k}}.
</math>

As predicted, the curl points in the negative {{Math|''z''}} direction when {{Math|''x''}} is positive and vice versa. In this field, the intensity of rotation would be greater as the object moves away from the plane {{Math|''x'' {{=}} 0}}.

The plot describing the curl of {{math|'''F'''}}:

[[Image:Curl of nonuniform curl.png|center|250px]]

===Descriptive examples===

* In a vector field describing the linear velocities of each part of a rotating disk, the curl has the same value at all points.
* Of the four [[Maxwell's equations]], two—[[Faraday's law of induction|Faraday's law]] and [[Ampère's circuital law|Ampère's law]]—can be compactly expressed using curl. Faraday's law states that the curl of an electric field is equal to the opposite of the time rate of change of the magnetic field, while Ampère's law relates the curl of the magnetic field to the current and rate of change of the electric field.

<br />

== Identities ==
{{Main|Vector calculus identities}}
In general [[curvilinear coordinates]] (not only in Cartesian coordinates), the curl of a cross product of vector fields {{math|'''v'''}} and {{math|'''F'''}} can be shown to be

:<math>\nabla \times \left( \mathbf{v \times F} \right) = \Big( \left( \mathbf{ \nabla \cdot F } \right) + \mathbf{F \cdot \nabla} \Big) \mathbf{v}- \Big( \left( \mathbf{ \nabla \cdot v } \right) + \mathbf{v \cdot \nabla} \Big) \mathbf{F} \ . </math>

Interchanging the vector field {{math|'''v'''}} and {{math|∇}} operator, we arrive at the cross product of a vector field with curl of a vector field:

:<math> \mathbf{v \ \times } \left( \mathbf{ \nabla \times F} \right) =\nabla_\mathbf{F} \left( \mathbf{v \cdot F } \right) - \left( \mathbf{v \cdot \nabla } \right) \mathbf{ F} \ , </math>

where {{math|∇<sub>'''F'''</sub>}} is the Feynman subscript notation, which considers only the variation due to the vector field {{math|'''F'''}} (i.e., in this case, {{math|'''v'''}} is treated as being constant in space).

Another example is the curl of a curl of a vector field. It can be shown that in general coordinates

:<math> \nabla \times \left( \mathbf{\nabla \times F} \right) = \mathbf{\nabla} (\mathbf{\nabla \cdot F}) - \nabla^2 \mathbf{F} \ , </math>

and this identity defines the [[vector Laplacian]] of {{math|'''F'''}}, symbolized as {{math|∇<sup>2</sup>'''F'''}}.

The curl of the [[gradient]] of ''any'' [[scalar field]] {{mvar|φ}} is always the [[zero vector]] field

:<math>\nabla \times ( \nabla \varphi ) = \boldsymbol{0}</math>

which follows from the [[antisymmetric tensor|antisymmetry]] in the definition of the curl, and the [[symmetry of second derivatives]].

If {{mvar|φ}} is a scalar valued function and {{math|'''F'''}} is a vector field, then

:<math>\nabla \times ( \varphi \mathbf{F}) = \nabla \varphi \times \mathbf{F} + \varphi \nabla \times \mathbf{F} </math>

== Generalizations ==
The vector calculus operations of [[gradient|grad]], curl, and [[divergence|div]] are most easily generalized and understood in the context of differential forms, which involves a number of steps. In a nutshell, they correspond to the derivatives of 0-forms, 1-forms, and 2-forms, respectively. The geometric interpretation of curl as rotation corresponds to identifying [[bivector]]s (2-vectors) in 3 dimensions with the [[special orthogonal Lie algebra]] {{math|<math>\mathfrak{so}</math>(3)}} of infinitesimal rotations (in coordinates, skew-symmetric 3&nbsp;×&nbsp;3 matrices), while representing rotations by vectors corresponds to identifying 1-vectors (equivalently, 2-vectors) and {{math|<math>\mathfrak{so}</math>(3)}}, these all being 3-dimensional spaces.

=== Differential forms ===
{{main|Differential form}}
In 3 dimensions, a differential 0-form is simply a function {{math|''f''(''x'', ''y'', ''z'')}}; a differential 1-form is the following expression:
:<math>a_1\,dx + a_2\,dy + a_3\,dz;</math>
a differential 2-form is the formal sum:
:<math>a_{12}\,dx\wedge dy + a_{13}\,dx\wedge dz + a_{23}\,dy\wedge dz;</math>
and a differential 3-form is defined by a single term:
:<math>a_{123}\,dx\wedge dy\wedge dz.</math>
(Here the {{mvar|a}}-coefficients are real functions; the "wedge products", e.g. {{math|''dx'' ∧ ''dy''}}, can be interpreted as some kind of oriented area elements, {{math|''dx'' ∧ ''dy'' {{=}} −''dy'' ∧ ''dx''}}, etc.)

The [[exterior derivative]] of a {{math|''k''}}-form in {{math|ℝ<sup>3</sup>}} is defined as the {{math|(''k'' + 1)}}-form from above—and in {{math|ℝ<sup>''n''</sup>}} if, e.g.,

:<math>\omega^{(k)}=\sum_{\scriptstyle{i_1<i_2<\cdots<i_k} \atop \forall \scriptstyle{i_\nu\in 1,\ldots,n}} a_{i_1,\ldots,i_k}\,dx_{i_1}\wedge \cdots\wedge dx_{i_k},</math>

then the exterior derivative {{math|''d''}} leads to

:<math> d\omega^{(k)}=\sum_{\scriptstyle{j=1} \atop \scriptstyle{i_1<\cdots<i_k}}^n\frac{\partial a_{i_1,\ldots,i_k}}{\partial x_j}\,dx_j \wedge dx_{i_1}\wedge \cdots \wedge dx_{i_k}.</math>

The exterior derivative of a 1-form is therefore a 2-form, and that of a 2-form is a 3-form. On the other hand, because of the interchangeability of mixed derivatives, e.g. because of

:<math>\frac{\partial^2}{\partial x\,\partial y}=\frac{\partial^2}{\partial y\,\partial x},</math>

the twofold application of the exterior derivative leads to 0.

Thus, denoting the space of {{math|''k''}}-forms by {{math|Ω<sup>''k''</sup>(ℝ<sup>3</sup>)}} and the exterior derivative by {{math|''d''}} one gets a sequence:

:<math>0 \, \overset{d}{\longrightarrow} \, \Omega^0(\mathbf{R}^3) \, \overset{d}{\longrightarrow} \, \Omega^1(\mathbf{R}^3) \, \overset{d}{\longrightarrow} \, \Omega^2(\mathbf{R}^3) \, \overset{d}{\longrightarrow} \, \Omega^3(\mathbf{R}^3) \, \overset{d}{\longrightarrow} \, 0.</math>

Here {{math|Ω<sup>''k''</sup>(ℝ<sup>''n''</sup>)}} is the space of sections of the [[exterior algebra]] {{math|Λ<sup>''k''</sup>(ℝ<sup>''n''</sup>)}} [[vector bundle]] over ℝ<sup>''n''</sup>, whose dimension is the [[binomial coefficient]] {{math|<big><big>(</big></big>{{su|p=''n''|b=''k''}}<big><big>)</big></big>}}; note that {{math|Ω<sup>''k''</sup>(ℝ<sup>3</sup>) {{=}} 0}} for {{math|''k'' > 3}} or {{math|''k'' < 0}}. Writing only dimensions, one obtains a row of [[Pascal's triangle]]:

:0 → 1 → 3 → 3 → 1 → 0;

the 1-dimensional fibers correspond to scalar fields, and the 3-dimensional fibers to vector fields, as described below. Modulo suitable identifications, the three nontrivial occurrences of the exterior derivative correspond to grad, curl, and div.

Differential forms and the differential can be defined on any Euclidean space, or indeed any manifold, without any notion of a Riemannian metric. On a [[Riemannian manifold]], or more generally [[pseudo-Riemannian manifold]], {{math|''k''}}-forms can be identified with [[p-vector|{{math|''k''}}-vector]] fields ({{math|''k''}}-forms are {{math|''k''}}-covector fields, and a pseudo-Riemannian metric gives an isomorphism between vectors and covectors), and on an ''oriented'' vector space with a [[nondegenerate form]] (an isomorphism between vectors and covectors), there is an isomorphism between {{math|''k''}}-vectors and {{math|(''n'' − ''k'')}}-vectors; in particular on (the tangent space of) an oriented pseudo-Riemannian manifold. Thus on an oriented pseudo-Riemannian manifold, one can interchange {{math|''k''}}-forms, {{math|''k''}}-vector fields, {{math|(''n'' − ''k'')}}-forms, and {{math|(''n'' − ''k'')}}-vector fields; this is known as [[Hodge duality]]. Concretely, on {{math|ℝ<sup>3</sup>}} this is given by:
* 1-forms and 1-vector fields: the 1-form {{math|''a<sub>x</sub> dx'' + ''a<sub>y</sub> dy'' + ''a<sub>z</sub> dz''}} corresponds to the vector field {{math|(''a<sub>x</sub>'', ''a<sub>y</sub>'', ''a<sub>z</sub>'')}}.
* 1-forms and 2-forms: one replaces {{math|''dx''}} by the dual quantity {{math|''dy'' ∧ ''dz''}} (i.e., omit {{math|''dx''}}), and likewise, taking care of orientation: {{math|''dy''}} corresponds to {{math|''dz'' ∧ ''dx'' {{=}} −''dx'' ∧ ''dz''}}, and {{math|''dz''}} corresponds to {{math|''dx'' ∧ ''dy''}}. Thus the form {{math|''a<sub>x</sub> dx'' + ''a<sub>y</sub> dy'' + ''a<sub>z</sub> dz''}} corresponds to the "dual form" {{math|''a<sub>z</sub> dx'' ∧ ''dy'' + ''a<sub>y</sub> dz'' ∧ ''dx'' + ''a<sub>x</sub> dy'' ∧ ''dz''}}.

Thus, identifying 0-forms and 3-forms with scalar fields, and 1-forms and 2-forms with vector fields:
* grad takes a scalar field (0-form) to a vector field (1-form);
* curl takes a vector field (1-form) to a pseudovector field (2-form);
* div takes a pseudovector field (2-form) to a pseudoscalar field (3-form)

On the other hand, the fact that {{math|''d''{{isup|2}} {{=}} 0}} corresponds to the identities
:<math>\nabla\times(\nabla f) = 0</math>
for any scalar field {{mvar|f}}, and
:<math>\nabla \cdot (\nabla \times\mathbf v)=0</math>
for any vector field {{math|'''v'''}}.

Grad and div generalize to all oriented pseudo-Riemannian manifolds, with the same geometric interpretation, because the spaces of 0-forms and {{math|''n''}}-forms is always (fiberwise) 1-dimensional and can be identified with scalar fields, while the spaces of 1-forms and {{math|(''n'' − 1)}}-forms are always fiberwise {{math|''n''}}-dimensional and can be identified with vector fields.

Curl does not generalize in this way to 4 or more dimensions (or down to 2 or fewer dimensions); in 4 dimensions the dimensions are

:0 → 1 → 4 → 6 → 4 → 1 → 0;

so the curl of a 1-vector field (fiberwise 4-dimensional) is a ''2-vector field'', which is fiberwise 6-dimensional, one has

:<math>\omega^{(2)}=\sum_{i<k=1,2,3,4}a_{i,k}\,dx_i\wedge dx_k,</math>

which yields a sum of six independent terms, and cannot be identified with a 1-vector field. Nor can one meaningfully go from a 1-vector field to a 2-vector field to a 3-vector field (4 → 6 → 4), as taking the differential twice yields zero ({{math|''d''{{isup|2}} {{=}} 0}}). Thus there is no curl function from vector fields to vector fields in other dimensions arising in this way.

However, one can define a curl of a vector field as a ''2-vector field'' in general, as described below.

=== Curl geometrically ===
2-vectors correspond to the exterior power {{math|Λ<sup>2</sup>''V''}}; in the presence of an inner product, in coordinates these are the skew-symmetric matrices, which are geometrically considered as the [[special orthogonal Lie algebra]] {{math|<math>\mathfrak{so}</math>(''V'')}} of infinitesimal rotations. This has {{math|<big><big>(</big></big>{{su|p=''n''|b=2}}<big><big>)</big></big> {{=}} {{sfrac|1|2}}''n''(''n'' − 1)}} dimensions, and allows one to interpret the differential of a 1-vector field as its infinitesimal rotations. Only in 3 dimensions (or trivially in 0 dimensions) does {{math|''n'' {{=}} {{sfrac|1|2}}''n''(''n'' − 1)}}, which is the most elegant and common case. In 2 dimensions the curl of a vector field is not a vector field but a function, as 2-dimensional rotations are given by an angle (a scalar – an orientation is required to choose whether one counts clockwise or counterclockwise rotations as positive); this is not the div, but is rather perpendicular to it. In 3 dimensions the curl of a vector field is a vector field as is familiar (in 1 and 0 dimensions the curl of a vector field is 0, because there are no non-trivial 2-vectors), while in 4 dimensions the curl of a vector field is, geometrically, at each point an element of the 6-dimensional Lie algebra {{math|<math>\mathfrak{so}</math>(4)}}.

Note also that the curl of a 3-dimensional vector field which only depends on 2 coordinates (say {{math|''x''}} and {{math|''y''}}) is simply a vertical vector field (in the {{math|''z''}} direction) whose magnitude is the curl of the 2-dimensional vector field, as in the examples on this page.

Considering curl as a 2-vector field (an antisymmetric 2-tensor) has been used to generalize vector calculus and associated physics to higher dimensions.<ref>[https://arxiv.org/abs/hep-ph/0609260 Generalizing Cross Products and Maxwell's Equations to Universal Extra Dimensions], A.W. McDavid, C.D. McMullen, 2006</ref>

== See also ==
*[[Vector cross product#Triple product expansion|Cross product]]
*[[Del]]
*[[Divergence]]
*[[Gradient]]
*[[Helmholtz decomposition]]
*[[Del in cylindrical and spherical coordinates]]
*[[Vorticity]]

==References==
{{Reflist|30em}}

==Sources==
* [[George B. Arfken|Arfken, George B.]] and Hans J. Weber. ''Mathematical Methods For Physicists'', Academic Press; 6 edition (June 21, 2005). {{isbn|978-0-12-059876-2}}.
* {{cite book |author=Korn, Granino Arthur and Theresa M. Korn |title=Mathematical Handbook for Scientists and Engineers: Definitions, Theorems, and Formulas for Reference and Review |publisher=Dover Publications |location=New York |year= |pages=157–160 |isbn=0-486-41147-8 |oclc= |doi=}}
* {{cite book |title=Div, Grad, Curl, and All That: An Informal Text on Vector Calculus |first=H. M. |last=Schey |year=1997 |location=New York |publisher=Norton |isbn=0-393-96997-5 }}

== External links ==
* {{springer|title=Curl|id=p/c027310}}
* [http://mathinsight.org/curl_idea The idea of curl of a vector field]
* [http://betterexplained.com/articles/vector-calculus-understanding-circulation-and-curl Curl BetterExplained]
* {{cite web |title=Divergence and Curl: The Language of Maxwell's Equations, Fluid Flow, and More |work=3Blue1Brown |date=June 21, 2018 |url=https://www.youtube.com/watch?v=rB83DpBJQsE |via=[[YouTube]] }}

[[Category:Differential operators]]
[[Category:Linear operators in calculus]]
[[Category:Vector calculus]]
[[Category:Analytic geometry]]