# Stokes' Theorem

Stokes' Theorem expresses the surface integral of the curl of vector field in terms of its easier-to-compute circulation:



Stated another way, Stokes' Theorem equates the line integral of a vector fields to a surface integral of the same vector field. For this identity to be true, the direction of the vector normal n must obey the right-hand rule for the direction of the contour, i.e., when walking along the contour the surface must be on your left.

This is an extension of Green's Theorem to surface integrals, and is also the analog in two dimensions of the Divergence Theorem. The above formulation is also called as the "Curl Theorem," to distinguish it from the more general form of the Stokes' Theorem described below.

Stokes' Theorem is useful in calculating circulation in mechanical engineering. A conservative field has a circulation (line integral on a simple, closed curve) of zero, and application of the Stokes' Theorem to such a field proves that the curl of a conservative field over the enclosed surface must also be zero.

## General Form

In its most general form, this theorem is the fundamental theorem of Exterior Calculus, and is a generalization of the Fundamental Theorem of Calculus. It states that if M is an oriented piecewise smooth manifold of dimension k and  is a smooth (k−1)-form with compact support on M, and ∂M denotes the boundary of M with its induced orientation, then

,

where d is the exterior derivative.

There are a number of well-known special cases of Stokes' theorem, including one that is referred to simply as "Stokes' theorem" in less advanced treatments of mathematics, physics, and engineering:

• When k=1, and the terms appearing in the theorem are translated into their simpler form, this is just the Fundamental Theorem of Calculus.
• When k=3, this is often called Gauss' Theorem or the Divergence Theorem and is useful in vector calculus:


Where R is some region of 3-space, S is the boundary surface of R, the triple integral denotes volume integration over R with dV as the volume element, and the double integral denotes surface integration over S with  as the oriented normal of the surface element. The  on the left side is the divergence operator, and the  on the right side is the vector dot product.

• When k=2, this is often just called Stokes' Theorem:


Here S is a surface, E is the boundary path of S, and the single integral denotes path integration around E with  as the length element. The  on the left side is the curl operator.

These last two examples (and Stokes' theorem in general) are are the subject of vector calculus. They play important roles in electrodynamics. The divergence and curl operations are cornerstones of Maxwell's Equations.

Stokes' Theorem is a lower-dimension version of the Divergence Theorem, and a higher-dimension version of Green's Theorem. Green’s Theorem relates a line integral to a double integral over a region, while Stokes' Theorem relates a surface integral of the curl of a function to its line integral.