Line integral

In mathematics, a line integral is an integral where the function to be integrated is evaluated along a curve. The terms "path integral", "curve integral", and "curvilinear integral" are also used; "contour integral" is used as well, although that is typically reserved for line integrals in the complex plane.
The function to be integrated may be a scalar field or a vector field. The value of the line integral is the sum of values of the field at all points on the curve, weighted by some scalar function on the curve (commonly arc length or, for a vector field, the scalar product of the vector field with a differential vector in the curve). This weighting distinguishes the line integral from simpler integrals defined on intervals. Many simple formulae in physics, such as the definition of work as formula_1, have natural continuous analogues in terms of line integrals, in this case formula_2, which computes the work done on an object moving through an electric or gravitational field F along a path formula_3.
Vector calculus.
In qualitative terms, a line integral in vector calculus can be thought of as a measure of the total effect of a given tensor field along a given curve. For example, the line integral over a scalar field (rank 0 tensor) can be interpreted as the area under the field carved out by a particular curve. This can be visualized as the surface created by "z" = "f"("x","y") and a curve "C" in the "xy" plane. The line integral of "f" would be the area of the "curtain" created—when the points of the surface that are directly over "C" are carved out.
Line integral of a scalar field.
Definition.
For some scalar field formula_4 where formula_5, the line integral along a piecewise smooth curve formula_6 is defined as
where formula_8 is an arbitrary bijective parametrization of the curve formula_9 such that and give the endpoints of formula_9 and . Here, and in the rest of the article, the absolute value bars denote the standard (Euclidean) norm of a vector.
The function is called the integrand, the curve formula_9 is the domain of integration, and the symbol may be intuitively interpreted as an elementary arc length. Line integrals of scalar fields over a curve formula_9 do not depend on the chosen parametrization of formula_9.
Geometrically, when the scalar field is defined over a plane , its graph is a surface in space, and the line integral gives the (signed) cross-sectional area bounded by the curve formula_9 and the graph of . See the animation to the right.
Derivation.
For a line integral over a scalar field, the integral can be constructed from a Riemann sum using the above definitions of , and a parametrization of . This can be done by partitioning the interval into sub-intervals of length , then denotes some point, call it a sample point, on the curve . We can use the set of sample points to approximate the curve by a polygonal path by introducing a straight line piece between each of the sample points and . We then label the distance between each of the sample points on the curve as . The product of and can be associated with the signed area of a rectangle with a height and width of and , respectively. Taking the limit of the sum of the terms as the length of the partitions approaches zero gives us
By the mean value theorem, the distance between subsequent points on the curve, is
Substituting this in the above Riemann sum yields
which is the Riemann sum for the integral
Line integral of a vector field.
Definition.
For a vector field F : "U" ⊆ R"n" → R"n", the line integral along a piecewise smooth curve "C" ⊂ "U", in the direction of r, is defined as
where · is the dot product, and r: ["a", "b"] → "C" is a bijective parametrization of the curve "C" such that r("a") and r("b") give the endpoints of "C".
A line integral of a scalar field is thus a line integral of a vector field, where the vectors are always tangential to the line.
Line integrals of vector fields are independent of the parametrization r in absolute value, but they do depend on its orientation. Specifically, a reversal in the orientation of the parametrization changes the sign of the line integral.
From the viewpoint of differential geometry, the line integral of a vector field along a curve is the integral of the corresponding 1-form under the musical isomorphism (which takes the vector field to the corresponding covector field), over the curve considered as an immersed 1-manifold.
Derivation.
The line integral of a vector field can be derived in a manner very similar to the case of a scalar field, but this time with the inclusion of a dot product. Again using the above definitions of , and its parametrization , we construct the integral from a Riemann sum. We partition the interval (which is the range of the values of the parameter ) into intervals of length . Letting be the th point on , then gives us the position of the th point on the curve. However, instead of calculating up the distances between subsequent points, we need to calculate their displacement vectors, . As before, evaluating at all the points on the curve and taking the dot product with each displacement vector gives us the infinitesimal contribution of each partition of on . Letting the size of the partitions go to zero gives us a sum
By the mean value theorem, we see that the displacement vector between adjacent points on the curve is
Substituting this in the above Riemann sum yields
which is the Riemann sum for the integral defined above.
Path independence.
If a vector field F is the gradient of a scalar field "G" (i.e. if F is conservative), that is,
then by the multivariable chain rule the derivative of the composition of "G" and r("t") is
which happens to be the integrand for the line integral of F on r("t"). It follows, given a path "C ", that
In other words, the integral of F over "C" depends solely on the values of "G" at the points r("b") and r("a"), and is thus independent of the path between them. For this reason, a line integral of a conservative vector field is called "path independent".
Applications.
The line integral has many uses in physics. For example, the work done on a particle traveling on a curve "C" inside a force field represented as a vector field F is the line integral of F on "C".
Flow across a curve.
For a vector field formula_26, , the line integral across a curve "C" ⊂ "U", also called the "flux integral", is defined in terms of a piecewise smooth parametrization , , as:
Here • is the dot product, and formula_28 is the clockwise perpendicular of the velocity vector formula_29.
The flow is computed in an oriented sense: the curve has a specified forward direction from to , and the flow is counted as positive when is on the clockwise side of the forward velocity vector .
Complex line integral.
In complex analysis, the line integral is defined in terms of multiplication and addition of complex numbers. Suppose "U" is an open subset of the complex plane C, is a function, and formula_30 is a curve of finite length, parametrized by , where . The line integral
may be defined by subdividing the interval ["a", "b"] into and considering the expression
The integral is then the limit of this Riemann sum as the lengths of the subdivision intervals approach zero.
If the parametrization is continuously differentiable, the line integral can be evaluated as an integral of a function of a real variable:
When is a closed curve (initial and final points coincide), the line integral is often denoted formula_34 sometimes referred to in engineering as a "cyclic integral".
The line integral with respect to the conjugate complex differential formula_35 is defined to be
The line integrals of complex functions can be evaluated using a number of techniques. The most direct is to split into real and imaginary parts, reducing the problem to evaluating two real-valued line integrals. The Cauchy integral theorem may be used to equate the line integral of an analytic function to the same integral over a more convenient curve. It also implies that over a closed curve enclosing a region where is analytic without singularities, the value of the integral is simply zero, or in case the region includes singularities, the residue theorem computes the integral in terms of the singularities.
Example.
Consider the function "f"("z") = 1/"z", and let the contour "L" be the counterclockwise unit circle about 0, parametrized by z("t") = "e""it" with "t" in [0, 2π] using the complex exponential. Substituting, we find:
This is a typical result of Cauchy's integral formula and the residue theorem.
Relation of complex line integral and line integral of vector field.
Viewing complex numbers as 2-dimensional vectors, the line integral of a complex-valued function formula_38 has real and complex parts equal to the line integral and the flux integral of the vector field corresponding to the conjugate function formula_39 Specifically, if formula_40 parametrizes "L", and formula_41 corresponds to the vector field formula_42 then:
By Cauchy's theorem, the left-hand integral is zero when formula_44 is analytic (satisfying the Cauchy–Riemann equations) for any smooth closed curve L. Correspondingly, by Green's theorem, the right-hand integrals are zero when formula_45 is irrotational (curl-free) and incompressible (divergence-free). In fact, the Cauchy-Riemann equations for formula_44 are identical to the vanishing of curl and divergence for F.
By Green's theorem, the area of a region enclosed by a smooth, closed, positively oriented curve formula_3 is given by the integral formula_48 This fact is used, for example, in the proof of the area theorem.
Quantum mechanics.
The path integral formulation of quantum mechanics actually refers not to path integrals in this sense but to functional integrals, that is, integrals over a space of paths, of a function "of" a possible path. However, path integrals in the sense of this article are important in quantum mechanics; for example, complex contour integration is often used in evaluating probability amplitudes in quantum scattering theory.