Vector field

Vector field given by vectors of the form (-y, x)

In mathematics a vector field is a construction in vector calculus which associates a vector to every point in space.

Vector fields are often used in physics to model for example the speed and direction of a moving fluid throughout space, or the strength and direction of some force, such as the magnetic or gravitational force, as it changes from point to point.

Table of contents

1 Definition
2 Notes
3 Examples

3.1 Gradient field
3.2 Central field

4 Curve integral
5 Integral curves
6 See also
7 External links

Definition

Given an open and connected subset X in Rⁿ a vector field is a vector valued function

We say F is a C^k vector field if F is k times continuously differentiable in X.

A point x in X is called stationary if

The vector field can be visualized as a n-dimensional space with a n-dimensional vector attached to each point in X.

Given two C^k-vector fields F,G defined over X and a real valued C^k-function f defined over X

defines the module of C^k-vector fields over the ring of C^k-functions.

Notes

In the rigorous mathematical treatment, vector fields are defined on manifolds: a vector field is a section of the manifold's tangent bundle. While the underlying manifold is often the 2-dimensional or 3-dimensional Euclidean space (in which case any tangent fiber is equal to the same Euclidean space), other manifolds are also useful: describing the wind distribution on the surface of the Earth for instance requires a vector field on the sphere, a 2-dimensional manifold; the spacetime of relativity is a 4-dimensional manifold; and phase spaces of complicated physical systems are often modeled as high dimensional manifolds with a vector field indicating how the system changes over time.

Vector fields should be compared to scalar fields, which associate a number or scalar to every point in space (or every point of some manifold).

The derivatives of a vector field, resulting in a scalar field or another vector field, are called the divergence and curl respectively.

Examples

A vector field for the movement of air on Earth will associate for every point on the surface of the Earth a vector with the wind speed and direction for that point. This can be drawn using arrows to represent the wind; the length (magnitude) of the arrow will be an indication of the wind speed. A "high" on the usual barometric pressure map would then act as a source (arrows pointing away), and a "low" would be a sink (arrows pointing towards), since air tends to move from high pressure areas to low pressure areas.
Velocity field of a moving fluid. In this case, a velocity vector is associated to each point in the fluid. In wind tunnels, the fieldlines can be revealed using smoke.
Magnetic fields. The fieldlines can be revealed using small iron filings.
Maxwell's equations allow us to use a given set of initial conditions to deduce, for every point in Euclidean space, a magnitude and direction for the force experienced by a charged test particle at that point; the resulting vector field is the electromagnetic field.

Gradient field

Vector fields can be constructed out of scalar fields using the vector operator gradient which gives rise to the following definition.

A C^k-vector field F over X is called a gradient field or conservative field if there exists a real valued C^k+1-function f : X → R (a scalar field) so that

The curve integral along any closed curve (c(a) = c(b)) in a gradient field is always zero.

Central field

A C^∞-vector field over Rⁿ \\ {0} is called central field if

Where O(n, R) is the orthogonal group. We say central fields are invariant under orthogonal transformations around 0.

The point 0 is called the center of the field.

A central field is always a gradient field.

Curve integral

A common technique in physics is to integrate a vector field along a curve. Given a particle in a gravitational vector field, where each vector represents the force acting on the particle at this point in space, the curve integral is the work done on the particle when it travels along a certain path.

The curve integral is constructed analogously to the Riemann integral and it exists if the curve is rectifiable (has finite length) and the vector field is continuous.

Given a vector field F(x) and a curve c(t) from a to b the curve integral is defined as

A few simple rules for calculation of curve integrals are

Integral curves

Vector fields have a nice interpretation in terms of autonomous, first order ordinary differential equations.

Given a C⁰ vector field F defined over X.

we can try to define curves c(t) over X so that for a each t in an interval I

and

Put in our vector field equation we get

which is the definition of an explicit first order ordinary differential equation with the curves c(t) as solutions.

If \F is Lipschitz continuous we can find a unique C¹-curve c_x for each point x in X so that

The curves c_x are called integral curves of the vector field F and partition X into equivalence classes. It is not always possible to extend the interval (-ε, +ε) to the whole real number line. The flow may for example reach the edge of X in a finite time.

Integrating the vector field along any integral curve c yields

In 2 or three dimension one can visualize the vector field as given rise to a flow on X. If we drop a particle into this flow at point p it will move along the a curve c_p in the flow depending on the inital point p. If p is a stationary point in F then the particle will remain stationary.

Typical applications are streamline in fluid, geodesic flow, and one-parameter subgroups and the exponential map in Lie groups.