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# Vector field

A vector field associates a vector to every point in space; the vectors may change from point to point. Vector fields are often used in physics, for instance to indicate 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.

In the rigorous mathematical treatment, vector fields are defined on differentiable manifolds: a vector field is a function that associates to every point of the manifold an element from the manifold's tangent space at that point.

While the underlying manifold is often the 2-dimensional or 3-dimensional Euclidean space (in which case the tangent space 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).

• 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.

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