A surface integral is a double integral applied over a surface. It is, in some ways, the higher-dimensional equivalent of a line integral. A surface integral can be applied over a scalar or vector field, the latter of which is sometimes called a flux integral.
Types of surface integrals
Over a scalar field
In its most basic form, a surface integral of over the surface takes the form
where is the surface and is the projection of onto the plane. Note that if , this integral becomes the surface area of .
The more general form is found by parameterizing the surface to a vector function , in which case it would become:
Over a vector field
Over a vector field, a surface or flux integral is equal to
where is a vector-valued function, is the unit normal vector to a surface, and is the vector-valued parametric representation of the surface being integrated over. In the special case that the surface can be represented as , it can be shown that this formula reduces to
where D is the projection of S onto the xy-plane.
Whether the value is positive or negative is arbitrary depending on which was the unit normal vectors should be pointing. By convention, for any closed surface, the normal vectors point outwards.