A flux integral is a special case of a surface integral.
A surface integral is integrating the value of *any* function over a surface. A regular one-dimension line integral can be thought of as a “cumulative” value, like a running sum. A surface integral is the same: the cumulative value of a function over an area. A trivial example of a surface is to calculate the surface area itself.
A “flux integral” is a surface integral, where the value being integrated represents the movement of a conserved value through the surface. The flow of fluid through a filter, or a magnetic field passing through the surface of an object.
In particular, the “flux” function being integrated needs to be a *vector-valued* function. Surface integrals can also apply to scalar-valued functions.
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