Flux

Flux is two separate simple and ubiquitous concepts throughout physics and applied mathematics. Within a discipline, the term is generally used consistently, but care must be taken when comparing phenomena from different disciplines. Both concepts have mathematical rigor, enabling comparison of the underlying math when the terminology is unclear. For transport phenomena, flux is a vector quantity, describing the magnitude and direction of the flow of a substance or property. In electromagnetism, flux is a scalar quantity, defined as the surface integral of the component of a vector field perpendicular to the surface at each point. As will be made clear, the easiest way to relate the two concepts is that the surface integral of a flux according to the first definition is a flux according to the second definition.