Fluxrate

Fluxrate is a term used in physics and engineering to describe the time-dependent rate at which a flux passes through a boundary. A flux measures the amount of a quantity crossing a surface per unit area; the flux rate therefore describes how this crossing changes with time. It is used to analyze transient behavior in electromagnetism, fluid dynamics, and radiative transfer.

For a fixed surface S and a field F representing the crossing quantity, the flux is Φ(t) = ∮_S F · n dA, and the flux rate is R(t) = dΦ/dt. If the boundary moves or the surface changes shape, the Reynolds transport theorem provides the general expression for the rate of change of flux, combining local production, convection, and boundary motion.

Fluxrate appears in applications such as the time variation of electric or magnetic flux in circuits, mass and energy transport in fluids, traffic or pollutant fluxes across boundaries, and radiative transfer problems in engineering and atmospheric science. It is a helpful descriptor of transient phenomena, especially when a system’s boundary or sources vary with time.

Units depend on the quantity and surface considered; in general the flux has units of quantity per area, and the flux rate has those units per time. In practice, practitioners report flux rate in the relevant quantity units, such as kg/m^2/s for mass flux rate or W/m^2/s for changing radiant flux density.

See also: flux, flux density, continuity equation, Reynolds transport theorem.