Where does flux collide?

In physics, flux is the amount of a field or quantity passing through a surface. In Introduction to Electrodynamics, Griffiths defines electric flux as the surface integral of the electric field over an area, and Gauss's law ties the flux through any closed surface to the charge it encloses. The same word covers magnetic flux, measured in webers, and heat flux, measured in watts per square metre. The referent is a rate of flow across a boundary, a number that depends on both the field and the geometry of the surface. It is not a substance you can hold, and it is not a doctrine about change; it is a measurable quantity fixed by the field and the surface it crosses.[1]

Unit

The amount of a field crossing a surface (webers for magnetic flux, watts per square metre for heat flux)

Recovery means

Not applicable

Measured by

The surface integral of the field over an area; Gauss's law relates the flux through a closed surface to the enclosed source

In philosophy, flux is the doctrine that everything is in ceaseless change, summed up in the phrase panta rhei, all things flow. As Graham sets out in his account of Heraclitus, the view is ascribed to the Ephesian philosopher, who is reported to have held that one cannot step into the same river twice. Here flux is not a measured quantity but a metaphysical claim about the nature of reality: that stability is an appearance and that becoming, not being, is fundamental. It shares only the image of flowing with the physicist's flux and the biochemist's; there is no surface, no unit, and nothing to integrate, only a thesis about how the world is.[2]

Unit

Not a measured quantity; a metaphysical claim about the world

Recovery means

Not applicable

Measured by

Not measured; argued from the observation that things never stay the same

In biochemistry and systems biology, flux is the rate at which molecules are turned over through a metabolic pathway. In What is flux balance analysis?, Orth, Thiele, and Palsson treat the flux through each reaction as an unknown and solve for the distribution a network can sustain at steady state, given the stoichiometry and a chosen objective. The referent is a throughput, typically in millimoles per gram dry weight per hour, describing the traffic through a living network. It is a rate like the physicist's flux, but what moves is matter through a chain of reactions rather than a field across a surface, and it is not the soldering agent nor the doctrine of change.[3]

Unit

Rate of turnover through a pathway (millimoles per gram dry weight per hour)

Recovery means

Not applicable

Measured by

Estimated by flux balance analysis over a stoichiometric network, or by isotope labelling