Flux
From Simple English Wikipedia, the free encyclopedia
This article is about the concept of flux in science and mathematics. For other uses of the word, see flux (disambiguation).
The flux visualized. The rings show the surface boundaries. The red arrows stand for the flow of charges, fluid particles, subatomic particles, photons, etc. The number of arrows that pass through each ring is the flux.
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[edit] Flux in Biology
In Biology there are many fluxes used in the study of transport phenomena. Each type of flux has its own distinct unit of measurement along with distinct physical constants. Six of the most common forms of flux from the transport literature are defined as:
- Momentum flux, the rate of transfer of momentum across a unit area (N·s·m-2·s-1). (Newtonian fluid, viscous flow)
- Heat flux, the rate of heat flow across a unit area (J·m-2·s-1). (Fourier's Law)[1] (This definition of heat flux fits Maxwell's original definition.[2])
- Chemical flux, the rate of movement of molecules across a unit area (mol·m-2·s-1). (Fick's law of diffusion)
- Volumetric flux, the rate of volume flow across a unit area (m3·m-2·s-1). (Darcy's law)
- Mass flux, the rate of mass flow across a unit area (kg·m-2·s-1). (Either an alternate form of Fick's law that includes the molecular mass, or an alternate form of Darcy's law that includes the density)
- Radiative flux, the amount of energy moving in the form of photons at a certain distance from the source per steradian per second (J·m-2·s-1). Used in astronomy to determining the magnitude and spectral class of a star. Also acts as a generalization of heat flux, which is equal to the radiative flux when restricted to the infrared spectrum.
[edit] Flux in Physics
In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.
- In the study of transport phenomena (heat transfer, mass transfer and fluid dynamics), flux is defined as the amount that flows through a unit area per unit time, the volumetric flow rate.[3] Flux in this definition is a vector.
- In the field of electromagnetism, flux is usually the integral of a vector quantity over a finite surface. The result of this integration is a scalar quantity.[4] The magnetic flux is thus the integral of the magnetic vector field over a surface, and the electric flux is defined similarly. Using this definition, the flux of the Poynting vector over a specified surface is the rate at which electromagnetic energy flows through that surface. Confusingly, the Poynting vector is sometimes called the power flux, which is an example of the first usage of flux, above.[5] It has units of watts per square metre (W/m2)
[edit] See also
[edit] References
- ↑ Carslaw, H.S.; and Jaeger, J.C. (1959). Conduction of Heat in Solids, Second Edition, Oxford University Press. ISBN 0-19-853303-9.
- ↑ Maxwell, James Clerk (1892). Treatise on Electricity and Magnetism.
- ↑ Bird, R. Byron; Stewart, Warren E., and Lightfoot, Edwin N. (1960). Transport Phenomena, Wiley. ISBN 0-471-07392-X.
- ↑ Lorrain, Paul; and Corson, Dale (1962). Electromagnetic Fields and Waves.
- ↑ Wangsness, Roald K. (1986). Electromagnetic Fields, 2nd ed., Wiley. ISBN 0-471-81186-6. p.357
[edit] Further reading
- Stauffer, P.H. (2006). "Flux Flummoxed: A Proposal for Consistent Usage". Ground Water 44 (2): 125–128.
