This article is about the concept of flux in physics and mathematics. For other uses of the word, see flux (disambiguation).

Flux is defined as the rate at which a given quantity passes through a fixed boundary per unit time.

In simplest terms, flux is represented mathematically by the surface integral of a vector field.

<math>\Phi_f = K \int_s F\,dA<math>

Where <math>\Phi<math> is the flux, K is a constant, F a given force, and dA the respective area.

Based on the mathematic definition, we can apply it to other disciplines when looking for forces applied through areas.

Contents

1 What is Flux? 2 Flux in Chemistry 2.1 Diffusion 2.2 Thermal Systems 3 Flux in Electrical Engineering 4 Flux in Physics 4.1 Electromagnetic Radiation 4.2 Fluid Systems

What is Flux?

A general example

To better understand the concept of flux, imagine a butterfly net. The amount of air moving through the net at any given instant in time is the flux. If the wind is blowing hard then the flux through the net is larger than before. If the net is made bigger then the flux would be larger. For the most air to move through the net, the opening of the net must be facing the direction the wind is blowing. If the net is paralell to the wind then no wind will be moving through the net, it will all be moving past it.

Flux in Chemistry

Diffusion

Flux, or diffusion for gaseous molecules can be related to the function:

<math>\Phi = 4\pi\sigma_{ab}^2\sqrt{\frac{8k_bT}{\pi N}}<math>

Where N is the total number of gaseous particles, k is Boltzmann's Constant, T is the relative temperature in Kelvins and <math>\sigma_{ab}<math> is the mean free path between molecules a and b.

Thermal Systems

In thermal systems the flux is the rate of heat flow.

Flux in Electrical Engineering

In electrical engineering the flux is the electrical current or the magnetic current.

Flux in Physics

In Physics, flux is commonly used when working with electromagnetic fields and inductance. For example one can induce a current in a loop of wire by changing the magnetic flux through the wire.

Energy Flux, one very common type, has units of W/m^2.

Electromagnetic Radiation

For electromagnetic radiation, flux signifies the energy per unit time (or power) passing through a surface. (See also flux unit).

Fluid Systems

In fluid systems the flux is the rate of fluid flow.

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