The flux of a quantity is defined as the rate at which this quantity passes through a fixed boundary per unit time. In various contexts, flux is called the exitance, irradiance, or radiancy. Energy flux, one of the most common types encountered, has units of energy per unit area per unit time (in MKS, J m-2 s-1, or W m-2). Flux can also be defined as a surface integral Eric Weisstein's World of Math of a vector field Eric Weisstein's World of Math F,
For an ideal gas, the flux of gas due to random motion is
where n is the number density, P is the gas pressure, m is the mass of a particle of the gas, k is Boltzmann's constant, T is the temperature, N is the number of particles, and V is the volume.
Energy Flux, Flux Density, Planck Law
