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Energy flux is the rate of transfer of energy through a surface. The quantity is defined in two different ways, depending on the context: Total rate of energy transfer (not per unit area); [1] SI units: W = J⋅s −1. Specific rate of energy transfer (total normalized per unit area); [2] SI units: W⋅m −2 = J⋅m −2 ⋅s −1:
However, any type of energy has its direction of movement in space, as well as its density, so energy flux vectors can be defined for other types of energy as well, e.g., for mechanical energy. The Umov–Poynting vector [11] discovered by Nikolay Umov in 1874 describes energy flux in liquid and elastic media in a completely generalized view.
Energy flux, the rate of transfer of energy through a unit area (J·m −2 ·s −1). The radiative flux and heat flux are specific cases of energy flux. Particle flux, the rate of transfer of particles through a unit area ([number of particles] m −2 ·s −1) These fluxes are vectors at each point in space, and have a definite magnitude and ...
Flux F through a surface, dS is the differential vector area element, n is the unit normal to the surface. Left: No flux passes in the surface, the maximum amount flows normal to the surface. Right: The reduction in flux passing through a surface can be visualized by reduction in F or dS equivalently (resolved into components, θ is angle to ...
In physics and many other areas of science and engineering the intensity or flux of radiant energy is the power transferred per unit area, where the area is measured on the plane perpendicular to the direction of propagation of the energy. [a] In the SI system, it has units watts per square metre (W/m 2), or kg⋅s −3 in base units.
where: is the rate of change of the energy density in the volume. ∇•S is the energy flow out of the volume, given by the divergence of the Poynting vector S. J•E is the rate at which the fields do work on charges in the volume (J is the current density corresponding to the motion of charge, E is the electric field, and • is the dot product).
The SI unit of electric flux is the volt-meter (V·m), or, equivalently, newton-meter squared per coulomb (N·m 2 ·C −1). Thus, the unit of electric flux expressed in terms of SI base units is kg·m 3 ·s −3 ·A −1. Its dimensional formula is L 3 M T −3 I −1.
The dimension of power is energy divided by time. In the International System of Units (SI), the unit of power is the watt (W), which is equal to one joule per second. Other common and traditional measures are horsepower (hp), comparing to the power of a horse; one mechanical horsepower equals about 745.7 watts.