- Intensity (physics)
In

physics ,**intensity**is a measure of the time-averagedenergy flux . The word "intensity" here is not synonymous with "", "", or "", as it sometimes is in colloquial speech. For example, "the intensity of pressure" is meaningless, since the parameters of those variables do not match.To find the intensity, take the

energy density (that is, the energy per unit volume) and multiply it by thevelocity at which the energy is moving. The resulting vector has the units of power divided byarea (i.e. watt/m²). It is possible to define the intensity of the water coming from a garden sprinkler, but intensity is used most frequently withwave s (i.e.sound orlight ).**Mathematical description**If a point source is radiating energy in three dimensions and there is no energy lost to the medium, then the intensity decreases in proportion to distance from the object squared. This is due to physics and geometry. Physically,

conservation of energy applies. The consequence of this is that the net power coming from the source must be constant, thus::$P\; =\; int\; I,\; dA$

where "P" is the net power radiated, "I" is the intensity as a function of position, and "dA" is a differential element of a closed surface that contains the source. That "P" is a constant. If we integrate over a surface of uniform intensity "I", for instance, over a sphere centered around a point source radiating equally in all directions, the equation becomes:

:$P\; =\; |I|\; cdot\; A\_\{surf\}\; =\; |I|\; cdot\; 4pi\; r^2\; ,$

where "I" is the intensity at the surface of the sphere, and "r" is the radius of the sphere. ($A\_\{surf\}\; =\; 4pi\; r^2$ is the expression for the surface area of a sphere). Solving for "I", we get:

:$|I|\; =\; frac\{P\}\{A\_\{surf\; =\; frac\{P\}\{4pi\; r^2\}$

If the medium is damped, then the intensity drops off more quickly than the above equation suggests.

Anything that can carry energy can have an intensity associated with it. For an

electromagnetic wave , if "E" is the complexamplitude of theelectric field , then theenergy density of the wave is given by:$U\; =\; frac\{n^2\; epsilon\_0\}\{2\}\; |E|^2$,and the intensity is obtained multiplying this expression by the velocity of the wave, $c/n$::$I\; =\; frac\{c\; n\; epsilon\_0\}\{2\}\; |E|^2$,where "n" is the

refractive index , $c$ is thespeed of light invacuum and $epsilon\_0$ is theelectric permittivity in vacuum.**Intensity in photometry and radiometry**In photometry and

radiometry , "intensity" has a different meaning: it is the luminous or radiant power "per unitsolid angle ". This can cause confusion inoptics , where "intensity" can mean any ofradiant intensity ,luminous intensity orirradiance , depending on the background of the person using the term.Radiance is also sometimes called "intensity", especially by astronomers and astrophysicists.**ee also***

Sound intensity

*Magnitude

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