Editing Intensity (physics) (section)

{{Short description|Power transferred per unit area}}
{{Other uses|Intensity (disambiguation)}}
In [[physics]] and many other areas of science and engineering the '''intensity''' or '''flux''' of [[radiant energy]] is the [[Power (physics)|power]] transferred per unit [[area]], where the area is measured on the plane perpendicular to the direction of propagation of the energy.{{efn|The terms ''intensity'' and ''flux'' have multiple, inconsistent, definitions in physics and related fields. This article covers the concept of power per unit area, whatever one calls it. In [[radiometry]] the terms ''intensity'' and ''flux'' have different meanings, not covered here.}} In the [[SI]] system, it has units [[watt]]s per [[square metre]] (W/m<sup>2</sup>), or [[kilogram|kg]]⋅[[second|s]]<sup>−3</sup> in [[SI base unit|base units]]. Intensity is used most frequently with [[wave]]s such as acoustic waves ([[sound]]), [[matter wave]]s such as electrons in [[electron microscope]]s, and [[electromagnetic wave]]s such as [[light]] or [[radio wave]]s, in which case the [[time averaging|''average'']] power transfer over one [[Period (physics)|period]] of the wave is used. ''Intensity'' can be applied to other circumstances where energy is transferred. For example, one could calculate the intensity of the [[kinetic energy]] carried by drops of water from a [[garden sprinkler]].

The word "intensity" as used here is not synonymous with "[[wikt:strength|strength]]", "[[wikt:amplitude|amplitude]]", "[[wikt:magnitude|magnitude]]", or "[[wikt:level|level]]", as it sometimes is in colloquial speech.

Intensity can be found by taking the [[energy density]] (energy per unit volume) at a point in space and multiplying it by the [[velocity]] at which the energy is moving. The resulting [[Vector (geometry)|vector]] has the units of power divided by area (i.e., [[surface power density]]). The intensity of a wave is proportional to the square of its amplitude. For example, the intensity of an electromagnetic wave is proportional to the square of the wave's [[electric field]] amplitude.