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In physics, intensity is the power transferred per unit area, where the area is measured on the plane perpendicular to the direction of propagation of the energy. In the SI system, it has units watts per square metre (W/m2). It is used most frequently with waves (example - sound or light), in which case the average power transfer over one 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. In physics, intensity is the power transferred per unit area, where the area is measured on the plane perpendicular to the direction of propagation of the energy. In the SI system, it has units watts per square metre (W/m2). It is used most frequently with waves (example - sound or light), in which case the average power transfer over one 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 'strength', 'amplitude', 'magnitude', or '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 has the units of power divided by area (i.e., surface power density). If a point source is radiating energy in all directions (producing a spherical wave), and no energy is absorbed or scattered by the medium, then the intensity decreases in proportion to the distance from the object squared. This is an example of the inverse-square law. Applying the law of conservation of energy, if the net power emanating is constant, 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. If one integrates over a surface of uniform intensity I, for instance over a sphere centered around the point source, the equation becomes where I is the intensity at the surface of the sphere, and r is the radius of the sphere. ( A s u r f = 4 π r 2 {displaystyle A_{mathrm {surf} }=4pi r^{2}} is the expression for the surface area of a sphere). Solving for I gives

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