Inverse Square Law
An inverse square law is a law that states that the influence (such as sound) of a point source (such as radio) is inversely proportional to the square of the distance from the source of the particular quantity. In other words:
where I is a physical quantity or intensity and r is an arbitrary distance. This law applies to several areas studied in mathematics and physics, such as gravity, electric field, light, sound, and radiation. The formulas of the aforementioned concepts that obey the inverse law are as follows:
Light, Sound, and Radiation:
I=P4 π r2
This law generally applies when some form of conserved quantity (such as force and energy) is radiated outwardly and uniformly in 3-D space from a point source.
If the power of a point source is 10 W, what is the intensity of the radiation at 5 m?
Enter 3 values at which point sources are radiating power between 0 and 100 W. Adjust the slider to change the radius at which you are measuring the intensity of the radiation. Click 'Draw!' to see the relationship between intensity and radius. Click the check box to see the intensity calculations for the three respective power values.
You can use the formula in the previous section for light, sound, and radiation to determine the intensity 5 m away from a point source radiating at a rate of 10 W.
I=10 W4 π 5 m2
Therefore, the intensity 5.0 m away from a point source radiating at a rate of 10 W is 0.03 W/m2.
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