Waves
Inverse Square Law (Intensity)
Intensity from point source decreases as 1/r².
Variables
IW/m²
Intensity
At distance r
In SI units: kg/s³
Allowed values: greater than 0
PW
Power
Source power
In SI units: kg·m²/s³
Allowed values: greater than 0
rm
Distance
From point source
Allowed values: greater than 0
Derivation
Power spreads over sphere: I = P/(4πr²).
Your notes
0/5000
Check value
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- P = 2 W (Power)
- r = 3 m (Distance)
Result: I = 0.01768388 W/m²
Other waves formulas with the same quantities
- Intensity-Amplitude Relation (Wave Properties) shares Intensity
- Sound Level (Decibels) (Sound Waves) shares Intensity
Where this formula is listed
- Waves formulas, under Wave Properties, alongside Wave Equation, Period and Frequency, Angular Frequency, Wave Number and Wave Speed on String.
- In the waves list it comes after Intensity-Amplitude Relation and before Wave Speed on String.
- Waves formula sheet, one of 37 formulas printed with symbol keys and SI units.
- The full formula index, grouped by category and topic.
Common questions
What is the Inverse Square Law (Intensity) formula?
Inverse Square Law (Intensity) is written I = P/(4πr²). Intensity from point source decreases as 1/r². It belongs to Waves, under Wave Properties.
What are the units in I = P/(4πr²)?
In SI units, I is intensity, measured in kg/s³; P is power, measured in kg·m²/s³; r is distance, measured in m.
How is the Inverse Square Law (Intensity) formula derived?
Power spreads over sphere: I = P/(4πr²). The result is I = P/(4πr²).