Waves: Intensity

Intensity: of a progressive wave is the radiant power passing through a surface per unit area

Intensity (I) has the unit Wm^-2

Equation: I=P/A

Where P is power passing through the surface and A is the cross-sectional area of the surface

As we get further from a source of waves, the intensity decreases ( I is directly proportional to 1/r² for a wave falling on sphere)- This is inverse square law. Doubling distance reduces intensity by 4

E.g. Avg. distance from Earth to Sun is 150 million km. Intensity of radiation received by upper atmosphere is 1400 Wm^(-2). Calculate total power output of Sun:
Total Power = Intensity × Area = 1400 Wm^(-2) × 4π(150 million km)² = 3.846 × 10²⁶ W.

Can measure intensity with an LDR. Can measure R of LDR at different distances from source. Then use a calibration curve to convert measured resistance to an intensity value. (Make graph of R against I)

When waves spread, the intensity drops as energy becomes more spread out - causes amplitude of wave to decrease

Avg. speed of oscillations of particles is proportional to amplitude of wave: halving amplitude results in particle oscillations with 0.5 speed and 0.25 the KE (KE directly proportional to v²)

For any wave, intensity is proportional to amplitude²