11.5 Intensity Flashcards
What is the intensity of a progressive wave defined as?
The radiant power passing through a surface per unit area.
Intensity has units watts per square metre.
Intensity (I) = Power (P) / cross-sectional area (A)
What happens when a wave travels out from a source?
The radiant power spreads out, reducing the intensity.
The total radiant power P at a distance r from the source is spread out over an area equal to the surface area of a sphere (A = 4pi x r squared)
What happens when ripples travel out across the surface of pond?
The intensity drops as the energy becomes more spread out. This causes a drop in amplitude. That is a drop in ripple height the further the wave is from the source.
Decreased amplitude means a reduced average speed of the oscillating particles as they travel less of a distance.
Therefore, halving amplitude results in particles oscillating with half the speed, and a quarter of the kinetic energy.
Intensity is directly proportional to the square of the amplitude.
