Energy and Transport

Wave energy density, average power flow, and acoustic Poynting intensity vector.

General idea of the topic

This topic studies how a wave stores energy and how that energy moves. The key distinction is between energy contained in the disturbance, energy flux, and intensity.

What you will truly understand

You will understand that amplitude does not only change the appearance of the wave: it also changes its energy. You will also see why total energy, energy per length or volume, transported power, and intensity per area are not the same.

Recommended learning route

Start with wave energy to establish the relation with amplitude. Continue with energy flux to introduce the rate of transport. Finish with wave intensity to connect the model with detectors, areas, and distances.

Physical criterion you must acquire

You should learn to always ask "energy in which region" and "energy crossing which surface". This distinction prevents mixing extensive quantities, densities, powers, and intensities.

Connection with real problems

This topic applies to sound, ultrasound, water waves, vibrations, radiation, acoustic insulation, and energy safety. It helps estimate how much actually reaches a receiver or exposed region.

Final goal of the topic

The final goal is to interpret and calculate energy transport in a wave, correctly distinguishing amplitude, energy, flux, and intensity according to the geometry of the problem.