Waves as energy transfer

Tsunami buoy

‘Wave’ is a common term for a number of different ways in which energy is transferred:

In electromagnetic waves, energy is transferred through vibrations of electric and magnetic fields.
In sound waves, energy is transferred through vibration of air particles or particles of a solid through which the sound travels.
In water waves, energy is transferred through the vibration of the water particles.

Waves transfer energy but not mass. Buoys like this DART® tsunami buoy are moved up and down by the waves that pass by them but don’t move directionally across the water.

Waves transfer energy but not mass

When we watch surf waves coming into shore, it’s easy to think that individual water particles are moving towards us, but that’s not actually the case. The particles involved in waves move back and forth perpendicularly to the way the wave is going, but don’t move significantly in the direction of the wave. The particles ‘take part’ in the wave by bumping into one another and transferring energy. This is why energy can be transferred, even though the average position of the particles doesn’t change.

How does this work? It can help to think of a buoy bobbing in the ocean. The buoy is moved up and down by the waves that pass by it, but doesn’t move directionally across the water.

You could also think about a Mexican wave at a sports match. The wave moves around the arena, but the audience members don’t move around with it – they only stand up and sit down (a perpendicular movement to the wave direction).

Particles in a water wave exchange kinetic energy for potential energy

When particles in water become part of a wave, they start to move up or down. This means that kinetic energy (energy of movement) has been transferred to them. As the particles move further away from their normal position (up towards the wave crest or down towards the trough), they slow down. This means that some of their kinetic energy has been converted into potential energy – the energy of particles in a wave oscillates between kinetic and potential energy.

Mexican wave

As each person stands up and sits down, the ‘Mexican wave’ moves around the arena. The average position of each person doesn’t change.

Thinking about potential energy can help us understand why tsunamis can be so damaging. When a tsunami approaches the shore, it shoals (becomes much higher), so the water particles are displaced further from equilibrium. They acquire a lot of potential energy, and this is released when the wave interacts with land.

Measuring the energy in a wave

Why do some waves have more energy than others? A wave’s frequency and wavelength are both indicators of its energy, but this differs for different types for waves.

For water waves, those with a high speed and long wavelength (like a tsunami) have the most energy. For electromagnetic waves, speed is constant, so waves with a high frequency and a short wavelength (like X-rays) are the most energetic.

For all waves, a greater amplitude means more energy.

Harnessing wave energy

Scientists in New Zealand and elsewhere are looking at how to turn the energy of water waves into electricity. The oceans around New Zealand are promising places to generate wave power because we have large waves and strong currents. Generating wave power would involve an underwater device (like a paddle, for example) that would move in response to waves and drive a turbine that would produce electricity.

The idea of wave power is appealing because waves are a sustainable resource – they can’t be used up (unlike other resources, like coal, that are used for making electricity in New Zealand). However, they are quite inefficient – they need a lot of coastal space to generate useful quantities of energy. Using mathematical modelling and physical model building, Kiwi scientists are investigating how to harness wave power, but it will be some time before we’re using electricity from wave power in our homes.