Waves carry energy without carrying matter
Definition
Wave
An oscillation that transfers energy and information from one place to another without transferring matter.
- A wave begins with an oscillating source, such as a loudspeaker cone, a vibrating string or a dipper in a ripple tank.
- The source transfers energy to nearby particles or fields, and the disturbance passes from one place to the next.
- Particles in a material medium oscillate about fixed positions and have no net movement with the wave.
- A wave can carry information when variations in the wave form a signal, such as speech in sound or digital data in radio waves.
Key Idea
The disturbance transfers energy and information, while matter in the medium only oscillates locally.
Water waves leave the water behind
- A floating cork moves up and down as crests and troughs pass, but remains in approximately the same horizontal position.
- If the water travelled with the wave, the cork would be carried continuously across the tank.
- The travelling crests and the stationary average position of the cork show that the wave transfers energy without a net transfer of water.
Sound waves leave the air behind
- A loudspeaker cone moves backwards and forwards, producing compressions and rarefactions in the surrounding air.
- Each air particle oscillates about a fixed position and transfers energy to neighbouring particles.
- Sound reaches a listener without producing a continuous wind or moving all the air towards one wall.
- This shows that the sound disturbance travels while the air has no net movement from speaker to listener.
Common Mistake
Do not state that particles travel with the wave; state that they oscillate about fixed positions while energy is transferred.
Exam technique
- Name the observation and link it directly to the conclusion.
- For water, state that a float oscillates but is not carried across; for sound, state that sound arrives without a flow of air.
Self review
- What does a wave transfer?
- What happens to particles as a wave passes?
- How does a floating cork show that water is not transferred?
- How does sound arriving without a wind show that air is not transferred?
