Describe evidence for the cases of ripples on water surfaces and for sound…

The idea

A wave transfers energy from one place to another without transferring the medium itself. For ripples on water, a floating object such as a cork bobs up and down as the ripple passes but stays in roughly the same place, so the water oscillates while the ripple pattern moves outward. For sound in air, a vibrating source pushes air molecules back and forth; each molecule oscillates about a fixed position and passes the disturbance to its neighbour, so a compression travels through the air while the air itself is not carried across the room. In both cases the evidence is that the medium oscillates locally while the wave pattern and its energy travel.

What earns the marks

• States that a wave transfers energy through a medium without transferring the medium itself.
• Describes a floating object on water bobbing as a ripple passes while staying in roughly the same position.
• Explains that air molecules oscillate about fixed positions and pass the disturbance to neighbouring molecules.
• Identifies that the compression or ripple pattern travels while the water or air does not travel with it.

Common mistakes and how to put them right

• Claims that the water moves outward with the ripple.

  Put it right by noting the floating object bobs in place while the pattern travels.

• Asserts that air is carried from the source to the listener.

  Put it right by explaining that molecules oscillate locally and pass on the disturbance.

• Confuses the medium with the wave.

  Put it right by separating the oscillating particles from the travelling energy transfer.

Exam tips

• Name the medium and the wave separately in your answer so the distinction is clear.
• Use a concrete observation, such as a cork on water, to support the claim.
• Link the evidence to energy transfer rather than to movement of the medium.