A standing wave is established in air in a pipe with one closed and one open end.

The air molecules near X are

A.     always at the centre of a compression.

B.     always at the centre of a rarefaction.

C.     sometimes at the centre of a compression and sometimes at the centre of a rarefaction.

D.     never at the centre of a compression or a rarefaction.

C

The majority response of A showed a basic misconception of what these standing wave patterns represent. Indeed, the correct response was chosen by the fewest candidates! The standing wave is longitudinal with air molecules moving parallel to the sides of the pipe. At the antinodes they are moving with maximum amplitude, while at the nodes (X) they are not moving (amplitude is zero).

At adjacent antinodes the molecules are out of phase such that when a molecule to the left of X is moving to the left, then a particle to the right of X will be moving to the right creating a rarefaction at X; half a time period later the directions are reversed and a compression will be formed at X.

Teachers should emphasise the dynamic nature of the standing wave with the traditional representation being essentially a graph of amplitude against distance.