| Date | May 2015 | Marks available | 2 | Reference code | 15M.3.SL.TZ2.2 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | Identify and State | Question number | 2 | Adapted from | N/A |
Question
This question is about standing (stationary) waves in a tube.
A thin tube is immersed in a container of water. A length L of the tube extends above the surface of water.
A tuning fork is sounded above the tube. For particular values of L, a standing wave is established in the tube.
(i) Explain how a standing wave is formed in this tube.
(ii) The frequency of the tuning fork is 256 Hz. The smallest length L for which a standing wave is established in the tube is 33.0 cm. Estimate the speed of sound in the tube.
The diagram shows an enlarged view of the tube shown in (a). X, Y and Z are three molecules of air in the tube.
The length L is 33.0 cm.
(i) State the direction of oscillation of molecule Y.
(ii) Identify the molecule that has the greatest amplitude.
Markscheme
(i) travelling waves move down the tube;
which then interfere with the reflected waves (from the closed end of the tube/surface of the water);
Accept superposition as an alternative to interference.
(ii) \(\begin{array}{l}
\lambda = \left( {4L = 4 \times 0.33 = } \right)1.32\left( {\rm{m}} \right);\\
v = \left( {f\lambda = 256 \times 1.32 = } \right)338\left( {{\rm{m}}{{\rm{s}}^{ - 1}}} \right)
\end{array}\);
(i) vertical;
(ii) X;
Examiners report
Part (a) was answered well by many, but the idea of superposition of incident and reflected waves was often expressed poorly. Candidates seemed to have memorised the definition of how a standing wave is formed but often struggled to see how it applied to this situation. Part (ii) was easy if the candidate knew that the wavelength was 4L. Many just used L or other multiples of L.
Part (b) was also an easy 2 marks as long as it was remembered that the waves were longitudinal.
