(a)(i) What is a wave motion? (ii) State two differences between a radio wave and a sound wave. (b)(i) Given that you are provided with a tuning fork, a bur...
Question 1 Report
(a)(i) What is a wave motion?
(ii) State two differences between a radio wave and a sound wave.
(b)(i) Given that you are provided with a tuning fork, a burette and other necessary apparatus, describe with the aid of a diagram, an experiment to determine the frequency of a note emitted by a source of sound. [assume the velocity of sound in air is known]
(ii)State two precautions necessary to obtain accurate result in the experiment described in (b)(i) above
(c) A pipe closed at one end is 100 cm long. If the air in the pipe is set into vibration and a fundamental note is produced, calculate the frequency of the note. [ velocity of sound in air = 340 ms\(^{-1}\)]
Answer Details
(a)(i) Wave motion. A wave motion is a disturbance that travels through a medium (or through space) transferring energy from one point to another without any net transfer of the particles of the medium.
(a)(ii) Two differences between a radio wave and a sound wave.
| Radio wave | Sound wave |
|---|---|
| It is an electromagnetic (transverse) wave. | It is a mechanical (longitudinal) wave. |
| Can travel through a vacuum (needs no material medium). | Requires a material medium for propagation. |
| Travels at the speed of light, \(3\times10^{8}\,\text{ms}^{-1}\). | Travels much more slowly, about \(340\,\text{ms}^{-1}\) in air. |
(b)(i) Experiment to determine the frequency of a note (resonance method using a burette).
The burette is clamped vertically and filled with water, so that the water and the tube walls enclose a short column of air at the open top. The vibrating tuning fork is held horizontally, with its prongs just above (not touching) the open mouth of the burette, as shown below.
The tap of the burette is opened so that water runs out slowly, gradually lengthening the air column. As the length increases, a position is reached where the sound suddenly becomes loudest; this is the first resonance. The length of the air column from the water surface to the open mouth is measured and recorded as \(L_1\).
Water is run out further, keeping the fork sounding over the mouth, until the sound is again loudest; this is the second resonance. This longer air column is measured and recorded as \(L_2\). The whole procedure is repeated a few times and the mean values of \(L_1\) and \(L_2\) taken.
Between the first and second resonances the air column has increased by exactly half a wavelength, so end-correction is eliminated:
\[ L_2-L_1=\frac{\lambda}{2}\;\Rightarrow\;\lambda=2\left(L_2-L_1\right) \]The frequency of the note is then obtained from \(f=\dfrac{v}{\lambda}\), giving
\[ f=\frac{v}{2\left(L_2-L_1\right)} \]where \(v\) is the known velocity of sound in air.
(b)(ii) Two precautions.
- The tuning fork should be struck gently on a soft rubber pad (not on a hard surface) and held with its prongs just above the mouth of the burette without touching it.
- The resonance lengths should be read at the position of loudest sound with the eye level with the water meniscus to avoid parallax error; the readings should be repeated and averaged.
(c) Fundamental frequency of a pipe closed at one end.
For a pipe closed at one end, the fundamental note has an air column equal to a quarter of a wavelength: \(L=\dfrac{\lambda}{4}\), so \(\lambda=4L\). With \(L=100\,\text{cm}=1.0\,\text{m}\),
\[ \lambda=4L=4\times1.0=4.0\,\text{m} \] \[ f=\frac{v}{\lambda}=\frac{340}{4.0}=85\,\text{Hz} \]The frequency of the fundamental note is 85 Hz.
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