A museum is making an interactive display about old telephone technology. A visitor speaks into a moving-coil microphone, and an oscilloscope shows the pote...
Question 1 Report
A museum is making an interactive display about old telephone technology. A visitor speaks into a moving-coil microphone, and an oscilloscope shows the potential difference produced at its terminals. Fig. 2 includes a simplified section through the microphone and the trace for one steady note. Sound waves push the diaphragm backwards and forwards. The diaphragm is attached to a light coil placed between the poles of a permanent magnet. The horizontal width of the oscilloscope screen is 20 ms.
Fig. 2
(a) Name the part in Fig. 2 that provides a magnetic field without a current from a power supply. [1]
(b) Describe how the microphone produces an alternating potential difference when the diaphragm vibrates. [3]
(c) Use the trace in Fig. 2 to calculate the frequency of the note. [3]
(d) Explain why speaking more loudly produces a larger potential difference at the microphone output. [3]
Answer Details
(a) The permanent magnet provides a magnetic field without a current from a power supply. [1]
(b) Sound makes the diaphragm vibrate. [1] The attached coil moves in the magnetic field. [1] The changing magnetic field linkage induces a potential difference; it reverses as the coil reverses direction, producing an alternating potential difference. [1]
(c) Four complete waves are shown across \(20\ \text{ms}\). [1]
\[T=\frac{20\ \text{ms}}{4}=5.0\ \text{ms}=0.0050\ \text{s}\] [1]
\[f=\frac{1}{T}=\frac{1}{0.0050\ \text{s}}=200\ \text{Hz}\] [1]
(d) Louder sound has greater amplitude. [1] This makes the diaphragm and coil move further or faster. [1] The magnetic field linkage then changes more rapidly, producing a larger induced potential difference. [1]
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