A hospital uses a device that converts sound into electrical signals to monitor a patient's heartbeat sounds. Inside the device, a coil of wire is free to move around a fixed permanent magnet. Explain how this device produces an electrical current when sound waves reach it.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A microphone is placed against a patient's chest to detect heartbeat sounds. The microphone contains a fixed permanent magnet surrounded by a coil of wire that is attached to a diaphragm.
Model answer (5 marks)
Sound waves create alternating pressure variations that make the diaphragm vibrate. The diaphragm is attached to a coil, so the coil moves back and forth with the diaphragm. The coil moves through the magnetic field of the fixed permanent magnet, giving relative motion between the coil and the field. This motion induces a potential difference across the ends of the coil (generator effect). Because the coil is part of a complete electrical circuit, a current flows. The current is alternating, changing direction each time the coil reverses its motion.
Examiner tips
- Use the word ‘induce’ for the potential difference and ‘generator effect’ to show understanding of Faraday’s law.
- Show the alternation of current by noting the coil’s back‑and‑forth motion and the reversal of the induced emf.
Mark scheme (5 marks)
- Sound waves cause pressure variations that make the diaphragm (and attached coil) vibrate / move back and forth
- The moving coil cuts through the magnetic field of the permanent magnet / there is relative motion between the coil and the magnetic field
- A potential difference is induced across the ends of the coil (generator effect)
- Because the coil is part of a complete circuit, a current flows
- The current produced is alternating / changes direction because the coil moves back and forth / the direction of motion repeatedly reverses
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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