A-LevelPhysicsMedical physics9702/41 · Oct/Nov 2020 · Q48 Marks
Explain the principles of the generation of ultrasound waves for use in medical diagnosis.
- (a)Explain the principles of the generation of ultrasound waves for use in medical diagnosis.[4]
- (b)The linear attenuation (absorption) coefficient for a parallel beam of ultrasound waves in air is 1.2 cm⁻¹. The parallel beam passes through a layer of air of thickness 3.5 cm. Calculate the ratio, in dB, intensity of beam after passing through the layer of air / intensity of beam entering the layer of air[4]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.
- [B1] quartz crystal
- [B1] alternating p.d. across crystal causes it to vibrate
- [B1] resonance occurs when frequency of p.d. matches natural frequency of crystal
- [B1] natural frequency of crystal is in ultrasound range
- [C1] I = I₀e⁻μx
- [C1] I/ I₀ = e⁻¹·² × ³·⁵
- [C1] ratio/dB = -10 lg (1/0.015) or 10 lg (0.015)
- [A1] = -18 dB
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