A-LevelPhysicsMedical physics9702/42 · Oct/Nov 2021 · Q117 Marks
A piezoelectric transducer containing a quartz crystal is used to obtain diagnostic information about internal structures.
- (a)Describe the function of the quartz crystal.[3]
- (b)(i)Define specific acoustic impedance.[2]
- (b)(ii)Describe, qualitatively, how the specific acoustic impedances of two materials affect the intensity reflection coefficient at a boundary between the materials.[2]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.
- generates ultrasound
- detects reflected ultrasound
- applied p.d. causes crystal to vibrate
- or vibrations cause crystal to generate an e.m.f.
- product of density and speed
- speed of ultrasound in medium
- difference between (the specific acoustic impedances)
- if similar/same then reflection coefficient is zero/very low
- if very different then reflection coefficient is (nearly) 1
- the lower the difference means lower the reflection coefficient
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