Explain piezoelectric crystals and ultrasound transducers
Connect the piezoelectric effect to generating and detecting short ultrasound pulses.
Section 24 · Medical physics
A Level · Cambridge International Physics 9702, 2025–2027 syllabus. Topic 24 was checked against Cambridge’s 2028–2030 syllabus; the other remaining syllabus-year changes are still being reconciled. Online study supports the theory but cannot replace supervised practical work, full timed assessment or human subject-teacher review.
Before you begin
- Use exponential functions, wave speed, energy conservation and SI units; convert between joules and electronvolts when required.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens to a piezoelectric crystal when a p.d. is applied?
What you will learn
- Explain the direct and inverse piezoelectric effects
- Describe how a transducer generates and detects ultrasound
- Distinguish the transmitting and receiving stages of a pulse
One crystal can convert between electrical and mechanical changes
When a potential difference is applied across a piezoelectric crystal, its shape changes slightly. This is the inverse piezoelectric effect, and an alternating potential difference can make the crystal vibrate and launch ultrasound into a medium.
When an ultrasound echo deforms the crystal, the changing shape produces an e.m.f. This is the direct piezoelectric effect. The received electrical signal can then be measured by the same transducer.
A transducer sends a pulse and listens for echoes
A short electrical pulse excites the crystal, producing a short ultrasound pulse. The crystal then acts as a detector while the reflected pulse returns. Separating transmit and receive times lets the system associate an echo with a depth inside the body.
The word ultrasound describes sound above the human hearing range. In medical imaging it is sent in pulses so that reflected echoes can be distinguished by their arrival times.
Pause and explain
Which effect produces an e.m.f. when an ultrasound echo changes the crystal’s shape?
Worked example
A transducer first sends an ultrasound pulse and then produces a voltage when an echo arrives. Which piezoelectric effect describes each action?
Show the worked solution
- Applying a potential difference makes the crystal change shape, so this is the inverse effect.
- The echo changes the crystal’s shape.
- The crystal then generates an e.m.f., so this is the direct effect.
Answer Transmit: inverse piezoelectric effect. Receive: direct piezoelectric effect.
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An alternating p.d. is applied to a piezoelectric crystal in a probe. Explain how this can launch ultrasound.
Give me a hint
Follow the energy conversion from electrical input to crystal motion.
Compare my reasoning
- The alternating p.d. makes the crystal change shape repeatedly.
- The crystal vibrates at the driving frequency.
- Its vibrations create pressure oscillations that travel as ultrasound in the tissue.
The inverse piezoelectric effect makes the crystal vibrate, producing an ultrasound wave.
Look for these in your work
- Names the inverse effect
- Links p.d. to repeated shape change
- Connects vibration to an ultrasound wave
An echo reaches the same probe. Describe how the returning wave becomes a recorded signal.
Give me a hint
Trace the conversion in the opposite direction.
Compare my reasoning
- The echo exerts a changing pressure on the crystal.
- The crystal changes shape as the wave arrives.
- The direct piezoelectric effect generates a varying e.m.f. that the receiver records.
The echo deforms the crystal, which generates a measurable e.m.f. by the direct piezoelectric effect.
Look for these in your work
- Connects the echo to deformation
- Identifies the direct effect
- States that an electrical signal is recorded
A technician sees a transmitted pulse but no returning voltage after a phantom is inserted. Give one transducer-related cause that could explain the observation.
Give me a hint
Consider the two different operating directions of the same crystal.
Compare my reasoning
- The crystal must be driven to transmit by an applied p.d.
- It must also convert echo deformation to an e.m.f. when receiving.
- A failed receive circuit or damaged crystal can therefore leave a transmitted pulse but no detected echo.
A fault in the receive connection or crystal could prevent the direct piezoelectric signal from being detected.
Look for these in your work
- Separates transmit and receive functions
- Identifies a receive-side failure
- Does not confuse the fault with the absence of ultrasound generation
Common mistakes
- Reversing the direct and inverse effects
- Saying the crystal creates an echo rather than detects a returning pulse
- Assuming the transducer only sends or only receives
What can you explain now?
What happens to a piezoelectric crystal when a p.d. is applied?
Compare with the explanation
Its shape changes
The inverse effect turns an electrical input into a small deformation.
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Make it stick
Which direction of conversion is the direct piezoelectric effect?
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