The Piezoelectric Effect & the Ultrasound Transducer (OCR A Level Physics): Revision Note
Exam code: H556
The Piezoelectric Effect & Generating Ultrasound
The piezoelectric effect is defined as:
The ability of particular materials to generate a potential difference (p.d.) by transferring mechanical energy to electrical energy
This effect enables ultrasound generation and detection technology to exist in medicine
In the piezoelectric effect, an applied voltage causes a piezo-crystal to contract or expand, and vice versa
Piezoelectric Crystals
At the heart of a piezoelectric transducer is a piezoelectric crystal
A transducer is any device that converts energy from one form to another
Piezoelectric crystals are materials which produce a p.d. when they are deformed
This deformation can be by compression or stretching
If a p.d. is applied to a piezoelectric crystal, then it deforms, and if the p.d. is reversed, then it expands
If this is an alternating p.d. then the crystal will vibrate at the same frequency as the alternating voltage
Crystals must be cut to a certain size in order to induce resonance
One of the most common piezoelectric crystals is quartz, which is made from a lattice of silicon dioxide atoms
When the lattice is distorted, the structure becomes charged creating an electric field and, as a result, an electric current
If an electric current is applied to the crystal, then this causes the shape of the lattice to alternate which produces a sound wave
Due to the conventional direction of electric current, it will flow from the positive to the negative region of the crystal
A molecule in a quartz crystal. When the compression and stretching alternates, an alternating e.m.f. is induced
Generating & Detecting Ultrasound
An ultrasound transducer is made up of a piezoelectric crystal and electrodes which produce an alternating p.d.
The crystal is heavily damped, usually with epoxy resin, to stop the crystal from vibrating too much
This produces short pulses and increases the resolution of the ultrasound device
The structure of an ultrasound transducer
A piezoelectric crystal can act as both a receiver or transmitter of ultrasound
When it is receiving ultrasound, it converts the sound waves into an alternating p.d.
When it is transmitting ultrasound, it converts an alternating p.d. into sound waves
A piezoelectric crystal can act as both a receiver or transmitter of ultrasound
Worked Example
Explain the principles of the generation and detection of ultrasound waves.
Answer:
Generation:
An alternating p.d. is applied across a piezoelectric crystal, causing it to change shape
The alternating p.d. causes the crystal to vibrate and produce pulses of ultrasound waves
The crystal vibrates at the frequency of the alternating p.d., so, the crystal must be cut to a specific size in order to produce resonance
Detection:
The ultrasound pulse is reflected at the boundary of the tissue and returns to the transducer
When the ultrasound wave returns, the crystal vibrates which produces an alternating p.d. across the crystal
This received signal can then be processed and used for medical diagnosis
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