CIE A Level Physics复习笔记24.1.1 The Piezoelectric Transducer

• 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

• A transducer is any device that converts energy from one form to another

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
• 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

Applications of the Piezoelectric Transducer

• Microphone
  • A piezoelectric microphone detects pressure variations in sound waves
  • These can then be converted to an electrical signal for processing
• Ultrasound
  • In a piezoelectric transducer, an alternating p.d. is applied to produce ultrasound waves and sent into the patient’s body
  • The returning ultrasound waves induce a p.d. in the transducer for analysis by a healthcare professional