---
title: "Piezoelectric materials and piezoelectric effect"
subject: "Electrical Engineering Material"
chapter: 3
key: "semester_3/electrical_engineering_material:piezoelectric-effect-polarization"
source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/piezoelectric-effect-polarization
site: BCT Engineering
---

# Piezoelectric materials and piezoelectric effect

Electrical Engineering Material · Chapter 3: Dielectric Materials

- Asked in: 2082 Kartik (new course), 4 marks; 2074 Chaitra, 4 marks (asked 2 times)
- Video: [Piezoelectricity - why hitting crystals makes electricity](https://www.youtube.com/watch?v=wcJXA8IqYl8) — Steve Mould
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/piezoelectric-effect-polarization
- Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-3

## Question

What do you mean by piezo-electric materials? Explain piezoelectric effect in terms of polarization.

## Answer

**Piezoelectric materials** are crystals that become electrically polarized when mechanically stressed, and change dimensions when an electric field is applied. The crystal must lack a centre of symmetry. Examples: quartz (SiO₂), Rochelle salt, barium titanate, PZT (lead zirconate titanate), ZnO.

#### Piezoelectric effect in terms of polarization

In an unstressed piezoelectric crystal, the positive and negative ions are arranged so that the centres of positive and negative charge in each unit cell coincide. The dipole moments cancel and the net polarization is zero.

When a stress $T$ is applied, the ions are displaced unequally. Because the cell has no centre of symmetry, the centre of positive charge moves away from the centre of negative charge. Each cell then has a net dipole moment, and the crystal acquires a polarization $P$. Bound charges appear on opposite faces and a voltage can be measured between electrodes.

```text
 Unstressed         Compressed (force down)
     +                  +
   -   -              -   -
     +    P=0         -+-     P != 0
   +   +             + . +   (+ centre moves)
     -                  -
```

For small stresses the induced polarization is proportional to stress:

$$
P = d\,T
$$

where $d$ is the **piezoelectric coefficient** (C/N). Reversing the stress (tension instead of compression) reverses $P$.

**Converse effect:** an applied field $E$ shifts the ions and produces a strain $S = dE$; the crystal expands or contracts.

#### Applications

- Direct effect: gas lighters and spark igniters, microphones, pressure and vibration sensors, accelerometers.
- Converse effect: quartz crystal oscillators (in watches and computers), ultrasonic transducers, buzzers, precision actuators.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In an unstressed piezoelectric crystal, the net polarization is zero because:
   A. The centres of positive and negative charge in each cell coincide
   B. There are no ions
   C. The crystal has a centre of symmetry
   D. Ions are free to move
   Answer: A. Coinciding charge centres cancel the dipole moments. (Key point: Unstressed state)

2. Under stress, a net dipole appears because:
   A. The crystal gains charge from outside
   B. Ions are displaced unequally and, with no centre of symmetry, the positive and negative charge centres separate
   C. Electrons are removed
   D. The temperature rises
   Answer: B. The lack of symmetry lets the stress shift the charge centres apart, giving polarization and voltage. (Key point: Effect of stress)

3. For small stress T, the polarization is:
   A. P = d/T
   B. P = dT^2
   C. P = dT, with d the piezoelectric coefficient
   D. P = T/d
   Answer: C. P is proportional to stress; reversing the stress reverses P. (Key point: Linear law)

4. An applied field E produces a strain S = dE. This is:
   A. The direct effect
   B. Dielectric breakdown
   C. The Curie effect
   D. The converse effect, used in quartz oscillators and ultrasonic transducers
   Answer: D. The field shifts ions so the crystal changes shape. (Key point: Converse effect)

## Related in Electrical Engineering Material

- [Hard vs soft magnetic materials for relays](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Ahard-soft-relay) — Electrical Engineering Material, chapter 4
- [Dipole moment and Clausius-Mossotti equation](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Adipole-moment-clausius-mossotti) — Electrical Engineering Material, chapter 3, asked 2×

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- [Define electric dipole and polarization](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Adefine-dipole-and-polarization) — Electromagnetism, chapter 2
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