Piezoelectric effect – response of a piezoelectric crystal to mechanical stress When a mechanical force is applied to a piezoelectric crystal, which primary effect is observed?

Introduction / Context:
Piezoceramics and single-crystal piezoelectrics are widely used in sensors, actuators, and frequency control devices. The direct piezoelectric effect converts mechanical stress into electric polarization, enabling force, acceleration, and pressure measurements with high sensitivity and bandwidth.

Given Data / Assumptions:

• Crystal lacks a center of symmetry (necessary for piezoelectricity).
• Mechanical stress is applied within elastic limits.
• Electrodes allow measurement of generated charge or voltage.

Concept / Approach:

In a piezoelectric material, stress displaces charge centers within the unit cell, creating a net dipole moment per unit volume (polarization). This gives rise to bound surface charges, measurable as a voltage across electrodes under open-circuit conditions or as a current under short-circuit conditions. The converse effect also holds: an applied electric field produces strain, useful in actuators and ultrasonic transducers.

Step-by-Step Solution:

Verification / Alternative check:

Calibration with known forces yields linear charge output Q = d * F for many materials (d is piezoelectric coefficient) within the elastic regime.

Why Other Options Are Wrong:

(a) Piezoelectric operation is elastic, not plastic. (b) Magnetic dipoles are unrelated to piezoelectricity. (d) Fermi level shifts concern semiconductors under doping or bias, not mechanical stress in insulators used for piezo sensors.

Common Pitfalls:

Confusing pyroelectric or ferroelectric effects with piezoelectricity; although related classes exist, the key here is stress-induced polarization.

Final Answer:

Electrical polarization and the development of surface charges (voltage)