Piezoelectric Effect
The generation of electric polarization charge in non-centrosymmetric crystals when subjected to mechanical stress (and vice versa).
Governing formula
Direct: P_i = d_ijk σ_jk; Reverse: ε_jk = d_ijk E_i
SI unit
C/N or m/V (Piezoelectric Coefficient d)
In depth
Occurs in crystal structures lacking inversion symmetry. Mechanical compression distorts the unit cell, shifting positive and negative ion centers to produce net electric dipole moments.
Examples in the real world
Quartz crystal resonators keeping precise time in wristwatches, and ultrasound transducer transducers.