Definition of piezoelectric effect
The piezoelectric body is electrically polarized under the action of external mechanical force, and leads to the opposite sign of bound charges appear on the two ends of the piezoelectric body. The charge density is proportional to the external mechanical force. This phenomenon is called the positive piezoelectric effect. The electric body is deformed by the action of an external electric field, and its deformation is proportional to the strength of the external electric field. This phenomenon is called the inverse piezoelectric effect. A solid with a positive piezoelectric effect must also have an inverse piezoelectric effect, and vice versa. The positive piezoelectric effect and the inverse piezoelectric effect are collectively referred to as the piezoelectric effect. Whether a crystal has a piezoelectric effect is determined by the symmetry of the crystal structure.