When some crystals are deformed by the application of an external stress, their crystalline structure is such that charge separation between the positive and negative ions establishes an polarization phenomeno, and the equal electrical charges appear across its two opposite faces. When the force is removed, it will return to the uncharged state. This phenomenon is called piezoelectric effect, also referred to direct piezoelectric effect. In addition, the polarity of the charge also changes along with the direction of the force, the amount of charge is proportional to the applied force value.
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