If one pair of opposite faces of a quartz crystal is subjected to pressure, the other pair of opposite faces develop equal and opposite electric charges on them (Fig.). The Sign of the charges is reversed when the faces are subjected to tension instead of pressure. The electric charge developed is proportional to the amount of pressure or tension. This phenomenon is called Piezoelectric effect
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The effect is reversible i.e., if an electric field is applied across one pair of faces of the crystal, contraction or expansion occurs across the other pair.
When the two opposite faces of a quartz crystal, their faces being cut perpendicular to the optic axis, are subjected to alternating voltage, the other pair of opposite faces experiences stresses and strains. The quartz crystal will continuously contract and expand Elastic vibrations are set Up in the crystal.
When the frequency of the alternating voltage 18 equal to the natural frequency of vibration of the crystal or its simple higher multiples, the crystal is thrown into resonant vibrations The amplitude' is large. These vibrations are longitudinal in nature.
Consider a X-cut crystal plate of thickness t. The fundamental frequency of vibration is given by n=1/2t √E/P
E is the Young’s modulus and p is the density of the material of the crystal plate.
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