Quartz

Piezoelectric effect
A thin panel of quartz (C) is covered on each side with a conductive material (a) and (b) (see Figure). If a pulling or pressure force is applied to the sheet, an electric voltage is created between points (d) and (e). The polarity of the voltage depends on the direction of the force (pressure/pulling).

Reversed piezoelectric effect
If a voltage is applied to points (d) and (e), the quartz panel lengthens or shortens.

Task
Using the piezoelectric effect, mechanical vibrations can be converted into electric vibrations, e.g. for a microphone, knock sensor, or pressure sensor.

The reversed piezoelectric effect can be used to convert electric vibrations into mechanical vibrations or movements, for example piezo loud speakers, actuators.

Both effects are combined in the oscillator crystal. If alternating current is applied to points (d) and (e), with the same frequency as the mechanical resonant frequency (fundamental or harmonic components) of the quartz panel, the quartz oscillates with sympathetic oscillations which stabilize the frequency of the electric vibrations. Quartz crystals are therefore often used in applications where high frequency stability is required, for example in clock pulse generators (oscillators) for electronic clocks and microprocessors.