Piezo-Electricity

PIEZO-ELECTRICITY

These materials have the property of becoming electrically polarized when mechanical stress is applied. This property, known as piezoelectric effect and has an inverse.

According to inverse piezoelectric effect when an electric stress (voltage) is applied, the material becomes strained. The strain is directly proportional to the applied field E.

When piezoelectric crystals are subjected to compression or tension, opposite kinds of charges are developed at the opposite faces perpendicular to the direction of the applied force. The charges produced are proportional to the applied force.

This property is utilized in conversion of mechanical energy into electrical energy and vice versa. The quartz crystal is a very good example of one such transducer material. Crystal faces are to be cut in proper direction and shaped to achieve maximum efficiency.

Explanation

A rectangular slab is cut from a quartz crystal such that two opposite faces ABCD and EFGH are perpendicular to optic axis.

The other two opposite faces AEHD and BFGC are parallel to optic axis.

When the mechanical stress is applied to two opposite faces ABCD and EFGH, then opposite electrical charges (electrical field) are developed on the other pair of opposite faces AEHD and BFGC. This is known as piezo-electric effect. (Fig. 1.29 (a)).

When the electrical field is applied across the opposite faces parallel to optic axis AEHD and BFGC (fig. 1.29 (b)), then thickness of the crystal (faces ABCD and EFGH) along the optic axis changes ie., there is expansion or contraction of the crystal. This is known as inverse piezo-electric effect.

• The electric charges produced is directly proportional to the stress applied.

• Piezo-electric effect is well exhibited on natural crystals cut in a suitable direction with respect to optic axis.

• Piezo Electricity

Electricity produced by means of piezo-electric effect is known as piezoelectricity.