On heating some polar crystals, weak electric current is produced. It is termed as: (a) Superconductivity (b) Piezoelectricity (c) Photoelectric current (d) None of these

Understanding polar crystals is key to grasping how piezoelectricity works. A crystal structure is considered polar if it inherently has a polarization due to the arrangement of its ions – that is, there is a geometric separation of positive and negative charges within the material. Polar crystals are unique because their crystal lattices are asymmetrical which results in electrical dipoles.

These dipoles align in a particular direction within the material. However, when the crystal lattice is subject to changes, such as mechanical stress or changes in temperature, the alignment of these dipoles changes. This variation can result in a temporary unbalance of charge within the material, causing the generation of an electric field. Thus, under the right conditions, these polar crystals can generate an electric charge, which is an important aspect of piezoelectricity.

Electric current is the flow of electric charge carriers, typically electrons or ions. In the context of piezoelectric materials, the current generation happens without the need for an external voltage source. Instead, the mechanical stress applied to the crystal causes a shift in the positions of the ions within the crystal lattice, leading to uneven charge distribution.

This can produce an electric potential across certain areas of the material. Consequently, if electrodes are attached to the material, the electric potential can drive a current from one electrode to another, translating the mechanical energy into electric current. The discovery of this process has led to numerous applications, including the conversion of kinetic energy from vibrations or pressure into electrical energy, used in sensors, actuators, and energy harvesting devices.

The mechanical stress electrical response is the underlying principle of piezoelectricity. When mechanical stress, such as pressure, force, or vibrations, is applied to a piezoelectric material, it undergoes a physical deformation. This deformation leads to a realignment of the electrical dipole moments within the crystal structure which creates a voltage across the material.

In practical applications, this mechanical-to-electrical transduction enables these materials to act as sensors that convert mechanical signals into electrical signals. An example is the piezoelectric disk in an electronic buzzer, wherein pressure is converted into an audible tone. Understanding this conversion allows us to harness the electrical response of materials under stress for various technological innovations, emphasizing the importance of piezoelectric materials in modern science and engineering.