The piezoelectric effect occurs in several
crystalline substances, such as barium titanate and tourmaline.
The effect is explained by the displacement of ions in crystals that have a nonsymmetrical
unit cell, the simplest polyhedron that makes up the crystal structure (see Crystal).
When the crystal is compressed, the ions in each unit cell are displaced,
causing the electric polarization of the unit cell. Because of
the regularity of crystalline structure, these effects accumulate,
causing the appearance of an electric potential difference between certain
faces of the crystal. When an external electric field is applied to
the crystal, the ions in each unit cell are displaced by
electrostatic forces, resulting in the mechanical deformation of the whole
crystal
The Hall effect occurs when a conductor or
semiconductor carrying an electric current is placed in a
magnetic field. A voltage, called the Hall voltage, is created across the
conductor or semiconductor perpendicular to both the current and
the magnetic field. This voltage arises because the magnetic field distorts the flow of
electrons or other charge carriers that constitute the
current, pushing the charged particles to one side of the conductor.
The Hall
voltage is proportional to the current and magnetic field and inversely
proportional to the number of electrons or other charged particles. For
instance, the Hall voltage across a metal is much smaller than across
a semiconductor carrying the same current in the same magnetic field because
the metal contains more charged particles than the semiconductor.