Qhhha abhhha hahhabba hauuua hhhab.

1. Piezoelectricity
• The piezoelectricity is the ability of certain materials to
generate AC voltage when subjected to mechanical stress or
vibration or to vibrate when subjected to AC voltage or both.

2. • The required force to spark
the gap must be applied
quickly because the
piezoelectric charge
generated will leak away. If
the charge generation takes
a longer time, spark ignition
uses mechanical impact.
The energy in the spark
depends on the amount of
charge generated. This can
be increased by using two
piezoelectric crystals back
to back.
Working principle of piezoelectric
spark generator

3. Example 7.14
• Consider a piezoelectric sample in the form of a cylinder.
Suppose that the piezoelectric co-efficient d= 250*10-12mV-1
and εR= 1000. The piezoelectric cylinder has a length of
10mm and a diameter of 3mm. The breakdown voltage is
about 3.5KV. What is the force required to spark the gap and
what is the mass ?
This force can be applied by squeezing by hand an appropriate lever
arrangement; it is the weight of (87.6/9.81) = 9kg

4. Electromechanical coupling factor
• It is the ratio of electrical energy converted to
mechanical energy to applied electrical energy
or is the It is the ratio of mechanical energy
converted to electrical energy to applied
mechanical energy .

5. • Paramagnetic:
• An atom in which the effects of the electron spin and orbital motion do
not quite cancel. The atom as a whole has a small magnetic moment, but
the random orientation of the atoms in a larger sample produces an
average magnetic momentof zero. The material shows no magnetic effects
in the absence of an external field.
• When an external field Bis applied, there is a small torque on each atomic
moment, and these moments tend to become aligned with the external
field. This alignment acts to increase the value of Bwithin the material
over the external value. The material is termed paramagnetic.
• However, the diamagnetic effect is still operating on the orbiting electrons
and may counteract the increase. If the net result is a decrease in B, the
material is still called diamagnetic.
• Potassium, oxygen, tungsten, and the rare earth elements and many of
their salts, such as erbium chloride, neodymium oxide, and yttrium oxide,
one of the materials used in masers, are examples of paramagnetic
substances.
Nature of different magnetic materials

6. Nature of magnetic material
• Anti-ferromagnetic:
• In anti-ferromagnetic materials, the forces between adjacent atoms cause the
atomic moments to line up in an anti-parallel fashion.
• The net magnetic moment is zero, and anti-ferromagnetic materials are affected
only slightly by the presence of an external magnetic field.
• Anti-ferromagnetism is only present at relatively low temperatures, often well
below room temperature.
• Ferri-magnetic:
• The ferri-magnetic substances also show an anti-parallel alignment of adjacent
atomic moments, but the moments are not equal.
• A large response to an external magnetic field therefore occurs, although not as
large as that in ferromagnetic materials.
• Super-paramagnetic:
• Super-paramagnetic materials are composed of an assembly of ferromagnetic
particles in a non-ferromagnetic matrix. Although domains exist within the
individual particles, the domain walls cannot penetrate the intervening matrix
material to the adjacent particle.

7. Nature of magnetic material