Q.2. What are polar and non polar molecules? (AKTU. 2014 - 15)
Ans. Based
on the presence or absense of electric dipoles, material in general can be
classified into polar or non polar. The distinction made in the following way.
Matter consists of either atoms, molecules or ions each having a definite
electric charge - positive or negative. If all the positive and negative
charges of a molecule are equivalently represented by one positive and one
negative charge mutually equal in absolute magnitude and located at the centre
of gravity, these summary charges may or may not coincide in space. In the
first case a non-polar molecule results whereas in the latter a polar molecule
results. It is further seen that the chemical structure of the molecule plays
an important role in elucidating the polar and non polar nature. In case of
symmetrically arranged gas moleucles like CO2, CCl4 and CH4, etc. the centres of gravity of both positive and
negative charges of the molecule coincide with the centre of symmetry of the
moleucle and therefore with each other. These molecules are called non-polar.
On the other hand, assymmetry moleucles like CH3Cl, CH2Cl2, etc are polar as the centre of gravity of positive
charges is separated from the centre of gravityof negative charge, thereby
inducing permanent dipole moments in them.
The dipole moment of an
atom/molecule is equal to the product of the summary positive (or negative)
electric charge multiplied by the distance between the two charges. In most of
the polar materials the electric dipole moment is of the order of 10-30 C-m. It must be
remembered that the polrization effect in polar molecules is strongly
temperature dependent and vice vesra in non polar molecules.
Q.3. What
is internal field in dielectric. (AKTU. 2015 - 16)
Ans. In
dielectric solids, the atoms or molecules experience not only the external
applied electric field but also the electric field produced by the dipoles. The
resultant electric field acting on the atoms or molecules of dielectric
substance is called the local field or an internal field.
Q.4. Explain the behaviour of dielectric in an
alternating electric field. What is relaxation time? (AKTU. 2015 - 16)
Ans. Behaviour Of Dielectrics In Alternating
Field: -
The study of the behaviour of the dielectric in alternating electric
field shows that the dielectric constant becomes complex. The imaginary part of
the complex dielectric constant accounts for the dielectric losses of the
material.
If a dielectric material is placed in an alternating
field the orientation of the dipoles and hence the polarization will tend to
reverse when the polarity of the field changes. So long as the frequency
remains low (< 106 Hz) there is no significant lag in
polarization with alternations of field. The permitivity is independent of
frequency and has same magnitude as in a static field. As the frequency
increases, the dipoles will not be able to rotate rapidly and their oscillators
will lag behind those of the field. With more increase in frequency, the
permanent dipoles in the medium will be unable to follow the field and the
contribution to the static permitivity from this molecular process i.e. the
orientation polarization stops. This generally happens in the radio frequency
range (106 - 1011 Hz) of
electromagnetic spectrum. At still more frequencies i.e. in the infra-red range
(1011 - 1014 Hz), the
relatively heavy positive and negative ions cannot follow the field reversals
and the contribution to permivity from atomic or ionic polarization ceases and
only electronic polarization persists.
So the permitivity of the dielectric material
decreases with increase in frequency and this phenomenon is called anomalous
dielectric dispersion.
Dielectric Absorption: Dispersion coming into play during the transition from
full atomic polarization at radio frequencies to negligible atomic polarization
at optical frequency is referred to as dielectric absorption.
Dielectric Relaxation: Dispersion due to the transition from full
orientational polarization at zero or low frequencies to negligble
orientational polarization at high frequency is known as dielectric relaxation.
Relaxation Time: -
It is
defined as the time in which the amplitude of the damped oscillations falls to
1/e of its original value. It is usually denoted by the symbol t. Form the expression
A = A0e-kt
where A = the amplitude of the damped oscillator
at any time t.
A0 = the original amplitude of a damped
oscillator.
If t is the relaxation time, then A = A0e-kt = A0.e-1
The heavier the mass or
smaller the damping, the larger is the relaxation time and slower is the rate
of fall of amplitude. Hence slower is the rate of dissipation of energy for
higher relaxation time.
