Capacitors, Dielectrics,
The Role of Dielectrics in the Capacitor
Presented by
Ramu Yarra,
Lecturer In Physics,
Govt College (A) Rajahmundry
Contents:
 Capacitor: i) Working Principle of Capacitor
ii) Construction and working of capacitor
 Dielectrics , Types of Dielectrics
 The role of Dielectric in the capacitor
Capacitor
 Capacitor is an electronic Device has the
ability to hold the charges at the given
potential.
 Capacitor consists of two metal plates
separated by insulating material (dielectric)
between them.
Capacitor symbol
Working Principle Of Capacitor:
 The capacitance of a capacitor can be increased by reducing the
potential between the two metal plates having opposite charges.
C=
𝑄
𝑉
Construction and Working of Capacitor:
Construction and Working of Capacitor:
A
In metal plate, the electrons are
freely moving around the +ve Ions
Due to electrostatic induction, - vely
charged electrons come towords
metal plate A (having +ve charges)
The + ve charges move away from
metal plate A
The electrostatic attraction between +ve charges on plate A and -ve charges on plate B,reduces the effect of repulsion
among +ve charges on plate A. so we can accommodate more charges on plate A with less amount of work (potential)
Construction and Working of Capacitor:
The capacitance of capacitor is also depends on geometry.
 If the area between the metal plate is large, we can store more number of
charges
𝐶 ∝𝐴
 If distance (d) between the plates is less, then more attraction between +ve
charges on plate A and -ve induced charges on plate B, it leads to reduce the
repulsion among the +ve charges of plate A
𝐶 ∝ 1/𝑑
𝜖 𝐴
Therefore
𝐶= 0
𝑑
Where 𝜖0 is propositional constant called permittivity of free space.
Note :if the distance between the plates further reduces, there should be electric
discharge.so we should keep maintain certain limiting distance between the metal
plates of a capacitor.
Dielectrics
 Dielectrics are insulators, plain and simple.
The two words refer to the same class of materials, but are of
different origin and are used preferentially in different contexts.
The origin of the word Dielectric comes from Greek. The Greek
prefix di or dia means "across". The material placed across the plates
of a capacitor like a little nonconducting bridge is a dielectric.
In a dielectric the charges are bounded, not free to move throughout
the material
Types of Dielectrics
Polar
Eg : heterogeneous
molecules like H2O, HCl
Non-Polar
ex: materials consists of
homonuclear molecules like H2,N2,
symmetric molecules like C6H6,CO2
Non-Polar Dielectrics
𝐸=0
Non polar Dielectric material
consists of molecules, whose
centre of gravity of +ve charge and
centre of gravity of –ve charge
coincide (dipolemoment P =0)
𝐸≠0
Induced electric field
In presence of electric field, each
molecule behaves like a dipole and
having induced dipolemomentum.
Non-Polar Dielectrics
 Under the influence of Electric field, The molecules of dielectric
behaves like a tiny electric dipole, in turn got induced dipole moment
(Pind).
 This net separation of charge on the surfaces of the dielectric
molecules gives rise to an induced electric field opposed to the
applied external field
𝑬net = 𝑬applied - 𝑬induced
Polar Dielectrics
𝐸=0
Polar Dielectric materials having
permanent dipoles, without an electric
field
𝐸≠0
If we apply electric field on polar
dielectrics, the dipoles are oriented
along the field direction
Polar Dielectrics
 The net permanent dipole moment (Pp) is increases due to the
orientation of dipoles along the field.
The net separation of charge on opposing surfaces gives rise to an
induced electric field( 𝑬𝒊 𝒏𝒅 ) within the bulk of the dielectric material
which is opposed to the external field.
𝑬net = 𝑬applied - 𝑬induced
Inserting a Dielectric into a Capacitor
Case 1: Capacitor without Dielectric :
There exit a uniform electric field between
the plates of a capacitor
Case 2: Capacitor with Dielectric :
Uniform Electric Field
Induced Electric Field
Note : Net Electric Field across the dielectric in between
the plates of a capacitor reduces
 If we insert dielectric between the capacitor plates, then net electric field reduces across the
capacitor.
Enet = Eapplied − Einduced = E0 – Ei
 The capacitance of a capacitor increases can be explained in two ways
i.
w.k.t
V = E/d
The potential across the plates of a capacitor reduces.
and
C = Q/V
so capacitance of a capacitor increases.
ii.
Due to the insert of the dielectric, the distance (d) between the +ve charge on metal plate
𝜖 𝐴
and induced -ve charge on dielectric decreases so capacitance of a capacitor increases ( 𝐶 = 0 )
𝑑
Thankyou All