05-09-2016, 12:15 PM
1452797881-CapacitorsAndDielectrics.pdf (Size: 256.08 KB / Downloads: 8)
In this small e-book we’ll learn about capacitors and
dielectrics in short and then we’ll have some questions
discussed along with their solutions. I’ll also give you a
practices test series which you can solve on your own.
Capacitors And Capacitance
• A capacitor (formerly known as condenser) is a device
that can store electronic charge and energy.
• Capacitors are important components used in
electronics and telecommunication devices for example
radio , television receivers, transmitter circuits etc.
• Capacitor is a device used for storing electronic charge.
• All capacitors consists of two metal plates (or
conductors) separated by an insulator (air, vacuum or
any other dielectric medium).
• Figure 1 below shows the symbol used to represent a
capacitor.
Capacitor gets charged when a battery is connected to
it.
• So when there is a potential difference between two
metal plates of the capacitor shown below in the figure.
• Capacitor gets discharged on joining two of its plates.
• If V is the potential difference between two plates of
the capacitor and q is the amount of charge developed
on each plate then q/V is constant for the capacitor
since q is proportional to V.
• The ratio of charge on either plate to the potential
difference between the plates is called capacitance C of
the capacitor. Thus,
C=q/V
or,
q=CV
• Unit of capacitance is Farads (F) or CV-1
.
• 1F is very large unit of capacitance. Practically
capacitors with capacitance of the order of micro farads
(µF) are used in circuits of radio receivers, transmitters
etc. Thus,
1µF=10-6 (micro)
1nF=10-9 (nano)
1pF=10-12 (pico)
• For any capacitor it's capacitance is constant and
depends on shape , size , separation f the two
conductors and also on insulating medium being used
for making capacitor.
• Capacitance of parallel plate capacitor having vacuum
or air acting as dielectric or insulating medium is
C= (ε0A)/d
where,
C= capacitance of capacitor
A= area of conducting plate
d= distance between plates of the capacitor
ε0=8.854× 10-12 and is known as electric permittivity in
vacuum.
• If k is the relative permittivity of the dielectric medium
then
ε=ε0k
thus capacitance of parallel plate air capacitor in
presence of dielectric medium of electric permittivity ε
is
C=εA/d
• Capacitance of spherical capacitor having radii a, b
(b>a) with
(a) air as dielectric between them
C=(4πε0ab)/(b-a)
(b) dielectric with relative permittivity ε
C=(4πεab)/(b-a)
Series And Parallel Combination Of
Capacitors
(A) Parallel combination of capacitors
• Capacitors connected in parallel combination have
same potential difference across their terminals. Figure
below shows two capacitors connected in parallel
between two points A and B
• Right hand side plate of capacitors would be at same
common potential VA. Similarly left hand side plates of
capacitors would also be at same common potential VB.
• Thus in this case potential difference VAB=VA-VB would
be same for both the capacitors, and charges Q1 and Q2
on both the capacitors are not necessarily equal. So,
Q1=C1V and Q2=C2V
• Thus charge stored is divided amongst both the
capacitors in direct proportion to their capacitance.
• Total charge on both the capacitors is,
Q=Q1+Q2
=V(C1+C2)
and
Q/V=C1+C2