Transcript Capacitors

CAPACITORS &
CAPACITANCE
End of the lesson, students
should be ;
• Understand capacitors and capacitance
• Understand capacitance equivalent circuits
for series and parallel connections
• Understand circuits with capacitive load
• Understand the process of charging and
discharging in a capacitor
The Capacitor
• Capacitors are one of the fundamental
passive components. In its most basic
form, it is composed of two plates
separated by a dielectric.
• The ability to store charge is the definition
of capacitance.
Capacitance
Q
C
V
Rearranging, the amount of charge on a
capacitor is determined by the size of the
capacitor (C) and the voltage (V).
Q  CV
Q for charge = unit Coulomb,
C for Capacitor = unit Farad
V for voltage = unit Volts
Construction of capacitor
Construction of capacitor
Conductors
Dielectric
Types of capacitors
• Fixed
• Variable
Fixed Capacitor
• Is a capacitor with a fixed value of
capacitance which cannot be adjusted.
• Is classified according to the type of
material used as its dielectric, such as
paper, oil, mica or electrolyte.
Example of Fixed Capacitor
• Mica
Mica capacitors are small with high
working voltage. The working voltage is
the voltage limit that cannot be exceeded.
Foil
Mic a
Foil
Mic a
Foil
Mic a
Foil
Ceramic disk
• Ceramic disks are small nonpolarized
capacitors They have relatively high
capacitance due to high er.
Lead wire soldered
to silver elec trode
Solder
Ceramic
dielectric
Dipped phenolic c oating
Silv er elec trodes deposited on
top and bottom of ceram ic disk
Plastic Film
Plastic film capacitors are small and nonpolarized.
They have relatively high capacitance due to larger
plate area.
High-purity
foil electrodes
Plastic film
dielec tric
Outer wrap of
polyester film
Capacitor section
(alternate strips of
film dielectric and
Lead wire
foil electrodes)
Solder coated end
Electrolytic (two types)
• Electrolytic capacitors have very high
capacitance but they are not as precise as
other types and tend to have more
leakage current. Electrolytic types are
polarized.
+
_
Symbol for any electrolytic capacitor
Variable Capacitor
• Is a capacitor that is constructed with an
adjustable value of capacitance.
• E.g :
• Example :rotor-stator type, trimmer
capacitor
Capacitance
• A capacitor stores energy in the form of an
electric field that is established by the
opposite charges on the two plates. The
energy of a charged capacitor is given by
the equation
1
2
WatauE  CV
2
where
W = the energy in joules
C = the capacitance in farads
V = the voltage in volts
• If a 0.001 mF capacitor is connected in
series with an 800 pF capacitor, the total
capacitance is
C
1
C
2
Answer :
444.44pF
0
.0
0
1µ
F
8
0
0p
F
• If a 0.001 mF capacitor is connected in
parallel with an 800 pF capacitor, the total
capacitance is
C
1
C
2
0
.0
0
1
µ
F
8
0
0
p
F
Answer :
1.8 x10-9 F
Series-parallel Capacitors
C1
C2
C3
E
CT = C1 Siri (C2 // C3)
CT = C1 Siri (C2 + C3)
1
1
1
---- = ----- + --------------CT
C1
(C2 + C3)
• When a metal wire is connected across the two terminals of a
DC voltage source such as a battery, the source places an
electric field across the conductor.
• The moment contact is made, the free electrons of the
conductor are forced to drift toward the positive terminal under
the influence of this field.
• The free electrons are therefore the charge carrier in a typical
solid conductor.
• For an electric current of 1 ampere, 1 coulomb of electric
charge (which consists of about 6.242 × 1018 elementary
charges) drifts every second through any plane through which
the conductor passes.
•For a steady flow of charge through a surface, the current I
(in amperes) can be calculated with the following equation:
where Q is the electric charge transferred through the surface
over some time t. If Q and t are measured in coulombs and
seconds respectively, I is in amperes.
More generally, electric current can be represented as the rate
at which charge flows through a given surface as:
•Current is the time rate of change of charge,
Electric flux, 
• Flux, a scientific term describing the rate of flow
of something through a surface.
• Electric flux, a measure of quantity of electricity.
• In electromagnetism, electric flux is the flux of
the electric field.
• Electric flux is proportional to the number of
electric field lines going through a virtual surface.
• is measured in coulombs.
Electric flux density, D
• is the amount of flux passing through a
defined area A that is perpendicular to the
direction of the flux:
• also called charge density, σ
Electric field strength,E
• also called potential gradient
• Electric field strength is a quantitative expression of the
intensity of an electric field at a particular location.
• The standard unit is the volt per meter (v/m or v · m -1 ).
• A field strength of 1 v/m represents a potential difference of one
volt between points separated by one meter.
• Any electrically charged object produces an electric field. This
field has an effect on other charged objects in the vicinity.
• The field strength at a particular distance from an object is
directly proportional to the electric charge, in coulomb s, on that
object.
• The field strength is inversely proportional to the distance from
a charged object.
E = V/d
Dielectric
• The insulating medium separating charged
surfaces is called a dielectric.
• Compared with conductors, dielectric
materials have very high resistivities.
• They are therefore used to separate
conductors at different potentials, such as
capacitor plates or electric power lines.
Factors affecting capacitance..