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Capacitance
Capacitor
•A capacitor is a
device that stores
electric charge.
Capacitor applications:
Computer RAM memory and keyboards.
Capacitor applications:
Electronic
flashes for
cameras.
Capacitor applications:
Electric
power
surge
protectors.
Capacitor applications:
Radios and electronic circuits.
Capacitor applications:
Electric
Power
Supplies
Capacitor applications:
Timing
Circuits
Capacitor applications:
Voltage
Storage
Device
For
Car
Stereos
Capacitors
A basic capacitor has two parallel plates
separated by an insulating material
 A capacitor stores an electrical charge
between the two plates
 The unit of capacitance is Farads (F)
 Capacitance values are normally smaller,
such as µF, nF or pF

Capacitors

Basic capacitor construction
Plate 2
Dielectric
material
Plate 1
The dielectric
material is an
insulator therefore
no current flows
through the
capacitor
Capacitors
Storing a charge between
the plates
 Electrons on the left
plate are attracted
toward the positive
terminal of the voltage
source
 This leaves an excess of
positively charged holes
 The electrons are
pushed toward the right
plate
 Excess electrons leave a
negative charge
+
+
+
_
_
-
Capacitors
Types of capacitors
 The dielectric material
determines the type of
capacitor
 Common types of
capacitors are:
 Mica
 Ceramic
 Plastic film
Capacitors


Some capacitors are
polarised, they can
only be connected
one way around
Electrolytic
capacitors are
polarised
Capacitors



Variable capacitors are
used in communication
equipment, radios,
televisions and VCRs
They can be adjusted by
consumers by tuning
controls
Trimmers are internal
adjusted capacitors
that a consumer cannot
adjust
Capacitors


These variable
capacitors would be
difficult to squeeze
into your mobile
phone and iPod
Current technology
uses semi-conductor
variable capacitors
called varactors
(varicaps)
Capacitor
A capacitor consists of two conductors separated by an
insulator, which could be air or even a vacuum.
Metal Plates
Insulating Material
(Dielectric)
Some Capacitors
conductor
insulator
Capacitance : Definition

Take two chunks of
conductor



Apply a potential V
between them
Charge (Q) will appear on
the conductors,



Separated by insulator
Q+ = +CV
Q- = -CV
C depends upon both:


“geometry”
Insulator material (dielectric)
Q+ = +CV
+++++++++++
+++++++++++
+++++++++++
V
V
0
+
e
e
e
-
e
e
e
ee
e e- e- e-
V
e e e eeee e
ee
Types of Capacitors
Parallel-Plate Capacitor
Cylindrical Capacitor
A cylindrical capacitor is a parallel-plate capacitor
that has been rolled up with an insulating layer
between the plates.
Dielectrics - Insulator
Air - Often used in radio tuning circuits
Dielectrics
Ceramic - Used for high frequency purposes
like antennas, X-ray and MRI machines
Dielectrics
Mylar - Most commonly used for timer circuits
like clocks, alarms and counters
Dielectrics

Glass - Good for high voltage applications
Dielectrics
Super capacitor - Powers electric and hybrid
cars
Example 1 : Parallel-Plate
Capacitor
1.
2.
3.
Calculate field
strength E as a
function of charge
±Q on the plates
Integrate field to
calculate potential V
between the plates
Q=CV,
C = V/Q
Area A
+Q
Dielectric constant e
Separation d
Area A
-Q
E
V
Example 1 : Parallel-Plate
Capacitor
 Qd 
V 

 eA 
Q eA
C 
V
d
+Q
Area A
e
d
Area A
dl
âz
E
-Q
RC Time Constant
T = RC
RC Time Constant
The time it takes for a
capacitor to charge to
63.2% or discharge to
36.8% of the maximum
voltage.
Capacitance in AC Circuits
Capacitive Reactance
Find the current flowing in a circuit when a
4uF capacitor is connected across a 880v,
60Hz supply.