Definition & Determination of Capacitance
• Definition of Capacitance
• Uses for Capacitors
• Capacitor Symbology
• Properties of a Capacitor
• Determination of Capacitance
Capacitors
• A capacitor is a device which
stores electric charge on two
conducting plates, separated
by a finite gap.
• Electric potential energy is stored in a uniform
electric field, generated by the charge
distribution on the two plates
Uses for Capacitors
• Capacitors are used to dissipate electric
energy to run a camera flash
• A capacitor can provide energy to back-up
computer power supplies
• Capacitors can block low frequency AC
signals, and act as power supply filters
Capacitor Symbology
• Capacitor schematic symbols
• vs. Battery schematic symbol
Properties of a Capacitor
• When a capacitor is connected to a source of
potential difference (battery), the electrons are
drawn to the (+) terminal of the battery & flow to
the other plate, until equilibrium has been
reached
Properties of a Capacitor
• The electron debt on the LH plate leaves the
plate with a net (+) charge. The excess electrons
on the RH plate leaves the plate with a net (-)
charge.
•
+Q
-Q
• The establishment of this charge
distribution leads to the development
of a uniform electric field between the
plates
Capacitance Parameters
• The amount of potential difference (ΔV) that can
be applied across the plates is limited by the
amount of charge that can be accumulated on
the plates & is given by the relationship:
Q
C
V
• SI Units: Farads (1μF = 1.0 x 10-6 F)
Determination of Capacitance
• The electric field between the plates
Is mostly uniform

1 Q
ch arg e
E
  where  is
0 0 A
area
Qd
V  E  d 
0 A
Q
A
C
 0
V
d
• The capacitance does not depend on Q or ΔV
but only on the geometry of the device
The Coaxial Capacitor
20l
C
 Ra 
ln  
 Rb 
Capacitance of Parallel Conducting
Wires
• 2 conducting wires, of radius R, and length
l
C
0l
d 
ln  
R