Capacitance and Dielectric
Capacitance and Dielectric
Capacitance and Dielectric
Capacitance
and
Dielectrics
Capacitors
Capacitors are devices that store
electric charge
Examples of where capacitors are used
include:
radio receivers
filters in power supplies
energy-storing devices in electronic flashes
Definition of Capacitance
The capacitance, C, of a capacitor is
defined as the ratio of the magnitude of the
charge on either conductor to the potential
difference between the conductors
Q
C
V
The SI unit of capacitance is the farad (F)
Makeup of a Capacitor
A capacitor consists of
two conductors
These conductors are
called plates
When the conductor is
charged, the plates carry
charges of equal
magnitude and opposite
directions
A potential difference
exists between the plates
due to the charge
More About Capacitance
Capacitance will always be a positive quantity
The capacitance of a given capacitor is
constant
The capacitance is a measure of the
capacitor’s ability to store charge
The farad is a large unit, typically you will see
microfarads (mF) and picofarads (pF)
Parallel Plate Capacitor
Each plate is
connected to a
terminal of the
battery
If the capacitor is
initially uncharged,
the battery
establishes an
electric field in the
connecting wires
Parallel Plate Capacitor, cont
This field applies a force on electrons in the
wire just outside of the plates
The force causes the electrons to move onto
the negative plate
This continues until equilibrium is achieved
The plate, the wire and the terminal are all at the
same potential
At this point, there is no field present in the
wire and the movement of the electrons
ceases
Parallel Plate Capacitor, final
The plate is now negatively charged
A similar process occurs at the other
plate, electrons moving away from the
plate and leaving it positively charged
In its final configuration, the potential
difference across the capacitor plates is
the same as that between the terminals
of the battery
Capacitance – Isolated
Sphere
Assume a spherical charged conductor
Assume V = 0 at infinity
Q Q R
C 4πεoR
V keQ / R ke
Note, this is independent of the charge
and the potential difference
Capacitance – Parallel Plates
The charge density on the plates is
σ = Q/A
A is the area of each plate, which are equal
Q is the charge on each plate, equal with
opposite signs
The electric field is uniform between the
plates and zero elsewhere
Capacitance – Parallel Plates,
cont.
The capacitance is proportional to the
area of its plates and inversely
proportional to the distance between the
plates
Q Q Q εo A
C
V Ed Qd / εo A d
Parallel Plate Assumptions