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Honors Physics Power-Point 6-5
CAPACITORS
WHAT IS A CAPACITOR?
Capacitors store electric charge and decay
through exponential means.
 They are made out of two parallel plates with a
material (typically waxed paper) between them.
This material is called a dielectric.

APPLICATIONS OF CAPACITANCE
It all began with the Leyden jar (demonstrated
earlier).
 It is now used in computers, radios, televisions,
and camera flashes.

THE DEFINITION OF CAPACITANCE

It is the charge stored per unit voltage. It is
measured in a unit called a Farad. (It is named
after Michael Faraday (1791).
Q Coulombs
C 
 Farad
V
Volt
EXAMPLE #1: USE OF DEFINITIONS

A 12.0 V battery is connected to a parallel plate
capacitor which is rated at 1.0 micro-Farads.
The plates are separated by 0.00600 m of dry air.
(a) What is the charge on each plate?
(b) What is the electric field exactly between the
plates?
FACTORS AFFECTING CAPACITANCE

The relative permittivity (K) indicates the
degree to which a capacitor can hold its charge.
EXAMPLE 2: FACTORS AFFECTING CAPACITANCE

A parallel plate capacitor consists of two metal
plates separated by 0.00600 m of dry air. The
capacitor is connected to a 100 V source. The
area of each plate is 0.0400 m2.
C
 o A
C
d

 o A
d
Where k= relative permittivity (k=1 here)
The permittivity constant is 8.85 X 10-12
(a) What is the value of the capacitance?
(b) What happens when the area of the plates is increased?
(c) What happens when the distance between the plates is increased?
EXAMPLE 3: RELATIONSHIP TO ENERGY
To derive expressions for energy stored in a
capacitor, we must appeal to integral calculus.

(We only need to use the results here!)
EXAMPLE 3: ENERGY IN A CAPACITOR
A 1.0 pF capacitor is hooked up to a 9.0 V
battery.
 (a) What is the charge stored?
 (b) What Is the energy stored?

EXAMPLE 4: CAPACITOR DECAY

Derive the exponential decay equation for
Voltage in a capacitor.
EXPRESSION FOR EXPONENTIAL DECAY
V  Voe
kt
The constant k is the time constant for a circuit
and is equal to 1/RC (proven through
differential equations—well beyond the scope of
the course.

EXAMPLE 4: CAPACITOR DECAY
A 9 V battery is connected to a 100 kilo-ohm
resistor and a 1 micro-Farad Capacitor.
 (a) What is the time constant for this circuit?
 (b) What is the time that half the value of the
voltage decays?
