Capacitance
Chapter 17 Lesson 2
p. 602
Capacitor
• A device that is used to store electrical
potential energy.
• Its many uses include tuning the
frequency of radio receivers, eliminating
sparks in automobile ignition systems,
and storing energy in electronic flash
units.
How a Capacitor Works
• Check the links below for more information.
http://www.howstuffworks.com/capacitor.htm
http://www.howstuffworks.com/capacitor1.htm
* A typical design for a capacitor consists of two
parallel metal plates separated by a small
distance. This type of capacitor is called a
parallel-plate capacitor.
The First Capacitor (Leyden Jar)
• Ewald Georg von Kleist ,
(Nov 1745, German
scientist)
• Pieter van
Musschenbroek
(several months later,
Dutch scientist)
• Benjamin Franklin
• Michael Faraday
Capacitance
• The ability of a conductor to store energy
in the form of electrically separated
charges
• The ratio of the net charge on each plate
to the potential difference created by the
separated charges
• SI unit for capacitance is farad, F = 1 C/V
Formula for Capacitance
Q
C
V
Capacitance =magnitude of charge on each plate
potential difference
1 µF = 1 x 10-6 F
;
1 pF = 1 x 10-12 F
• Capacitance depends on the size and shape of
the capacitor.
• This means that for a given potential difference
∆V, the charge on a plate is proportional to the
area of the plates and inversely proportional to
the separation of the plates.
• Real World Application: Because Earth is so large,
it has extremely large capacitance. The Earth can
provide a large amount of capacitance without its
electric potential changing too much. This is often
used as a reference point for measuring potential
differences in electric circuits.
• The material between a capacitor’s
plates can change its capacitance.
• In many parallel-plate capacitors, the
space is filled with a material called a
dielectric.
• Dielectric – an insulating material, such
as air, rubber, glass, or waxed paper.
• When a dielectric is inserted between
the plates of a capacitor, the capacitance
increases.
Energy and Capacitors
• A charged capacitor stores electrical
potential energy because it requires work
to move charges through a circuit to the
opposite plates of a capacitor.
• The work done on these charges is a
measure of the transfer of energy.
Electrical Potential Energy Stored in a
Charged Capacitor
1
PEelectric  QV
2
electric potential energy =
½ (charge on one plate) (final potential
difference)
Practice Problems
1. A capacitor, connected to a 12V battery,
holds 36µC of charge on each plate.
What is the capacitance of the
capacitor? How much electrical
potential energy is stored in the
capacitor?
2. Practice B, # 1, p. 607.
3. Practice B, # 3, p. 607.
Homework
1. Practice B, #2, p. 607.
2. Section Review, #3, p. 607.
Due: April 27, 2013