Tuesday, February 28, 2012
LESSON 12:
CAPACITORS AND
CAPACITANCE
Announcements
HW #10 due today
HW #11 and 12 due Tuesday
Physics Labs this week
 200 point grade
 Miss: 0/200
 Make it up: 150/200
Photoelectric Effect lab simulation
 Due Thursday
AP Physics B Course Objectives
III.B.2. Capacitance
a) Students should understand the definition and function of
capacitance, so they can:
(1) Relate stored charge and voltage for a capacitor.
(2) Relate voltage, charge, and stored energy for a capacitor.
(3) Recognize situations in which energy stored in a capacitor is
converted to other forms.
b) Students should understand the physics of the parallelplate capacitor, so they can:
(1) Describe the electric field inside the capacitor, and relate the strength
of this field to the potential difference between the plates and the plate
separation.
(4) Determine how changes in dimension will affect the value of the
capacitance.
Student Objectives
Students will be able to:
1)
2)
3)
Derive equation for capacitance from basic understanding of
charge.
Determine the charge stored on a capacitor for a given
potential.
Calculate the energy stored in a charged capacitor.
Introduction to Capacitors
Electric Potential Energy
and Parallel Plates
A positive charge placed between
two oppositely charged plates
increases in potential energy
when brought closer to the
positive plate by an outside force.
A negative charge placed between
the same plates would decrease in
potential energy when brought
closer to the positive plate.
(think… what is really doing the
work to move the charge?)
Relationship between Field
and Potential
-
+
+
+
+
+
+
+
+
+
+
+
The electric potential of the positive
plate is higher than that of the
negative plate.
Equipotential lines can be drawn
perpendicularly to the electric field.
If V between the plates is 10V, it is
convenient to set the negative plate
to V=0 and the positive to
V = 10V.
ΔV = -Ed
Connecting Potential and Field
Slide 21-24
Potential and Field for Three Important Cases
Slide 21-25
The Capacitance of a Parallel-Plate Capacitor and
Units of Capacitance
C
0 A
d
Conceptual Questions
If the distance between two plates is doubled,
what happens to the capacitance?
If the radius of two parallel plates is doubled,
what happens to the capacitance?
Charging a Capacitor
Charging a Capacitor
Capacitance and Potential
The charge ±Q on each electrode
is proportional to the potential
difference ∆VC between the
electrodes:
Q  CVC
Sample Problem 12.1:
A 2.2 F capacitor is first charged so that the electric
potential difference is 6.0 V. How much additional charge is
needed to increase the electric potential difference to 15.0
V?
Energy Stored in a
Capacitor
 Uc = q2 / 2C and q = C V

Uc = ½ C
2
V
Sample Problem 12.2:
The capacitance of an electronic photoflash in a strobe light is
10.0 F. It is charged to 300V. How much energy is stored in the
capacitor?