Download electric potential energy

Friday, February 15, 2013
LESSON 6:
ELECTRIC POTENTIAL
ENERGY
AP Physics B Course Objectives
III.A.2. Electric Field and Electric Potential
Students should understand the concept of
electric potential, so they can:
(5) Calculate how much work is required to move a test charge
from one location to another in the field of fixed point charges.
(6) Calculate the electrostatic potential energy of a system of
two or more point charges, and calculate how much work is
required to establish the charge system.
Student Objectives
Students will be able to:
1) Calculate electrostatic work.
2) Relate electrostatic work to electric potential energy.
Announcements
Exam Repair TODAY
 7am, 4th, 5th, 3pm
HW #5 due today
HW #6 due tomorrow
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Monday to the office.
Review
Energy
Wc = - ∆U
W = F ∆r cos θ
Electric Potential Energy
Electrical potential energy is the energy
contained in a configuration of charges.
Like all potential energies,
 when it goes up the configuration is less stable;
 when it goes down, the configuration is more
stable.
The unit is the Joule (J).
Electric Potential Energy
For spherically symmetric or point charges:
Electric Potential Energy
Electrical potential energy increases
when charges are brought into less
favorable configurations
ΔU > 0
+
+
-
+
Electric Potential Energy
Electrical potential energy decreases
when charges are brought into more
favorable configurations.
ΔU < 0
+
-
+
+
Electric Potential Energy
+
+
+
ΔU is ____
+
–
ΔU is ____
-
WORK by an external force must be done on the charge
to increase the electric potential energy.
Work and Charge
+
d
+
F
E
Move a positive
charge “upfield”. The
electric force does
negative work and an
external force does
positive work.
The electric potential
energy has increased
and ∆U is positive
(U2 > U1)
Work and Charge
d
F
E
Move a negative
charge “upfield”. The
electric force does
positive work and an
external force does
negative work.
The change in the
electric potential energy
∆U is negative
(U2 < U1)