Do Now (9/13/13):
1. How does the electric force change when
distance is increased?
2. How does the electric force change when
either or both of the charges are increased?
Electric Field
Physics II
Analogy
The electric field is the space
around an electrical charge
just like
a gravitational field is the space
around a mass.
Electric Field
 Space around a charge.
What is the difference?
Van de Graaff Generator
 Builds up static electric
charges.
Electric field
 Any charge placed in an electric field will
experience a electrical force.
 The field strength is the electric force per unit
charge on any “test charge” placed in the
field: E = F /q.
 E: vector that points in the direction of the net
electric force on a positive charge; its units
are N / C.
 F : vector force on the test charge
 q : test charge, a scalar.
Electric Field Vector, E
 E = F/ qo
 qo , positive test charge
 E: electric field strength (vector
quantity)
 Unit: N/C
 E is analogous to the gravitational
field, g, where g=F/m
Example 1
 A charge of 3µC is used to test the electric
field of a central charge of 6C that causes a
force of 800N. What is the magnitude of the
electric field?
 Answer: 2.7 x108 N/C
Electric Field
http://higheredbcs.wiley.com/legacy/colleg
e/halliday/0471320005/simulations6e/inde
x.htm?newwindow=true
Electric Field of Point Charge
Practice:
 Use the rest of class to work on your
homework
Do Now (9/16/13):
 A charge of 9µC is used to test the electric
field created by a charge of 6C that causes a
force of 800N. What is the magnitude of the
electric field?
Do Now (9/17/13):
A proton is placed in a uniform electric field of
200 N/C. Determine its:
1. Force
2. Acceleration
Potentially useful info:
mp=1.67x10-27kg
me=9.1x10-31kg
Electric Field Lines of a positive
and negative charge
Electric Field Lines of two Positive
Charges
Electric Field Lines
 Lines that indicate the strength and
direction of the electric field.
 The more dense the lines, the stronger the
field.
 Electric field vectors are tangent to the
curve.
 Simulation
http://online.cctt.org/physicslab/content/applets/
pointcharges/elefi_z.htm
Field Lines should be evenly spaced!
Which field is stronger?
A
B
Consider the electric field lines shown in the
diagram below. From the diagram, it is apparent
that object A is ____ and object B is ____.
a. +, +
b. -, -
c. +, -
d. -, +
e.
insufficient
info
Practice:
 Use the rest of class to complete the paper
“Electric Field Lines.”
 When you finish, submit to Ms. Timson
 If you finish early, please work on the bonus
or your quiz review.
Do Now (9/18/13):
 Use your understanding of electric field lines to
identify the charges on the objects in the following
configurations
Practice:
 Use the rest of class to work on your quiz
review and notecard!
Conductors and Electric Fields
(under electrostatic conditions)
 “The electric field is zero inside a charged
conductor”.
 “Excess charge on an isolated conductor
resides on the surface”.
 “Excess charge accumulates on sharp points”.
 Electric field lines meet the conductor
perpendicular to the surface of the conductor.
Shielding
 The electric field is zero inside a charged
conductor.
Where are you safe during a
thunderstorm?
A) In a car
B) Outdoors
or
Why can you not get radio
reception in a tunnel or in a steel
bridge?
Electric Field for a Point Charge
Using E=F/qo and Coulomb’s Law prove:
E=k Q
______
r2
where Q is the central charge.
Example 2
 A test charge of +3µC is located 5m to the
east of a -4µC charge.
 A) Find the electric force felt by the test
charge.
 B) Find the electric field at that location.
 Answer: 4.32x10-3 N, 1.44 x 103 N/C along
the –x axis.
Example 3
 If a test charge is moved to a location three
times as far as its original location, how does
the electric field change?
Example 4
 Calculate the electric field felt by a test charge
located half way between a charge of +1C and
a charge of -3C, that are 2m apart.
 Answer: 1.8 x 1010 N/C