Download chapter 33 electric field

Chapter 33: Electric field.
• Define an electric field.
– An electric field is the space around an
electric charge where electric force of
attraction or repulsion can be felt by
another charge.
– Electric field around a charge is
represented by electric field lines
-
+
The direction of an electric field?
The direction of the electric field at any point,
is the direction of the electrical force on a
small positive test charge that is placed in the
field.
+q0
Small positive Test
Charge will either be
attracted or repelled.
q
Charged
object
What is an electric field line?
• Google Image
Result for
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csclassroom.com
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8l4c13.gif
What is electric
shielding?
• Electric shielding is the absence of electric field
within a conductor because the conductor is
holding static charge.
• The electrons within the conductor are not moving
and are evenly distributed.
• The net force would be Zero and therefore there
will be zero electric field.
100% Shielded
Zero
charge
electric shielding
• Explain why it’s
wise to remain in
your car when
there is thunder
storm and lightning
outside.
Class work/ homework
Class work: Textbook page 529,# 1-10
Homework: textbook pg 686 # 1-10
Lesson 2: Describing Electric Field
• Consider a test charge qo inside an electric field
around a large charged object as shown below. Charged
Force
experienced by
a test charge
(F)
object
F
qo
d
q
Positive Test
Charge qo
Distance between center of
small Test Charge and The
charged object
Electric field (E)
around the charged
object.
Electric field formula
electric Force
Electric Field 
Test charge
F
E 
q
o
Electric field formula # 2
• Electric field can also be calculated
using the formula
Kq
E
d
2
example
•
What is the electric
field at a location
where an electron
experiences a force
of 4.5 x 10-8 N?
F
E
q
o
example
•
A moving car
• Soln
accumulates positive
charge of 4.0 x 10-6 C.
What is the magnitude
and direction of the
electric field 0.2 m
away from the car?
Kq
E
d
2
Class work
• Workbook page 202 Exercises 6-9
and page 206 #A5-A10.
Lesson 3: Electric potential Energy
• Warm-up:
– A load weighing 400 N is raised a height
of 5m. Calculate the work done. What is
the gravitational potential energy?
Electric potential Energy
• Consider a charged particle being moved
towards a positively charged object.
Question:
+ qo
+
+
+
+ + + +
+
+
+
+
+
+
Push
Separation distance
between charged
objects
+q
Why is it not easy
to reduce the
separation
distance between
two charged
objects?
• Answer:
– The small charged particle will be repelled by
the large object.
• In order to move it, External work has to
be done.
• When work is done on a charge, it acquires
Electric potential Energy. (energy of
position)
– Think of charging your cell phone before using
it.
– Work Done = Electrical potential Energy.
Electric kinetic Energy
• When a positive charge close to a large positively
charged object is released, all the electrical potential
Energy becomes Kinetic Energy.
– (Think of using your already charged cell
phone).
Motion due to Repelling
force
+ qo
+
+
+
+q
+ +
+
+
+
Work done to move charge = Electric potential energy = Kinetic Energy
Charging cell phone → Charged cell phone → using cell phone
( law of conservation of energy)
Electric potential energy Vs
Gravitational-PE
• Pushing charges
close to a large
charged object is
like raising water
up the cliff.
+
+
+ ++ ++
+ +
+
+
+
+
+ +
+
+
Electric potential
• It is the electric potential energy per charge.
electric potential energy
electric potential (V) 
ch arg e
V 
Energy or work
q
• Also called potential difference or voltage (V)
• The units of electric potential is Volts
Electric potential is constant
• Pushing more charges requires more work, but
more electrical potential energy is generated
Does the ratio of
electric potential
energy acquired,
to the number of
charge moved,
change?
_____________
______________
Push
+ qo
+q
Electric potential
• Any charged body has an electric potential
within its electric field.
• Charge must not be present for there to
be electric potential.
• When a charge is introduced in the field,
that charge acquires;
• Electric __________ __________
Exercise
• Class work page 530, #31 and 32
• Homework: Textbook pg 530 # 11-18
Lesson 4: Electric potential, electric field and
distance between charges objects.
• Electric potential is proportional to the product
of electric field of the charged object and the
distance of separation between the test charge
and the charged object.
• Electric potential ~ electric field
• separation between
charged objects (Plates)
Electric potential  electric field  separation between Plates
potential difference
electric field (E) 
separation between plates
v
E 
d
• Video on Demand - The
Mechanical Universe...and Beyond
- The Millikan Experiment
Class work:
1. workbooks, Pg 203, Exercises 10-14,
A10- A13.