Download Superposition of forces

Chapter 19
Electric Charges, Forces, and
Fields (Cont.)
Dr. Jie Zou
PHY 1161
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Outline
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Superposition of electric forces
Electric field
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Electric field of a point charge
Superposition of electric fields
Electric field lines
Dr. Jie Zou
PHY 1161
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Superposition of forces
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Dr. Jie Zou
When a charge
experiences forces due to
two or more other
charges, the net force on
it is simply the vector
sum of the forces taken
individually.
Superposition
 
of 
forces: F1  F12  F13  F14
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Example 19-3: superposition
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Three charges, each equal
to +2.90 C, are placed at
three corners of a square
0.500 m on a side, as shown
in the diagram.
Find the magnitude and
direction of the net force on
charge number 3.
Dr. Jie Zou
PHY 1161
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The electric field
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Definition of the electric field
F
q0
q0 is a test charge and F is the
electric force exerted on q0 by the
electric field.
Vector quantity
E
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The electric force and the electric
field: F  qE
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A positive charge experiences a
force in the same direction of E .
A negative charge experiences a
force in the opposite direction of E .
Dr. Jie Zou
PHY 1161
“Force field” associated
with charge q
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Example 19-4: force field
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In a certain region of
space, a uniform electric
field has a magnitude of
4.60x104 N/C and points in
the positive x direction.
Find the magnitude and
direction of the force this
field exerts on a charge of
(a) +2.80 C and (b) -9.30
C.
Dr. Jie Zou
PHY 1161
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Electric field of a point charge
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Magnitude of the electric
field due to a point charge:
q
Ek 2
r
Direction of the electric
field of a point charge
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If the charge q is “+”, its
field points radially outward.
If the charge q is “-”, its field
points radially inward.
Dr. Jie Zou
PHY 1161
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Superposition of electric fields
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The electric field due to a system of
charges can be found by superposition.
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The total electric field is found by
calculating the vector sum of the electric
fields due to each charge separately.
Dr. Jie Zou
PHY 1161
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Example 19-5: Superposition
in the Field
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In the figure,
q1=q2=+2.90 C.
The square has a
size of 0.500 m on
each side.
Find the magnitude
and direction of the
net electric field at
point 3.
Dr. Jie Zou
PHY 1161
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Electric Field Lines
Electric field lines: Imagined
pictorial representation of the
electric field.
Rules for drawing electric field
lines
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Point in the direction of the electric
field vector at every point.
Start at positive (+) charges or at
infinity.
End at negative (-) charges or at
infinity.
Are more dense where the electric field
has a greater magnitude.
The number of lines entering or leaving
a charge is proportional to the
magnitude of the charge.
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PHY 1161
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More Examples
An electric dipole
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PHY 1161
Uniform electric
field produced by
a parallel-plate
capacitor
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Homework #1
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Chapter 19, P. 684-685, Problems:
#31, 35, 48 (from Walker, 4th ed.)
Dr. Jie Zou
PHY 1161
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