Chapter 26. Electric Charges and Forces

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Transcript Chapter 26. Electric Charges and Forces

Chapter 26. Electric Charges and
The electric force is one of
the fundamental forces of
nature. Controlled electricity
is the cornerstone of our
modern, technological
Chapter Goal: To develop
a basic understanding of
electric phenomena in terms
of charges, forces, and
Chapter 26. Electric Charges and
• Developing a Charge Model
• Charge
• Insulators and Conductors
• Coulomb’s Law
• The Field Model
Stop to think 26.1
Stop to think 26.2
Stop to think 26.3
Stop to think 26.4
Stop to think 26.5
Stop to think 26.6
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Charge Model, Part I
• Quantization of Electric
• Charges
The electric charge, q, is said to be quantized
q is the standard symbol used for charge as a variable
Electric charge exists as discrete packets
q = Ne
N is an integer
e is the fundamental unit of charge
|e| = 1.6 x 10-19 C
Electron: q = -e
Proton: q = +e
Rank in order, from most positive to
most negative, the charges qa to qe
of these five systems.
qa = qb > qe > qc > qd
qa > qe > qd > qc > qb
qe > qa > qd > qb > qc
qd > qc > qe > qa = qb
qd > qc > qe > qa > qb
Electrical conductors are materials in which some of
the electrons are free electrons
Free electrons are not bound to the atoms
These electrons can move relatively freely through the
Examples of good conductors include copper, aluminum
and silver
When a good conductor is charged in a small region, the
charge readily distributes itself over the entire surface of
the material
Electrical insulators are materials in which all of the
electrons are bound to atoms
These electrons can not move relatively freely through
the material
Examples of good insulators include glass, rubber and
When a good insulator is charged in a small region, the
charge is unable to move to other regions of the
Charging an electroscope
Charge Polarization
Charge Polarization
The Electric Dipole
The Electric Dipole
Charging by Induction, Step 1
Charging by Induction, Step 2
Charging by Induction, Step 3
Metal spheres A and B are initially neutral and are
A positively charge rod is brought near A, but not
Is A now positive, Negative or neutral. If keeping rod
near the
A, separate A and B, are they still neutral or charged?
Coulomb’s Law
Charles Coulomb measured the magnitudes of electric
forces between two small charged spheres
He found the force depended on the charges and the
distance between them
In SI units K = 8.99 × 109 N m2/C2. K = 1/4πε0 ,ε 0 = 8.85x10 -12 C2 /Nm2
Charges A and B exert repulsive
forces on each other. qA = 4qB. Which
statement is true?
A. FA on B > FB on A
B. FA on B < FB on A
C. FA on B = FB on A
Vector Nature of Electrical Forces
Electrical forces obey Newton’s Third Law
The force on q1 is equal in magnitude and
opposite in direction to the force on q2
F21 = -F12
With like signs for the charges, the product
q1q2 is positive and the force is repulsive
Example 26.4 The point of zero force.
Two positively charged particles q1 and q2 = 3q1 are 10 cm apart.
Where(other than at infinity) could a third charge q3 be placed so as to
experience no net force.
From the figure, you can see: At point A, above the axis, and at B,
outside the charges, cannot possibly add to zero. However, at point C
on the x-axis between the charges, the two forces are oppositely
Important Concepts
The Electric Field
We begin our investigation of electric fields by postulating
a field model that describes how charges interact:
1.Some charges, which we will call the source
charges, alter the space around them by creating an
electric field.
2.A separate charge in the electric field experiences a
force exerted by the field.
Suppose probe charge q experiences an electric force
Fon q due to other charges.
The units of the electric field are N/C. The magnitude
E of the electric field is called the electric field
Important Concepts
An electron is
placed at the
position marked
by the dot. The
force on the
electron is the right. the left.
D.There’s not enough information to tell.
Rank in order, from largest to
smallest, the electric field strengths
E1 to E4 at points 1 to 4.
A. E2 > E4 > E1 > E3
B. E1 = E2 > E3 = E4
C. E2 > E1 = E4 > E3
D. E2 > E1 > E4 > E3
E. E1 > E2 > E3 > E4