1. Chapter 26. Electric Charges and Forces
The electric force is one of the fundamental forces of nature. Controlled electricity is the cornerstone of our modern, technological society.
Chapter Goal: To develop a basic understanding of electric phenomena in terms of charges, forces, and fields.

2. Chapter 26. Electric Charges and Forces
Topics:
Developing a Charge Model
Charge
Insulators and Conductors
Coulomb’s Law
The Field Model

3. Stop to think 26. 1. page 792 Stop to think 26. 2
Stop to think page 792 Stop to think page 795 Stop to think page 799 Stop to think page 805 Stop to think page 808 Stop to think page 810

4. Charge Model, Part I

6. Charges 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 C
Electron: q = -e
Proton: q = +e

8. 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
STT25.2

9. Conductors
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
Material
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

11. Insulators
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
wood
When a good insulator is charged in a small region, the
charge is unable to move to other regions of the material

13. Charging an electroscope

14. Charge Polarization

15. Charge Polarization

16. The Electric Dipole

17. The Electric Dipole

18. Charging by Induction, Step 1

19. Charging by Induction, Step 2

20. Charging by Induction, Step 3

21. Metal spheres A and B are initially neutral and are touching.
A positively charge rod is brought near A, but not touching.
Is A now positive, Negative or neutral. If keeping rod near the
A, separate A and B, are they still neutral or charged?

22. Charles Coulomb measured the magnitudes of electric
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 = 8.85x C2 /Nm2

24. Charges A and B exert repulsive forces on each other. qA = 4qB
Charges A and B exert repulsive forces on each other. qA = 4qB. Which statement is true?
STT25.4
FA on B > FB on A
FA on B < FB on A
FA on B = FB on A

25. 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

27. Example 26. 4 The point of zero force
Example 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 directed

28. Important Concepts

29. The Electric Field
We begin our investigation of electric fields by postulating a field model that describes how charges interact:
Some charges, which we will call the source charges,    alter the space around them by creating an electric field.
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 strength.

30. Important Concepts

31. An electron is placed at the position marked by the dot
An electron is placed at the position marked by the dot. The force on the electron is
to the right.
to the left.
zero.
There’s not enough information to tell.
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32. Rank in order, from largest to smallest, the electric field strengths E1 to E4 at points 1 to 4.
E2 > E4 > E1 > E3
E1 = E2 > E3 = E4
E2 > E1 = E4 > E3
E2 > E1 > E4 > E3
E1 > E2 > E3 > E4
STT25.6