1. Chapter 20
Electric Charges,
and Forces,

2. Units of Chapter 20 Electric Charge Insulators and Conductors
Coulomb’s Law

3. Structure of Matter All material objects are composed of _____.
Atom gain or lose its _______ and become a/an ______.
There are ____ types of _____ such as ______ and _______ _____. 3 3

4. Structure of atom
An atom consists of a _______ and a vast region of ______ outside the nucleus. ________ are present in the region of space outside the nucleus. They are _________ charged and weakly bound to the atom. _________ are often removed from and added to an atom by normal everyday occurrences. 4 4

5. Summary of Subatomic Particles
Proton Neutron Electron
In nucleus In nucleus Outside nucleus
Tightly Bound Tightly Bound Weakly Bound
Positive Charge No Charge Negative Charge
Massive Massive Not very massive
Charged versus Uncharged Particles
Positively Charged Negatively Charged Uncharged
protons > electrons electrons > protons protons = electrons

6. The study of the behavior of stationary charges.
ELECTROSTATICS:
The study of the behavior of stationary charges.
ELECTRIC CHARGE
There are two types of electric charge.
(positive and negative)
Unlike charges - attractive
Like charges - repulsive

7. What will happen? Between positive charged object and neutral object
Between negative charged object and neutral object 7

8. Methods of charging By Friction - two objects are rubbed together
By Induction - without touching
By Conduction - with touching
Grounding (the removal of a charge) - the process of removing the excess charge on an object by another object of substantial size. 8

9. Electric charge is conserved.
CONSERVATION OF ELECTRIC CHARGE
The total electric charge of the universe is a constant:
Electric charge is conserved.
electric charge cannot be created or destroyed, however, it can be transferred from one object to another.

10. Charge is quantity The SI unit of charge is the coulomb (C).
Represented by:
- the electron is -e,
- the proton is +e.
e = 1.6 x C
melectron = 9.11 x kg
mproton = x kg
mneutron = x kg.

11. Insulators and Conductors
Conductor: A material whose conduction electrons are free to move throughout. Most metals are conductors.
Insulator: A material whose electrons seldom move from atom to atom. Most insulators are non-metals.

12. Insulators and Conductors
If a conductor carries excess charge, the excess is distributed over the surface of the conductor.

13. Check your understanding
After a comb is rubbed on a wool, it is able to pick up small pieces of paper. Why does the comb lose that ability after a few minutes?
During a lighting storm, what is it about staying inside a car that makes it safe? 11

14. Electric Force
The electrical force is a push or pull exerted upon an object as a result of an interaction between electrical charges.
the electrical force is a non-contact force.
The electrical force is vector quantity 12

15. COULOMB’S LAW
Coulomb’s Law states that two point charges exert a force (F) on one another that is directly proportional to the product of the magnitudes of the charges (q) and inversely proportional to the square of the distance (r) between their centers. The equation is:
F = electrostatic force (N)
q = charge (C)
k = 9x109 N. m2/C2
r = separation between charges (m)

16. The value of k can also be expressed in terms of the permittivity of free space (εo):
9x109 N. m2/C2
k is a proportionality constant (the Coulomb's law constant)
The value of this constant is dependent upon the medium that the charged objects are immersed in.
If the charged objects are present in water, the value of k can be reduced by as much as a factor of 80.

17. Coulomb’s Law
The forces on the two charges are action-reaction forces.

18. Coulomb’s Law
If there are multiple point charges, the forces add by superposition.

19. Coulomb’s Law example problem
Sphere A, with a charge of +6.0 μC, is located near another charged sphere B. It has a charge of – 3.0 μC and is located 4.0 cm to the right of A.
What is the force of sphere A on sphere B?

20. Coulomb’s Law example problem
A third sphere C, with a μC charge, is added to the configuration. If it is located 5 cm directly below sphere B. What is the net force on sphere B? 18

21. Practice problems Page 552 # 9-13
Page 558 # # 26, 27, 31, 36, 40, 43, 48, 49,51 and 60 19

22. Summary
Electrons have a negative charge, and protons a positive charge, of magnitude
Unit of charge: Coulomb, C
Charge is conserved, and quantized in units of e
Insulators do not allow electrons to move between atoms; conductors allow conduction electrons to flow freely

23. Summary
The force between electric charges is along the line connecting them
Like charges repel, opposites attract
Coulomb’s law gives the magnitude of the force:
Forces exerted by several charges add as vectors