1. Electrostatics Electric charges at rest (static electricity)
Involves electric charges, the forces between them, and their behavior in materials

2. Electric Forces Arise from the particles in atoms
Alone: Billions and billions of times as strong as the force of gravity
In Pairs: Cancel each other out and have no noticeable effect

3. Charge
In an atom, a positively charged nucleus is surrounded by electrons
The protons (+) in the nucleus attract the electrons (-), while the electrons repel each other
This attraction and repulsion behavior gives an object its charge

4. Properties of Atoms
All electrons are identical, with the same mass and same quantity of charge
The nucleus is composed of protons (all of which are also identical) and neutrons (neutral)
Atoms usually have as many electrons as protons, so the atom has a zero net charge

5. Like charges repel; opposite charges attract
The Fundamental Rule…
Like charges repel; opposite charges attract

6. Ions
If the total positive charge balances the total negative charge, the atom is neutral
If a particle is removed from the atom, it becomes an ion
Positive Ion: Has lost one or more electrons and has a net positive charge
Negative Ion: Has gained one or more electrons and has a net negative charge

7. Removal of Electrons
The inner electrons are bound very tightly to the atom
The outermost electrons of many atoms are bound loosely and can be easily dislodged

8. Conductors and Insulators
Conductor: Materials which allow electric charge to flow freely
Metals are good conductors because their outer electrons are not bound tightly
Insulator: Materials which do not allow electric charge to flow freely (i.e. glass, rubber)

9. Conductors and Insulators
Semiconductor: Materials that can be made to behave as either a conductor or an insulator of electricity
i.e.) germanium, silicon
Superconductor: Material that has infinite conductivity at low temperatures so that charge flows through it without resistance

10. Conservation of Charge
Electrons are never created nor destroyed, but are simply transferred from one material to another
No case of the creation or destruction of net electric charge has ever been found
Electrons are always transferred in whole – they cannot be divided into fractions of electrons

11. Coulomb’s Law
The force between two charges depends on the magnitude of the charges and the distance between them
F = k
q1q2
d 2

12. Coulomb’s Law q1q2 F = k d 2 Units of charge = Coulomb (C)
k is a proportionality constant called the Coulomb constant
k  9,000,000,000 N·m2 or 9·109 N·m2 C2 C2

13. Other Useful Numbers Charge of an Electron qe = -1.6 · 10-19 C
Charge of a Proton
qp = +1.6 · C

14. Coulomb’s Law
The force between two charges depends on the magnitude of the charges and the distance between them
F = k
q1q2
d 2

15. Two electrons are a meter apart. What is the force between them?
Sample Problem
Two electrons are a meter apart.
What is the force between them?
What direction is it in?

16. Sample Problem q1q2 F = k d 2 F = 2.3*10-28N d = 1m
qp = -1.6*10-19C qe = -1.6*10-19C
F = k
q1q2
d 2
F = (9*109N*m2/C2)
(-1.6*10-19C)* (-1.6*10-19C)
(1 m)2
F = 2.3*10-28N

17. Two electrons are two meters apart. What is the force between them?
Sample Problem
Two electrons are two meters apart.
What is the force between them?
What direction is it in?

18. The force between the two particles falls off as 1/d2
Coulomb’s Law
The force between the two particles falls off as 1/d2
F = k
q1q2
d 2

19. Charging Items may be charged by friction
Electrons are moved from one object to another by being scraped away
Items may be charged by contact
Electrons are moved without being scraped off

20. Charging by Induction
Induction: The charging of an object without direct contact

21. Charging by Induction
Grounding: Allowing charges to move freely along a connection between a conductor and the ground
(shown in sketch c)

22. Induction and Lightning
The bottom of the negatively charged cloud induces a positive charge at the surface of the ground below.

23. Objects are electrically charged in one of three ways:
In Summary. . .
Objects are electrically charged in one of three ways:
By friction, when electrons are rubbed from one object to another
By contact, when electrons are transferred through direct contact without rubbing
Through induction when electrons are gathered or dispersed by the presence of a nearby charge (without physical contact)

24. Field Lines Electric Fields have a magnitude and direction
Vector Quantities
Lines go away from positive
Lines go toward negative

25. Drawing Field Lines From + to –
Lines start perpendicular to the surface of the charge
Field strength is shown by the density of the field lines

26. Electric Shielding All charge on a conductor gathers on the outside
If a charge is contained inside a conductor the electric field is zero
If a charge is outside a conductor the inside of the container will not be affected by the outside charge
Example: Faraday Cage

27. Faraday Cage

28. Electrical Polarization:
Charge Polarization
Electrical Polarization:
When one side of a molecule is induced to be slightly more positive than the other side

29. Capacitor A capacitor stores difference in electric charge
Usually stored in parallel plates
Consists of two conductors with an insulator in between

30. Capacitors
Can hold this separation of charge for large periods of time
This is why it is not smart to take apart an Old Television Set