1. Chapter 17: Electrostatics
Methods of Charging
Coulomb’s Law
Electric Fields

2. Types of Electrical Charge:
two kinds
positive (+)
negative (-)
unlike attract
like repel
there is a force (FE) between electrically charged particles
called the electrostatic force

3. the study of electrical charges at rest
Electrostatics:
the study of electrical charges at rest

4. Charged Objects: an atom is electrically neutral
same number of protons (+) as electrons (-)
objects are charged by
adding/removing electrons
charge is quantized:
all charges occur as a multiple of the fundamental charge (e)
e = 1.60210-19 C

5. Charge Is Conserved: no charge is created or destroyed negative charge
a buildup of electrons
more electrons than protons
classical definition
charge accumulated by
a glass rod
rubbed with silk or wool
positive charge
a loss of electrons
fewer electrons than protons
a hard, rubber rod
rubbed with fur

6. Vocabulary: Conductor Insulator
a substance that allows electrons to move easily throughout
metals are good conductors because
freely-flowing electrons
metallic bonds
Insulator
a substance that does not allow electrons to move freely (electrons stay in one place)
glass and plastic

7. Vocabulary: Semiconductor Superconductor Electroscope
partially conductive, partially insulative
most metalloids
Superconductor
as temperatures decreases, conductivity increases
close to 0K, conductivity is infinite
some metals do this closer to 100K !
Electroscope
an instrument used to detect the presence of an electrostatic charge

8. Electroscope:

9. Methods of Charging: Conduction
a charged object touches another object
charge
divides equally
same sign

10. Methods of Charging: Induction
a charged object is brought near, but not touching, another object
opposites attract and likes repel
with a ground, the other object acquires an opposite charge

11. Methods of Charging: Polarization
a charged object is brought near an insulator
if the compound is polar
there will be an attraction/repulsion
ex. water

12. Coulomb’s Law:
describes the electrostatic force between two charged objects
(another inverse square law relationship!)

13. Coulomb’s Law: , called the Coulomb constant
q is the magnitude (NO sign) of each charge (C)
r is the center-to-center distance (m)
F is the electrostatic force (N)

14. Coulomb’s Law: attractive or repulsive
your job to decide which direction
toward/away
“radially”

15. Charge: charges are small usually expressed in non-SI units
microcoulombs 1C = 110-6C
nanocoulombs 1 nC = 110-9 C
picocoulombs 1 pC = 110-12 C
convert for calculations

16. Elementary Charge (qo):
the charge of either a proton or an electron
charge on protons/electrons
equal in magnitude
opposite signs
qo =  1.60210-19 C

17. Coulomb's Law Problems:
don’t forget to convert…
C and cm are not SI
forces are vectors…
Coulomb's Law only calculates the magnitude
no +/- signs
electrostatic force
between two charges
free body diagrams determine the direction
opposites attract, likes repel

18. Coulomb's Law Problems:

19. More Than Two Charges: draw a force diagram add the vectors
Law of Superposition
draw a force diagram
add the vectors
calculate the resultant force

20. Law of Superposition: Step 1: Draw a picture.
Example: A sphere with a charge of 6.0C is located near two other charged spheres. A –3.0C charge is located 4.00cm to the right and a 1.5C charge is located 3.00cm directly underneath. Determine the net force on the 6.0C sphere.
Step 1:
Draw a picture.

21. Law of Superposition: Step 2:
Example: A sphere with a charge of 6.0C is located near two other charged spheres. A –3.0C charge is located 4.00cm to the right and a 1.5C charge is located 3.00cm directly underneath. Determine the net force on the 6.0C sphere.
Step 2:
Determine which forces are attractive and repulsive.
6.0C
-3.0C
4.00cm
3.00cm
1.5C

22. Law of Superposition: Step 3:
Example: A sphere with a charge of 6.0C is located near two other charged spheres. A –3.0C charge is located 4.00cm to the right and a 1.5C charge is located 3.00cm directly underneath. Determine the net force on the 6.0C sphere.
Step 3:
Determine each force using Coulomb's Law.
6.0C
-3.0C
4.00cm
3.00cm
1.5C

23. Law of Superposition: Step 4:
Example: A sphere with a charge of 6.0C is located near two other charged spheres. A –3.0C charge is located 4.00cm to the right and a 1.5C charge is located 3.00cm directly underneath. Determine the net force on the 6.0C sphere.
Step 4:
Draw a force diagram of the two forces and determine the resultant force.

24. More Practice Problems:
A charge of 3.0C (q1) is near two charges, a
-2.0C charge (q2) 0.050m to the north and a
-4.0C charge (q3) 0.030m to the south. What total force is exerted on q1?
Three particles are placed in a line. The left particle has a charge of –67C, the middle particle +45C, and the right particle -83C. The middle particle is 2cm from each of the others. Determine the net force acting on
the middle particle.
the right particle.

25. Electric Fields: exists in the space surrounding a charged object
exerts a force on nearby charged objects
force decreases as distance increases

26. Electric Fields: vector quantity magnitude given by equation at right
force per unit charge
SI unit is N/C
also V/m
direction given by
direction of FE on a test charge
positive test charge

27. Test Charge: a (+) charge of very small magnitude
determines direction of
effect on is negligible
diagram on pg 643

28. Electric Field Strength:
E is electric field (N/C)
k is Coulomb’s constant (9x109 Nm2/C2)
d is center-to-center distance (m)

29. Electric Field Direction:
If q is positive
field is “radially outward”
If q is negative
field is “radially inward”
In other words…
field lines travel
FROM positive
TO negative

30. Sample Problem:
An electric field around a charged object is 5.95106 N/C at a distance of 10.0cm. Find the charge on the object.

31. Electric Field Lines: direction is that of a positive test charge
The electric field can be represented with electric field lines.
direction is that of a positive test charge
from (+) to (-)
density is a measure of the strength of
# of lines 

32. Rules For Drawing: Electric field lines
begin on (+) charges or 
end on (-) charges or 
The number of lines drawn is proportional to the magnitude of the charge.
leaving a positive charge or
approaching a negative charge
Field lines may not cross.
See pg

40. Properties of Conductors:
Excess charge resides entirely on the conductor's outer surface.
Charge accumulates where the radius of curvature of the surface is smallest (sharp points).
irregularly shaped objects

41. Properties of Conductors:
Inside a charged conductor, the electric field is zero.
Just outside a charged conductor, the electric field is perpendicular to the surface.
so that