Electrostatics is the study of electric charges at rest and the forces between them. Electrical forces act over short distances and opposite charges attract while like charges repel based on Coulomb's law. Materials are classified as conductors or insulators based on how tightly bound their electrons are. Objects can be charged through contact/friction or induction and develop a non-uniform distribution of charge known as polarization. An electric field is created by a charged object and defined as the region in which a test charge would experience force, with field lines indicating direction. Electric potential energy is the energy a charge has based on its position in an electric field. Capacitors can store electric energy by separating opposite charges on conducting plates.

1. Chapter 22: Electrostatics

2. Electrostatics
Electrostatics is electricity at rest.
The study of electric charges and the forces between
them.

3. Electrical Forces
• Electrical forces are strong forces that act over short
distances.
• Positive and negative forces are balanced in every atom.
• Opposite forces attract.
• Negative attracts positive, positive attracts negative.
•Same forces repel.
Same
• Negative repels negative, positive repels positive.
q1q 2
Fk 2
d

4. Electric Charge
• electric charge: a property of particles that is important to
charge
the structure of atoms and molecules and acts as the source
of a fundamental force of nature.
• protons: positively charged particles found in atomic nuclei
protons
that possess the smallest measurable electric charge.
• electrons: negatively charged particles found in atoms that
electrons
possess the small measurable electric charge.
electrons
protons &
neutrons

5. Law of Conservation of Charge
•Charge cannot be destroyed nor created, only transferred from
one object to another.
•Ion: charged particle.
•Positive ion: net positive charge.
• An atom that has lost one or more electrons.
•Negative ion: net negative charge.
• An atom that has gained one or more electrons.
Outermost electrons are the easiest to dislodge, creating a net
charge.
Ex: rubber and plastic hold their electrons more firmly than hair,
so when you comb your hair, electrons are excited and transfer
from your hair to the comb.
Question: What is the net charge of your hair? The comb?

6. Coulomb’s Law
q1q 2
Fk 2
• Used to calculate electrical force.
d
• d = distance between charged particles
• q1 = charge of particle 1
• Unit of charge is the Coulomb
• q2 = charge of particle 2
• k=9.00 x 109 N*m2/C2

7. Conductors and Insulators
• Conductors: Atoms / Molecules with several loosely bound
electrons in outer energy levels
• Ex’s: Gold, Silver, Copper, Aluminum, … (metals)
• Insulators: Atoms / Molecules whose electrons are more
“tightly” bound to the nucleus.
• Ex’s: Hydrocarbons (plastic, acrylic, rubber, ..), Air, Wood

8. Charging
• Charging by Contact / Friction:
Friction Triboelectric Series
(-)
• electrons are “scrapped” off
Celluloid
one material and adhere to
Sulfur
another
Copper, Brass
• Rubbing brings electrons of
Amber
one substance close to the
Wood
nuclei of another substance
Human Skin
• Electrons are taken by the Silk
substance that wants electrons Cat Fur
Wool
Glass
Rabbit Fur
(+)

9. Charging
• Charging by Induction:
Induction
• A charged object is used to separate the charges in a nearby
conductor.
The presence of a – Sphere B is separated from The excess charge
Two metal
(negative) charge sphere A. The two spheres distributes itself
spheres are
induces e- to move from have opposite charges. uniformly over the
mounted on
sphere A to B. surface of the
insulating stands
spheres.

10. Charge Polarization
• Charge Polarization
• Separating opposite charges within an object
• A charged object is held near another object and
“polarizes” the atoms / molecules in the other object.
A neutral soda can With a negative object held nearby, Charge within the can is
resting on an electrons are repelled and induced into polarized – separated into
insulating stand moving to the opposite side of the can. opposites.

12. Electric Field
Electric forces act over a distance. Ex: moving stream of water
with rod. Does not need to make contact.
• Definition: an area of “space” in which if a charged
object were placed in, it would experience a force.
• Magnitude:
• E = F / qtest
• Direction: the direction of the field is taken to be the
direction of the force felt by a positive test charge.

13. Electric Field Lines
Electric field is a vector with
both a magnitude and
direction. Electric field lines
point the direction a positive
test charge would accelerate
if placed on the line.
The quantity of the charge is represented by the
density of the field lines.

14. Electric Potential Energy
• Electric Potential Energy: energy of a particle due to its
location.
• So the closer a charge is to particle of the same charge,
the more electric potential energy it is.
• Electric Potential = electric potential energy / charge
• Similar to gravitational PE (mgh)
• Work done to lift and object
• Work done to move a charge towards a repellent
charge
• Volt: unit for electric potential
Volt
• Voltage: another term for electric potential
Voltage
• 1 volt = 1 Joule / coulomb
• Consider this – an electron has a charge of 1.60 × 10-19
coulombs

15. Electric Energy Storage
• Energy is stored in devices called capacitors
• Capacitors can be charged and can store charge.
• Charging occurs when attached to a battery
• The positive battery terminal pulls electrons from the plate
attached to it, moving them to the plate connected to the
negative terminal
• This creates a potential difference and energy is released
when a conducting path is created
+
-

16. Homework
Review Questions: 26, 27, 31, 32
Exercises: 30, 35, 40, 42
Problems: 1, 2, 3