Download lectures from Chapter 26

What Are Electric Charges and How
Do They Interact ?
(Chapter 26)
Where do charges come from ?
•
•
•
Charge is a intrinsic property of all basic building blocks of the universe.
These building blocks are known as particles.
There are three fundamental particles that make up all of the matter in our
world
– Protons: Positive (+) charge. Form the nucleus of atoms.
– Neutrons: Neutral (0) charge. Form the nucleus of atoms with Protons.
– Electrons: Negative (-) charge. They orbit the nucleus.
Other particles (neutrinos, muons, tauons, mesons, pions, etc…) can only
be found under exotic conditions: radioactive materials, nuclear reactors,
particle accelerators, inside of stars and other astronomical bodies.
An Atom
Charge quantization
• A macroscopic object has net charge
Proton weighs 1800 times
more than electron but has
the same amount of charge!
Neutrons have same mass
as protons.
• Where Np and Ne are the number of protons and electrons
contained in the object.
• The process of removing an electron from the electron
cloud of an atom is called ionization.
• An atom that is missing an electron is called a positive
ion. Its net charge is q = +e.
Charge Conservation
• The total amount of charge is always conserved!
• In other words, in a closed system (like the
universe), the sum of the positive and negative
charges is always the same!
• Charges move around OR can be created/destroyed
in balanced pairs.
Example of charge creation and destruction in nuclear reactions:
The net charge is unchanged by the reactions.
PET (Positron Emission Tomography) Scanner
Because protons and neutrons
are much heavier than
electrons, they do not move.
Charge is transferred by the
motion of electrons.
An atom is a neutral object
(no. electrons= no. protons).
An ion is an atom that has lost
or received an extra electron.
EXAMPLE: Table salt when
dissolved in water creates
negative Cl and positive Na
ions.
Insulators
• The electrons in an insulator
are all tightly bound to the
positive nuclei and not free to
move around.
• Charging an insulator by
friction leaves patches of
molecular ions on the surface
• These patches are immobile.
Chemistry: Insulators have a nearly full valence shell
Conductors
• Metals: the outer atomic
electrons are weakly bound to
the nuclei.
• Valence electrons become
detached from their parent
nuclei and are free to wander
about through the entire solid.
• Electrons form negatively
charged “liquid” permeating an
array of positively charged ion
cores.
Chemistry: Metals are atoms
with a nearly empty valence shell
What else besides metals is a conductor???
How Do you Charge Objects??
• Friction (rubbing plastic rod with wool)
• Transfer (touching a charged rod to a piece of
metal)
• INDUCTION: Charging without physical
contact
Polarization Force: Metals
The Electric Dipole
Dipoles are created whenever a charged object is placed
near atoms or molecules.
An electric dipole is a charge neutral object where
one side has +q positive charge and the opposite
side has -q negative charge.
Charge Polarization: Insulators
Coulomb’s Law: Force Between Two Point Charges
In SI units K = 8.99 × 109 N m2/C2.
The Electric Force (~Gravity)
• The electric force between two charges is
– Attractive if they have opposite charges
– Repulsive if they have the same charge
Limitations of Coulomb’s Law
• Coulomb’s Law applies to point charges.
OR
• Size of the charged objects is very small compared
to the distance separating them.
The Superposition Principle
• Electric forces like other forces can be
superimposed.
Coulomb’s Law: Example
What is total force
on q2 ???
Equilteral triangle- all sides 1m
Solution....
!
F32 = ?
"1ˆ
3 %
!
" ˆ
"
r32 = cos(60 )i ! sin(60 ) ĵ = $ i !
ĵ ' m
2 &
#2
r32 = 1m
"1ˆ
3 %
$# 2 i ! 2 ĵ '& m
!
q3q2 !
F32 = ke 3 r32 = 9 ( 10 9 Nm 2 / C 2 (3C)(3C)
r32
(1m)3
!
F32 = 4 ( 1010 iˆ ! 3 ĵ N
(
(
)
)
ˆ
!
q1q2 !
1im
9
2
2
F12 = ke 3 r12 = 9 ( 10 Nm / C (6C)(3C)
r12
(1m)3
!
F12 = 1.6 ( 1011 Niˆ
!
!
!
F2 = F32 + F12
(
)
What is a Field ????
Fields
• Fields are mathematical functions that have a value
at all points in the space that they occupy
• They describe some physical quantity that is
changing from point to point.
• The physical quantity at each point is either an
ordinary number or a vector.
What are examples of fields encountered in everyday life ????
Scalar Field: Each point has a numerical value
• Temperature Map: Each location on the map has temperature
(scalar quantity).
Vector Field: Each point
has both a magnitude and direction
• Example: Wind Velocity Vector Field
Example of a Vector Field: The Gravitational
Field
The Electric Field
Source charges: alter the space around them by creating
an electric field.
Probe charge: a single point charge in the electric field that
experiences a force exerted by the field.
Probe charge q experiences an electric force Fon q due to
other charges. The electric field is DEFINED by
Units of E ~ N/C
Comparison Between Gravity and Electricity
Field of Source Mass:
Field of Source Charge:
s
Force of Source Mass
on Probe Mass:
Force of Source Charge
on Probe Charge:
p
Positive Charge
Negative Charge