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Unit 3
Electricity and Simple Circuits
Electrical forces arise from particles in atoms.
The protons in the nucleus attract the electrons and hold
them in orbit. Electrons are attracted to protons, but
electrons repel other electrons.
Structure of the atom
Atoms consist of a nucleus
and electron orbital shells
Particles in the nucleus
Neutrons – neutral charge
Protons – positive charge
Particles in the shells
Electron – negative charge
What is a charge?
The fundamental electrical property to which the mutual
attractions or repulsions between electrons or protons is
attributed is called charge.
• By convention, electrons are negatively charged and
protons positively charged.
• Neutrons have no charge, and are neither attracted
nor repelled by charged particles.
Important facts about atoms
1. Every atom has a positively charged nucleus
surrounded by negatively charged electrons.
2. All electrons are identical.
3. The nucleus is composed of protons and neutrons. All
protons are identical; similarly, all neutrons are
identical.
4. Atoms usually have as many electrons as protons, so
the atom has zero net charge.
Note: A proton has nearly 2000 times the mass of an
electron, but its positive charge is equal in magnitude to
the negative charge of the electron.
The fundamental rule of all electrical phenomena
Like charges repel
Opposite charges attract
Neutral atom
Electrons and protons have electric charge.
In a neutral atom, there are as many electrons as
protons, so there is no net charge.
When is an object electrically charged?
An object that has unequal numbers of electrons and protons
is electrically charged.
If an electron is removed from an atom, the atom is no longer
neutral. It has one more positive charge than negative
charge. There is an imbalance of charges.
A charged atom is called an ion.
A positive ion has a net positive charge; it has
lost one or more electrons.
A negative ion has a net negative charge; it has
gained one or more extra electrons.
How electrons are transferred
Electrons are being transferred by friction when one
material rubs against another.
Electrons can also be transferred from one material to
another by simply touching.
Show Balloon and Static Electricity pHet
Coulomb’s Law
The electrical force between any two objects obeys a
similar inverse-square relationship with distance.
The relationship among electrical force, charges, and
distance—Coulomb’s law—was discovered by the
French physicist Charles Coulomb in the eighteenth
century.
Coulomb’s Law
For charged objects, the force between the charges
varies directly as the product of the charges and
inversely as the square of the distance between them.
Where:
d is the distance between the charged particles.
q1 represents the quantity of charge of one particle.
q2 is the quantity of charge of the other particle.
k is the proportionality constant.
The SI unit of charge is the coulomb, abbreviated C.
A charge of 1 C is the charge of 6.24 × 1018 electrons.
A coulomb represents the amount of charge that
passes through a common 100-W light bulb in
about one second.
Conductor and Insulator
Materials through which electric charge can flow are
called conductors.
Outer electrons of the atoms in a metal are not
anchored to the nuclei of particular atoms, but are free
to roam in the material.
Metals are good conductors for the motion of electric
charges because their electrons are “loose.”
Conductor and Insulator
Insulators are materials that tightly bound their electrons
to the nucleus and are not free to wander.
Materials such as rubber or glass.
These materials are poor conductors of electricity.
Conductor and Insulator
A substance is classified as a
conductor or an insulator based
on how tightly the atoms of the
substance hold their electrons.
The conductivity of a metal can
be more than a million trillion
times greater than the
conductivity of an insulator such
as glass.
In power lines, charge flows
much more easily through
hundreds of kilometers of metal
wire than through the few
centimeters of insulating
material that separates the wire
from the supporting tower.
Conductor and Insulator
Electrons move easily in good conductors and poorly in
good insulators.
Induction
If a charged object is brought near a conducting
surface, even without physical contact, electrons will
move in the conducting surface.
Charging by induction can be illustrated using two
insulated metal spheres.
Uncharged insulated metal spheres touching each
other, in effect, form a single noncharged conductor.
Induction
When a negatively charged rod is held near one sphere,
electrons in the metal are repelled by the rod.
Excess negative charge has moved to the other sphere,
leaving the first sphere with an excess positive charge.
The charge on the spheres has been redistributed, or
induced.
Induction
When the spheres are separated and the rod removed,
the spheres are charged equally and oppositely.
They have been charged by induction, which is the
charging of an object without direct contact.
What is an electric field?
It is a region around a charged particle or object within
which a force would be exerted on other charged
particles or objects.
Force field
The space around a concentration of electric charge is
different from how it would be if the charge were not
there. If you walk by the charged dome of an
electrostatic machine—a Van de Graaff generator, for
example—you can sense the charge. Hair on your body
stands out—just a tiny bit if you’re more than a meter
away, and more if you’re closer. The space is said to
contain a force field.
Van de Graaff Generator
An electric field has both magnitude and direction. The
magnitude can be measured by its effect on charges
located in the field.
Imagine a small positive “test charge” placed in an
electric field.
Where the force is greatest on the test charge, the
field is strongest.
Where the force on the test charge is weak, the field
is small.
Direction of field
The direction of an electric field at any point, by
convention, is the direction of the electrical force on a
small positive test charge.
● If the charge that sets up the field is positive, the
field points away from that charge.
● If the charge that sets up the field is negative, the
field points toward that charge.
Positive charge
Negative charge
Neutral charge
a. In a vector representation
of an electric field, the
length of the vectors
indicates the magnitude of
the field.
b. In a lines-of-force
representation, the
distance between field
lines indicates magnitudes.
Electric Field Lines
a.
The field lines around a single positive charge extend to infinity.
Electric Field Lines
a.
b.
The field lines around a single positive charge extend to infinity.
For a pair of equal but opposite charges, the field lines emanate
from the positive charge and terminate on the negative charge.
Electric Field Lines
a.
b.
c.
The field lines around a single positive charge extend to infinity.
For a pair of equal but opposite charges, the field lines emanate
from the positive charge and terminate on the negative charge.
Field lines are evenly spaced between two oppositely charged
capacitor plates.
Show electric field demo.
Electric Field equation
E = electric field strength
Q = charge
d = distance
k = 9.0 x 109 Nm2 / C2
Examples
The electric field strength in a region is 2,200 N/C. What is
the force on an object with a charge of 0.0040 C?
What equation are we going to use?
E=F/q
E = 2200 N/C
q = 0.0040 C
F=?
Rearrange the equation and we get F = Eq
F = (2200)(0.0040) = 8.8 N
Example
If two charges (q1= 2.3mC & q2=1.0mC) are placed
0.50m apart what force is experienced by q1? By q2?
q1q2
q1q2
F k 2
F k 2
d
d
k  9.0 109 Nm 2 / C 2
9  (0.0023)(0.0010) 
F  9.0 10 

2
0.5
in air


F  82800 N
q1  2.3mC

q2  1.0mC
d  0.5m

Example
What is the field strength 2.0m away from a -0.060C
charge? Is the field directed towards or away from the
charge?
kQ
E 2
d
k  9.0 109 Nm 2 / C 2
Q  0.060C
d  2m
kQ
E 2
d
(9.0 109 )(0.060)
E
22
8
E  1.35 10 N / C
Since the charge is negative, the direction would be towards the charge.