ELECTRICITY
Electric Charge
 Atoms are made up of 3 particles
– Neutrons have a neutral charge.
– Protons have a positive charge.
– Electrons have a negative charge.
 An atom is electrically neutral if
the number of electrons EQUALS
the number of protons
 However, atoms sometimes
LOSE or GAIN electrons.
 When excess (or extra) electrons
are stored, an overall NEGATIVE
(-) charge is achieved
“Electrons” clip
Charges
 Handy rule : Opposites attract
And likes repel
Electric fields
 Every electric charge is surrounded by an
electric field which can exert a force on
objects which come into it.
These fields can push or pull over distances
2 types of electricity
 Static electricity
Electric current
 Static electricity is an accumulation
of electric charges on an object.
Neutral Charge
Negative Charge
transferred
-
Charging objects
 Objects can become charged by a variety of
methods
 2 ways are
– Charging by CONTACT
– Charging by INDUCTION
Charging by Contact
 Objects touch or rub
against each other
(contact) and charge is
transferred
Charging by Induction
 Occurs when a
charged object is
brought near a
neutral object. The
charges in the
neutral object
rearrange
themselves and
present the illusion
of a charge.
 Once objects are charged, they can then
transfer their charge…sometimes, suddenly, in
what is called a STATIC DISCHARGE.
“Balloons on the Wall” clip
Conductors and Insulators
Conductors =
materials which allow excess
electrons to easily move through them.
Examples : metals, water
The Earth also functions as a conductor.
Conductors and Insulators
Insulator = a material that doesn’t allow electrons
to move through it easily
Examples : plastic around wires, wood doors, fiberglass, rubbers
Notice what the
handles are made of.
Why?
“Conductors and insulators” clip
 A device made up of
two thin metal leaves
attached to a
knobbed metal rod.
The leaves can move
and hang straight
down when not
charged. This device
is used to detect
electric charges.
Electroscope
Lightning
Large static discharge
Air masses move and swirl
Top +
Bottom -
Negative charge induces a + charge
on ground (repel e-)
When electrons get close to ground
Bottom accumulates enough Electrons in cloud attracted
to ground
Positive charges are attracted
Connection made
= lightning
Electric charges move through air  collide with atoms/molecules
Collisions cause atoms/molecules to emit light
Electrons are ripped off atoms
in the air
Produce great amounts of heat
Powerful sound waves
Warms surrounding air to 25,000 C (2.5x hotter than Sun surface)
Heat causes air
in path to
expand rapidly
 sound waves
= Thunder
Earth is a large neutral object
Connection (conductor) will
transfer any excess electric
charge
“Grounding” – the act of making a
connection for an electric charge to
go to the Earth
(Lightning rods)
“Lightning” clip
“Van de Graff” clip with safety from static discharge
Video quiz static electricity
Resistance
 RESISTANCE is the
tendency for a material
to oppose the flow of
electrons.
 Causes the
ELECTRICAL energy
to change into
THERMAL energy and
LIGHT.
 Filament in light-bulb
has high resistance so
it heats up and glows
 The amount of resistance is different for
each type of conductor.
 Resistance is measured in ohms. Its symbol
is
Ω.
This is the Greek letter omega
Resistance in wires
 Resistance is caused by electrons running
into each other or the walls of the wire.
 Resistance is caused by friction which
means heat will be present.
Longer, thin wires have MORE resistance than
short, thick wires
 The resistance of a material will increase (↑)
as the temperature increases (↑) in most
conductors.
 This is why electrical equipment does not
work as well if it has been on for an extended
time.
Electric Current
 Similar to HEAT which flows from areas
of high temps to areas of low temps,
ELECTRONS flow from places of high
potential energy to places of lower PE.
 A negatively charged object has more PE
than an uncharged one
 When electrons do get to move (like in a
spark of static discharge) the potentials
become equal and the flow stops.
Comparison of GPE to potential
difference
Potential Energy
(height)
1.5 V
Potential Difference
1.5 Meters
0.0 V
To Lower Energy
The potential difference
(voltage) is 1.5 V.
Electrical Energy Moving electrons in a path is
electricity
 Electrical Potential Difference (v) is measured
in Volts
 The rate of moving electric charges, Electric
Current (I), is measured in Amperes
 Resistance or opposition to the movement of
the energy is called Resistance (R).
 I=V
R
Circuits – 2 types
 Series circuits are
the most simple.
 One COMPLETE (1)
path for the current
to travel.
 Contains an energy
source, a path, and a
load (something for it
to do, like a lamp)
Circuits – 2 types
 Parallel circuits
provide more
than one path
for the current
to travel.
 Most circuits
are parallel,
since if one
lamp goes out,
the others can
stay lit.
Which switches, if opened, will
cause the light bulb to stop
glowing?
F. Q
G.
R
It is the only
H. S
switch in
J. T
series to
both the
battery and
light.
USE THE FORMULA SHEET!!
 What is the current in a copper wire that
has a resistance of 2 ohms and is
connected to a 9 volt electrical source?
A.
B.
C.
D.
0.22 amp
4.5 amps
11.0 amps
18.0 amps
V = I R so,
9V = I x 2 ohms
or 4.5 amps
Electrocution
Historical perspective on Electricity
 600 B.C. Thales of
Miletus writes that the
Greeks had discovered
that rubbing fur on certain
substances such as
amber would cause an
attraction between the
two. The amber would
attract light things such as
hair and if rubbed long
enough produced a spark.
250 B.C.
 Baghdad Battery
– Object found in Iraq in
1938 (and dated back
to 250 BC) resembles
a galvanic cell and is
thought to have been
used for electroplating
– Electroplating is the
use of electricity to
cover something with
certain metals
1550 A.D.
 Italian physician Girolamo Cardano in his De
Subtilitate distinguishes difference between
electrical and magnetic forces
1600 A.D.
 English scientist
William Gilbert writes
De Magnate coined
the Latin word
electricus from the
Greek word ηλεκτρον
(elektron),
 Ηλεκτρον means
“amber” in Greek
 English words electric
and electricity came
from
 1660 A.D. Otto von
Guericke invents an
electrostatic generator
1675 A.D. Robert Boyle
determines that electric
forces can attract or repel
through a vacuum
1729 A.D. Stephen Gray classifies materials as
either conductors or insulators
1729 A.D. C.F. Du Fay identifies positive and
negative charges
1752 A.D.
 Benjamin Franklin legend says that
he conducted famous, yet extremely
dangerous experiment of flying kite
with metal key attached during
lightning storm linking lightning with
electricity.
He saw that the
fibers on the
string were
standing up and
also received a
shock from the
key when he
touched it.
 Franklin
invented the
lightning rod to
protect
structures by
directing the
charge to the
ground.
•Georg Wilhelm Richmann
of St. Petersburg, Russia
died performing kite
experiment in 1753
Nikola Tesla
Michael
Faraday
Thomas Edison