ELECTRICITY
TYPES OF ELECTRICITY
Electricity is the study of how charges interact. In
general, electricity refers to electric charge.
There are two types of electricity:
static electricity and current electricity.
How Electricity was discovered!
Discovered by Thales (a Greek philosopher) in 600 B.C.
Thales observed that a piece of amber rubbed with cat fur
could attract small pieces of straw and wood shavings. By
rubbing the amber and the fur, Thales had created an electric
charge on both objects (one positive, the other negative.).
Coincidentally, the Greek word for amber is “elektron”.
Because the charge does not leave the material, it is said to be
stationary or static. This led to the term static electricity.
STATIC ELECTRICITY
Electric Charge
Electric charge is a property of the particles of atoms.
Recall the structure of the atom, from chemistry:
NUCLEUS
PROTON,
Positive charge,
high mass
NEUTRON,
Neutral
charge, high
mass
ELECTRON,
Negative
charge, low
mass
BOHR-RUTHERFORD MODEL
OF A LITHIUM ATOM
SIMPLIFIED MODELS OF A
LITHIUM ATOM
Simplified Model of Atom used in
this Class!
THIS IS NOT A BOHR-RUTHERFORD
DIAGRAM OF AN ATOM! THIS IS A
SIMPLIFIED DIAGRAM OF AN ATOM TO
HELP EXPLAIN STATIC ELECTRICITY
FOR OUR LESSON.
Electric Charge of an Atom
Electrons can be added to, or removed from atoms.
http://library.thinkquest.org/27961/guide/4%20-%20high/elect/2%20-%20moving%20electrons.html
Neutrally
Charged Lithium
Atom
Negatively
Charged Lithium
Atom
Positively
Charged Lithium
Atom
Symbols used for charges
+
NEUTRAL
CHARGE
ATOM, (MAY
NOT BE
SHOWN ON A
DIAGRAM)
POSITIVELY
CHARGE ATOM
NEGATIVELY
CHARGED ATOM
DO NOT CONFUSE “+” AS THE PROTONS
IN A NUCLEUS, THE NUCLEUS REMAINS IN
ITS SAME POSITION INSIDE AN ATOM IN A
OBJECT !
.
Another Example of Electric
Charge of an Atom
Positively
Charged
Aluminum Atom
Neutrally
Charged
Aluminum Atom
Negatively
Charged
Aluminum Atom
DRAW A SIMPLIFIED ATOMIC MODEL FOR NEUTRAL, NEGATIVE AND
POSTIVE CHARGED ATOMS OF CARBON, C AND MAGNESIUM, Mg.
Charges on Objects
Neutral objects have equal numbers of protons and
electrons within a quantity of matter.
Charges on Objects
Negatively charged objects have more electrons than protons.
Notice there are more neutrally charge atoms than
negatively charged atoms! How can this object be
negatively charged?
Charges on Objects
Positively charged objects have fewer electrons than protons.
Notice there is a negatively charged atom in this object!
Is this object really positively charged?
Draw the charges for this object
What is the overall charge on this object?
What are the overall
charges of these objects?
Static Electricity
Static electricity is an imbalance of electric charge of an
object. Often charges move to the surface of an object.
REMEMBER! “+” is a positively charged atom, not a proton.
The nucleus of an atom always stays in the same position
and electrons move from atom to atom!
Static Electricity
When two different materials come in contact,
electrons can be transferred between them,
causing an imbalance in charge.
Charging by Friction
(contact i)
Charging by friction occurs when two neutral
objects made of different materials rub against or
touch each other and electrons are transferred
between them.
fur
BEFORE
The rod and fur
are both neutral.
Plastic rod
Charging by Friction
DURING
Plastic
rod
fur
Charging by Friction
Notice that some of the electrons from the fur moved to
the plastic rod and thus changed the number of charges
on the rod and fur.
AFTER
•What are the new charges on each
object?
•Are there still the same number of
“atoms” in each object?
•Are there still the same number of
+ , - and 0 charges from before we
rubbed the rod and fur?
Electrostatic Series
• When objects are charged by friction, one material is
more likely to accept electrons, while the other is
more likely to give up electrons; this is because some
kinds of atoms are more strongly attracted to
electrons than others.
• The electrostatic series is a list that ranks the
tendency of different materials to gain electrons. It
can be used to predict the charge that will be gained
by two objects (made from different materials) when
they come in contact..
• An electrostatic series chart is arranged in terms of
the relative “hold” that different materials have on
their electrons.
Human Skin
Rabbit Fur
Acetate
+
(weaker tendency
to gain electrons)
Glass
Human Hair
Nylon
Wool
Cat fur
Silk
Paper
Cotton
Wood
Amber
Rubber Balloon
Vinyl
Polyester
Ebonite
(stronger tendency
to gain electrons)
-
Charging an object through Conduction
(contact ii)
Metal rodCharging by conduction occurs when
overall -5
two objects with different amounts of
charge
electric charge come in contact and
electrons are transferred from on
object to the other.
BEFORE
Metal ball- overall
neutral charge
Charging an object through Contact or
Conduction
Metal
•Metal rod losing 5
rod
electrons.
•Metals ball gaining 5
electrons
Metal
ball
During
Charging an object through Contact or
Conduction
A neutral object is charged by conduction when a charged
object touches it. The neutral object becomes charged with
the same charge as the object it touched.
AFTER
Metal
rod
•What are the new charges on
each object?
Metal
ball
•Are there still the same
number of “atoms” in each
object?
More about Charging by
Conduction
When two charged objects with different amounts of
electric charge come in contact, electrons are
transferred between them.
metal rod x- overall +3 charge
metal rod Y- overall charge +1
metal rod x- gaining an electron, metal rod Y- losing an electron
metal rod x- overall +2 charge
metal rod Y- overall charge +2
Classifying materials by their Electrical
Characteristics : Conductors & Insulators
Most materials let electrons flow through
them, though some let electrons flow more
easily than others.
A conductor is any material that will allow
the flow of charge (i.e. electrons). Electrons
distribute themselves evenly throughout the
object (ex. metals, solutions).
An insulator is any material that does NOT
allow the flow of charge. Electrons are not
free to move, causing a build-up of charge
in a part of the object (ex. rubber, glass,
paper).
What are conductors used
for?
• Conductors are used
whenever a flow of
electricity is needed.
• The inside of a wire, the tips
of batteries, and the rods of
a plug are conductors.
• Copper and aluminum are
the most common materials
used due to their low cost
and efficiency.
See Table 9.1, pg 308 of text
What are Insulators
used for?
• Rubber is an excellent insulator,
and it is used to protect people
from electricity.
• Insulators are used primarily for
safety.
• Common uses include gloves for
working on power lines and the
main body of a plug.
See Table 9.1, pg 308 of text
Grounding
Objects with excess electric charge-either positive or
negative- can have access charge removed by a
process called grounding.
When we ground an object, we transfer electrons
between the object and a large neutral object such
as Earth (the ground).
electrons
electrons
Symbol used for
grounding
Grounding
Before the discharge,
the hand is negatively
charged and the faucet
is neutral
During discharge,
excess electrons are
transferred from the
hand into the ground.
After discharge, the
negative charge on the
hand has been
removed.
Charging an Object through Induction
Induction – placing a charged object near a neutral
object without touching it. The neutral object will
end up with a temporary charge opposite of the
charged object.
Neutral sphere
grounded
Separation of Charge by bringing
a charged rod close by
Charging an Object through
Induction
Keep the charged rod near
sphere and remove
connection to ground.
Remove the
charged rod
THE LAW OF ELECTRIC CHARGE
Opposite charges attract (law of attraction)
Positive, + and Negative,Same charges repel (law of repulsion)
Positive, + and Positive, +
or
Negative, – and Negative, –
Neutral and charged objects attract
Neutral,0 and Positive, +
or
Neutral,0 and Negative, –
Neutral and Neutral objects do nothing!
Detecting Charge: The Electroscope
An electroscope can be used to detect the
presence of an electric charge.
Static Electricity:
Applications & Phenomena
•
•
•
•
•
•
Lightning
Photocopying Machine
Painting with charges
Gasoline Truck
Electrostatic Precipitator
Appliances
Sections 9.3-9.4 of Textbook