Download electric field

STATIC
…the word static comes from the
ancient Greek for “not moving”
Static electricity
is a stationary
electrical charge
that is built up on
the surface of a
material.
Where do electrical charges
come from?
Matter is made up of atoms.
Proton (positive charge)
Neutron (neutral, no charge)
Electron (negative charge)
atom
nucleus
A NEUTRAL charge occurs when the
number of electrons equals the number of
protons.
A NEGATIVE charge occurs when there are
more electrons than protons, the object is
gaining electrons.
A POSITIVE charge occurs when there are
less electrons than protons, the object is
losing electrons.
•  The difference between a positive
and a negative charge has to do
with how they behave and the
forces they exert. A FORCE is a
push or a pull on an object.
•  When charged particles come near
each other two different types of
forces can occur…
•  A force that pulls objects
together is a force of
ATTRACTION. This force
exists between two objects
that have opposite charges,
ex: one object has a
positive charge and the
other has a negative
charge.
•  A force that pushes objects
apart is a force of
REPULSION. A force of
repulsion happens when
two objects have the same
charge, ex: both objects
have a positive charge or
both objects have a
negative charge.
Electric charges behave
according to this simple rule:
Like charges repel each other,
unlike charges attract each other.
ELECTRIC FIELDS
•  The attraction or repulsion of
charged particles occurs
because charged particles have
electric fields around them. An
ELECTRIC FIELD is an area
over which an electric charge
exerts a force.
•  The electric field is strongest
near the charged particle. As
the distance from the charged
particle increases, the strength
of the electric field decreases.
•  When a charged particle moves
into the electric field of another
charged particle, it is either
pushed or pulled depending on
the charge of the two particles.
•  A neutral object acquires an
electric charge when it either
gains or loses electrons.
How do you give an object an
electric charge?
...by rubbing two objects together, such
as a balloon being rubbed by a cloth.
This method of giving an object an
electric charge is called the FRICTION
method.
When a balloon rubs a piece of wool...
Electrons are pulled from
the wool to the balloon.
The balloon has more
electrons than usual.
wool
The balloon: – charged,
The wool: +charged
When a charged rod is brought close to
an uncharged scrap of paper...
- - - - - - -
paper
molecules of paper align.
⇒ attraction between the
attraction
rod and + charge > repulsion
between the rod and charge.
repulsion
•  Rubbing materials does NOT
create electric charges. It just
transfers electrons from one
material to the other.
•  …another method of charging an
object is by CONDUCTION. In
conduction, a charged object is
touched by an uncharged object.
and electrons flow from the charged
object to the uncharged object.
Materials that permit electric charges to
flow freely are called CONDUCTORS.
The Earth and certain metals are good
conductors because they allow electrons
to flow through them easily.
A material that does not allow electrons to flow
through it easily is called an INSULATOR.
Insulators do not conduct electric charge well
because the electrons in the atoms of
insulators are tightly bound and cannot move
throughout the material.
Good insulators
include rubber,
glass, wood,
plastic and air.
…a third method of charging is by
INDUCTION. Induction involves a
rearrangement of electric charges.
For induction to occur, a neutral
object needs to only come close
to a charged object. No contact is
necessary.
When a negatively charged rod is put near a
metal can...
electrons of the can are
- - - - - - pushed away from the rod.
induced
charges
metal
can
attraction ⇒ top of the can: positive
++ +
+
+
-
-
-
repulsion
& attraction > repulsion
buttom of the can: negative
Quick Quiz:
A balloon gains a negative charge when
rubbed by a woolen cloth.
The charge on the woolen cloth is now…..
A. 
Neutral
B.
Positive
C.
Negative
During rubbing, what has been
transferred from the woolen cloth to
the balloon?
A. 
Electrons
B.
Protons
C.
Neutrons
Static Discharge
The loss of static electricity as electric charges
move off an object is called ELECTRIC
DISCHARGE. Lightning is the most dramatic
example of electric discharge.
How can a person be safe from
electric discharge?
One way to be safe is to remove the excess
positive or negative charge by GROUNDING.
When a charged object is grounded, the
excess charge is balanced by the transfer of
electrons between the charged object and a
ground.
A ground is simply an object that serves as a
reservoir of electrons.
To ground a positively charged
object….
You have to balance the positive charge
with negative electrons from a ground
(which can be another object, a person or
the Earth)
GROUNDING…
When we touch a metal ball of
positive charge...
electrons flow from the
earth to the metal ball to
neutralize the metal ball.
Metal ball becomes neutral.
To ground a negatively
charged object….
•  You have to remove the extra electrons
from the object. This is usually done by
having a “conducting pathway”, that is
somewhere for the electrons to go.
Again this can be another object, a
person or the Earth.
GROUNDING…
Similarly, if the metal ball is of
negative charge...
extra electrons flow
from the metal ball to
the earth and the ball
becomes neutral.
Lightning rods are used to protect houses
from the energy of a lightning strike by
grounding it.
DO NOW…
Write a paragraph
about the cartoon. Tell
what he is trying to
do. Will it work and
why? What do you
think will happen?
Use the following
words, static
electricity, lightning,
electrons, positive
and negative charges.
The End