Pre AP Physics
STATIC
ELECTRICITY
•Electrostatic – the study of electrical
charges that can be collected and held in
one area.
•They flow in no particular direction
(trapped in a body)
Electricity of the Atom
They are ordinarily neutral in charge
# of protons = # of electrons
Neutrons = no charge
Protons = + 1 charge
Electrons = -1 charge
The actual charge of an
electron
-1.60217657 X 10-19
coulombs
Little History: Ben Franklin
Proved lightning was
static electricity
Invented lightning rod
Built a static electricity
generator
Coined the terms
positive and negative
for electrical charge
Charged objects
Atoms become charged by gaining or losing
electrons
An object that exhibits electrical interaction
after rubbing is said to be
charged
Rules of Attraction
Opposite will Like will repel
attract
Protons will move but they
Only the electrons will don’t leave the nucleus.
move
Interaction between a
charged object and a neutral
object:
Any charged object (positive or negative) will
have an attraction interaction with a neutral
object.
Why are protons never lost or
gained?
Protons are held in the nucleus by the
strongest forces in the universe.
Ions
Are electrically charged particles
Charge is unbalanced
Negative Charge
#e>#p
Positive charge
#p>#e
Neutral State
Charged objects will eventually return to their
NEUTRAL state
What/How does this happen?
The charge “leaks off” onto the water molecules
in the air.
On humid/rain days it would be difficult to hold a
charge for long.
Electro-negativity
Relative
electro-negativity +++++ Glass
ranking for some ++++ Human Hair
common materials +++ Nylon
from electron Silk
++
donating Fur
+
Aluminum
materials (+, glass) to
Paper
electron accepting - Cotton
materials (-, Teflon) -- Copper
--- Rubber
---- PVC
----- Teflon
Grounded - neutral
Charged objects lose their charge when
grounded.
"What is the
bottom prong of
a plug called?"
The bottom prong of a plug is called the
ground.
It discharges any excess
charge that may develop
in an electrical device
When grounded:
Negatively charged
objects
.... Lose electrons and
become neutral
Positively charged
objects
... Gain electrons and
become neutral
Electroscope
Used to test the presence of a charged object.
If no charge is present,
leaves point straight down.
Leaves separate when charge
is present.
Where do charges come from?
Rubbing materials does NOT create
electric charges. It just transfers
electrons from one material to the
other.
Transfer of charge
Charge can be transferred between neutral
objects by FRICTION.
Ex: Rubber rod rubbed with fur or wool will cause
electrons to transfer to the rod
Rubber Rod gains electrons and becomes negative
Fur/wool loses electrons and becomes positive
- + - + +
+ - -
-
+ + -
- +
+ -
-
+
-
+
- + - + +
+ - -
-
+ + -
- +
+ -
-
+
-
+
- + - + +
+ - -
-
+ + -
- +
+ -
-
+
-
+
- + - + +
+ - -
-
+ + -
- +
+ -
-
+
-
+
- + - + +
+ - -
-
+ + -
- +
+ -
-
+
-
+
-
- - + - + +
+ - -
- + + -
- +
+ -
+
+
Conduction
Charging by contact – direct touching
EX: Conduction with a negative rod
How do the electrons flow
during conduction?
When a negative charged rod comes in
contact with the knob the electrons flow into
the electroscope – charge negative
When a postive charged rod comes in contact
with the knob the electrons flow into the rod
– charge positive
Charging by
Induction
Charging by
Induction
Induction: The production of a charge
in an uncharged body by bringing a
charged object close to it
When negatively charged rod is put near a metal can...
electrons of the can are pushed
- - - - - - - away from the rod.
induced
attraction top of the can: positive
charges
++ + buttom of the can: negative
+ +
attraction > repulsion
- -
- -
metal can -
repulsion
Attraction of uncharged
objects
Similarly, when charged rod is close to paper scrap..
molecules of paper align.
- - - - - - -
attraction between the rod
attraction
++ + + and + charge > repulsion
–––– between the rod and - charge.
paper ++ + +
––––
repulsion
Electricity, Height, and Women
https://www.youtube.com/watch?v=FGoaXZwF
lJ4&nohtml5=False
Static Discharge
Human body can not feel less than
2,000 volts of static discharge
Static charge built up by scuffing
shoes on a carpet can exceed 20,000
volts!
Lightning
What causes lightning?
You need cold air
and warm air.
It is a discharge of
the static electricity
that builds up in
cloud in certain
weather conditions.
Usually when the
ground is warm and
the air is humid.
Interesting facts
How hot is a lightning bolt? Only about 54,000 degrees Fahrenheit —
roughly five times hotter than the surface of the sun!
Lightning flashes more than 3 million times a day worldwide — that’s
about 40 times a second. Not all those flashes hit the ground —
some happen between or inside clouds.
An average lightning bolt can release enough energy to operate a
100-watt light bulb for more than three months straight (about 250
kilowatt-hours of energy).
Lightning starts in cumulonimbus clouds — aka thunderheads —
which have a positive charge up top and a negative charge below.
We don’t know how the charges start, but water droplets and ice
crystals carry them.
Lightning
Head
straight to
your car
Move to a
lower
location
Electric Fields
We visualize the field by drawing the
field lines
Lines point in the same direction as the field.
Density of lines gives the magnitude of the
field.
Lines begin on + charges; end on – charges.
Field Lines
Point direction a + charge
would move
More lines = stronger field
Two positive charges..
A new force…
Electric force
Between 2 charged objects
Depends on distance and
amt. of charge
Coulomb’s Law
qAqB
F=K 2
d
K=9x 109 2
N·m /C 2
Coulomb’s Law
Units
Charge - coulomb (C)
Force – Newtons (N)
Distance – meter (m)
Electric force is a vector
Van de Graaff Generator
The globe of the generator builds up excess
electrons through conduction.
The built electrons are repelling since they
are like charges and we know that like
charges repel so they are spread out as much
as possible.
The charges are spread out because the globe
is a conductor, unlike electrons that are
rubbed onto the balloon, which stay in the
general area where you rub the balloon.
