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Electrostatics
Textbook Chp 16
What happened
here?
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Electric Charges
Electric Fields
Electrostatic Charging
Hazards of Electrostatic Charging
Application of Electrostatic Charging –
Photocopying Machine
Topics
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Electric Charge is a physical quantity
Units of charge is Coulomb (C)
It is NOT an object (e.g. an electron)
There are two types of electric charge –
positive charge & negative charge
Note: charge is a scalar quantity, even
though there is positive and negative!
Electric Charges
An object is said to be neutral when it has
equal number of positive and negative
charges (i.e. no net charge)
 An object is said to be positively charged
when it has more positive charges than
negative charges
 An object is said to be negatively charged
when it has more negative charges than
positive charges
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Electric Charges
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[not in syllabus]
Particles which carry charges are called
charge carriers.
The electron is the most common charge
carrier
Aqueous ions are also charge carriers
The charge of an electron is -1.60 x 10-19 C
This number is represented by the symbol
“e”
Electric Charges
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When two objects which have net charge
are brought close together:
They attract each other if their charges
are opposite
They repel each other if their charges are
alike
E.g. electrons and protons attract each
other; electrons repel each other, protons
repel each other
This force of attraction / repulsion is
called an Electric Force
Electric Force
Recall Gravitational Field - a region in
which a mass experiences a force due to
gravitational attraction
 Electric Field is a region in which a charge
experiences a force
 Electric Field Worksheet
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Electric Fields
You need to be familiar with two methods
of electrostatic charging
 1) by rubbing
 2) by induction
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Electrostatic Charging
Electrostatic charging by rubbing involves
two non-conductors
 When charging by rubbing occurs, ONLY
electrons are transferred from one object
to the other
 The object receiving electrons becomes
negatively charged
 The object donating electrons becomes
positively charged
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Charging by Rubbing
When wool is used to rub against a
perspex rod, wool becomes negatively
charged
 When wool is used to rub against a
polythene rod, wool becomes positively
charged
 In both cases ONLY electrons are
transferred, NOT protons.
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Charging by Rubbing
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Unlike insulators, you cannot charge
conductors by rubbing
Conductors can be charged by another
process called induction
3 Principles of Charging by Induction:
a) Charges are free to move about within
a conductor, or across conductors
b) Unlike charges attract, like charges
repel
c) The Earth has an unlimited supply of
charges
Charging by Induction
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Charging by Induction Worksheet
Charging by Induction
pg 314 of your textbook describes the
formation of lightning
 Warning: this is a simplified model.
Nobody really knows for sure how
lightning works….yet.
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Hazards and Applications
During a thunderstorm, bottom layer of
clouds are negatively charged
 This induces the ground to be positively
charged
 Charges continue to build up on both
sides until air molecules are forced to split
and become ions
 A current of electricity flows and this is
the lightning bolt
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Lightning (Simplified version)
That’s why:
 sometimes during a thunderstorm, your
house circuit breaker goes off
 Buildings have lightning conductors on
them to protect them from lightning
 In a thunderstorm, do NOT seek shelter
under a tree!
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Lightning (Simplified version)
On a smaller scale, electrostatic discharge
also occur as sparks
 Most of the time sparks are harmless, but
they can be dangerous, e.g. when near
flammable fluids (ref: Ironman 3)
 Old oil tanker trucks dangle chains at the
back of the truck to prevent such sparks
from occurring
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Sparking
Not in Syllabus, but in your MC textbook:
 Electrostatic Precipitators
 Spray Painting / Crop Spraying
 Van de Graff generator
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Applications of Electrostatics
Charging
How do we explain
these?
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key component of the photocopier:
electrostatic drum:
Photocopier
Step 1) The entire drum is positively
charged
 Step 2) light shines on the original and
image is reflected unto the drum (recall
lenses)
 [in laser printers, lasers are used to
discharge the drum]
 Step 3) light areas are discharged, while
dark areas remain charged
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Photocopier
Step 4) The positively charged portions of
the drum attract toner (black powder)
while the discharged portions do not
 Step 5) Toner powder (in the shape of the
image) is transferred over to positively
charged paper
 Step 6) Paper is heated, causing toner to
fuse with the paper
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Photocopier
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Electric Charges
Electric Fields
Charging by Rubbing
Charging by Induction
Hazards of Electrostatic Charging
Application of Electrostatic Charging –
Photocopying Machine
Summary
Quiz