Transcript Static Electricity and Electric Fields

Static Electricity and Electric
Fields
Static Electricity Review
• Static electrical charge is created when
insulators ( or conductors insulated from
Earth) are subjected to friction.
• There are two types of static electrical
charge positive charge and negative
charge.
Static electricity
A polythene rod rubbed on a duster acquires a negative charge while
an acetate rod rubbed on a duster gets a positive charge
A negative charge is produced when excess electrons are present.
A positive charge is produced when there are too few electrons leaving
an excess of positive ions.
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What is happening is that some
negatively charged electrons
from the surfaces of atoms are
being redistributed from the rod
to the cloth.
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CHARGE IS CONSERVED
When a plastic rod are rubbed with a dry cloth. It acquires an
electrostatic charge. In the case above the rod has acquired a positive
charge. The cloth has acquired a corresponding negative charge
The sort of charge acquired depends on the material of the rod
and the cloth. E.g. a polythene rod rubbed with a cloth acquires a
negative charge
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Like charges attract and unlike charges repel
Test this Unlike charged materials attract each
other and like charged materials repel each
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Conductors and non conductors
With a poor conductor, the charge acquired
is localized
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With a metal conductor the
charge can be distributed. This
is becausemetals have
electrons which can move
freely about the structure
Insulating
plastic stand
Conductors and non conductors
Bringing charged rods next to a metal surface results in the
movement of charge on the conducting surface. This is
known as INDUCTION.
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Conductors and non conductors
Inducing charge on a conductor works even
if there is no initial overall charge on it.
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Induced charge and picking up
paper
If a positively charged rod is brought near to a piece
of paper, the paper sticks to the rod.
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As the positively charged rod approaches the paper,
the electrons in the paper are attracted towards the rod.
This makes one side of the paper negative and one side
positive. A charge has been induced on the paper and the
negative side of the paper is attracted to the positive rod.
Electric Fields
Just as bar magnets have an associated
magnetic field objects with an electrical
charge have an associated “electric field”.
0-5000 V
EHT
Observing Electric Fields
Grains of semolina
in castor oil.
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The grains of semolina behave like electric compass needles (electric dipoles),
and line up to show the direction of the electric field.
Observing Electric Fields
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Convention:
The field lines are given a direction away from a
positive charge and towards a negative charge.
Field lines never cross each other.