Electrostatics

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Electrostatics
From our previous discussions in class you do
remember the four fundamental interactions in the
universe, Can you name them?
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Four Kinds of Forces

• Strong Nuclear Force

A strong force that holds the particles of the
nucleus of an atom together. Short range
attractive force that is much larger in magnitude
to the gravitational or the electromagnetic
forces.
Weak Force
Force involved in transmutation of particles
within the nucleus. Only observed/viewed in
radioactive decay. Stronger only than the
gravitational force.
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Four Kinds of Forces

• Gravitational Force

Attractive force that exists between all objects.
The gravitational force between the Earth and the
moon keeps the moon in orbit. It may be the most
evident but it is the weakest of all the forces.
Electromagnetic Force
Charged particles at rest or in motion exert
electric forces on each other. They give
materials their strength, their ability to bend,
squeeze, stretch or shatter. When charged
particles are in motion they produce magnetic
forces on each other. Electric and magnetic
forces are both considered to be aspects of this
single force.
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Electromagnetic Force
We are going to study electromagnetic force and
its effects and importance on our world for the
next couple of months
To start we look at electric charges at rest
called.
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Electrostatics

• Study of properties and results of electric
  charges at rest

Atom is Neutrally Charged Positive charge on
the nucleus is exactly balanced by the negative
charge of electrons
Atom
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An Electron can be removed from an atom to create
a positive ion
Electrons
Freed electron can be
Unattached and Free Creating a negative charged
particle
Attached to an atom Creating a negative Ion
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Electric Forces

• There are two kinds of electric charges
• Positive protons p
• Negative electrons e-
• Charges exert a force on other charges over a
  distance.
• Like charges repel.
• Unlike charges attract.
• Electroscope Instrument to determine charge.

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Materials can be of three kinds
Insulators Materials that inhibit the flow of
free charged particles
Examples wood, air rubber
Conductors Materials that allow the flow of
free charged particles
Examples metal water
Semiconductors Intermediate class, conduction
between an insulator and conductor
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Charging of Objects
An object can be charged either through
conduction or induction
Charging by Conduction
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Charging a neutral body by touching it with a
charged body
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Charging of Objects
An object can be charged either through
conduction or induction
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Induction
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Charging a neutral object by bring a charged body
close to but not touching the object.


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Charging of Objects
An object can be charged either through
conduction or induction
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Charging by Induction
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Charging of Objects
An object can be charged either through
conduction or induction
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Charging by Induction


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Charles Coulomb

• Charles Coulomb (1738-1806) was a French
  physicist and military engineer. Because of his
  expertise with simple machines, he was able to
  build an apparatus to measure the electrical
  force between two charged objects. He derived a
  law, Coulombs Law, which gives the relationship
  between charges, their separation and the
  electrical force of attraction or repulsion.

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Coulombs Law
Coulombs Law states that the size of the electric
force between two charged particles depends on
the size of the charges and the distance between
them.
q is a unit of charge measured in coulombs C
r is the distance between the charged objects
K is a constant 9.0 x 109 Nm2/C2
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Coulombs Law
The charge on one electron or one proton is
called an elementary charge
e- -1.60 x 10-19 C
p 1.60 x 10-19 C
One coulomb of charge has
6.25 x 1018 electrons or protons
One lightning bolt may have 10 C of charge
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Millikan Oil Drop Experiment
Robert Millikan
1868-1953
American physicist who determined the charge on
an electron using charged oil drop experiments in
1909.
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Millikan Oil Drop Experiment
What Millikan did was to put a charge on a tiny
drop of oil, and measure how strong an applied
electric field had to be in order to stop the oil
drop from falling. Since he was able to work out
the mass of the oil drop, and he could calculate
the force of gravity on one drop, he could then
determine the electric charge that the drop must
have. By varying the charge on different drops,
he noticed that the charge was always a multiple
of -1.6 x 10 -19 C, the charge on a single
electron. This meant that it was electrons
carrying this unit charge.
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The Millikan oil-drop experiment was far superior
to previous determinations of the charge of an
electron. Where other workers had attempted to
measure the quantity by observing the effect of
an electric field on a cloud of water droplets,
Millikan used single drops, first of water and
then, when he found these evaporating, of oil.
The experiment had broader significance than a
simple refinement of a number. Millikan
emphasized that the very nature of his data
refuted conclusively the minority of scientists
who still held that electrons (and perhaps atoms
too) were not necessarily fundamental, discrete
particles. And he provided a value for the
electronic charge which, when inserted in Niels
Bohr's theoretical formula for the hydrogen
spectrum, accurately gave the Rydberg
constantthe first and most convincing proof of
Bohr's quantum theory of the atom.
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