What Do All These Pictures Have In Common

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What Do All These Pictures Have In Common?
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?Did You Figure It Out?
Electrostatics
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Intro to Electrostatics video
User Name rlharper_at_edu.pe.ca Password
485PCX
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Electrostatics
Electrostatics involves electrical charges at
rest and the forces that exist between them.
In order to understand these forces we first need
to discuss the basic parts of an atom.
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The Atom
The atom consists of a nucleus at the center made
up of protons and neutrons. The nucleus is
orbited by electrons.
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Protons are positively charged particles located
in nucleus of the atom.
Neutrons are neutral particles (particles that
have no charge) are located in the nucleus of an
atom. The neutron has a mass equal to that of a
proton.
Electrons are negatively charged particles
located in orbit around the nucleus of the atom.
The mass of an electron is approximately 1/1837
that proton.
The electrical charge on protons and electrons
are equal in magnitude but opposite in charge.
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Rules for working with charges
Like charges repel
Different charges attract
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Working with neutral charges
A neutral charge (no charge), is different than
both negative and positive charges.
Therefor a neutral charge will actually be
attracted to both positive charges and negative
charges.
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Total charge of a body
Because an electron and proton have equal and
opposite charges the net charge of atom is zero.
(2 negative charges)
(2 positive charges)
(2 neutral charges)


0
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However if and Atom somehow loses an electron,
the net charge of the atom is no longer balanced,
thus resulting in a net positive charge.
This positively charged atom is known as a
positive ion.
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A negatively charged atom is known as a negative
ion.
Because electrons are located very far away from
the nucleus of an atom it is reasonably easy to
remove them from the atom.
The same cannot be said for proton. It is very
difficult to remove a proton from an atom.
Therefore if a net negative charge is desired one
must add electrons to an atom, so that there is
an abundance of electrons.
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Negative Ion
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Conservation of Charge
The conservation of charge states that charge
cannot be created or destroyed simply transferred
from one location to another.
We now know that it is much easier to move an
electron than a proton.
Therefore electrostatics is the result of the
movement of electrons not protons.
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Electrons can move more easily through some
substances than others.
Substances where electrons can move very easily,
are called conductors.
Most metals are very good conductors because they
have many loosely attached electrons.
Substances where the electrons cannot move easily
are called insulators.
Some common insulators are glass, rubber, and
silicon.
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The question now is how do we transfer electrons
to or from a body
There are three ways
1) Charging by friction
2) Charging by conduction
3) Charging by Induction
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Charging by friction
When two objects are made of different materials,
their atoms will hold onto their electrons with
different strengths.
As they pass over each other the atoms with
weaker bonded electrons are ripped off of that
material and collect on the other material.
There is a net transfer of electrons from one
material to the other!
This is known as charging by friction.
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Example
Rub a glass rod with a piece of silk.
In this case the silk holds onto the electrons
more strongly than the glass.
Electrons are ripped off of the glass and go on
to the silk.
The glass is now positive and the silk is
negative.
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Sweater and Balloon
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Charging by Conduction
Conduction just means that the two objects will
come into actual physical contact with each other
(this is why it is sometimes called charging by
contact).
Charging by conduction involves the contact of a
charged object to a neutral object.
Once something is charged either positively or
negatively, it tends to lose its excess charge to
another object if they come in contact
If the two objects are brought close enough that
an arc of electricity jumps between them, it
counts as conduction also.
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Let's assume we have a negatively charged metal
object and an uncharged metal sphere. The
uncharged sphere is on an insulating stand so
that it will not interact with anything else.
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We bring the two objects close together. We will
see a separation of charge happen in the neutral
object as negative electrons are repelled to the
right hand side. At this time, they are not
touching and no charges have been transferred.
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We now allow the two objects to touch.
Some of the negative charge will transfer over to
the uncharged metal object.
This happens since the negative charges on the
first object are repelling each other... By
moving onto the second object they will be able
to spread away from each other.
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When the negative object is removed, it will not
be as negative as it was.
Both of the objects have some of the negative
charge how much depends on the size of the
objects and the materials they are made of.
If they are the same size, made of the same
materials, then the charge will be the same on
both.
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Charging by Conduction Video
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Charging by Induction
When a charged object is brought close to an
object without physical contact, the charged
object will induce a movement of electrons in the
uncharged object.
In this case the electrons in the neutral object
will will be repelled from the electrons in the
charged object.
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If the ground wire is then attached to the
neutral object electrons will flow to or from
ground in order to stabilize the electrical
forces surrounding the neutral object.
In this case the electrons will leave to an
object and go to ground.
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Once the electrons have been removed from the
neutral object, remove the ground wire and then
the negatively charged object.
It is crucial that the ground wire is removed
first, otherwise the electrons will simply return
to the metal sphere.
Thus leaving the originally neutral object with
fewer electrons than protons, and resulting in a
net positive charge.
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Charging by Induction Video
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Concept Development Hand Out (32-2)
Charging an Electroscope Computer Simulation
Whats the Charge Activity
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Charles Augustin de Coulomb

• Born in 1736, died in 1806
• Created a device that helped him develop his
  theories on charges and electric force and field.
• Discovered Coulombs Law

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What is Coulombs Law

• The magnitudes of the electrostatic force between
  two point electric charges are directly
  proportional to the product of the magnitudes of
  each charge and inversely proportional to the
  square of the distance between the charges.

Where k is Coulombs constant and is equal to
9.0x10 9 Nm2/C2
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When working with Coulombs law a negative
solution means that there is an attractive force.
This would be the case for finding the force
between two opposite charges such as a positive
and negative charge.
- -
A positive solution means that there is a
repulsive force. Such as two like charges.

- -
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For a two point source there is actually two
forces at play.
For example, you have the force that the first
charge exerts on the second (F12)
and the force that the second charge exerts on
the first (F21)
Each charged object exerts an equal but opposite
force on the other charged object.
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The Coulomb
The Coulomb is a SI unit of charge.
One Coulomb is the charge on 6.25 x 1018
electrons.
The magnitude of charge on an electron is called
the elementary charge and is equal to
1.6 x 10-19C.
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Example
Suppose that two point charges, each with a
charge of 1.00 Coulomb are separated by a
distance of 1.00 meter. Determine the magnitude
of the electrical force of repulsion between
them.
F 9.0 x 109 N
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Example
Two balloons with charges of 3.37 µC and -8.21
µC attract each other with a force of 0.0626 N.
Determine the separation distance between the two
balloons.
d 1.99 m
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Example
A small sphere, carrying a charge of 8.0 ?C,
exerts an attractive force of 0.50 N on another
sphere carrying a charge with a magnitude of 5.0
?C.
a) What is the sign of the second charge?
a) Positive
b) What is the distance of separation of the
center of the spheres?
b) 0.85 m
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Do s 1-5 Page 638
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Example
Three charges A(5.0 ?C), B(-2.0 ?C), and C(3.0
?C), are arranged at the corners of a right
triangle as shown. What is the net force on
charge C?
150 N _at_ 22 clockwise from the horizontal
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Do s 6-9 Page 640
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THE END