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Electrostatics
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.
The Atom
The atom consists of a
nucleus at the center
made up of protons
and neutrons. The
nucleus is orbited by
electrons.
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.
Rules for working with charges
Like charges repel
Different charges
attract
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.
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
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.
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.
Negative Ion
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.
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.
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
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.
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.
+
+
+
+
- -
-
Sweater and Balloon
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.
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.
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.
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.
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.
Charging by Conduction Video
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.
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.
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.
Charging by Induction Video
Do
Concept Development (32-2)
Charging an Electroscope Computer Simulation
What’s the Charge Activity
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 Coulomb’s
Law
What is Coulomb’s 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.
kq1q 2
F
2
r
Where “k” is Coulomb’s
constant and is equal to
9.0x10 9 Nm2/C2
When working with Coulomb’s 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.
+&+=+
-&-=+
For a two point source there is actually two forces at play.
q1
q2
For example, you have the force that the
first charge exerts on the second (F12)
F12
and the force that the second
charge exerts on the first (F21)
F21
Each charged object exerts an equal but opposite force
on the other charged object.
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.
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
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
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
Do
Concept Development (32-1)
Page 638 #’s 1-5 (pdf 81)
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 @ 22° clockwise
from the horizontal
Do
Page 640 #’s 6-10 (pdf 81)
THE END