Transcript Electrostatics - wths

Electrostatics
The study of electrical charges at
rest
The word electricity comes from the Greek
word elektron, which means “amber”
The Greeks observed that when the
amber was polished with a piece of fur, it
would attract small leaves and dust
particles
We now know that this is caused by static
electricity
Electrical Forces
Electrical forces arise from the interactions
between electrically charged particles
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Postive charges – protons (p+)
Negative charges – electrons (e-)
Opposite charges attract
Similar charges repel
Atoms
Atoms are made of a nucleus containing
protons and neutrons (n0) surrounded by
electrons
The atom as a whole usually contains the
same number of protons, neutrons, and
electrons.
If there are the same amount of positives
than negatives…the atom is electrically
neutral in charge.
Charging objects
If an electron is removed then it is no
longer neutral
The atom now has one more positive
charge than negative so it is positively
charged.
Positively and negatively charged atoms
are called ions.
Charging objects
When objects become charged, ONLY
electrons are either gained or lost.
Why would electrons be the ONLY particle
to be transferred?
Electrons are more easily stripped off of
atoms than protons because they are
found on the outside of the atom
Law of Conservation of Charge
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No electrons are created or destroyed when
objects become charged…only transferred
The net amount of electric charged produced
in any process is zero
Balloons
When objects become charged, they tend
to lose their charge over time.
Where does the charge go?
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The charge usually leaks off into water
molecules in the air
Water molecules polar, meaning their charge
is not distributed evenly and have a positive
and negative side (similar to a bar magnet)
Static Electricity
The word ‘static’ means not moving
So static electricity is defined as charges
that are not moving
It is easier to hold a static charge on a dry
day rather than a humid day since the
polar water molecule will steal the stored
charge when colliding with the charged
object
Insulators and Conductors
Insulators are materials that do not allow
electric charge to move through it easily

Examples?
Conductors are materials that allow
electric charge to move through it easily

Examples?
When a piece of metal is placed across
the two spheres, charge is allowed to
move easily into the other object.
The positive particles want to repel each
other and get as far away from each other
as possible forcing their way throught the
conductor.
The charges can not move through the air
or through the wood to spread out,
because they are not good conductors
What makes some materials good
conductors is the way the electrons are
bound very loosely to the atoms and can
move freely within the material
In insulators the electrons are tightly
bound to the atom
Charging by Contact
When two substances are in contact, one
of them has to give up some of its loose
electrons to the other
Usually charges are stripped away through
friction caused when two objects are
rubbed together
Charging by conduction
Any excess charges that are gained on a good
conductor always resided on its outer surface
regardless of shape.
Charges simply push each other away to the
very extremities of the object
John Travoltage
If the object is not a good conductor, the object
might have to be charged in various locations
along the surface to spread the charge
Charging by Conduction
Charging by Induction
An object can become charged without
contact, a charge can be ‘induced’.
Charging through induction occurs when
another charged object is brought next to a
neutral object and the charges separate
If the object is moved away the charges return
to normal
If the object was grounded, the electrons would
flow out of the object to leave a net positive
charge on the object
Charge Polarization
When a charged object is brought near an
insulator, there is no movement of free electrons
Instead, there is a rearrangement of charges
within the individual atoms so that the atoms are
electrically polarized (like the poles on a magnet)
Check your Understanding
from The Physics Classroom Tutorial
A negatively charged balloon is brought near a
neutral conducting sphere as shown below. As it
approaches, charge within the sphere will
distribute itself in a very specific manner. Which
one of the diagrams below properly depicts the
distribution of charge in the sphere?
Check your Understanding
Two neutral conducting pop cans are touching
each other. A positively charged balloon is
brought near one of the cans as shown below.
The cans are separated while the balloon is
nearby, as shown. After the balloon is removed
the cans are brought back together. When
touching again, can X is ____.
Check your Understanding
A positively charged balloon is brought near a
neutral conducting sphere as shown below.
While the balloon is near, the sphere is touched
(grounded).
At this point, there is a movement of electrons.
Electrons move ____ .
Electroscope
Electric Field
The space around every electrically
charged body creates an electric field.

Just as every object that has mass has a
gravitational field around it
Electric field is a vector quantity
If a body with charge q experiences a
force F at some point in space the Electric
field E is
F
E
q
If a positive test charge was placed near
the charges below, and vectors are
pointing in the direction of the electric
force on the test charge, what would be
the charge of each?
+
-
Electric Field
Electric Force
The electric field acts similar to a field of gravity,
so there will be a force acting on the charge
Coulomb’s law
q1q 2
F k 2
d
q represents the charge of the particle measure
in coulombs [C]
k is Boltzman’s constant equal to 9x109 N·m2/C2
Quantized charge
Charge in quantized so charge only exists
in discrete amounts (1q, 2q, 3q…nq)
You can never lose a fraction of a charge
The charge of an electron is

e = -1.602x10-19 C
The charge on a proton is the same
amount but (+)
Electric Potential
Just as mass has potential energy in a
gravitational field, charge has potential
energy in the presence of an electric field
A positive charge
placed in this electric
field will be pushed
away buy the positive
side and pulled
towards it by the
negative.
The potential will
change to kinetic as it
moves.
The opposite occurs if
the charge is changed
to a negative.
The negative charge
will move in the
opposite direction of
the electric field
What is going to happen?
Acts like a projectile on Earth
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Electric Potential Energy
The change in PE is equal to the work
done to move a charge
W=Fd
F
E
And force is equal to Eq from
q
So we get the equation
W=qEd which is electric
potential energy
A difference in electric potential energy is
called the potential difference
Potential difference is measured in volts
It measures the difference in PE between
two points per unit of charge
Equipotential Lines
All points between charged objects that
have the same potential
Hence Equal Potential
An equipotential line must be
perpendicular to the electric field
If theMoving
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