Download Electricity**EEElectricity

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Electrostatics, the study of electric charges
The evidence of electricity has been noted since
2700 BC
Lets review some basics:
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Like charges repel
Opposite charges attract
An insulator is an object that a charge or energy can
not move easily through
A conductor is an object that a charge or energy can
move easily through.
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The Atom
Composed of Protons (+), Electrons (-) and Neutrons
 Protons are found in the nucleus, along with
Neutrons. Electrons orbit the nucleus.
 In a neutral atom, the number of electrons equals the
number of protons
 Electric charges exert a force on other charges at a
distance
 This force is stronger when the charges are closer
together
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The charge possessed by a single electron and
proton are equal at 1.60 x 10-19 C.
The magnitude of the charge on an electron is
called the elementary charge
C = 1 Coulomb the SI unit for charge
1 Coulomb of charge is equal to 6.24 x 1018
electrons, about 2000 Amps
A typical lightning bolt has 5 – 350 C
A nickel can have up to 106 C of charge
balanced by positively charge protons.
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Typically two non conductive or neutral objects
in contact with each other will result in the
transfer of electrons.
Air is non conductive but lightning and sparks
fly across it. If the unbalance of electrons or
protons is large enough, almost anything can
become a conductor. In the case of air electrons
are forced to move across it creating a plasma
of charged atoms and freely moving electrons
we see as lightning.
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Electric charges can be move through
induction or conduction.
Induced charges are charges brought on
without physical contact. If a source of
electrons or negative charge is brought next to
two objects in contact the electrons will move
from one to the other creating a positively
charged object and a negatively charged one
The second way to charge something is via
INDUCTION, which requires NO PHYSICAL
CONTACT.
We bring a negatively charged rod near a neutral sphere. The protons in the sphere
localize near the rod, while the electrons are repelled to the other side of the sphere. A
wire can then be brought in contact with the negative side and allowed to touch the
GROUND. The electrons will always move towards a more massive objects to increase
separation from other electrons, leaving a NET positive sphere behind.
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Polarization is the separation of positive and
negative charges in a neutral object when a
charged object is brought close.
When any object/atom gains or loses a charge
it is ionized, aka an unequal amount of
electrons and protons.
This separation creates a dipole, any separation
is a dipole, created by another charge is called
an induced electric dipole.
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Charging by conduction is where charges are
brought by physical contact. A source of
negative charge brought in contact with
another object will transfer those excess
electrons.
Charges will separate based on their polarity.
Electrostatic forces can be useful and
destructive.
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As stated earlier. Force depends on distance
and is the inverse square of the distance
F = 1/r2
Force depends on charge F = qa qb where q
represents charge and (a) and (b) are charge if
two objects
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Coulomb was a French physicist around 1785
and established these relationships which are
summarized in the following equation:
F = K qaqb/ r2
 Which states the force (F) between two objects (q) is
equal to coulombs constant (K) times the product of
the two charges (q) in Coulombs, divided by the
square of the distance between them (r) in meters
 K = 9.0 x 109 N – m2/C2
Remember this electric forces are vector quantities
with magnitude and direction, sorry, more trig.
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Compare Coulombs law with Newton’s Law of
Universal Gravitation
These both equate force and will be treated the
same from a kinematics point of view.
Red neck speak = opposite forces cancel, same
or like forces add, forces at angles have vector
components
Electric Fields and Forces are ALL vectors, thus
all rules applying to vectors must be followed.
Consider three point charges, q1 = 6.00 x10-9 C (located at the origin),q3 =
5.00x10-9 C, and q2 = -2.00x10-9 C, located at the corners of a RIGHT triangle. q2
is located at y= 3 m while q3 is located 4m to the right of q2. Find the resultant
force on q3.
Which way does q2 push q3?
4m
q2
q3
Which way does q1 push q3?
3m
q1
q
Fon 3 due to 1
5m
Fon 3 due to 2
q= tan-1(3/4)
q3
q = 37
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Electricity behaves much like gravity, the same
equations actually just different variables.
As stated yesterday, the electric force varies
inversely with the square of the distance.
If a small charged object has an electric force on it
then there is an electric field, basically an electric
field is a force on a charged object.
Faraday theorized that the electric force changes
the properties of space and that the object acted
upon experiences this change in space
That object (charge) has to be considered a point
source.
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The electric field strength or force can be
summarized by the following:
E = F on q/q
E is in N/C
E is the electric field, divided by the force on q
divided by the strength of q
Lets try one:
An electric field is measured using a positive
test charge of 3.0 x 10-6 C. The test charge
experiences a force of 0.12 N. What is the
magnitude of the electric field strength on the
test charge
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Just like vectors, arrows can be used to
represent the magnitude and direction of the
electric field.
The electric field is represented by field lines.
These are drawn perpendicular to the point
source or charge. Positive charges have field
lines pointing away from the source. Negative
charges have field lines pointing towards the
source
JANKY PICTURES, I
KNOW
LIMITED BUDGET
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Field lines extend out straight unless there are
two or more charges then the field becomes the
vector sum of the fields , the field lines become
curved and more complex
Remember, field lines are simply a way of
representing the electric field around an object
or charge.
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What happens to the potential energy of an
object as you lift it into the air?
In the same way, as you perform work on a
charge by pulling it away from another one of
opposite charge you increase its potential
energy. The larger the test charge the greater
the potential energy because more work is
required.
V = W on q/q
This is the difference in electrical potential, the
ration of the work done on a charge/by the
strength of that charge
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The electric potential difference is measured in
J/C also known as a volt = 1 joule/1 coulomb
Can you move a charge to two different
positions and have the same potential
difference?
If the charge is moved in a circle. The distance
remains the same.
Equipotential is when the electrical potential
difference is the same in two or more positions.
Only differences in electric potential can be
measured
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Putting different equations together we get the
following :
V = Ed , remember
V = W on q/q as well, basically W/q
Where V is the potential difference, E is electric
field intensity, and d is the distance the charge
moves. E is in N/C and d is in m putting the
two together we get J/C which is 1 volt.
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Two charged parallel plates are 1.5 cm apart
with a single proton between them. The
magnitude of the electric field between the
plates is 1800 N/C
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What is the potential difference between the plates?
What work is required to move from the negative to
the positive plate?
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Uniform electric fields allowed for the charge
of an electron to be established which means
that an object can have only a charge with a
magnitude that is some integral multiple of the
charge of an electron (1.6 x 10-19 C).
Robert Millikan did this in 1909 with the oil
drop experiment.
REAL DEAL
EASIER VERSION
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All systems come to equilibrium when the
energy of the system is at a minimum. If there
excess charges on one charged object they will
migrate to another object in contact with it until
the balance of charges is equal on both sides.
After equilibrium is reached, work would have
to be performed to move any more charges.
At equilibrium there is no potential difference.
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Two metal objects (spheres) of different sizes
would result in an imbalance of charges at
equilibrium due the proximity of charges and
their repulsive forces
Any potential difference results in the
movement of electrons (charges) aka
electricity.
This is the purpose of grounding. Providing a
path for a potential difference to flow. The
book references gas trucks and electrical
devices.
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Sort of common sense, but charges on a
conductive surface will spread apart as far as
possible. A solid sphere or a hollow sphere for
example will have all the accumulated charges
migrate to the outside. This explains why a car
(hollow metal sphere) shields a driver from a
lightning strike or downed power line. Also
because the outside provides the easiest path to
ground.
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Charges accumulate more in smaller areas on
an irregularly shaped object, a point or
protrusion of some kind. For this reason
smooth, regularly shape objects are used to
reduce the electric fields and thus reduce the
chance of a spark. Lighting rods are pointed in
order to maximize the electric field and the
subsequent potential difference to form the
stepped leader (conducting path) for lightning.
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A capacitor is a device used to store electrical
charge. First created in 1746 by a Dutch dude,
the device was called the Leydon Jar.
Capacitance is the ratio of the stored charge in
an object to the electrical potential difference
C = q/ V
Capacitors vary widely in size and capacitance
are independent of the charge. If charge goes
up so does the potential difference.
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Capacitors are nothing more than two
conducting plates of equal and opposite charge
separated by an insulator, usually some type of
foil separated by thin plastic. The capacitance
is varied by manipulating the amount of
surface area of the conductors, the distance
between them, and the insulating material
used. Ceramic, polyester, and air are common
dielectrics or insulating materials.
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Capacitance is measured in farads (F) which is
one coulomb per volt.
A farad is a relatively large unit and is most
often measured in very small fractional
amounts.