Transcript Electricity

Electricity
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Lightning
• Stepped Leader: Path of ionized air.
– large number of quick jumps,
each up to 50 meters long.
– branch into a number of paths.
– takes (100’s of milliseconds) to
descend.
– almost invisible compared
• Positive Streamers arise as the
leader approaches the ground due to
the intense electric field created by
the approaching leaders.
• circuit is closed, and the main
lightning stroke follows with much
higher current.
• Negative lightning contains a
number of re-strikes along the same
channel creating a strobe effect.
Lightning
• Positive
– - 300 kiloamperes of current
(10 times more current than neg. lightning)
– transfers a charge of up to 300 coulombs
– potential difference up to 1 gigavolt
– lasts for hundreds of milliseconds
– discharge energy of up to 3x1011joule.
Lightning
• No, no, no. This
sucker's electrical,
but I need a nuclear
reaction to generate
the 1.21 gigawatts
of electricity I
need.
What is Electrical Charge?
• There are two types of electric charges
– positive charges
– negative charges
• LIKE charges REPEL each other.
(positive-positive; negative-negative)
• UNLIKE charges ATTRACT each other
(positive-negative)
Electric Forces
+
Opposites
charges
attract
+
-
-
Fundamentals of Electricity
• Protons carry a
fundamental positive
charge.
Proton
Quarks
– Baryon (3 quarks)
• Electrons carry a
fundamental negative
charge.
Electron
Lepton
+1
(+⅔ +⅔ -⅓)
u u d
-1
– Leptons
• Neutrons carry no net
charge.
Neutron
Quarks
0
(+⅔ - ⅓ - ⅓)
u d d
Atomic Structure
?
• Protons, neutrons and electrons are found
within atoms
• Protons are in the center with neutrons and
are tightly bound in the nucleus
• Electrons, however, are much smaller and
move around the nucleus, and therefore, can
be easily lost or gained.
• Therefore, when a negative charge is gained
(-) electrons are gained, and when a positive
charge is gained (+) electrons are lost
How is Charge Measured?
• The unit of charge in the SI system of
measurement is the coulomb (C)
• The charges of protons and neutrons are
denoted as (e) elementary charges
• 1 elementary charge is 1.6 x 10-19 C.
• 1 coulomb (C) = 6.25 x 1018 elementary
charges.
Fundamentals of Electricity
Proton
• The proton has a
positive elementary
charge.
Electron
• The electron has a
negative elementary
charge.
• The neutron has a net Neutron
charge of zero.
+ 1.6 x 10-19 C
- 1.6 x 10-19 C
0
Q is the
charge of
object (in
coulombs)
e is the
elementary
charge
Q = ne
n is the
number of
elementary
charges
How many excess electrons are on an object
with a charge of - 8.0 x 10-19 C
Q = ne
Transfer of Electric Charge
• Materials with a
mobile sea of free
electrons, are called
conductors.
– Metals, 1028 /m3
• Materials with few
free electrons are
called insulators
– rubber, silk, glass,
plastic
Law of conservation of Charge
During any charging process or
interaction between matter the
net charge of the system must
remain constant.
Electrification by Friction
• Two neutral objects (ex. hair and balloon)
are rubbed together
0
- 5pC
+ 5pC
0
Electrons are transferred from one object to another,
resulting in equal magnitude, but opposite, charges.
Charge by Contact
• A Negatively charged sphere touches a
positive sphere of the same surface area.
- 18pC
+ 4 pC
-7pC + 7pC
Electrons are transferred from the sphere with more
negatives to the sphere with less negatives, resulting in
equal magnitude of Charge.
Charge by Contact
• A Negatively charged rod touches a neutral
electroscope.
Electrons are transferred from the rod with more
negatives to the device with less negatives. The leaves of
the scope acquire a negative charge and repell.
Charging by Induction
• A Negatively charged rod is brought near a
neutral sphere.
The
Electrons
charged
are
rod
forced
is removed
from and
the
theleft
net side
positive
of charge
the sphere
is re-to
the
distributed
right side,
charge
separation
A Ground
connection is
made. And the
electrons
“escape” to
ground,
neutralizing the
right side of the
sphere.
Charging by Induction
• A neutral conducting plate is brought near
a negatively charged insulator plate.
Electrons
The plateare
is
removed
forced from
andthe
the
bottom
net positive
of the
charge
conductor
is reto distributed
the top side.
charge
separation
A Ground
connection is
made. And the
electrons
“escape” to
ground,
neutralizing the
top side of the
plate.
• Using two metal spheres and a charged
object, how could you charge one sphere
positive and one negative? Answer
Electric Forces
+
Opposites
charges
attract
+
-
-
Coulomb’s Law
• Between two point charges the force of
attraction is proportional to the product of
the two charges and inversely proportional
to the square of the distance between them.
r
q1
q2
Force of repulsion is equal in magnitude on both
charges regardless of the charge.
Coulomb’s Law
•
•
F = kq1q2 /r2
k = 8.99 x 109 Nm2/C2
Electrostatic constant
• Click on Link Above for Example.
r
q1
q2
Force of repulsion is equal in magnitude on both
charges regardless of the charge.
The Electric Field
The Electric
Force per
unit charge,
where the
test charge is
a positive
point charge.
E = F/q
+
+
TEST
CHARGE
Electric Field is
a vector
quantity
The Electric
Field around a
point charge
obeys an
INVERSE
SQUARE
LAW
E = kq/r2
E = F/q
The Electric Field
+
+
Field lines point
away from
positive charges
Electric Field Rules
+
test charge
Directed away
from positive
Line Density
shows relative
field strength
Start and end
on surface of a
conductor at
90o
+
Field lines
never cross
Electric Potential
Electric Field
lines
Electric potential lines
form right angles with
Electric lines of force.
Electric
potential is the
Energy per
unit charge.
V = W/q
+
Equipotential
lines
Electric
Potential is a
Scalar quantity
Electric Potential
Electric Field
lines
Equipotential lines
around a point charge
form right angles with
Electric lines of force.
+
Equipotential
lines
V = W/q
Electric
Potential is a
Scalar quantity
Electric Potential Energy
• W = F d,
• U = kq1q2 /r
r
q1
q2
The potential energy stored in the system of both
charges regardless of the charge.
Electric Potential Energy
• W = F d,
• U = kq1q2 /r
r
q1
q2
The potential energy stored in the system of both
charges regardless of the charge.
Electric Field Formulas
• F = kq1q2 /r2
• U = kq1q2 /r
• E = kq/r2
• V = kq/r
• E = F/q
• V = W/q
Vectors
Scalars
E = V/d
Experiments
Faraday’s Cage - Shielding
• An external electrical field causes the charges to
rearrange which cancels the field inside.
Coaxial Cables
• Wire mesh is
grounded.
• Copper wire is
shielded.
Faraday’s Cage - Shielding
• Airplanes act as Faraday cages, protecting
passengers from lightning strikes.
Millikan's Oil Drop Experiment
• Showed the existence
of an elementary
charge
• Click Link