Download Electrostatics

Electric Fields
and Potentials
Joey Multari, Shannon Burt, Katie Abbott
Electric Force
Electricity exerts a force similarly to gravity.
Fe = kq1q2
r2
where q1 and q2 represent the amount of charge in
Coulombs (6.24 x 1018), r is in meters and k is the
electrical constant (9 x 109 Nm2 /C2)
1 Coulomb of electrons travels through a 100-W lightbulb
in about one second
Review
• How many coulombs travel through a 100W light
bulb in 3 seconds?
• What type of behavior does charge have?
A-Attracting
B-repelling
C-both
D- Fast movement
E- Slow movement
Electric Fields
Just like gravity field, charges have a force
field (E) as well, measured in force per unit
charge
E = F = kQ
q
r2
where Q is a positive test charge
Direction of fields – away from a positive
charge, toward a negative charge
Force Field Lines
•
•
•
Fields have strength and
direction
Field is determined by the
force and direction of
motion of a positive test
charge
Field is strongest where the
force is the strongest –
where the lines are the most
concentrated
Electrical Potential
Just like gravity—the potential
(possibility) of falling to earth, charges
have the potential to move toward or
away from each other
Electrical Potential
• Force of attraction/repulsion causes the potential
• Potential is energy divided by charge—since
charge is usually small, potential can be relatively
large—5000 volts on a charged balloon
• A larger amount of charge makes larger potential
Voltage – Electrical Potential
Voltage = PE/Q
PE in Joules and Q in Coulombs
100 Volts
0.000001-J/0.00000001-C
100-J/ 1-C
1,000,000-J/10,000-C
Electric Shielding
Shielding is
important in
electronic devices
such as
televisions and
computers
Electrons repel toward the
outside of any conducting
surface
Net charge inside is zero
Electrons flow outward
evenly, but pile up on
sharp corners
Faraday Cage
• Faraday stated that the charge on a charged
conductor resided only on its exterior
• To demonstrate this fact he built a room coated
with metal foil, and allowed high-voltage
discharges from an electrostatic
generator to strike the outside
of the room
• He used an electroscope to
show that there was no excess
electric charge on the inside of the
room's walls.
Person in a car hit by artificial lightning. The lightning strikes the car and jumps to the
ground bypassing the front tire arcing from the axle to the ground.
Storing Charges
Capacitors can
store charges on
plates which are
separated — as
in Franklin’s
Leyden jars
Storing Charges
• A capacitor is a device that
stores electric charge
• A capacitor consists of two
conductors separated by an
insulator
capacitor
Capacitors and Capacitance
A capacitor in a simple
electric circuit.
Charge Q stored:
Q  CV
The stored charge Q is proportional to the potential difference
V between the plates. The capacitance C is the constant of
proportionality, measured in Farads.
Farad = Coulomb / Volt
Parallel-Plate Capacitor
A simple parallel-plate
capacitor consists of two
conducting plates of area A
separated by a distance d.
Charge +Q is placed on
one plate and –Q on the
other plate.
An electric field E is
created between the plates.
Capacitor
Applications
–.
• Computer RAM memory and
keyboards.
• Electronic flashes for cameras.
• Electric power surge protectors.
• Radios and electronic circuits.
• Power supplies
Van de Graaf Generator
This machine is capable of
producing very high
electrostatic potential
differences in the order of
millions of volts
It works by friction of the belt
with the rollers and separates
charges at combs which take
the charges to the dome and
picks them up from the ground
at the base
Van de Graff Generator
http://demoroom.physics.ncsu.edu/movies.html
Van de Graff Generator
http://demoroom.physics.ncsu.edu/movies.html
Van de Graff Generator
http://demoroom.physics.ncsu.edu/movies.html
Electric Forces and Charges
Like Signs Repel
Unlike Signs Attract
Electrical
Force
Woman is touching
negatively-charge
sphere
Electrical force is
more powerful than
gravity
Review
•
•
•
•
•
•
Which two signs attract? Which two repel?
A) Unlike signs attract, like signs repel
B) unlike signs repel, like signs attract
C) like signs repel and attract
D) vary depending on amount of charge
E) No charges attract, they only repel
Conservation of Charge
Structure of
the Atom
Electron
Energy Levels
or Orbits
Neutron
Proton
Charge
• Electrons and protons have an attribute
called charge
– Electrons have a negative charge
– Protons have a positive charge
• 1800 times more massive than electrons
– Neutrons have no charge
• 1800 times more massive than electrons
Charge Conservation
• Charge is neither created or destroyed.
• What we call charging is either
– Transfer of charges, or
– Internal rearrangement of charge carrying units
• Uncharged (neutral) objects have equal amounts
of positive and negative charge
• An object with unequal number of electrons and
protons is electrically charged
– Negative – Electrons > Protons
– Positive – Protons > Electrons
Removing Electrons from Atoms
Rubber scrapes electrons from fur atoms
Charge Quantization
• Charge is always an integer multiple of a
constant.
– Six billion billion electrons is - 1 Coulomb of
charge
– Six billion billion proton is + 1 Coulomb of
charge
• Q=Ne, where e is the unit electrical charge
• Electrons have –e charge, protons have +e.
• Millikan’s Oil Drop experiment
Coulomb’s Law
• One Coulomb = 6.24 x 1018 electrons
• Electrons have a negative charge
– qe = -1.6 x 10-19 Coulomb
• Protons have a positive charge
– qp = +1.6 x 10-19 Coulomb
• Electrical Force can be positive or negative
– Positive – repulsive force
– Negative – attractive force
Example
One pair of charges of 1 C each are 1 m apart
F = k q1 q2 / d2
F = (9 x 109 N m2/C2)(1 C)(1 C)/(1-m)2
F = 9 x 109 N m2/C2)(1 C2)/1-m2
F = 9 x 109 N (repulsive)
10 times the weight of a battleship
Review
• The charge on an electron is 1.6 X 10^-19
C. How many electrons make a charge of
1C?
What happens to the magnitude of the
force as the charges get farther apart?
Conductors
• An electrical conductor is a substance
through which electrical current flows
with small resistance
• Metals are generally excellent
electrical conductors
• The electrons in conductors lie in an
‘loose’ outer orbit – the so-called
"valence band"
Valence Bonds
• In a periodic table the columns represent
number of valence bonds
• Often times, the valence bonds in two
combining elements will add up to eight
with rare exceptions.
Insulators
• An electrical insulator is a substance
with an extremely high resistance to
the flow of charge
• Most nonmetals solids are generally
excellent insulators
• Most atoms hold on to their electrons
tightly and are insulators
Covalent Bonds
• In a tightly knit molecular bond the atoms
are held together and therefore hang onto
their electrons, creating excellent insulators.
Review
• 1. How much resistance does a conductor
have?
• A- large amount
• B- small amount
• C- None
• D- Average amount
• E- infinite
Review
• Where are valence electrons represented on
the periodic table?
A- columns
B-rows
C- atomic number
D- atomic mass
E- grouping
Net Charge Neutral
Net Charge Positive
Net Charge Negative
Charging by Touching
• Charging by actually touching object
Charging by Induction
• Two charged objects placed on opposite
sides create a charge in third object.
Polarization
Electrons surrounding
nucleus
may be thought of as
a cloud
in which the total
negative
charged is smeared
out.
Polarization
Unpolarized atom
Put negatively charged
rod on the right side...
Center of electron cloud
shifts to the left.
Neutral Objects are Attracted to Charged
Objects
Charged comb attracts neutral
bits of paper
Charge Distributions
Charge on Metals
Metal Ball
Charge on Metal Points
Lightning, lightning rods
Lightning
• As the negative charges collect at the
bottom of the cloud it forces the negative
charges in the ground to be forced away
from the surface. This leaves the ground
positive.
• A streamer of negative charges is repelled
by the bottom of the cloud and attracted by
the ground.
• As this streamer of negative charges
approaches the ground, a streamer of
positive charges is repelled by the ground
and attracted to the negative streamer.
Lightning
• When the two streamers connect, they have
created a fairly conductive path which allows a
sudden down surge of electrons to jump to the
ground. This is the lightning.
• The rapidly moving electrons excite the air along
the path so much that it emits light. It also heats
the air so intensely that it rapidly expands creating
thunder.
• One thing to notice is that the positive charges that
make up both the cloud and the ground do not
move. Even the positive streamer launched by the
ground is really only made up of positively
charged air particles because the electron(s) left
the particle.
Electroscope
Conductor
Insulator
Metal
Leaves
Bibliography
• The textbook
FIN