Download (EPE) is stored when a charge is moved within an electric field

Electrostatics
GIRL SAFELY CHARGED TO
SEVERAL HUNDRED
THOUSAND VOLTS
GIRL IN GREAT DANGER AT
SEVERAL THOUSAND VOLTS
The Nature of Electric Charge
Discovery of charge
The Greeks first noticed electric charged by
rubbing amber with fur, then picking up bits of
matter. The Greek word for amber is elektron.
Benjamin Franklin arbitrarily called the two
kinds of charge positive and negative. In most
cases, only the negative charge is mobile.
Properties of charge
Like charges repel, and unlike charges
attract.
Charge is conserved, meaning it cannot be
created or destroyed, only transferred from
one location to another.
In all atoms, electrons (qe) have negative charge
and protons (qp) have positive charge.
Charge is quantized, meaning it comes in
discrete amounts (like money).
total charge = integer x fundamental unit of charge
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Insulators and Conductors
Insulators
In insulators, electrons are bound in
“orbit” to the nucleus in each atom.
When charge is placed on an insulator, it
stays in one region and does not distribute.
Wood, plastic, glass, air, and cloth
are good insulators.
Conductors
In conductors electrons can move from
atom to atom, thus electricity can “flow”.
CHARGED INSULATOR
When charge is placed on a conductor, it
redistributes to the outer surface.
Metals (copper, gold, and aluminum)
are good conductors.
CHARGED CONDUCTOR
Polarization
Polarization is the separation of charge
In a conductor, “free” electrons can move around the surface of
the material, leaving one side positive and the other side negative.
In an insulator, the electrons “realign” themselves within the
atom (or molecule), leaving one side of the atom positive and
the other side of the atom negative.
Polarization is not necessarily a charge imbalance!
Charging by Friction
POSITIVE
Rabbit's fur
Glass
Mica
Nylon
Wool
Cat's fur
Silk
Paper
Cotton
Wood
Lucite
Wax
Amber
Polystyrene
Polyethylene
Rubber ballon
Sulfur
Celluloid
Hard Rubber
Vinylite
Saran Wrap
NEGATIVE
When insulators are rubbed together, one
gives up electrons and becomes positively
charged, while the other gains electrons
and becomes negatively charged.
Materials have different affinities for
electrons. A triboelectric series rates this
relative affinity.
A material will give up electrons to
another material below it on a
triboelectric series.
Common examples of charging by friction:
• small shocks from a doorknob after walking
on carpet with rubber-soled shoes
• plastic foodwrap that sticks to a container
• sweater pulled over your head that sparks
• laundry from the dryer that clings
• balloon rubbed with hair sticks that to a wall
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Charging by Conduction
When a charged conductor makes contact with a
neutral conductor there is a transfer of charge.
CHARGING NEGATIVELY
CHARGING POSITIVELY
Electrons are transferred from
the rod to the ball, leaving them
both negatively charged.
Electrons are transferred from
the ball to the rod, leaving
them both positively charged.
Remember, only electrons are free to move in solids.
Notice that the original charged object loses some charge.
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Charging by Induction
Induction uses the influence of one charged object to
“coerce” charge flow.
Step 1. A charged rod is brought
near an isolated conductor. The
influence of the charge object
polarizes the conductor but does
not yet charge it.
Step 2. The conductor is
grounded to the Earth,
allowing charge to flow out
between it and the Earth.
Charging by Induction (cont.)
Step 3. The ground is removed
while the charge rod is still
nearby the conductor.
Step 4. The rod is removed
and the conductor is now
charge (opposite of rod).
An object charged by induction has the opposite sign
of the influencing body.
Notice that the original charged object does not lose charge.
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Electric Forces and Electric Fields
CHARLES COULOMB
(1736-1806)
MICHAEL FARADAY
(1791-1867)
Electrostatic Charges
A New Fundamental Physics Quantity
Electrostatic charge is a
fundamental quantity like
length, mass, and time.
The symbol for charge is q.
The SI unit for charge is
called the coulomb (C).
ATTRACTION AND REPULSION
The charge of an electron (qe) is -1.6 x 10-19 C
The charge of an proton (qp) is 1.6 x 10-19 C
Common electrostatic charges are small:
millicoulomb = mC = 10-3 C
microcoulomb = C = 10-6 C
nanocoulomb = nC = 10-9 C
The Electrostatic Force
Charles Coulomb’s Torsion Balance
A torsion balance measures the
force between small charges.
The electrostatic force depends directly
on the magnitude of the charges.
The force depends inversely on the
square of distance between charges
(another “inverse square law”)!
COULOMB’S LAW OF
ELECTROSTATIC FORCE
constant
charges
kq1q2
Fe  2
r
electrostatic
force
distance
TORSION BALANCE
The constant of proportionality, k,
is equal to 9.0 x 109 Nm2/C2.
A negative force is attractive,
and a positive force is repulsive.
The sign (+ or –) is different from
a vector direction (left or right)
The Electrostatic Force
EXAMPLE 1 - Find the force between these two charges
9.0  10 5  10


9
Fe
6

C 8  10 6 C
0.04 m 2

Fe  225 N
The negative signs means force of attraction,
but does not indicate left or right direction
EXAMPLE 2 - Find the net force on the left charge
9.0  10 5  10


9
Fe
Fe  360 N
6

C 5  10 6 C
0.025 m 2

(force of repulsion)
Fnet  Fleft  Fright
Fnet  360 N  225 N  135 N, to the left
Electric Field Strength
Field Theory Visualizes Force At A Distance
DEFINITION OF
GRAVITATIONAL
FIELD
DEFINITION OF
ELECTRIC
FIELD
force
g field 
mass
g
E field 
force
charge
Fe
E
q0
Fg
m
q0 is a small, positive test charge
Electric field is a vector quantity
E field points toward negative charges
E field points away from positive charges
SI unit of electric field
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newton
N

coulomb C
Electric Field Lines
Single Point Charges
POSITIVE CHARGE
NEGATIVE CHARGE
Density of field
lines indicates
electric field
strength
Inverse square
law obeyed
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Definition of E Field for single point charge
Fe kq0 q / r 2
E

q0
q0
electric
field
constant
charge
kq
E 2
r
distance
Electric Field Lines
Electric fields for multiple point charges
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POSITIVE AND NEGATIVE POINT CHARGES
TWO POSITIVE POINT CHARGES
OPPOSITE MAGNETIC POLES
ALIKE MAGNETIC POLES
Electric Potential Energy
Electric Potential Energy versus Gravitational Potential Energy
FALLING MASS VS. FALLING CHARGE
Electric potential energy (EPE) is
stored when a charge is moved
within an electric field.
EPE can be converted to kinetic
energy, heat, light, sound etc.
EPE is a scalar quantity
measured in joules (J).
STORING POTENTIAL ENERGY
POTENTIAL ENERGY GAIN OR LOSS
positive (+)
charge
negative (–)
charge
toward E
loses PE
gains PE
opposite E
gains PE
loses PE
EPE for Constant Electric Field
CONSTANT GRAVITATIONAL FIELD
CONSTANT ELECTRIC FIELD
PE  mgh
PE  qEd
electric
potential
energy charge
E field
distance
PE for Two Point Charges
Potential Energy is force times distance (like WORK!)
constant
kq1q2
PE  Fed  2  r
r
electric
potential
energy
charges
kq1q2
PE 
r
Potential energy is positive for like charges
Potential energy is negative for opposite charges
Potential energy is zero at infinite distance
distance
Potential Difference (Voltage)
Electric potential is average energy per charge.
Energy
Potential 
Charge
Potential difference is the difference in energy
values of the charge in two separate locations in
the electric field.
PE
V 
q
Potential difference is often called voltage.
Voltage is only dangerous when a
lot of energy is transferred.
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SI Units
Voltage, like energy, is a scalar.
A volt (v) is the unit for voltage
named in honor of Alessandro Volta,
inventor of the first battery.
1 joule
J
1 volt 
V
1 coulomb
C
source
voltage (V)
common dry cell
1.5
car battery
12
household (US)
120
comb through hair
500
utility pole
4,400
transmission line
120,000
Van de Graaff
400,000
lightning
1,000,000,000
Potential Difference (Voltage)
A SEVERAL THOUSAND VOLT POWERLINE
CAN ILLUMINATE A FLUORESCENT LIGHT
A PARACHUTE ACCIDENT LANDED THIS
MAN ON A 138,000 THOUSAND VOLT LINE,
BUT HE SUFFERED ONLY MINOR BURNS
Potential Difference (Voltage)
Batteries are sources of potential
difference.
Inside a battery, a chemical reaction
occurs that creates a separation of
charge: + at one end, - at the other.
This separation is maintained as
long as the terminals of the batteries
are not connected.
When the terminals are connected
by a conductor, the electrons,
attracted to the positive terminal,
flow through the conductor.
Summary of Electrostatic Equations
Electrostatic Force
kq1q2
Fe  2
r
force between two charges
Electric Field
Fe
E
q0
Potential Energy
PE  qEd
for constant E field
kq1q2
PE 
r
for two charges
Potential Difference
definition
PE
V 
q
kq
E 2
r
kq
V 
r
definition
for point charge
for point charge
V  Ed
for constant
E field