1. Electrostatics GIRL SAFELY CHARGED TO SEVERAL HUNDRED THOUSAND VOLTS
GIRL IN GREAT DANGER AT SEVERAL THOUSAND VOLTS

2. 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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3. Insulators and Conductors
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
CHARGED INSULATOR
In conductors electrons can move from atom to atom, thus electricity can “flow”.
When charge is placed on a conductor, it redistributes to the outer surface.
Metals (copper, gold, and aluminum) are good conductors.
CHARGED CONDUCTOR

4. 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!

5. 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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6. 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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7. 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.

8. 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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9. Electric Forces and Electric Fields
CHARLES COULOMB
( )
MICHAEL FARADAY
( )

10. Electrostatic Charges ATTRACTION AND REPULSION
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 C
The charge of an proton (qp) is 1.6 x C
Common electrostatic charges are small:
millicoulomb = mC = 10-3 C
microcoulomb = C = 10-6 C
nanocoulomb = nC = 10-9 C

11. The Electrostatic Force COULOMB’S LAW OF 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”)!
TORSION BALANCE
COULOMB’S LAW OF ELECTROSTATIC FORCE
The constant of proportionality, k, is equal to 9.0 x 109 Nm2/C2.
constant
charges
A negative force is attractive, and a positive force is repulsive.
electrostatic force
distance
The sign (+ or –) is different from a vector direction (left or right)

12. The Electrostatic Force
EXAMPLE 1 - Find the force between these two charges
EXAMPLE 2 - Find the net force on the left charge

13. Electric Field Strength
Field Theory Visualizes Force At A Distance
DEFINITION OF GRAVITATIONAL FIELD
DEFINITION OF ELECTRIC FIELD
q0 is a small, positive test charge
Electric field is a vector quantity
SI unit of electric field
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14. 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
constant
charge
electric field
distance

15. 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

16. Electric Potential Energy
Electric Potential Energy versus Gravitational Potential Energy
FALLING MASS VS. FALLING CHARGE
STORING POTENTIAL ENERGY
Electric potential energy (EPE) is stored when a charge is moved within an electric field.
POTENTIAL ENERGY GAIN OR LOSS
positive (+)
charge
negative (–)
toward E
loses PE
gains PE
opposite E
EPE can be converted to kinetic energy, heat, light, sound etc.
EPE is a scalar quantity measured in joules (J).

17. EPE for Constant Electric Field
CONSTANT GRAVITATIONAL FIELD
CONSTANT ELECTRIC FIELD
electric
potential
energy
charge
distance
E field

18. PE for Two Point Charges
Potential Energy is force times distance (like WORK!)
constant
charges
electric
potential
energy
distance
Potential energy is positive for like charges
Potential energy is negative for opposite charges
Potential energy is zero at infinite distance

19. Potential Difference (Voltage)
Electric potential is average energy per charge.
Potential difference is the difference in energy values of the charge in two separate locations in the electric field.
Potential difference is often called voltage.
Voltage is only dangerous when a lot of energy is transferred.
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
SI Units
click for web page
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.

20. 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

21. 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.

22. Summary of Electrostatic Equations
Electrostatic Force
Potential Energy
for constant E field
force between two charges
Electric Field
for two charges
Potential Difference
definition
definition
for point charge
for point charge
for constant E field