Electrostatics

History

• The word electricity comes from the Greek elektron which means “amber”.

• The “amber effect” is what we call static electricity.

History

• Ben Franklin made the arbitrary choice of calling one of the demo situations positive and one negative.

• He also argued that when a certain amount of charge is produced on one body, an equal amount of the opposite charge is produced on the other body…

Charge Concepts

• Opposite charges attract, like charges repel.

• Law of Conservation of Charge:– The net amount of electric charge produced in

any process is zero. thanks Ben!!!

• Symbol: q, Q

• Unit: C, Coulomb

Elementary Particles

Particle Charge, (C) Mass, (kg)

electron -1.6x10-19 9.109x10-31

proton +1.6x10-19 1.673x10-27

neutron 0 1.675x10-27

• If an object has a…+ charge it has less electrons than normal

- charge it has more electrons than normal

Ions and Polarity

• If an atom loses or gains valence electrons to become + or - , that atom is now called an ion.

• If a molecule, such as H2O, has a net positive charge on one side and negative charge on the other it is said to be polar

Why does…

Chemistry work?

Physics!!!

The electrostatic forces between ions (within molecules) form bonds called ionic bonds…all bonds are ionic; others, like covalent, are to a much lesser degree so that you can ignore the ionic properties of that type of bond.

Why does…

Biology work?

Physics!!!

The intermolecular electrostatic forces between polar molecules make such things as the DNA double helix possible.

Types of materials

1. Conductor: a material that transfers charge easily (ex. Metals).

2. Insulator: a material that does not transfer charge easily (ex. Nonmetals)

3. Semiconductors: somewhere between 1 & 2 (ex. Silicon, carbon, germanium).

4. Superconductors: some metals become perfect conductors below certain temperatures

Ways to Charge

• By Friction: two objects rubbed against each other may cause a transfer of charge from one to another (triboelectrification)

• Result: each object has a net charge which is equal and opposite to the other

• By Conduction: contact occurs between charged object and neutral object.

• Result: two objects with same charge

• By induction: no contact occurs between charged object and neutral object.

• Result: two objects with opposite charge

• Credit Card: You may use Visa, Master Card, or American Express

• Result: Debt from high interest rates

Charging by Friction

Materials have different affinities for electrons. A triboelectric series rates this relative affinity.

POSITIVERabbit's furGlassMicaNylonWoolCat's furSilkPaperCottonWoodLuciteWaxAmberPolystyrenePolyethyleneRubber ballonSulfurCelluloidHard RubberVinyliteSaran Wrap

NEGATIVE

When insulators are rubbed together, one gives up electrons and becomes positively charged, while the other gains electrons and becomes negatively charged.

• plastic foodwrap that sticks to a container• sweater pulled over your head that sparks• laundry from the dryer that clings

Common examples of charging by friction:

A material will give up electrons to another material below it on a triboelectric series.

• small shocks from a doorknob after walking on carpet with rubber-soled shoes

• balloon rubbed with hair sticks that to a wall

click for applet

Charging by ConductionWhen a charged conductor makes

contact with a neutral conductor there is a transfer of charge.

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.

CHARGING NEGATIVELY CHARGING POSITIVELY

Notice that the original charged object loses some charge.

Conduction

Charging by Induction

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.

Induction uses the influence of one charged object to “coerce” charge flow.

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).

Charging by Induction (cont.)

An object charged by induction has the opposite sign of the influencing body.Notice that the original charged object does not lose charge.

Induction Polarization

Conduction or Induction

A B

LightningBecomes very “negative”

Becomes very “positive”

Lightning (1)

Lightning (2)

Lightning (3)

Lightning (4)

Lightning (5)

Lightning (6)

Lightning Video compliments ofTom A. Warner, ztresearch.com

640x480 pixels7,200 images per second0.15 seconds recording timeVisual aspects:-stepped leaders-dart leaders-return strokes-continuing current

Lightning Rod

Simulator

Lightning striking theEmpire State Building

Van de Graaff

electrostatic generator:

simulates lightning

from cloud to ground

Electric Forces and Electric Fields

CHARLES COULOMB

(1736-1806)

MICHAEL FARADAY

(1791-1867)

Electric Force

AKA: Coulomb’s LawUsing a torsion balance,

Coulomb found that: the electric force between two charges is proportional to the product of the two charges and inversely proportional to the square of the distance between the charges.

Electric Force

Electric Force

• q charge, C

• r distance between charges, m

• FE Electric Force, N VECTOR

• kc coulomb constant, 8.99x109Nm2/C2

1 22E c

q qF k

r

��������������

The Electrostatic Force

Fe 9.0 109 5 10 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 1 - Find the force between these two charges

EXAMPLE 2 - Find the net force on the left charge

Fe 9.0 109 5 10 6 C 5 10 6 C

0.025 m 2

Fe 360 N (force of repulsion)

Electric Field

The electric force is a field force, it applies force without touching (like the gravitational force)

In the region around a charged object, an Electric Field is said to exist

Electric Field

Rules for Drawing Electric Field Lines1. The lines must originate on a positive

charge (or infinity) and end on a negative charge (or infinity).

2. The number of lines drawn leaving a positive charge or approaching a negative charge is proportional to the magnitude of the charge.

3. No two field lines can cross each other.4. The line must be perpendicular to the

surface of the charge

Electric Field

• E electric field strength, N/C VECTOR• q0 + test charge, C• q charge producing field, C• r distance between charges, m• FE Electric Force, N VECTOR• kc coulomb constant, 8.99x109Nm2/C2

0

EFE

q

����������������������������

becomes 2c

qE k

r

E-Field vs g-field

0q

FE

FieldE

e

0m

Fg

fieldg

g

Electric Field Strength

g Fgm

E Feq0

DEFINITION OF GRAVITATIONAL

FIELD

DEFINITION OF ELECTRIC FIELD

Field Theory Visualizes Force At A Distance

g field force

massE field

force

charge

SI unit of electric field

Electric field is a vector quantity

q0 is a small, positive test charge

click for

applet

Van der Graff Generator

• When no net motion of charge occurs within a conductor, the conductor is said to be in electrostatic equilibrium

• There are four properties to consider when looking at conductors

Conductors in Electrostatic Equilibrium

Property 1

• The electric field is zero everywhere inside the conducting material– Consider if this were not true

• if there were an electric field inside the conductor, the free charge there would move and there would be a flow of charge

• If there were a movement of charge, the conductor would not be in equilibrium

Property 2

• Any excess charge on an isolated conductor resides entirely on its surface– If some excess of charge could be placed inside

the conductor, the repulsive forces would push them as far apart as possible, causing them to migrate to the surface

Property 3

• The electric field just outside a charged conductor is perpendicular to the conductor’s surface– Consider what would happen if this was not

true• The component along the surface would

cause the charge to move• It would not be in equilibrium

Property 4

• On an irregularly shaped conductor, the charge accumulates at locations where the radius of curvature of the surface is smallest (that is, at sharp points)

Conductors in Electrostatic Equilibrium

1. The electric field is zero everywhere inside a conductor.

2. Any excess charge on an isolated conductor resides entirely on the outside surface of the conductor.

3. The electric field just outside the charged conductor is perpendicular to the conductor’s surface.

4. On an irregularly shaped conductor, charge tends to accumulate where the radius of curvature is the smallest, i.e. AT SHARP POINTS.

E=0

Otherwise charges would be moved around

(Not equilibrium)

Charge resides along the surface

=> Charges try to get as far away as possible

Perpendicular otherwise there would be a force acting on the charges along the surface

Charge accumulates at smallest curvature

Electric Potential Difference (a.k.a. Voltage, Potential Difference)

The difference of potential between two points is defined as the work done to move a charge from a point of lower potential to a point of higher potential

Potential Difference

q

WV Where:

V is the difference in potential between two points

W is the work done in moving a charge in Joules

q is the charge being moved in coulombsUnits:

voltsCoulombs

Joules

Electronvolts (eV)

The Joule is a large unit of energy…much too large a unit to use when moving elementary charges around.

The amount of work done to move a single elementary charge across a potential difference of one volt is called an electronvolt

q

WV

)106.1(1

19Cx

Wvolt

JxW 19106.1

JxeV 19106.11