Electric Fields Electric Fields and Potentialsand Potentials

Joey Multari, Shannon Burt, Katie Abbott

Electric ForceElectric ForceElectricity 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-AttractingB-repellingC-bothD- Fast movementE- Slow movement

Electric FieldsElectric FieldsJust 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 LinesForce Field Lines• Fields have strength and

direction

• Field is determined by the force and direction of motion of a positivepositive test charge

• Field is strongest where the force is the strongest – where the lines are the most concentrated

http://www.glenbrook.k12.il.us/gbssci/phys/Class/estatics/u8l4c.html

http://www.glenbrook.k12.il.us/gbssci/phys/Class/estatics/u8l4c.html

Electrical PotentialElectrical PotentialJust like gravity—the potential (possibility) of falling to earth, charges have the potential to move toward or away from each other

Electrical PotentialElectrical 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 PotentialVoltage – Electrical Potential

Voltage = PE/QVoltage = PE/Q PE in Joules and Q in Coulombs

100 Volts0.000001-J/0.00000001-C

100-J/ 1-C1,000,000-J/10,000-C

Electric ShieldingElectric ShieldingElectrons repel toward the outside of any conducting surfaceNet charge inside is zeroElectrons flow outward evenly, but pile up on sharp corners

Shielding is important in electronic devices such as televisions and computers

Faraday CageFaraday 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 ChargesStoring ChargesCapacitors can store charges on plates which are separated — as in Franklin’s Leyden jars

Storing ChargesStoring Charges• A capacitor is a device that

stores electric charge• A capacitor consists of two

conductors separated by an insulator

capacitorcapacitor

Capacitors and CapacitanceCapacitors and Capacitance

Charge Q stored:

CVQ 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

A capacitor in a simpleelectric circuit.

Parallel-Plate CapacitorParallel-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 ApplicationsCapacitor Applications• Computer RAM memory and

keyboards.

• Electronic flashes for cameras.

• Electric power surge protectors.

• Radios and electronic circuits.

• Power supplies

–.

Van de Graaf GeneratorVan de Graaf GeneratorThis 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 GeneratorVan de Graff Generatorhttp://demoroom.physics.ncsu.edu/movies.html

Van de Graff GeneratorVan de Graff Generatorhttp://demoroom.physics.ncsu.edu/movies.html

Van de Graff GeneratorVan de Graff Generatorhttp://demoroom.physics.ncsu.edu/movies.html

Electric Forces and Charges

Like Signs RepelLike Signs Repel

Unlike Signs AttractUnlike 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

ProtonNeutron

Electron

Energy Levels or Orbits

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 atomsRubber 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

ExampleOne pair of charges of 1 C each are 1 m apart

F = F = kk qq11 qq22 / / dd22

F = (F = (9 x 109 x 109 9 N mN m22/C/C22)()(1 C1 C)()(1 C1 C)/()/(1-m1-m))22

F = 9 x 10F = 9 x 109 9 N mN m22/C/C22)(1 C)(1 C22)/1-m)/1-m22

F = 9 x 10F = 9 x 109 9 N (repulsive)N (repulsive)

10 times the weight of a battleship10 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 What happens to the magnitude of the force as the charges get farther apart?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 Electrons surrounding nucleusnucleus

may be thought of as may be thought of as a clouda cloud

in which the total in which the total negativenegative

charged is smeared charged is smeared out.out.

Polarization

Unpolarized atomUnpolarized atom

Put negatively charged Put negatively charged rod on the right side... rod on the right side...

Center of electron cloud Center of electron cloud shifts to the left.shifts to the left.

Neutral Objects are Attracted to Charged Objects

Charged comb attracts neutralbits of paper

Charge on Metals              

             Metal Ball

Charge Distributions

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.  

ElectroscopeElectroscope

InsulatorInsulator

Metal Metal LeavesLeaves

ConductorConductor

Bibliography

• The textbook

FIN