ElectricityElectricity

The Electric ChargeThe Electric ChargeThe ElectronThe ElectronInsulators and ConductorsInsulators and ConductorsThe Gold-Leaf ElectroscopeThe Gold-Leaf ElectroscopeRedistribution of ChargeRedistribution of ChargeCharging by InductionCharging by InductionCoulomb’s Law of Electric ForceCoulomb’s Law of Electric Force

ElectricityElectricity

ObjectivesObjectives

Define the nature of electric charge, Define the nature of electric charge, and describe how objects can obtain a and describe how objects can obtain a negative or positive chargenegative or positive chargeDistinguish between insulators, Distinguish between insulators, conductors, and semi-conductorsconductors, and semi-conductorsExplain how the gold-leaf electroscope Explain how the gold-leaf electroscope can be used to determine a chargecan be used to determine a chargeState and apply Coulomb’s LawState and apply Coulomb’s Law

ElectrostaticsElectrostatics

StaticStatic electricityelectricityStationary electricity in the form of a Stationary electricity in the form of a electric charge “at rest”electric charge “at rest”Symbol for charge is “q” or “Q”Symbol for charge is “q” or “Q”

Unit Unit of of chargechargeCoulombCoulombSymbol for coulomb is “C”Symbol for coulomb is “C”

Elementary ChargeElementary Charge

SmallestSmallest unit of unit of chargechargeCharge is quantizedCharge is quantized

AtomsAtoms contain contain elementaryelementary charges chargesElectron (-e) = -1.6 x 10Electron (-e) = -1.6 x 10-19-19 C CProton (+e) = 1.6 x 10Proton (+e) = 1.6 x 10-19-19 C CNeutron (0) = no chargeNeutron (0) = no charge

The Electric ChargeThe Electric Charge

NeutralNeutral objects objectsNumber of negative charges equals the Number of negative charges equals the number of positive chargesnumber of positive chargesCharge is balanced (equilibrium)Charge is balanced (equilibrium)

ChargedCharged objects objectsCharge is transferred from one object Charge is transferred from one object to anotherto anotherCharge is separated within an objectCharge is separated within an object

The Electric ChargeThe Electric Charge

First Law of Electrostatics:First Law of Electrostatics:Like charges repel and unlike charges attract.Like charges repel and unlike charges attract.

A A force of repulsionforce of repulsion exists between two exists between two substances that are electrified in the substances that are electrified in the same waysame way..

The ElectronThe Electron

•An object that has an An object that has an excess excess of electronsof electrons is is negatively negatively chargedcharged..

•An object that has a An object that has a deficiency of electronsdeficiency of electrons is is positively chargedpositively charged..

Insulators and ConductorsInsulators and Conductors

•A A conductorconductor is a material through which charges can be is a material through which charges can be easily transferredeasily transferred..

•Such as metals, that allow charges to move about freelySuch as metals, that allow charges to move about freely

•Charges spread out over the surfaceCharges spread out over the surface

•An An insulatorinsulator is a material that is a material that resistsresists the transfer of charge. the transfer of charge.

•Materials through which charges will not move easilyMaterials through which charges will not move easily

•Charges stay where they are placedCharges stay where they are placed

•A A semiconductorsemiconductor is a material that transfers charges is a material that transfers charges better better than an insulator but not as well as a conductorthan an insulator but not as well as a conductor..

Charging by ConductionCharging by Conduction

•Charging a neutral object by Charging a neutral object by touchingtouching it with a charged body it with a charged body

•Charges are Charges are transferredtransferred to the object to the object

•Charges Charges spreadspread over all metal surfaces over all metal surfaces

Charging by InductionCharging by Induction

•A charged body is brought near another object A charged body is brought near another object withoutwithout touchingtouching it it

•Charges are Charges are separatedseparated in the object in the object

•Like charges within the object are forced to Like charges within the object are forced to move awaymove away from a charged body to another part of the objectfrom a charged body to another part of the object

OROR

•Opposite charges within the object are forced to Opposite charges within the object are forced to move move towardtoward a charged body leaving like charges in another a charged body leaving like charges in another part of the objectpart of the object

Charging by InductionCharging by Induction

•Dipole moment of water moleculeDipole moment of water molecule

•Water bends toward positive chargeWater bends toward positive charge

H

O

H

Charging by InductionCharging by Induction

•Dipole moment of water moleculeDipole moment of water molecule

•Water bends toward negative chargeWater bends toward negative charge

H

O

H

The Gold-Leaf ElectroscopeThe Gold-Leaf Electroscope

Redistribution of ChargeRedistribution of Charge

•When a When a charged rodcharged rod is brought is brought close to an close to an uncharged pith balluncharged pith ball there is an initial there is an initial attractionattraction. .

•No charge is gained or lostNo charge is gained or lost during during this process--the charge on the this process--the charge on the neutral body is simply neutral body is simply redistributed. redistributed.

Charging by InductionCharging by Induction

Charging by induction:Charging by induction:The The redistribution of chargeredistribution of charge due to the presence of a nearby due to the presence of a nearby charged object.charged object.

Charging by induction can be accomplished Charging by induction can be accomplished without any loss of without any loss of chargecharge from the charging body. from the charging body.

Coulomb’s LawCoulomb’s Law

Coulomb’s LawCoulomb’s Law

The force of attraction or repulsion The force of attraction or repulsion between two point charges is directly between two point charges is directly proportional to the proportional to the product of the two product of the two chargescharges and inversely proportional to the and inversely proportional to the square of the square of the distance between themdistance between them..

221

r

qkqF 2

21

r

qkqF

One coulombOne coulomb is the charge transferred through any cross- is the charge transferred through any cross-section of a conductor in section of a conductor in one secondone second by a constant current of by a constant current of one ampereone ampere..

1 C = 6.25 x 1018 electrons1 C = 6.25 x 1018 electrons

k = 8.99 x 109 N·m2/C2k = 8.99 x 109 N·m2/C2

Summary of New TermsSummary of New Terms

•electrostaticselectrostatics•chargingcharging•electronelectron•negative chargenegative charge•positive chargepositive charge•ionion•induced chargeinduced charge

•conductorconductor•insulatorinsulator•semiconductorsemiconductor•Coulomb’s lawCoulomb’s law•coulombcoulomb•microcoulombmicrocoulomb•electroscopeelectroscope

ExampleExample

3.0 cm

q = 6 μC q = 3 μC

A B

What is the electrical force on A?

AnswerAnswer

F kq q

d

x NmC

x C x C

x m

x x

xN

x x N

x N

F N

A B

2

9 2

2

6 6

2 2

9 12

4

12 13

1

9 106 10 3 10

3 0 10

9 10 18 10

9 10

18 10 10

18 10

180

c hc hc h

c hc h

c h

.