ElectrostaticsElectrostatics

Ch 20Ch 20

ElectrostaticsElectrostatics

Study of electrical charges that can be collected Study of electrical charges that can be collected and held in one placeand held in one place

Electrical BasicsElectrical Basics

All atoms have a positively charged All atoms have a positively charged nucleus (made up of protons (+) and nucleus (made up of protons (+) and neutrons (0) that are surrounded by neutrons (0) that are surrounded by electrons (-))electrons (-))

When the positive charge equals the When the positive charge equals the negative charge the atom is neutralnegative charge the atom is neutral

Electrons can be removed or transferred Electrons can be removed or transferred by the addition of energy.by the addition of energy.

Conservation of ChargeConservation of Charge

Electric charge is never created or Electric charge is never created or destroyed, just transferred by the destroyed, just transferred by the separation of positive and negative chargeseparation of positive and negative charge

Like charges repelLike charges repelOpposite charges attractOpposite charges attractA neutral object will be attracted to a A neutral object will be attracted to a

charged object charged object

Electrical MaterialsElectrical Materials

Conductors – materials through which Conductors – materials through which charges will move about easily because charges will move about easily because they have loosely bound electrons; Ex: they have loosely bound electrons; Ex: most metalsmost metals

Insulators - materials through which Insulators - materials through which charges will not move about easily charges will not move about easily because they have tightly bound electrons; because they have tightly bound electrons; Ex: glass, wood, plasticEx: glass, wood, plastic

Charging a neutral objectCharging a neutral object

Conduction – charging a neutral object by Conduction – charging a neutral object by touching it with a charged objecttouching it with a charged object

Induction – charging a neutral object Induction – charging a neutral object without touching it, but by the separation without touching it, but by the separation of chargesof charges

Coulomb’s LawCoulomb’s Law

The electrical force between 2 charged objects The electrical force between 2 charged objects varies directly with the product of the charges varies directly with the product of the charges and inversely with the square of the distance and inversely with the square of the distance between thembetween them

F = F = kqqkqq’’ k = 9EE9 Nmk = 9EE9 Nm22/C/C22

dd22

A repulsive force is positive because both A repulsive force is positive because both charges are alikecharges are alike

An attractive force is negative because the An attractive force is negative because the charges are opposites charges are opposites

Electric FieldsElectric Fields

Ch 21Ch 21

Electric FieldsElectric Fields

A charge creates an electric field in all A charge creates an electric field in all directionsdirections

The electric field intensity is the force per The electric field intensity is the force per unit chargeunit charge

E = F/qE = F/q units are N/Cunits are N/C

Electric Field LinesElectric Field Lines

Provide a model for Provide a model for the electric field. The the electric field. The strength of the field is strength of the field is indicated by the indicated by the spacing between spacing between lines. The closer the lines. The closer the lines the stronger the lines the stronger the field. The field always field. The field always points away from the points away from the positive towards the positive towards the negative charge.negative charge.

Electric Potential DifferenceElectric Potential Difference

Change in Potential Energy per unit Change in Potential Energy per unit charge; this is often called voltage charge; this is often called voltage differencedifference

ΔΔV = V = ΔΔPE/q or PE/q or ΔΔV = Work/qV = Work/q

units are J/C = voltsunits are J/C = volts

Also Also ΔΔV = EdV = Ed

GroundingGrounding

Eliminating the Eliminating the excess charge by excess charge by touching a charged touching a charged object to the earth object to the earth (low voltage). The (low voltage). The charge moves from charge moves from higher potential to higher potential to lower potential until lower potential until they are equal.they are equal.

All charged objects All charged objects are on the surface of are on the surface of a solid conductora solid conductor

Excess charge will Excess charge will move to the outer move to the outer surface of a hollow surface of a hollow conductorconductor

CapacitorCapacitor

Stores charge; made up Stores charge; made up of two oppositely charged of two oppositely charged conducting plates conducting plates separated by an insulatorseparated by an insulator

Capacitance – ratio of Capacitance – ratio of charge stored to potential charge stored to potential differencedifference

C = q/VC = q/V

units are C/V = 1 Farad (F)units are C/V = 1 Farad (F)

Millikan ExperimentMillikan Experiment

He determined the He determined the charge of an electron charge of an electron to be 1.6 EE -19 Cto be 1.6 EE -19 C

Eq = mgEq = mg