Chapter 20Chapter 20

Static ElectricityStatic Electricity

20.1 Electrical Charge20.1 Electrical Charge

Charged objectsCharged objects– Like chargesLike charges– Opposite chargesOpposite charges– Experimenting with chargeExperimenting with charge– Types of chargeTypes of charge

Microscopic View of ChargeMicroscopic View of Charge– Exist within atomsExist within atoms– Separation of chargeSeparation of charge– Charge is never created or destroyedCharge is never created or destroyed

Charges are only separated due to movement of Charges are only separated due to movement of electrons.electrons.

Conductors and InsulatorsConductors and Insulators– InsulatorsInsulators

Materials in which charges will not move easilyMaterials in which charges will not move easily

Glass, dry wood, most plastics, cloth, and dry air Glass, dry wood, most plastics, cloth, and dry air are all good insulators.are all good insulators.

– ConductorsConductorsMaterial which allow the free movement of Material which allow the free movement of electrons.electrons.

Electrons carry, or conduct, electric charge through Electrons carry, or conduct, electric charge through the conductor.the conductor.

Metals are good conductors because at least one Metals are good conductors because at least one electron on each atom can be removed easily.electron on each atom can be removed easily.

When air becomes a conductorWhen air becomes a conductor– Under certain conditions air can become a Under certain conditions air can become a

conductor.conductor.– Lightning is one example.Lightning is one example.– Your finger on a doorknob after walking Your finger on a doorknob after walking

across a carpet is another.across a carpet is another.– Plasma forms as the air becomes charged.Plasma forms as the air becomes charged.

20.2 Electrical Force20.2 Electrical Force

Forces on charged bodiesForces on charged bodies– There are two kinds of charges, positive and There are two kinds of charges, positive and

negative.negative.– Charges can exert force on other charges Charges can exert force on other charges

over a distance.over a distance.– The force is stronger when the charges are The force is stronger when the charges are

close together.close together.– Like charges repel; opposites attract.Like charges repel; opposites attract.

Charging by conductionCharging by conduction– Charging a neutral body by touching it with a Charging a neutral body by touching it with a

charged body.charged body.

Separation of charge on a neutral objectSeparation of charge on a neutral object– Recall the electroscope leaves moving as a Recall the electroscope leaves moving as a

charged object is brought near.charged object is brought near.

Charging by inductionCharging by induction– Charge separation can be used to charge an Charge separation can be used to charge an

object without touching it with the charged object without touching it with the charged object.object.

Coulomb’s LawCoulomb’s Law

Coulomb found how the force between two Coulomb found how the force between two charged spheres depended on the charged spheres depended on the distance between them.distance between them.– The symbol for charge is The symbol for charge is qq– Coulomb showed that the force Coulomb showed that the force FF varied with varied with

the inverse square of the distance between the inverse square of the distance between the spheres.the spheres.

– He also found that the force varied directly He also found that the force varied directly with the charge of the bodies.with the charge of the bodies.

One coulomb is the charge of 6.25 X 10One coulomb is the charge of 6.25 X 1018 18

electrons or protons.electrons or protons.

The charge on an individual electron is The charge on an individual electron is 1.60 X 101.60 X 10-19-19

According to Coulomb’s law, the According to Coulomb’s law, the magnitude of the force on a charge magnitude of the force on a charge qqAA

caused by charge caused by charge qqBB a distance a distance dd away away

can be written as followscan be written as follows

The unit of charge: The coulombThe unit of charge: The coulomb

Coulomb’s LawCoulomb’s Law– When the charges are measured in coulombs, When the charges are measured in coulombs,

the distance is measured in meters, and the the distance is measured in meters, and the force is measured in newtons, the constant , force is measured in newtons, the constant , K, is K, is

9.0 X 109.0 X 1099 N N••mm22/C/C22

F = K F = K qqAAqqBB

dd22

PSSPSS

Sketch the system showing all Sketch the system showing all distances and anglesdistances and angles

Diagram the vectors of the system; Diagram the vectors of the system; include derived vectors using dashed include derived vectors using dashed lineslines

Use Coulomb’s law to find the Use Coulomb’s law to find the magnitude of the forcemagnitude of the force

Use your diagram along with the Use your diagram along with the trigonometric relations to find the trigonometric relations to find the direction of the force.direction of the force.

Perform all algebraic operations as Perform all algebraic operations as well as the numbers. Make sure the well as the numbers. Make sure the units match the variable in question.units match the variable in question.

Consider the magnitude of the Consider the magnitude of the answer. Is it reasonable?answer. Is it reasonable?