Electronic Air CleanersElectronic Air Cleaners(Static Electricity)(Static Electricity)

Electronic Air CleanersElectronic Air Cleaners(Static Electricity)(Static Electricity)

Physics 10Physics 10

Kenneth N. BarishKenneth N. Barish

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Introductory Question:Introductory Question:A woman rubs her feet on the A woman rubs her feet on the carpet and gives a shock to her carpet and gives a shock to her identical twin. If the twin also identical twin. If the twin also rubs her feet on the carpet rubs her feet on the carpet before being touched, the shock before being touched, the shock will bewill be1. larger.2. smaller.3. the same size.

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Observations About Air Observations About Air CleanersCleaners

o Dust doesn’t settle quickly on its ownDust doesn’t settle quickly on its owno Mechanical filters gradually plug upMechanical filters gradually plug upo Dust clings to things with static Dust clings to things with static

electricityelectricityo Air cleaners could be based on static Air cleaners could be based on static

electricityelectricity

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Questions about Questions about electronic air cleanerselectronic air cleanerso What keeps dust in the air? Why doesn’t What keeps dust in the air? Why doesn’t

gravity remove it?gravity remove it?o If gravity can’t clear the air, is there If gravity can’t clear the air, is there

another force around that can succeed?another force around that can succeed?o Is there an electrostatic potential energy Is there an electrostatic potential energy

associated with electronic air cleaners?associated with electronic air cleaners?o How can dust acquire charge? Why is How can dust acquire charge? Why is

charged dust attracted to walls?charged dust attracted to walls?

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Questions about Questions about electronic air cleanerselectronic air cleanerso What keeps dust in the air? Why What keeps dust in the air? Why

doesn’t gravity remove it?doesn’t gravity remove it?

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Air ResistanceAir Resistanceo Different from buoyancyDifferent from buoyancyo Opposes the relative motion of air and objectOpposes the relative motion of air and objecto Acts to bring both to one velocityActs to bring both to one velocityo Consists of a matched pair of forces:Consists of a matched pair of forces:

– The air pushes on the object– The object pushes on the air– Forces have equal magnitudes, opposite directions

o Increases with relative speed and cross sectionIncreases with relative speed and cross section

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Terminal VelocityTerminal Velocityo A real falling object experiences two forces:A real falling object experiences two forces:

– Weight downward– Air resistance upward (while dropping in still

air)o Air resistance affects descentAir resistance affects descent

– Object accelerates downward initially– Upward air resistance gradually increases– Terminal velocity occurs when net force is zero

o Small objects have small terminal velocitiesSmall objects have small terminal velocities

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Questions about Questions about electronic air cleanerselectronic air cleanerso If gravity can’t clear the air, is there If gravity can’t clear the air, is there

another force around that can another force around that can succeed?succeed?

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Electric Charge Electric Charge (Part 1)(Part 1)

o Two types: positive & negativeTwo types: positive & negativeo Like charges repel, opposites attractLike charges repel, opposites attract

– Forces consist of a matched pair:• Each charge pushes or pulls on the other• Forces have equal magnitudes, opposite

directions

– Forces increase with decreasing separation

o Charge is measured in coulombsCharge is measured in coulombs

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Electric Charge Electric Charge (Part 2)(Part 2)

o Charge is conservedCharge is conservedo Charge is quantizedCharge is quantized

– One fundamental charge is 1.6 x 10-19 coulomb

o Charge is an intrinsic property of Charge is an intrinsic property of mattermatter– Electrons are negatively charged– Protons are positively charged– Each has one fundamental charge

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Net ChargeNet Charge

o Net charge is the sum of an object’s Net charge is the sum of an object’s chargescharges

o Most objects have zero net charge Most objects have zero net charge (neutral)(neutral)

o Neutral objects contain equal + & – Neutral objects contain equal + & – chargescharges

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A woman rubs her feet on the A woman rubs her feet on the carpet and gives a shock to her carpet and gives a shock to her identical twin. If the twin also identical twin. If the twin also rubs her feet on the carpet rubs her feet on the carpet before being touched, the shock before being touched, the shock will bewill be1. larger.2. smaller.3. the same size.

Introductory Question:Introductory Question:

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Questions about Questions about electronic air cleanerselectronic air cleanerso Is there an electrostatic potential Is there an electrostatic potential

energy associated with electronic air energy associated with electronic air cleaners?cleaners?

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VoltageVoltageo Charge has electrostatic potential Charge has electrostatic potential

energy (EPS)energy (EPS)o Voltage measures the electrostatic

potential energy per unit of charge– Raising the voltage of positive charge takes work– Lowering the voltage of negative charge takes

work

o Voltage is measured in joules/coulomb or volts rf

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Questions about Questions about electronic air cleanerselectronic air cleanerso How can dust acquire charge? Why is How can dust acquire charge? Why is

charged dust attracted to walls?charged dust attracted to walls?

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Charging ObjectsCharging Objects

o Like charge separate whenever possibleLike charge separate whenever possible– They disperse on the outside of a conductor– They accumulate on an outside point or thin

wire

o Charging is limited by charge escapeCharging is limited by charge escape– Severe repulsion leads to corona discharge– Charges leap onto air molecules and escape– Air molecules often glow during corona

discharge

o If ionization occurs, a spark or arc formsIf ionization occurs, a spark or arc forms

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Polarizing ObjectsPolarizing Objects

o Nearby charges can shift an object’s chargesNearby charges can shift an object’s chargeso When negatively charged dust nears a wall,When negatively charged dust nears a wall,

– the wall’s positive charges move toward dust

– the wall’s negative charges move away from dust

– the wall becomes electrically polarizedo Charged dust clings to walls and surfacesCharged dust clings to walls and surfaces

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Questions about Questions about electronic air cleanerselectronic air cleanerso What keeps dust in the air? Why doesn’t gravity What keeps dust in the air? Why doesn’t gravity

remove it?remove it?– Air resistance & terminal velocity

o If gravity can’t clear the air, is there another force If gravity can’t clear the air, is there another force around that can succeed?around that can succeed?– Yes, the electric force

o Is there an electrostatic potential energy associated Is there an electrostatic potential energy associated with electronic air cleaners?with electronic air cleaners?– Yes, Voltage is energy held by each unit of charge.

o How can dust acquire charge? Why is charged dust How can dust acquire charge? Why is charged dust attracted to walls?attracted to walls?– Corona discharge and polarization.

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Quiz QuestionQuiz QuestionYou are standing on a plastic bench that insulates You are standing on a plastic bench that insulates you from your surroundings. Both you and the you from your surroundings. Both you and the helium balloon you are holding are electrically helium balloon you are holding are electrically neutral. You now rub the balloon against your neutral. You now rub the balloon against your sweater, so that the balloon becomes negatively sweater, so that the balloon becomes negatively charged, and then let the balloon float away. You charged, and then let the balloon float away. You are left are left

a)a) with a negative electric charge. with a negative electric charge.

b)b) with no negative electrically charged particles with no negative electrically charged particles in your body. in your body.

c)c) with a positive electric charge. with a positive electric charge.

d)d) electrically neutral.electrically neutral.

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