Presentation on theme: "The Physics of Electrostatic Air Cleaners"— Presentation transcript:
1 The Physics of Electrostatic Air Cleaners and Xerox Machines
2 Preliminaries….Not all things can be explained by gravity, mechanical forces:Shocking your self by touching doorknobs, car doors.Hair-raising experiencesLighting a flourescent lamp while walking on a carpetObserved ‘Repulsion’ & Strength of Attraction’Postulate:1. Presence of ‘charges’(2 types: positive & negative)that flow from 1 object to another2. Opposite charges attract (pull). Like charges repel (push)3. Forces increase with decreasing separation.
3 What we know about ‘electric charges’ Charge is conservedCharge is quantized in fundamental units ofe = 1.6 x CoulombsCharge is intrinsic to matter:Sub-atomic particles: electrons have q = - eprotons have q = + eWhat does it mean to have a net charge ?Net charge is the sum of an object’s +,- charges.Just because an object is negatively-charged doesn’t meanit has no + chargesNormally, objects have neutral charge (equal +, - charges)How does one normally ‘charge’ an object ?By rubbing against a different materialConnecting to one side of a battery
4 Electrostatic Force between two Point Charges r = m+q1+q2proportional to the magnitude of chargesinversely proportional to the square of the separationF = k q1 q2 / r2Coulomb’s Lawwhere k = 9 x109 N-m2/C2 (Coulomb’s constant)Electrostatic force is much stronger than gravitational force!Example:F(electrostatic between 2 electrons)= (9x109N-m2/C2)(1.6x10-19C) )(1.6x10-19C)/(10-10m)2= 23 x 10-9 N (electrostatic)Fgravitational= (6.67x10-11 N-m2/kg2)(9.1x10-31kg)2/(10-10m)2= 55 x 10-52N (grav)
5 Charges in Conductors (metals) Charges in metals move to the surface and disperse from each otherApplications: Shielded RoomsE = 0Charges can discharge to the environment:- Charges on sharp corners can leap, escape onto air molecules- ‘Corona discharge’ – accompanied by ‘glowing’, happenswith high humidity.- can be discharged by ‘touching’- Ionization: spark of arc formsEveryday Applications:Lightning Rods
6 Can objects attract/repel even if they are neutral ? + chargedstill neutralinducedpolarizationattraction !+-+Yes, the opposite charges are closer than the like chargesand the effect is thus, attraction !Everyday examples:neutral hair close to a charged combnegatively-charged dust sticking to a neutral wall or surface
7 Problem: Undesirable Air Particles in Factories, Hospitals, etc. Solution: Use an Electrostatic Precipitator or Filter
8 Application 1: Electrostatic Air Cleaners Dust particles chargednegatively in air chargedusing high voltage andcollected on positively-charged metal plates
9 Application 2 : The Xerox Machine First Photocopy1938 Chester Carlson madethe prototype photocopierFirst commercial photocopier
10 What are Electric Fields ? Two Ways of Viewing Charge – Charge Interactions:q2q11. Charge q1 feels a force due to charge q2.+2. Charge q1 feels a force due its interaction withan electric field E set up by charge q2.E – magnitude proportional to the generating charge q2direction at a point is in the direction of the force feltby a unit + test charge at point++q1Eq2
11 Particles in Nature Fermions: Bosons: Photons electrons, protons, neutrons- 1 indistinguishable fermion/wave- follow’s Pauli’s exclusion principleBosons: Photons- indistinguishable bosonscan share wavesApplications:lasers, superconductorsLet’s look at electrons flowing in solidstravel like waves in a solid, w/ specific energy levelsoccupy each level two at a time: Spin up and spin down electronslevels filled from lowest to highest energylevels form ‘bands’:valence band (highest level is Fermi level)beyond valence band is conduction band
12 Metals vs. Insulators vs. Semiconductors Metals - have empty levels above Fermi energy levelsAnalogy of electron flow in metals:Like guests in a partly-filled 1-floor theatre, electrons readilymove, responding to applied electric fieldsEnergyFermi level(ground floor)Energyfilled levelvacant level2 Insulators – no empty levels near Fermi levelAnalogy: Ground floor is full. High balcony.electrons can’t respond to forcesConduction bands(high balcony)Valence band: filled(ground floor)
13 3. Semiconductors – narrow gap between valence & conduction 3. Semiconductors – narrow gap between valence & conduction bands; poor insulators/conductors at room temp.Analogy: Guests in a theatre with low balconyConduction band(low balcony)Energy(light orHeat)Valence bandElectrons can hop into the low ‘balcony’ and move.(gap is smaller)Application: Photoconductors in Xerox Machinesinsulating in the darkconducting in the light: Light in the form of photons, giveenergy for electrons to bridge gap.
14 The Xerox Process - Corona discharge - - - - - - - - - - photoconductorPhotoconductor is coatedwith negative chargeLightLight- -Exposure to light fromoriginal erases charge toform a charge image.charge imagetoner particlesCharge image attracts+ charged toner particles
15 LightCharge image is erased torelease the toner particlesNegatively chargedpaperThe toner is transferred to negatively-charged paperHeatToner is fused to paper by heat.Copy is now done.Cycle is then repeated.