Presentation on theme: "The Physics of Electrostatic Air Cleaners"— Presentation transcript:
1The Physics of Electrostatic Air Cleaners and Xerox Machines
2Preliminaries….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.
3What 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
4Electrostatic 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)
5Charges 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
6Can 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
7Problem: Undesirable Air Particles in Factories, Hospitals, etc. Solution: Use an Electrostatic Precipitator or Filter
8Application 1: Electrostatic Air Cleaners Dust particles chargednegatively in air chargedusing high voltage andcollected on positively-charged metal plates
9Application 2 : The Xerox Machine First Photocopy1938 Chester Carlson madethe prototype photocopierFirst commercial photocopier
10What 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
11Particles 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
12Metals 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)
133. 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.
14The 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
15LightCharge 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.