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The Physics of Electrostatic Air Cleaners and Xerox Machines. Preliminaries…. Not all things can be explained by gravity, mechanical forces: Shocking your self by touching doorknobs, car doors. Hair-raising experiences Lighting a flourescent lamp while walking on a carpet .

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The Physics of Electrostatic Air Cleaners and Xerox Machines


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    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 experiences • Lighting a flourescent lamp while walking on a carpet Observed ‘Repulsion’ & Strength of Attraction’ Postulate: 1. Presence of ‘charges’(2 types: positive & negative) that flow from 1 object to another 2. Opposite charges attract (pull). Like charges repel (push) 3. Forces increase with decreasing separation.

    3. What we know about ‘electric charges’ • Charge is conserved • Charge is quantized in fundamental units of • e = 1.6 x 10 -19 Coulombs • Charge is intrinsic to matter: • Sub-atomic particles: electrons have q = - e • protons have q = + e • What 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 mean • it has no + charges • Normally, objects have neutral charge (equal +, - charges) • How does one normally ‘charge’ an object ? • By rubbing against a different material • Connecting to one side of a battery

    4. Electrostatic Force between two Point Charges r = 10 -10m +q1 +q2 • proportional to the magnitude of charges • inversely proportional to the square of the separation F = k q1 q2 / r2 Coulomb’s Law where 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) E = 0 • Charges in metals move to the surface and disperse from each other Applications: Shielded Rooms • Charges can discharge to the environment: • - Charges on sharp corners can leap, escape onto air molecules • - ‘Corona discharge’ – accompanied by ‘glowing’, happens • with high humidity. • - can be discharged by ‘touching’ • - Ionization: spark of arc forms Everyday Applications: Lightning Rods

    6. Can objects attract/repel even if they are neutral ? A B induced polarization neutral + charged + charged still neutral attraction ! + + + + - - Yes, the opposite charges are closer than the like charges and the effect is thus, attraction ! • Everyday examples: • neutral hair close to a charged comb • negatively-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 charged negatively in air charged using high voltage and collected on positively- charged metal plates

    9. Application 2 : The Xerox Machine First Photocopy 1938 Chester Carlson made the prototype photocopier First commercial photocopier

    10. + + What are Electric Fields ? Two Ways of Viewing Charge – Charge Interactions: q2 q1 1. Charge q1 feels a force due to charge q2. 2. Charge q1 feels a force due its interaction with an electric field E set up by charge q2. E – magnitude proportional to the generating charge q2 direction at a point is in the direction of the force felt by a unit + test charge at point + + q1 E q2

    11. Particles in Nature Fermions: electrons, protons, neutrons - 1 indistinguishable fermion/wave - follow’s Pauli’s exclusion principle Bosons: Photons - indistinguishable bosons can share waves Applications: lasers, superconductors Let’s look at electrons flowing in solids • travel like waves in a solid, w/ specific energy levels • occupy each level two at a time: Spin up and spin down electrons • levels filled from lowest to highest energy • levels 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 levels • Analogy of electron flow in metals: • Like guests in a partly-filled 1-floor theatre, electrons readily • move, responding to applied electric fields Energy Fermi level (ground floor) Energy filled level vacant level 2 Insulators – no empty levels near Fermi level Analogy: Ground floor is full. High balcony. electrons can’t respond to forces Conduction bands (high balcony) Valence band: filled (ground floor)

    13. 3. Semiconductors – narrow gap between valence & conduction bands; poor insulators/conductors at room temp. Analogy: Guests in a theatre with low balcony Conduction band (low balcony) Energy (light or Heat) Valence band Electrons can hop into the low ‘balcony’ and move. (gap is smaller) • Application: Photoconductors in Xerox Machines • insulating in the dark • conducting in the light: Light in the form of photons, give • energy for electrons to bridge gap.

    14. - - - - - - - - - - + + + + + + + + - - - - - - - - - - - + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + The Xerox Process - Corona discharge photoconductor Photoconductor is coated with negative charge Light Light Exposure to light from original erases charge to form a charge image. charge image toner particles Charge image attracts + charged toner particles

    15. - - - - - - - - - - - - - - - - - - - - - Light Charge image is erased to release the toner particles Negatively charged paper - - - - - - - - - - - - The toner is transferred to negatively- charged paper - - - - - - - - - - - - Heat Toner is fused to paper by heat. Copy is now done. Cycle is then repeated.

    16. Inside a Photocopier