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The Physics of Electrostatic Air Cleaners

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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 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 = m +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)
Charges in metals move to the surface and disperse from each other Applications: Shielded Rooms E = 0 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 ?
+ charged still neutral induced polarization 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: Bosons: Photons
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
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

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