xerographic copiers physics 10 kenneth n. barish

Xerographic CopiersXerographic CopiersXerographic CopiersXerographic Copiers

Physics 10Physics 10

Kenneth N. BarishKenneth N. Barish

K. Barish, Xerographic Copiers 2

Introdutory Question:Introdutory Question:

If you were to cover the original If you were to cover the original document with a red transparent document with a red transparent filter, would the copier still be be filter, would the copier still be be able to produce reasonable able to produce reasonable copies?copies?

a) YESb) NO


Observations About Observations About CopiersCopiers

o Copies consist of black stuff stuck on paperCopies consist of black stuff stuck on papero After jams, the stuff sometimes wipes offAfter jams, the stuff sometimes wipes offo Copiers often run out of “toner”Copiers often run out of “toner”o Copies are often warm after being madeCopies are often warm after being madeo Copies are staticy, particular transparenciesCopies are staticy, particular transparencieso Some copies scan a light, some use a flashSome copies scan a light, some use a flash


its historyits historyo 1938 – Chester Carlson makes the 1938 – Chester Carlson makes the

first photocopy “10-22-38 Astroria”first photocopy “10-22-38 Astroria”o No one interested – rejected by IBM. No one interested – rejected by IBM.

GE (20 companies)GE (20 companies)– Battelle – non-profit helps to refine

process!o 1947 Haloid Corporation, buys patent1947 Haloid Corporation, buys patento 1949 – Haloid becomes Xerox1949 – Haloid becomes Xerox


Electric Fields Electric Fields (Part 1)(Part 1)

o Two views of charge forces:Two views of charge forces:o Charge/Charge:Charge/Charge:

– Charge 1 pushes directly on Charge 2

o Charge/Field/Charge:Charge/Field/Charge:– Charge 1 creates an “Electric Field”– Electric Field pushes on Charge 2

o Electric Fields are Electric Fields are Real!Real!


Electric Fields Electric Fields (Part 2)(Part 2)

o An electric field is a structure in An electric field is a structure in space that pushes on electric chargespace that pushes on electric charge

o The magnitude of the field is The magnitude of the field is proportional to the magnitude of the proportional to the magnitude of the force on a test chargeforce on a test charge

o The direction of the field is the The direction of the field is the direction of the force on a positive direction of the force on a positive test chargetest charge


Voltage Gradients and Voltage Gradients and Electric FieldsElectric Fields

o Objects always accelerate so as to reduce their Objects always accelerate so as to reduce their total potential energies as quickly as possibletotal potential energies as quickly as possible

o A chargeA charge– has a total potential energy proportional to

voltage– so it accelerates toward lower voltage– but it also accelerates with an electric field

o Therefore voltage gradients are electric fields!Therefore voltage gradients are electric fields!


Quantum Physics 1Quantum Physics 1o All things All things traveltravel as waves as waveso All things All things interactinteract as particles as particleso Example 1: LightExample 1: Light

– Travels as waves – electromagnetic waves– Emitted and absorbed as particles –

photonso Example 2: ElectronsExample 2: Electrons

– Detected as particles– Travel as waves


Quantum Physics 2Quantum Physics 2

o Bosons: PhotonsBosons: Photons– Many indistinguishable bosons can share a

wave– Such sharing leads to lasers &

superconductors

o Fermions: Electrons, Protons, NeutronsFermions: Electrons, Protons, Neutrons– One indistinguishable fermion allowed per

wave– “Pauli Exclusion Principle”


Electrons in SolidsElectrons in Solidso Only certain electron waves fit in a solidOnly certain electron waves fit in a solido Each allowed wave has an energy “level”Each allowed wave has an energy “level”o The electrons “occupy” levels two at a timeThe electrons “occupy” levels two at a time

– Electrons have two spin states: up and down– Spin-up is distinguishable from spin-down

o Levels are filled from lowest to highest Levels are filled from lowest to highest energyenergy

o Last (highest) filled level is the “Fermi level”Last (highest) filled level is the “Fermi level”


In and Around MetalsIn and Around Metalso Charges can move inside metalsCharges can move inside metals

– They move to minimize potential energies– They give the metal a uniform voltage– There is thus no electric field inside a

metal

o However, outside metalsHowever, outside metals– Charges can’t move so easily– so voltages can vary with location

– and there can thus be electric fields


MetalsMetals

o The Fermi level has empty levels just above itThe Fermi level has empty levels just above ito Like patrons in a partly fill theatre, electrons Like patrons in a partly fill theatre, electrons

can move in response to electric fieldscan move in response to electric fields


InsulatorsInsulatorso The Fermi level has no empty levels The Fermi level has no empty levels

nearbynearbyo Like patrons in a full theatre, electrons Like patrons in a full theatre, electrons

can’t move in response to forcescan’t move in response to forces


SemiconductorsSemiconductors

o Semiconductors are “poor insulators”Semiconductors are “poor insulators”o Valence & conduction bands have narrow gapValence & conduction bands have narrow gapo Like patrons in a theatre with a low balcony, Like patrons in a theatre with a low balcony,

electrons can hop into the balcony and moveelectrons can hop into the balcony and move


PhotoconductorsPhotoconductorso In the dark, a semiconductor is insulatingIn the dark, a semiconductor is insulating

– When polarized, it has an electric field in it

o In the light, a semiconductor may conductIn the light, a semiconductor may conduct– Will conduct if photon energy can bridge gap– Blue photons have more energy than red

photons– If conducting, electric field makes charges

move– In light, a “photoconductor” will depolarize


Corona Discharges Corona Discharges (Revisited)(Revisited)

o Outside a sharp or narrow charged metalOutside a sharp or narrow charged metal– the voltage can vary rapidly with position– the electric field is thus very strong– and it can push charges onto passing air particles

o Such charging of air is a corona dischargeSuch charging of air is a corona dischargeo The charged air allows electric charges to moveThe charged air allows electric charges to moveo Corona discharges can dissipate static electricityCorona discharges can dissipate static electricity


Copier StructureCopier Structure


the stepsthe steps

o charge photoconductorcharge photoconductoro expose to image expose to image

– where light hits (image) photoconductor will become neutral

o black toner sticks to charged areasblack toner sticks to charged areas– neutral areas white– charged areas black– Voila! – a xerox copy!


Xerographic ProcessXerographic Process


Introductory Question:Introductory Question:

If you were to cover the original If you were to cover the original document with a red transparent document with a red transparent filter, would the copier still be be filter, would the copier still be be able to produce reasonable able to produce reasonable copies?copies?

a) YESb) NO


Quiz QuestionQuiz QuestionYou have covered a grounded metal surface with a You have covered a grounded metal surface with a layer of photoconductor. Working in the dark, you layer of photoconductor. Working in the dark, you sprinkle negative charge onto this surface. If you sprinkle negative charge onto this surface. If you now expose only the left half of the now expose only the left half of the photoconductor to light, you will find that photoconductor to light, you will find that

(A)(A)the left half becomes neutral while the right the left half becomes neutral while the right half remains negatively charged.half remains negatively charged.

(B)(B)nothing happens because there is no nothing happens because there is no changing magnetic field. changing magnetic field.

(C)(C)negative charge flows from the right side of negative charge flows from the right side of the photoconductor to the left and both sides the photoconductor to the left and both sides become neutral.become neutral.

(D)(D)the right half becomes neutral while the left the right half becomes neutral while the left half remains negatively charged.half remains negatively charged.

xerographic copiers physics 10 kenneth n. barish

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