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Xerographic Copiers Physics 10 Kenneth N. Barish.

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1 Xerographic Copiers Physics 10 Kenneth N. Barish

2 K. Barish, Xerographic Copiers 2 Introdutory Question: If you were to cover the original document with a red transparent filter, would the copier still be be able to produce reasonable copies? a)YES b)NO

3 K. Barish, Xerographic Copiers 3 Observations About Copiers oCopies consist of black stuff stuck on paper oAfter jams, the stuff sometimes wipes off oCopiers often run out of “toner” oCopies are often warm after being made oCopies are staticy, particular transparencies oSome copies scan a light, some use a flash

4 K. Barish, Xerographic Copiers 4 its history o1938 – Chester Carlson makes the first photocopy “ Astroria” oNo one interested – rejected by IBM. GE (20 companies) –Battelle – non-profit helps to refine process! o1947 Haloid Corporation, buys patent o1949 – Haloid becomes Xerox

5 K. Barish, Xerographic Copiers 5 Electric Fields (Part 1) oTwo views of charge forces: oCharge/Charge: –Charge 1 pushes directly on Charge 2 oCharge/Field/Charge: –Charge 1 creates an “Electric Field” –Electric Field pushes on Charge 2 oElectric Fields are Real!

6 K. Barish, Xerographic Copiers 6 Electric Fields (Part 2) oAn electric field is a structure in space that pushes on electric charge oThe magnitude of the field is proportional to the magnitude of the force on a test charge oThe direction of the field is the direction of the force on a positive test charge

7 K. Barish, Xerographic Copiers 7 Voltage Gradients and Electric Fields oObjects always accelerate so as to reduce their total potential energies as quickly as possible oA charge –has a total potential energy proportional to voltage –so it accelerates toward lower voltage –but it also accelerates with an electric field oTherefore voltage gradients are electric fields!

8 K. Barish, Xerographic Copiers 8 Quantum Physics 1 oAll things travel as waves oAll things interact as particles oExample 1: Light –Travels as waves – electromagnetic waves –Emitted and absorbed as particles – photons oExample 2: Electrons –Detected as particles –Travel as waves

9 K. Barish, Xerographic Copiers 9 Quantum Physics 2 oBosons: Photons –Many indistinguishable bosons can share a wave –Such sharing leads to lasers & superconductors oFermions: Electrons, Protons, Neutrons –One indistinguishable fermion allowed per wave –“Pauli Exclusion Principle”

10 K. Barish, Xerographic Copiers 10 Electrons in Solids oOnly certain electron waves fit in a solid oEach allowed wave has an energy “level” oThe electrons “occupy” levels two at a time –Electrons have two spin states: up and down –Spin-up is distinguishable from spin-down oLevels are filled from lowest to highest energy oLast (highest) filled level is the “Fermi level”

11 K. Barish, Xerographic Copiers 11 In and Around Metals oCharges 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 oHowever, outside metals –Charges can’t move so easily –so voltages can vary with location –and there can thus be electric fields

12 K. Barish, Xerographic Copiers 12 Metals oThe Fermi level has empty levels just above it oLike patrons in a partly fill theatre, electrons can move in response to electric fields

13 K. Barish, Xerographic Copiers 13 Insulators oThe Fermi level has no empty levels nearby oLike patrons in a full theatre, electrons can’t move in response to forces

14 K. Barish, Xerographic Copiers 14 Semiconductors oSemiconductors are “poor insulators” oValence & conduction bands have narrow gap oLike patrons in a theatre with a low balcony, electrons can hop into the balcony and move

15 K. Barish, Xerographic Copiers 15 Photoconductors oIn the dark, a semiconductor is insulating –When polarized, it has an electric field in it oIn 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

16 K. Barish, Xerographic Copiers 16 Corona Discharges (Revisited) oOutside 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 oSuch charging of air is a corona discharge oThe charged air allows electric charges to move oCorona discharges can dissipate static electricity

17 K. Barish, Xerographic Copiers 17 Copier Structure

18 K. Barish, Xerographic Copiers 18 the steps ocharge photoconductor oexpose to image –where light hits (image) photoconductor will become neutral oblack toner sticks to charged areas –neutral areas  white –charged areas  black –Voila! – a xerox copy!

19 K. Barish, Xerographic Copiers 19 Xerographic Process

20 K. Barish, Xerographic Copiers 20 Introductory Question: If you were to cover the original document with a red transparent filter, would the copier still be be able to produce reasonable copies? a)YES b)NO

21 K. Barish, Xerographic Copiers 21 Quiz Question You have covered a grounded metal surface with a layer of photoconductor. Working in the dark, you sprinkle negative charge onto this surface. If you now expose only the left half of the photoconductor to light, you will find that (A)the left half becomes neutral while the right half remains negatively charged. (B)nothing happens because there is no changing magnetic field. (C)negative charge flows from the right side of the photoconductor to the left and both sides become neutral. (D)the right half becomes neutral while the left half remains negatively charged.


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