Xerographic Copiers

1. Xerographic Copiers Physics 10 Kenneth N. Barish

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? • YES • NO

3. Observations About Copiers • Copies consist of black stuff stuck on paper • After jams, the stuff sometimes wipes off • Copiers often run out of “toner” • Copies are often warm after being made • Copies are staticy, particular transparencies • Some copies scan a light, some use a flash

4. its history • 1938 – Chester Carlson makes the first photocopy “10-22-38 Astroria” • No one interested – rejected by IBM. GE (20 companies) • Battelle – non-profit helps to refine process! • 1947 Haloid Corporation, buys patent • 1949 – Haloid becomes Xerox

5. Electric Fields (Part 1) • Two views of charge forces: • Charge/Charge: • Charge 1 pushes directly on Charge 2 • Charge/Field/Charge: • Charge 1 creates an “Electric Field” • Electric Field pushes on Charge 2 • Electric Fields are Real!

6. Electric Fields (Part 2) • An electric field is a structure in space that pushes on electric charge • The magnitude of the field is proportional to the magnitude of the force on a test charge • The direction of the field is the direction of the force on a positive test charge

7. Voltage Gradients and Electric Fields • Objects always accelerate so as to reduce their total potential energies as quickly as possible • A charge • has a total potential energy proportional to voltage • so it accelerates toward lower voltage • but it also accelerates with an electric field • Therefore voltage gradients are electric fields!

8. Quantum Physics 1 • All things travel as waves • All things interact as particles • Example 1: Light • Travels as waves – electromagnetic waves • Emitted and absorbed as particles – photons • Example 2: Electrons • Detected as particles • Travel as waves

9. Quantum Physics 2 • Bosons: Photons • Many indistinguishable bosons can share a wave • Such sharing leads to lasers & superconductors • Fermions: Electrons, Protons, Neutrons • One indistinguishable fermion allowed per wave • “Pauli Exclusion Principle”

10. Electrons in Solids • Only certain electron waves fit in a solid • Each allowed wave has an energy “level” • The electrons “occupy” levels two at a time • Electrons have two spin states: up and down • Spin-up is distinguishable from spin-down • Levels are filled from lowest to highest energy • Last (highest) filled level is the “Fermi level”

11. In and Around Metals • Charges 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 • However, outside metals • Charges can’t move so easily • so voltages can vary with location • and there can thus be electric fields

12. Metals • The Fermi level has empty levels just above it • Like patrons in a partly fill theatre, electrons can move in response to electric fields

13. Insulators • The Fermi level has no empty levels nearby • Like patrons in a full theatre, electrons can’t move in response to forces

14. Semiconductors • Semiconductors are “poor insulators” • Valence & conduction bands have narrow gap • Like patrons in a theatre with a low balcony, electrons can hop into the balcony and move

15. Photoconductors • In the dark, a semiconductor is insulating • When polarized, it has an electric field in it • In 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. Corona Discharges (Revisited) • Outside 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 • Such charging of air is a corona discharge • The charged air allows electric charges to move • Corona discharges can dissipate static electricity

17. Copier Structure

18. the steps • charge photoconductor • expose to image • where light hits (image) photoconductor will become neutral • black toner sticks to charged areas • neutral areas  white • charged areas black • Voila! – a xerox copy!

19. Xerographic Process

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? • YES • NO

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 • the left half becomes neutral while the right half remains negatively charged. • nothing happens because there is no changing magnetic field. • negative charge flows from the right side of the photoconductor to the left and both sides become neutral. • the right half becomes neutral while the left half remains negatively charged.