A brief history

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Photocopiers
Reminders/Updates HW 1 due NOW! Phys 1010 notes
on website All info is on web (I think)
Day 3 Finish electric circuits Photocopiers
Bloomfield 10.2 Photoconductors (Blm 12.1
p.427-433).
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Photocopiers Static electricity and
semiconductors Learning goals 1. Explain how you
could write on paper with photoconductor, toner,
light, high voltage wire. (basic design of
copier) 2. Be able to explain why materials are
conductors, insulators, or semiconductors in
terms of energy levels and electron motion. 3.
Explain how heat or light changes resistance of
semiconductor.
Big picture 1. Coulomb attraction of
positively charged ink particles (toner) to
negatively charged surface. 2. Negative charges
on surface distributed to match image to be
copied. 3. Control charge distribution by shining
light on surface, where light hits, negative
charges leave, so no ink will stick.
The big challenge- how to get charges to leave
when light shines on surface?
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Photocopiers a brief history

• Chester Carlson says there must be a way to
  copy things!
• Recopying and photography too expensive and
  time-consuming.
• 1938 Chester Carlson produces first xerographic
  image in his lab in Astoria, Queens. 10-22-38
  ASTORIA.
• Materials
• Sulfer,
• handkerchief,
• bright light,
• spoors from a club moss lycopodium powder
• Wax paper and heat

• Turned down by 20 companies
• 1959 Launches the Xerox 914, the first automatic,
  plain-paper office copier--which becomes the
  top-selling industrial product of all time.
• http//en.wikipedia.org/wiki/Chester_Carlson
• http//en.wikipedia.org/wiki/Xerox_914

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Photocopying - step 1
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Overhead Transparencies (insulator)
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Copper electron comb
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• When I drag the copper comb across the
  transparencies
• Negative charges flow through the transparencies
  and into the floor,
• Nothing.
• Negative charges will build up on the
  transparencies just in the areas where the comb
  touches
• Negative charges will be everywhere on the
  transparencies, even the parts I dont touch with
  the comb.

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Photocopying step 2
What will happen if I draw on the plate with my
finger?

1. Nothing will happen
2. The entire plate will lose its negative charge.
3. Only the areas of the plate that I touch will
   lose their negative charge.

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Photocopying step 3 Toner sticking to charged
surface
HI

• Chalk only sticks to places with negative charge
  because the chalk is
• positively charged,
• negatively charged,
• uncharged (neutral),
• both a and c are correct
• both b and c are correct.

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Photocopying expts - summary

• We have demonstrated some important steps inside
  a photocopier
• Charging an insulating sheet, discharge certain
  areas
• Make a charge image
• Apply chalk/toner
• Electrostatic attraction produces visible image
• But where does light (the photo bit)
  come in?

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Photocopying 1 Making a Charge Image
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Document
Light reflected off document
Selective discharge


Charge image


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Photocopying 2 Transferring Charge Image to
Toner and Paper





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Photocopying 2 Transferring Charge Image to
Toner and Paper
Straightforward. All just good old physics of
attraction between opposite charges, plus
photoconductor physics.
Copy
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Semi-conductor physics
Photoconductor at heart of photocopier -
Behaves like an insulator when in the dark -
Where light hits, R low, electrons flow away, -
When add toner, sticks only where charges are
left.
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Photoconductor - Very special material
insulator except when light hits. - One type of
semiconductor First have to understand what
determines resistance of a material a)
insulators (wood, ceramic, plastic)- very high
resistance. b) conductors (metals)- very low
resistance c) Semiconductors - in the middle.
Resistance depends on temp., light, cleanliness.
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What determines resistance of a material

• - Charged particles (almost always electrons)
  carry
• current inside materials
• - Resistance of material depends on
• a) Number of charged particles that are
  free to move and carry
• current
• b) Number of obstacles that charge carriers might
  bump into

Can be controlled in a semiconductor At the heart
of all modern electronics!
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Atomic structure

• Nucleus
• Protons and neutrons
• Positively charged
• Very small and dense

Electron cloud - Negatively charged - MUCH larger
than nucleus

• Quantum Mechanics
• - Weird physics of very small things (like
  electrons)
• - Electrons behave as both particles and waves
• As waves, they can only move in certain ways and
  have certain
• amounts of energy
• - Only 2 electrons per energy level (Pauli
  Exclusion Principle)

wave
particle
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Atomic structure of solids and energy bands
many atoms
one atom
Energy
Electron energy levels get shifted and shared
between all atoms and electrons
Discreet energy levels for electrons
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In solid, billions of atoms, electrons, and
energy levels!!

• Levels get shifted and shared for all
• atoms and electrons
• So many individual levels, just talk
• about bands of levels.
• 2 electrons per level until run out of
• electrons

Higher energy levels are empty
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Electron energy
Lower energy levels mostly full of electrons
2
1
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Microscopic look at different materials
Conduction rule For electrons to move (when a
voltage is applied) there must be an empty energy
level immediately above them
Insulator Big jump to empties.
Conductor empty levels very close
Semiconductor Half way in between. Small jump to
empties
empty
gap- no levels
empty
empty
Small gap
full
full
full
electron like ball rolling on almost flat ground
Electron like a ball in pit.

• Electron like ball
• in shallow pit.
• Small boost
• required to move.

Cant move without big boost.
moves easily
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Which band structure goes with which material?
DDiamond CCopper GGermanium
(semi-conductor)
a. Dw, Cx, Gy b. Dz, Cw, Gy c. Dz,
Cy, Gx d. Dy, C w, Gy. e. Dw, Cx, Gy
25 eV
Energy
y
w
x
z
0
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Review of semiconductors.
empty
Like ball stuck in shallow pit
Little gap
full
e

• Small gap between filled and empty energy
  levels.
• Gap is big enough to stop current (high R).
• Gap is small enough so humans can find ways to
  boost electrons up.
• Concept behind all modern electronics!
• Use semiconductors to control electric currents.
  

Q Which will decrease resistance of a
semiconductor? add heat, light,
shake, add dirt a. T T
T T b T
T F F c T
T F T d
F T F F e
F T F
T
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Photo-conductor Kick electron to empty levels
with light
empty
e
Little gap
E energy of photon h x frequency (h?)
h x c / wavelength (hc/?)
full
Q If green light has just enough energy to
excite electron into empty level and allow it to
move (conduct electricity) a. both blue and red
light will also make it a conductor b. blue light
will, red light will not c. neither blue nor red
light will make it into a conductor
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Photocopier summary

• Spread charge on photoconductor
• Use light to selectively remove some charge and
  make a charge image of original
• Electrostatic attraction sticks toner to regions
  where charge remains
• Use heat to bond toner to new piece of paper
• Copying is complete!

We will return to semi-conductors and their use
in electronics in a couple of weeks