Photocopiers
Day 3:
Photocopiers:
Photoconductors
Reminders/Updates:
Lab today and Wed.
Print out script, complete prelab in advance
HW 2 available on D2L
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What to take to lab 1
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Completed pre-lab sheet
Lab script – read through in advance
Textbook and lecture notes
Calculator
Pen
• Pre-lab and script can be downloaded from
course website
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Current conventions
-
- Direction of ‘conventional’ current flow
- Assumes that current is carried by
positively charged particles
- Convention has stuck even though we
now know that electrons are negative
+
- Direction of electron motion
- Electrons are the real charge carriers in a metal
- Electrons are negatively charged
Summary of some important ideas
1. Current is conserved (electrons don’t disappear)
2. Change in V over circuit = V of battery, or energy source
3. V= I R (Ohm’s law)
- useful for whole circuit (R total, Vtotal, give I total)
- or individual component (e.g. Rbulb, Vbulb give I bulb),
…….Be careful about what R, and V are.
4. P = I V = I (IR) = I2R
= (V/R)V = V2/R
power dissipated across object R
5. Resistors in series:
Resistances add: Rtot = R1 + R2
Current through all resistors is the same
6. Resistors in parallel:
Voltage drop across parallel legs of circuit is same
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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. Coat a photoconducting sheet with a layer of negative charge
2. Use light to selectively remove some charge, creating a charge image of
original
3. Toner sticks to charged areas only, creating a visible image of original
4. Transfer toner to new piece of paper
The big challenge- how to selectively remove charge using light ?
5
Photocopiers – a brief history
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Chester Carlson says: there must be a way to copy things!
– Recopying and photography too expensive and time-consuming.
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1938 Chester Carlson produces first xerographic image in his lab in Astoria, Queens.
10-22-38 ASTORIA.
– Materials:
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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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3
Photocopiers – the top selling industrial product of all time
“Photocopying” - step 1:
-
-
- -
----------------------------------------------------------------------------------------------------
Copper
Overhead
- electron
Transparencies
- - comb
(insulator)
-- - - -
When I drag the copper comb across the transparencies:
a. Negative charges flow through the transparencies and
into the floor,
b. Nothing.
c. Negative charges will build up on the transparencies
just in the areas where the comb touches
d. Negative charges will be everywhere on the
transparencies, even the parts I don’t touch with the
comb.
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4
“Photocopying” step 2:
What will happen if I draw on the plate with my finger?
------------------------------------------------------------a. Nothing will happen
b. The entire plate will lose it’s negative charge.
c. 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
What will happen if I draw on the plate with my finger?
--------------------------------------------------- - - - - - - - - --
HI
Chalk dust sticks to places with negative charge.
Chalk does not stick to neutralized part of transparencies.
Chalk only sticks to places with negative charge because the chalk is
a. positively charged,
b. negatively charged,
c. uncharged (neutral),
d. both a and c could be correct
e. both b and c could be correct.
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Photocopying expts - summary
We have demonstrated some important steps inside a photocopier:
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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?
Photocopying 1:
Making a Charge Image
Photoconductor
Grounded metal
Charge photoconductor
Corona wire
velocity
+++
+++++
Document
Selective
discharge
Light
reflected
off
document
+++++
Charge image
+++++
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Photocopying 2:
Transferring Charge Image to Toner and Paper
Attract toner to
charge image
Roller and
brush
toner
+ ++ +
++ + +
+
++ +
+
+ +++ + +
Black image
Positive
toner particle
+++++
Charge image
+
+ + +
+
Release toner
Light
+
+
- - -++ + +
+++
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Photocopying 2:
Transferring Charge Image to Toner and Paper
Charge Paper
Toner
attracted to
paper
+
+
+ + +
+
+ ++
+
+ ++
Heat
+ ++
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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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 charged particles might bump into
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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!
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)
particle
wave
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Atomic structure
If the nucleus was the size of a ping-pong ball, how big
would the circumference of the atom be?
A) Size of desk
B) Size of physics building
C) Size of Campus
D) Size of Boulder
E) Size of USA
Atomic structure of solids and energy bands
many atoms
Energy
one atom
Discreet energy levels
for electrons
Electron energy levels get
shifted and shared between
all atoms and 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
Electron energy
Higher energy levels
are empty
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2
Lower energy levels –
mostly full of electrons
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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
Conductor:
empty levels
very close
Insulator:
Big jump
to empties.
Semiconductor:
Half way in between.
Small jump to empties
empty
gap- no levels
empty
full
electron like ball
rolling on almost
flat ground
moves easily
empty
Small gap
full
Electron
like a ball
in pit.
Can’t move
without big boost.
full
- Electron like ball
in shallow pit.
- Small boost
required to move.
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