Transcript How Things Work

Xerographic Copiers 1
Xerographic Copiers
Xerographic Copiers 2
Introductory Question
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You are covered with static electricity. If you
hold a sharp pin in your hand as you reach out
for a doorknob, the pin will
A.
make the shock you’ll get worse
make the shock you’ll get weaker
leave the shock you’ll get unchanged
B.
C.
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Observations About Copiers
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Copies consist of black powder stuck on paper
After jams, the powder sometimes wipes off
Copiers run out of black powder or “toner “
Copies are often warm after being made
Copies are staticy and clingy
Some copiers scan a light, some use a flash
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3 Questions about
Xerographic Copiers
How can light arrange black powder on paper?
How does a copier spray charge on a surface?
How does a copier make its copies permanent?
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Question 1
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How can light arrange black powder on paper?
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The Xerographic Concept
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The xerographic concept
projects an image of the document onto a surface
 that has been sprayed with electric charges.
 Wherever light hits that surface, the charges escape.
 The remaining charges attracts black toner particles
 which are then attached to paper to produce a copy.
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The surface is a photoconductor,
an insulator that turns into a conductor in the light,
 so illumination allows charges to move and escape.
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Question 2
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How does a copier spray charge on a surface?
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Electric Fields (Part 1)
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There are two ways to view electric forces
The first view is charge-on-charge,
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in which charge 1 pushes directly on charge 2.
The second view is charge-field-charge,
in which charge 1 creates an electric field
 and that electric field pushes on charge 2.
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Although this electric field sounds fictional,
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it actually exists!
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Electric Fields (Part 1)
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An electric field
is a structure in space that pushes on electric charge
 and is vector in character: magnitude and direction.
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At each point in space, the electric field’s
magnitude is proportional to force on + test charge
 and direction is direction of that force.
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Voltage Gradients and
Electric Fields
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Objects always accelerate so as to reduce their
total potential energies as quickly as possible
The total potential energy of a positive charge
is proportional to the voltage at its location in space,
 so it accelerates toward decreasing voltage.
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But a charge accelerates with an electric field,
so a voltage gradient (a slope) is an electric field!
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Electric Fields
In and Around Metals
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Inside a metal, charges can move freely, so
they can move to minimize their potential energies.
 At equilibrium, the metal has a uniform voltage,
 and there is no electric field inside the metal.
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Outside a metal, charges can’t move freely, so
they cannot minimize their potential energies.
 At equilibrium, voltages can vary with location,
 and there can be an electric field outside the metal.
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Corona Discharges
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Outside a sharp or narrow metal at high voltage,
the voltage varies rapidly with position,
 so that the electric field is very strong
 and it can push charges onto passing air particles.
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This phenomenon is a corona discharge
in which the narrow metal sprays charges
 and can easily deposit or dissipate static electricity.
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Introductory Question (revisited)

You are covered with static electricity. If you
hold a sharp pin in your hand as you reach out
for a doorknob, the pin will
A.
make the shock you’ll get worse
make the shock you’ll get weaker
leave the shock you’ll get unchanged
B.
C.
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Question 3
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How does a copier make its copies permanent?
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Xerographic Process
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Copier Structure
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Summary about
Xerographic Copiers
It sprays charges from a corona discharge
Those charges precoat a photoconductor
It projects a light onto the photoconductor
The charges escape from illuminated regions
The remaining charges attract toner particles
Those particles are fused to the paper as a copy