Xerographic Copiers 1 Xerographic Copiers 2 Observations About Copiers Observations About Copiers � Copies consist of colored powder stuck to paper Copies consist of colored powder stuck to paper Xerographic Copiers Xerographic Copiers � After jams, the powder sometimes wipes off After jams, the powder sometimes wipes off � Copiers run out of colored powder or “toner” Copiers run out of colored powder or “toner” � Copies are often warm after being made Copies are often warm after being made � Copies tend to be Copies tend to be staticy staticy and clingy and clingy � Some copiers scan a light, some use a flash Some copiers scan a light, some use a flash Xerographic Copiers 3 Xerographic Copiers 4 3 Questions about 3 Questions about Question 1 Question 1 Xerographic Copiers Xerographic Copiers � How can light arrange colored powder on paper? How can light arrange colored powder on paper? � How can light arrange colored powder on paper? How can light arrange colored powder on paper? � How does a copier spray charge onto a surface? How does a copier spray charge onto a surface? � � How does a copier make its copies permanent? How does a copier make its copies permanent? � Xerographic Copiers 5 Xerographic Copiers 6 The Xerographic Concept The Xerographic Concept Question 2 Question 2 � The xerographic concept The xerographic concept � How does a copier spray charge onto a surface? How does a copier spray charge onto a surface? � sprays electric charge onto a surface and sprays electric charge onto a surface and � projects an image of the document onto that surface. projects an image of the document onto that surface. � Wherever light hits the surface, the charge escapes. Wherever light hits the surface, the charge escapes. g g , , g g p p � The remaining charge attracts colored toner particles The remaining charge attracts colored toner particles � which are then bonded to paper to produce a copy. which are then bonded to paper to produce a copy. � The surface is a photoconductor, The surface is a photoconductor, � an insulator that turns into a conductor in the light, an insulator that turns into a conductor in the light, � so illumination allows charge to move and escape. so illumination allows charge to move and escape. 1
Xerographic Copiers 7 Xerographic Copiers 8 Electric Fields Electric Fields (Part 1) Electric Fields Electric Fields (Part 2) (Part 1) (Part 2) � Consider these two views of electric forces: Consider these two views of electric forces: � An electric field An electric field � � is a structure in space that pushes on electric charge is a structure in space that pushes on electric charge � The first view is The first view is charge charge- -on on- -charge charge � � is vector in character: it has magnitude and direction is vector in character: it has magnitude and direction � Charge Charge 1 pushes directly on Charge pushes directly on Charge 2 . . � m � may depend on position and time m may depend on position and time d p nd n p d p nd n p iti n nd tim iti n nd tim � The second view is The second view is charge charge- -electric field electric field- -charge charge � The electric field at a given position and time The electric field at a given position and time � Charge Charge 1 creates an electric field creates an electric field � is proportional to the force on a + test charge is proportional to the force on a + test charge � That electric field pushes on Charge That electric field pushes on Charge 2 . . � is often represented graphically by an arrow is often represented graphically by an arrow � This electric field isn’t a fiction; it actually exists! This electric field isn’t a fiction; it actually exists! � but is actually located at just one point on that arrow but is actually located at just one point on that arrow Xerographic Copiers 9 Xerographic Copiers 10 Electric Fields and… Electric Fields and… …Voltage Gradients …Voltage Gradients � Consider these two views of electric forces: Consider these two views of electric forces: � The second view is The second view is voltage gradient voltage gradient- -on on- -charge charge � The first view is The first view is electric field electric field- -on on- -charge charge � An object accelerates to reduce potential energy (PE) An object accelerates to reduce potential energy (PE) � so it experiences a force down the “gradient” of PE so it experiences a force down the “gradient” of PE � An electric field exerts a force on a + test charge An electric field exerts a force on a + test charge � + test charge has electrostatic potential energy (EPE) � + t t h r + test charge has electrostatic potential energy (EPE) + t t h r h h l l tr tr t ti p t nti l n r t ti p t nti l n r (EPE) (EPE) � + test charge’s EPE is proportional to its voltage + test charge’s EPE is proportional to its voltage � + test charge experiences force down voltage gradient + test charge experiences force down voltage gradient � A voltage gradient exerts a force on a + test charge A voltage gradient exerts a force on a + test charge � A voltage gradient A voltage gradient is is an electric field! an electric field! Xerographic Copiers 11 Xerographic Copiers 12 Electric Fields Electric Fields Corona Discharges Corona Discharges In and Around Metals In and Around Metals � Inside a metal, charges can move freely Inside a metal, charges can move freely � Outside a sharp or narrow metal at high voltage, Outside a sharp or narrow metal at high voltage, � and can rearrange to minimize their EPE. and can rearrange to minimize their EPE. � the voltage varies rapidly with position, the voltage varies rapidly with position, � At equilibrium, the metal has a uniform voltage, At equilibrium, the metal has a uniform voltage, � so that the electric field is very strong so that the electric field is very strong � nd th r i n � and there is no electric field inside the metal. and there is no electric field inside the metal. nd th r i n l l tri fi ld in id th m t l tri fi ld in id th m t l � and it can push charges onto passing air particles. � nd it and it can push charges onto passing air particles. nd it n p n p h h r h h r nt p nt p in in ir p rti l ir p rti l � Outside a metal, charges can’t move freely, Outside a metal, charges can’t move freely, � This phenomenon is a corona discharge This phenomenon is a corona discharge � so they cannot rearrange to minimize their EPE. so they cannot rearrange to minimize their EPE. � in which the narrow metal sprays charges in which the narrow metal sprays charges � At equilibrium, voltages can vary with location, At equilibrium, voltages can vary with location, � and can easily deposit or dissipate static electricity. and can easily deposit or dissipate static electricity. � and there can be an electric field outside the metal. and there can be an electric field outside the metal. 2
Xerographic Copiers 13 Xerographic Copiers 14 Question 3 Question 3 Xerographic Process Xerographic Process � How does a copier make its copies permanent? How does a copier make its copies permanent? Xerographic Copiers 15 Xerographic Copiers 16 Summary about Summary about Copier Structure Copier Structure Xerographic Copiers Xerographic Copiers � It sprays charge from a corona discharge It sprays charge from a corona discharge � That charge That charge precoats precoats a photoconductor a photoconductor � It projects a light onto the photoconductor It projects a light onto the photoconductor � The charge escapes from illuminated regions The charge escapes from illuminated regions � The remaining charge attract toner particles The remaining charge attract toner particles � Those particles are fused to the paper as a copy Those particles are fused to the paper as a copy 3
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