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Xerographic Copiers 1 Xerographic Copiers 2 Introductory Question Xerographic Copiers 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. B. C. make the shock you’ll get worse make the shock you’ll get weaker leave the shock you’ll get unchanged Turn off all electronic devices Xerographic Copiers 3 Xerographic Copiers 4 Observations About Copiers Copiers consume colored powder or “toner” After jams, you can sometimes wipe off the powder images Copies are often warm after being made Copies are sometimes clingy with static electricity Xerographic Copiers 5 1. 2. 3. How can light arrange colored powder on paper? How does a copier spray charge onto a surface? How does a copier make its copies permanent? Xerographic Copiers 6 Question 1 Q: How can light arrange colored powder on paper? A: That light can control static electricity. In a xerographic copier or printer, 3 Questions about Xerographic Copiers charge is sprayed onto an insulating layer opposite charge flows onto the layer’s back the layer acts as a charged capacitor light selectively erases the separated charge the remaining charge attracts toner particles the toner particles are then bonded to paper Question 2 Q: How does a copier spray charge onto a surface? A: It uses a corona discharge to charge the air A fine wire having a large voltage (either or ) The repulsive forces are so intense, they push charges into the air is covered with tightly packed “like” charges the charges are ferried by air particles (atoms, molecules, or even dust) this flow of charge into the air is a corona discharge That discharge is caused by a strong electric field 1 Xerographic Copiers 7 Xerographic Copiers 8 Electric Field Two views of electrostatic forces: Charge1 pushes on Charge2 Charge1 creates electric field that pushes Charge2 Electric field isn’t a fiction; it actually exists! Voltage Gradient A + test charge accelerates along a structure in space and time that pushes on charge a vector field: a vector at each point in space and time observed using a test charge at each point Voltage is electrostatic potential energy per charge Inside a metal, charge can move At equilibrium: voltage is uniform, electric field is zero Charge resides only on the metal’s surface Outside a metal, charge cannot move At equilibrium: both voltage and electric field can vary In the space near a thin wire or sharp point at large voltage, electric field points in the direction opposite the voltage gradient Xerographic Copiers 10 Metals, Fields, & Corona Discharges decreasing voltage is decreasing electrostatic potential energy the path of quickest electrostatic potential decrease is (–) voltage gradient voltage gradient is essentially the (rising) slope of voltage A voltage gradient is an electric field Xerographic Copiers 9 the electric field at the charge’s position the path that reduces the charge’s total potential energy quickest 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. B. C. make the shock you’ll get worse make the shock you’ll get weaker leave the shock you’ll get unchanged voltage varies rapidly with distance, so big electric field charge is pushed into the air: a corona discharge Xerographic Copiers 11 Xerographic Copiers 12 Question 3 Q: How does a copier make its copies permanent? A: It fuses or melts the powder onto the paper. Summary about Xerographic Copiers It sprays charge from a corona discharge That charge precoats a special insulating surface It projects a light onto surface The charge escapes from illuminated regions The remaining charge attracts toner particles Those particles are fused to the paper as a copy 2