What is the foundation of most modern technologies: phones, computers etc? This message was made with 100% recycled electrons Day 3: Questions? Finish circuit review Begin Copiers: Bloomfield 10.2 Photoconductors (Blm 12.1). Reminders/Updates: HW anyone start? Lab Starts this week! All info is on web (I think) 1
Reading Quiz: SILENT!!!! (45 seconds / question) 1. If I know the amount of voltage across a flashlight battery, I can always calculate the amount of electrical power the flashlight is consuming, no matter what else I know or do not know about the circuit. a. true b. false 2. If you could not measure the resistance, which property would be most useful to know in guessing whether an unknown material was a conductor or an insulator? 3. A photoconductor is: 2
1. Compare the brightness of identical bulbs, bulb 1 compared to bulb 2? a. brighter, b. dimmer, c. same 1.5 V + 1 2
2. How will brightness of bulb 1 in left circuit compare to brightness of the same bulb in the right circuit (no #2)? a. brighter, b. dimmer, c. same 1.5 V + 1.5 V + 1 2 1
case 1 3. All bulbs(r) and batteries (V) are identical. How does current flow through circuit in case 1 compare to case 2? a. Current in case 2 is 1/4 of current in case 1 b. Current in case 2 is 1/2 of current in case 1 c. Current in case 2 is same as current in case 1 d. Current in case 2 is 2 times current in case 1 e. Current in case 2 is 4 times current in case 1 case 2 + Bulb 2 Bulb 3
4. When close switch battery life a. same as when open b. longer than when open c. much shorter
Photocopiers learning goals (see web, abbreviated here) 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? 7
Photocopiers A brief history Chester Carlson says: there must be a way to copy things! Recopying and photography too expensive and timeconsuming. Others existed ditto, etc. 1938 Chester Carlson produces first xerographic image in his lab in Astoria, Queens. 10 2238 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, plainpaper office copierwhich becomes the topselling industrial product of all time. http://en.wikipedia.org/wiki/chester_carlson http://en.wikipedia.org/wiki/xerox_914 8
Photocopying step 1: Copper Electron comb Overhead Transparencies (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. 9
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. 10
Photocopying step 3: Toner sticking to charged surface What will happen if I draw on the plate with my finger? 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 are correct e. both b and c are correct. 11
Chalk only sticks to places with negative charge because the chalk is: Why? If positively charged, opposites attract. If neutral, can be polarized with negatives on chalk getting pushed to far side of dust particle, so attraction of positives is bigger than repulsion of negatives. Just like charged balloon and neutral wall. Either or + + + + + + + + (see sweater simulation) 12 http://phet.colorado.edu
Photoconductor Making a Copy Part I: a charge image Corona wire metal + + + + + + + + ++ + velocity + + + + + + + + ++ + Light Document Light Charge image + + + + + + + + ++ + + + + + + + + + + 13
Part II: Transferring Charge Image to Toner and Paper Just like Part I, in reverse. + + + + + + + + + Charge image toner Positive toner particle Roller and brush + + + + + + + + + Black image + + + + Release toner Light + + + + + + + Charge Paper + + + + + + + + + + + Heat 14
All Pretty Simple ChargePhysics (opposites attract), except Document Charge image + + + + + + + + + 15
Photoconductorsway to get charges to leak away when shine light on surface. (side view) 1. before light. e s want to go to bottom, but R too high, stuck conducting plate V 2. where light hits, R low, electrons flow away. V 16
Semiconductor physics where light hits, R low, electrons flow away, then add ink, sticks only where charges. Very special material low R ( conducts ) only when light hits. Photoconductor V To understand, have to understand what determines resistance of a material. insulators (wood, ceramic, plastic) very high resistance. conductors (metals) very low resistance semiconductors in the middle. Resistance depends on temp., light, cleanliness. 17
Resistance Conductors (low resistance) metals Insulators (high R) plastic, glass, stuff in between (semiconductors) measure stuff with ohmmeter examples. If moving charge determines conductors, insulators, etc, it must be something about electrons and their relations to atoms that matters. Resistance: 1) Amount of stuff to bump into (copper vs impure copper vs tungsten) 2) Number of electrons free to move. (biggest effect) 18
Energy Levels: # of electrons that can move depends on energy levels in material. Explain (different and better(?) than book) Only two electrons (1 spin up and 1 spin down) per energy level. (Simplify to 1 per level and always a spin up and a spin down sublevel for each level.) empty energy levels Energy filled energy levels 19
but if jam a whole bunch of atoms together in a solid, energy levels get combined/smeared out. Energy levels get shifted and shared for all atoms and electrons. 20
In solid, `10 22 atoms/cm 3, many!! electrons, and levels countless levels smeared together, individual levels indistinguishable. "bands" of levels. Each level filled with 2 electrons until run out. Energy atom level 3 2 1 more atoms empty conduction empty band band gap ~ few ev filled with electrons filled with electrons bands valence band 21
In book top is conduction band. lower is valence band, How many energy levels are in conduction band? a. none, b. 1 c. between 1 and 10, d. an enormous number How many electrons are in valence band? a. none, b. 1 c. between 1 and 10, d. an enormous number 22
Microscopic look at material conductor empty levels very close empty full electron like ball rolling on almost flat ground insulator big jump to empties. empty gap no levels full electron like ball in pit. move easily Can t move without big boost. semiconductor half way in between. empty Little gap to empty levels full like shallow pit. 23
If apply voltage to make electron move, it must go into a. higher energy level, b. lower energy level, c. does not matter, these are the energy levels of electrons in atoms, no connection with motion. 3 2 1 moving a. higher moving, more energy, really only slightly higher level. stationary If push on electrons (apply voltage to material) which ones will move? a. all of them in bands 1,2,3 b. none of them c. all of them in band 3 d. only the top one in band three d. only the top one in band 3. The others have no higher level they can move into, all filled with other electrons. After top one has gotten pushed up room to move next one below it. so maybe include billion or so really close ones. 24
E empty full The seating analogy How about these e s? full Easy to push to right? position (x) RULE: for electrons to move (with a voltage applied) There must be an empty energy level immediately above where they are 25
PhET conductivity sim on phet site (also semiconductor and diode sim there) http://phet.colorado.edu/simulations/sims.php?sim=conductivity 26