MUEN 0046

1. DOOR N MUEN 0046 DOOR to 0046 Basement Hallway Go down the outside steps and in through the west entrance of Muenzinger. Take the stairs all the way down to the basement (you’ll go past the first floor landing). Once in the hallway, turn left. There are signs to 0046 posted on the hallway wall. Walk all the way down the hall until you get to the double doors. 0046 is directly to your right, through another set of doors. There are more signs here to direct you. Stairs to basement Exam tonight in Muenzinger E-0046: 7:30 to 9:00

2. Exam: some essay, some calculations (like HW) best preparation- go over homework and solutions, also class notes (particularly questions in class). Most importantly, be sure you make sense of the answers. one 3 x 5 card. Important part of studying to make list of most important ideas and formulas needed. Group achievement reward-- if group average is higher than 80% on exam( = .8 x 30 =24 pts), each member of group gets +5 pts on exam. Reading assignment thurs. review 7.1, 7.2 sound, new 10.1 amplifiers

3. Topics: Electrostatic forces … will things attract, repel, and why. Circuits with wires, batteries or regular outlets, bulbs, heaters… -- think like an electron, how fast will you be able to flow and where will you lose your energy. Power, current, resistance, voltage drop. Conductors, insulators, and semiconductors … --how is conductivity influenced by how electrons are found in bands --how can you change conductivity of semiconductor, why does it change. Photocopiers, how they work…, role of photoconductors and forces between electric charges.Power Distribution … --power loss in wires, why AC?, why HV and LV combo? --transformers: how do they work, how would you design transformer for power system to raise or lower voltages, currents creating magnetic fields and CHANGING magnetic fields creating currents. one 3 x 5 card. Important part of studying to make list of most important ideas and formulas needed. Reading assignment thurs. review 7.1, 7.2 sound, new 10.1 amplifiers

4. current through coil gives Magnetic field, reverse current, reverse magnetic field. current in current out What happens if oscillating current in primary but there is no core? • The light bulb will not light because there is no conduction path for electrons to move from one coil to another. • The light bulb will not light because there is no changing magnetic field present. • The light bulb will be dimmer than with a core. • The light bulb will be the same brightness as with core. • The light bulb will be brighter than with core. ans. c. some field will miss secondary coil

5. Transformer construction detail. The core. Magnetic field is always produced from current through primary coil. Without core, magnetic field spreads out a lot. So field going through secondary coil is weaker, doesn’t push as hard on electrons, produces less current, less power transfer … power wasted. Vsec = Vpri (Nsec/Npri) current in B current out What will happen to light bulb? iron core concentrates field (sucks it in), more through second coil bigger current! (incredible graphics display…) Does not carry current!

6. power distribution system power plant substation 500,000 V (on towers) 5000V 5000 V running around town. Electric power generation 120 V short wires into houses How did I generate power in class?

7. S I, V out N S S N N N iron core S moving coil through magnetic field. so if moved coil or magnet could generate electric power. Power plants: use steam or water to spin magnets past coils (or vice-versa) magnets spinning turbine

8. hydroelectric turbine E = mgh, power = mass/sec x gh ~ 40% efficient Pelectrical out = .4 (mass water/s x gh) h steam plants same idea, boil water to make steam pressure to spin turbine.

9. turbine boiler I cooling pond

10. N N N N N N N N s s s s s s s s 1 2 3 4 5 6 7 8 time B change in B/change in t (slope) gives voltage-current out of coil so current out of coil is biggest at a. 1, b.2, c. 3, d. 4, e. 7 d. 4. Magnetic field biggest at 1, 7, but changing slowly V

11. N N N N N N N N s s s s s s s s 1 2 3 4 5 6 7 8 time B change in B/change in t (slope) gives voltage-current out of coil so current out of coil is biggest at a. 1, b.2, c. 3, d. 4, e. 7 d. 4. Magnetic field biggest at 1, 7, but changing slowly V

12. conduction of materials- energy levels and electron occupation. 3 2 1 How many energy levels are in band 1? a. none, b. 1, c. between 1 and 10, d. an enormous number

13. Band 3 Band 2 Band 1 How many energy levels are in band 1? a. none, b. 1, c. between 1 and 10, d. an enormous number d. an enormous number. Each band is a whole bunch of very close levels. Each Is filled with an electron. • How many empty and filled levels are there in band 3? • 1 filled, none empty, b. 1 of each, c. very many of each, • d. many filled, 1 empty

14. 3 many empty levels close together 2 1 How many empty and filled levels are there in band 3? Answer is c: very many of each 1. the upper electrons in band 3 can easily move because there are very close energy levels they can move into. 2. the upper electrons in band 2 can easily move because there are very close levels to move into. many filled levels close together. c. 1 is true but 2 is false. Material with top band like 3 is conductor a. 1T 2T, b. 1F 2F, c. 1T 2F, d. 1F 2T

15. conductor- empty levels very close full insulator- big jump to empties. empty empty full ENERGY gap- no ALLOWED levels full electron like ball rolling on almost flat ground electron like in pit. ? Can’t move without big boost. move easily semiconductor-- half way in between. Little gap to empty levels, shallow pit. empty full

16. Material A. What is it? empty Band 2 Band 1 full Fill in the blank: This material is a ______________. When hooked to a battery, electrons in Band 1 will ______________. When hooked to a battery, electrons in Band 2 will ______________. a. conductor, move, move b. semi-conductor in the dark, not move, move. c. semi-conductor in the light, move, move. d. semi-conductor in the light, not move, move. e. insulator, not move, not move.

17. Semi-conductor in the light. empty little gap empty full E = energy of photon = h x frequency (h) =h x c/(wavelength) = hc/ Fill in the blank: This material is a ______________. When hooked to a battery, electrons in Band 1 will ______________. When hooked to a battery, electrons in Band 2 will ______________. a. conductor, move, move b. semi-conductor in the dark, not move, move. c. semi-conductor in the light, move, move. d. semi-conductor in the light, not move, move. e. insulator, not move, not move. Small energy gap between band 1 and band 2. As shown, electrons are excited up to Band 2 as would be the case for a semi-conductor in the light. Both Band 1 and Band 2 have electrons with empty energy levels just above them so in both bands we have electrons that can move.

18. Where does the power go? Hair Dryer, Lights, and Stereo plugged into same outlet Wires Stereo Hair Dryer Lightbulb 1 10 Amps Lightbulb 2 0.5 Amps 2 Amps • What is the current through the wires? • 10 Amps b. 12.5 Amps c. 7.5 Amps d. more than 12.5 Amps. • If the resistance of eachlight bulb is 100 Ohms, how much power is going in to the two bulbs combined? • a. 25 Watts, b. 50 Watts c. 100 Watts d. 200 Watts Answer is b. 12.5 Amps. Currents flow together in wire. Wire carries total. • What is voltage drop across one bulb? • V_across bulb = I_bulb x R_bulb =0.5 amps x 100 ohms = 50 Volts • Power loss in each bulb = I_bulb x V_bulb = 0.5 amps x 50 Volts = 25 Watts • There are 2 bulbs. So total loss is 50 Watts. .. Answer is b.

19. which would make the best core for a transformer if strength did not matter? a.wood, b. copper, c. glass, d. iron wrapped in plastic insulator d. iron- concentrates magnetic field. Does not conduct electrons so does not matter if insulated. In fact cores are painted to insulate and avoid rust. Also note, can not use permanent magnets as cores, because cores have to be able to reverse back and forth! If I took a transformer used to convert 100 V up to 1000 V and I hooked the primary up to a 12 V car battery. If I then went to measure the voltage across the secondary coil, what would I find? a. 0 V, b. 12 V, c. 1200 V. d. 120 V, e. 1.2 V a. 0 V. Battery would make a constant magnetic field through the secondary. Need a changing magnetic field to give electrons push to get voltage and current out of secondary. What is ratio of turns on primary to secondary? a. 10 pri. to 1 sec., b. 1 to 10, c. 100 to 1, d. 1000 to 1, e. 1 to 1000 ans. 1 on primary for 10 sec.

20. e 120 V Why does light dim when heater on, how much less current through light? (need to think like an electron!) Job for electron man! (on rollerblades) boot camp for electrons. bunch of them going through obstacle course.

21. e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e glide down easily, just a few bumps. Hardly any energy. !?#%, bridge out, stuck. lots of energy at start. energy used up getting through course. Vigor (V) exhausted! three feet of mud! takes lots of energy to get through. R (rottenness) Can’t get past each other, ones slowed down getting through much pile up others behind. Almost all the energy is used up getting through the mud. Sets limit on how many can get through course each second. #/s = vigor/rottenness (V/R). Nearly all the rottenness is the mud. Going down wire is very small effect.

22. What happens when bridge gets fixed so have another route? e’s piled up down both routes, so still divide up and go down both, just end up faster on bridge route e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e lots of energy at start. pretty easy deep mud! energy used up getting through course. Vigor (V) exhausted! What happens now compared with bridge out? a. Can go easy path across bridge. Takes less energy, can go much faster. b. Have to go down entry and exit road much faster. Lose more energy hitting bumps at high speed than with no bridge. Energy used going down road not so small. c. Ones that pick wrong path and go through mud have a little less energy to get through it than they did without bridge path, so get through it slower. #/s (I) through mud= (V - Vroad)/R, but going faster so Vroad lot bigger than before, go through mud slower than if bridge out. I smaller. (Vroad = I Rroad)

23. why different voltages, why different plugs, why 3 wires? Are different voltages more dangerous? 230 more dangerous than 120- easier to fibrillate heart (but wastes less power in wires). Different plugs partly historical, but modern European plugs harder to touch “live” plugs than US. Probably designed that way because voltage more dangerous. 3rd wire- ground. Protection electric appliance +120 to –120 V ~0 V but goes up when current flowing back “ground”, always 0V protection if wire touches case