1. 1 Using Physics to Understand the World: The Physics of Everyday Life Physics 1020: Dr. Eleanor Hodby Day 2: Review of electric circuits Begin copiers / photoconductors Reminders: Fill out group seating chart Homework 1 due on Tuesday Reading quiz Tuesday: 10.1, 10.2, 10.3 Attend HW problem solving sessions

2. 2 Analyzing explanations (in-class, homework etc). People learn better if they frequently ask themselves “Why did I do this step?” “How does this fit into the solution as a whole?” “Does this step or solution make sense, can I relate it to something else I know or check in some other way that it makes sense?” Practice these questions during discussions in class, when solving problems together, looking over solutions. Research shows results in solving problems faster and getting more correct. Problem solving strategy:

3. 3 Car battery demo- paper clip across terminals. What will happen a)nothing, b)drain battery slightly, c)melt paper clip, d)melt wires, e) both c. and d. Review of circuits and moving charges 1. Ohms Law (V=IR) and Power (P = IV)

4. 4 Circuits – Think like an electron e This is a task for Electron man! + - ee eee e e eeeee ee ee e eee e e eeeee e e e ee ee e ee ee eee e ee ee ee e ee eee e e e ee e e eee

5. 5 Rules: a) no electron deaths b) no passing of electrons c) electrons have energy (high at start, low at end) d) Material has resistance (lets electrons pass easily or not) Circuits – Think like an electron See: resistor heating simulation: http://phet.colorado.edu or class website e

6. 6 + - ----- ------ e e e 12 V Battery piles up electrons at negative side. Pushes electrons to + terminal with 12 V of energy. Reasoning a little more mathematically

7. 7 Student volunteer grabs terminals with hands. A.Nothing. B.Will get zapped (flames etc.) C.will get mild jolt, D.other How to figure out? 1 st step in process? Car battery demo II

8. 8 1.5 V + Review of circuits and moving charges 2. Bulbs in series and parallel Given 2 bulbs and a battery, there are 2 different ways that you can connect the circuit 1.5 V + Series Parallel

9. 9 1.5 V + Compare the brightness of identical bulbs. How does bulb 1 compare to bulb 2? a. brighter, b. dimmer, c. same 1 2 Bulbs in series

10. 10 + + - e e e eeeee eeeee eeeee eee ee eeeee e e ee e e e e ee e ee e e ee eeee e e eeeee e e e e e e ee e e e e ee e e e e Remember rules: - No Passing - No electron man deaths or births on route What does Current of 1 Amp mean? (1 Coulomb of charge per second) = 6.25 x 10 18 electrons per second passing by Electron man picture of bulb problem

11. 11 case 1 + 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 Series circuit question - current +

12. 12 case 1 + case 2 Series circuit question - current + When resistors are strung together in SERIES then: a)total resistance (R) is just the sum of the individual resistances b)Same current through each resistor

13. 13 1.5 V + 1 2 How will brightness of bulb 1 in left circuit compare to brightness of the same bulb in the right circuit? a. brighter, b. dimmer, c. same 1 Series circuit question - power 1.5 V +

14. 14 Compare the brightness of identical bulbs. How does bulb 1 compare to bulb 2? a. brighter, b. dimmer, c. same Bulbs in parallel 1.5 V + 1 2

15. 15 Thinking like an electron – parallel circuits + - e e e eeeeeee ee eeee e e ee eeeee eee e e ee e e ee e ee ee ee e eee e e ee e ee ee eee eee e ee e ee e e e e e e e e No matter what the path, electrons lose all voltage (energy per electron) by the time they return to outlet.  V path1 = V path2 = V battery Path 1 Path 2

16. 16 Bulbs in parallel 1.5 V + 1 2 V1V1 V2V2 For bulbs wired in parallel: Same voltage across each bulb V 1 = V 2 (Assumes R wires ~ 0)

17. 17 How does brightness of bulb 1 in circuit A compare to the same bulb in B? a. brighter, b. dimmer, c. same Bulbs in parallel 1.5 V + 1 + 1 2 A B

18. 18 Bulbs in parallel How does size of I B compare to I 1 ? (Bulbs 1 and 2 are identical) a)I B is three times I 1 b)I B is twice I 1 c)I B is half I 1 d)I B is the same as I 1 1.5 V + IBIB I1I1 1 2

19. 19 60 W 125 W 120V Q: What is different between the 60 Watt bulb and the 125 Watt bulb? a.the voltage difference (amount of energy lost by each electron) across bulb b.the number of electrons passing through the filament each second c.Choice a and the resistance of the filament d.Choice b and the resistance of the filament e.Choice a and Choice b and the resistance of the filament Bulbs in parallel

20. 20 More bulb questions What if we wire the 60 and 125 W bulbs in series, which will be brighter? 60 W 125 W 120V a)60 W bulb is brighter b)125 W bulb is brighter c)Same brightness

21. 21 Summary of some important ideas 1. Current is conserved (electrons don’t disappear) 2. Change in V over circuit = V of battery, or energy source 3. V= I R (Ohm’s law) - useful for whole circuit (R total, V total, give I total ) - or individual component (e.g. R bulb, V bulb give I bulb ), …….Be careful about what R, and V are. 4. P = I V = I (IR) = I 2 R power dissipated across object R = (V/R)V = V 2 /R 5. Resistors in series: Resistances add: R tot = R 1 + R 2 Current through all resistors is the same 6. Resistors in parallel: Voltage drop across parallel legs of circuit is same

22. 22 Homework #1 due Tuesday at start of class: Goal is to refresh what you learned in 1010 about electrostatic forces (like charges repel) energy, and circuits. Resources available to you: 1. Phys 1020 Lecture Notes (from course homepage) 2. Phys 1010 Lecture Notes from last semester 3. The book. Review chapters: 10.1 Static Electricity, 10.3 Flashlights, 4. Problem Solving Sessions (G2B87): Monday 2-3.30pm Friday 4-5.30pm Late submission policy: No late homeworks will be accepted. Two lowest homework scores will be dropped. Save these for emergencies.