1. Phys. 122: Tuesday, 20 Oct. HW 8: due by 2:00 pm. Written HW 9: ch. 30, probs. 28, 40, and ch. 31, probs. 6, 20, 42, and 46. Due a week from Thursday (29 Oct.). Mast. Phys.: assign. 6 due in one week. Reading: Finish ch. 31 by Thursday. Exam 1 curve: All scores were boosted up by 0.5 point (on top of test correction and/or bonus problem points); this is NOT shown on your exam but it is in the gradebook. Exam 2: will cover chapters 25 through 28; after HW 7 has been returned (very likely Tuesday the 27 th ). Study guide and sample exam problems have been updated and are available. Midterm Grades: I will post detailed grade breakdowns on Canvas later this week; please be patient!

2. Clickers: to find the total energy stored in a capacitor, a) Use Q times V b) Use E ² times ε ₀, integrated over volume c) Add half of (a) to half of (b) d) Add all of (a) to all of (b) e) Use either half of (a) or half of (b), but not both

3. Energy storage in capacitors: Half what you might guess! U C = Q V/2 Combine with Q = C V and we get two other, equivalent forms. (These other forms are good for expressing the energy in terms of either the charge or the voltage.)

4. Another way to do the energy “book-keeping” in capacitors: (The reason for giving you this extra way of finding the energy is that it's ALWAYS TRUE, even for other electric fields besides capacitors! In fact, it's part of the energy of electromagnetic waves, including visible light.) Don't add this energy to the charge or voltage capacitor energy! It's the exact same thing.

5. Dielectric: An insulator which is placed in between the conductors of a capacitor (gives C = κ C 0 ) Two important numbers for a dielectric: the dielectric constant ( κ ) and the maximum E field strength before it sparks (breakdown E) Dielectrics are a great way to boost C (to κ C ₀ ), but they also lower the maximum E the capacitor can handle before sparking (which can ruin the capacitor!)

6. Current is the net motion of charge (positive charge moving along I, negative charge moving against I, or both). Water Analogy: Electrical current is just like current of a stream, river, or pipe. Both are measured in amount of stuff per time (that you would catch in a bucket).

8. Clickers: It's a constant trouble to keep track of the minus sign in the electron's charge! Whom should you blame?? a) Thomas Edison b) Knight (your textbook author) c) Thomas Jefferson d) Benjamin Franklin e) Nikola Tesla

9. Clickers: We've said that a conductor is something which has lots of charges that can flow freely. What is a superconductor? a) A metallic hero who keeps ordinary conductors safe from criminals b) A conductor whose charges are all free to move c) A conductor that can conduct current across both space and time d) A conductor held at dangerously high voltage e) Just what we've been calling a conductor so far

10. Actual motion in an imperfect conductor is very irregular; only the average motion (called the drift velocity) contributes to I.

11. (conductivity) σ = 1/ ρ (resistivity)

12. At first glance, Ohm's Law should bother you!

13. Clickers: the units of current density J are... a) Amps b) Coulombs per cubic meter c) Amps per cubic meter d) Amps per square meter e) Amps per meter

14. (conductivity) σ = 1/ ρ (resistivity) Let's figure out the macroscopic (total) version:

16. Some circuit diagram elements Rule: treat wires as perfect conductors in circuit diagrams. (Actual resistive wires can have their R put in series with the wire on the diagram.)

17. Clickers: What is the current through the 3 Ω resistor? a) 3 Amps b) 2 Amps c) 1 Amp d) 9/8 Amp e) 0 Amps

18. The fluid (water) analogy... updated Electrical Thing Charge Voltage Electric Field Superconductor Capacitor Current (in a wire)‏ Resistor Resistivity Fluid thing Fluid (water)‏ Pressure Pressure difference (Unrestricted) pipe Water tank Current (in a pipe)‏ Pipe filled with sand Density of sand

19. A very simple series network

20. Parallel and Series Resistors In Parallel, the voltage drop across any item is always the same as each of the others. Check for uninterrupted conductors connecting each side to identify things in parallel in a circuit diagram. Parallel: 1/R eq. = 1/R 1 + 1/R 2 +... (Currents in parallel add to give the total current.)‏ In Series, the current through each item is the same. (For capacitors, this meant the charges were equal after the capacitors became charged.) Look for a single wire broken only by the items, to identify a series network. Series: R eq. = R 1 + R 2 +... (Voltage drops in series add to give the total.)‏

21. Clickers: why are the series and parallel rules backward for resistors versus for capacitors? a) Because capacitors are like inverse resistors: larger C means smaller R b) Because capacitors are like negative resistors c) Because the voltage rules for R and C are flipped d) To confuse physics students e) To confuse physics professors

22. This is the formula for the power across anything in a circuit. It might stand for power being lost, being stored, or being used in some useful way.

23. Clickers: For a resistor in a circuit, what does the power represent in P = I Δ V ? a) Energy per time being used by the resistor b) Energy per time being stored in the resistor c) It depends upon the relative direction of I and the difference in potential d) The power being stored by capacitors in the same circuit e) The power gained by each electron

24. Clickers: For a capacitor in a circuit, what does the power represent in P = I Δ V ? a) Energy per time being stored in the capacitor b) Energy per time being released from it c) It depends upon the relative direction of I and the difference in potential d) The power is being used by resistors in the same circuit e) The power gained by each electron