Phys. 122: Thursday, 15 Oct. HW 6 returned: please pick up yours in front. Mast. Phys.: Assign. 5 due this evening. Assign. 6 due the Tues.after next. HW 8: due Tuesday. Reading: Should have finished ch. 30; read ch. 31 next. Exam 1: “Test Corrections” and/or bonus problem are DUE by today (2 pm if you don't have them here yet). Exam 2: will cover chapters 25 through 28; after HW 7 hasbeen returned (possibly next Thursday, or the Tues. afterward). Thursday “Tech Talks”: begin at 11 in Fidel; we may end class today a bit early. If you use the REEF clicker app, or have used more than 1clicker this semester, please email me the registration numbersby the end of today, or you won't get the proper midterm credit!

Which problems are eligible for “testcorrections?” 6(b), 7(ii), 8(a), 8(b)

Re-do at the OSL (Speare 110) beforeThursday. Turn your original exam backin to me by Thurs. with the corrections. If you had fewerthan 2 points off for these problems, youmay do a “bonus problem” for extra crediton the test (see our webpage or Canvas); turnin your answer with original exam by Thursday.

HW hints needed for today?

Clickers: A 2.0 pF capacitor is in parallelwith a 6.0 pF capacitor. If the 2.0 pF capacitorcarries a total charge of 1.0 nC, what is thecharge on the 6.0 pF capacitor?

a) also 1.0 nC b) 0.333 nC c) 6.0 nC d) 3.0 nC e) Need more information to decide.

Energy storage in capacitors:Half what you might guess!

UC = 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.)

Clickers: A 4.0 μF capacitor and a 2.0 μFcapacitor are connected in parallel, and thecombination is hooked to a 12.0 V battery.Which capacitor stores the most energy?

a) They have the same energy b) Neither has any energy c) the 4.0 μF one d) the 2.0 μF one e) I can't answer because I just earned a... LEVEL UP!!

Clickers: A 4.0 μF capacitor and a 2.0 μFcapacitor are connected in series, and thecombination is hooked to a 12.0 V battery.Which capacitor stores the most energy?

a) They have the same energy b) Neither has any energy c) the 4.0 μF one d) the 2.0 μF one e) I can't answer because someone I don't like just “liked” my Facebook post!

Another way to do the energy“book-keeping” in capacitors:

Another way to do the energy“book-keeping” in capacitors:

(The reason for giving you this extra way of finding theenergy is that it's ALWAYS TRUE, even for other electricfields besides capacitors! In fact, it's part of the energyof electromagnetic waves, including visible light.)

• Don't add this energy to the chargeor voltage capacitor energy! It's theexact same thing.

Dielectric: An insulator which is placedin between the conductors of a capacitor (gives C = κ C0)

Two important numbers for a dielectric: thedielectric constant (κ) and the maximum Efield strength before it sparks (breakdown E)

Dielectrics are a great way to boost C (to κ C₀),but they also lower the maximum E the capacitorcan handle before sparking (which can ruin the capacitor!)

Example: dielectric halfwayinserted into a capacitor:

Current is the net motion of charge (positivecharge moving along I, negative charge movingagainst I, or both).

Water Analogy: Electrical current is just like currentof a stream, river, or pipe. Both are measured in amountof stuff per time (that you would catch in a bucket).