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 has been 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 1 clicker this semester, please me the registration numbers by the end of today, or you won't get the proper midterm credit!

Which problems are eligible for “test corrections?” 6(b), 7(ii), 8(a), 8(b) Re-do at the OSL (Speare 110) before Thursday. Turn your original exam back in to me by Thurs. with the corrections. If you had fewer than 2 points off for these problems, you may do a “bonus problem” for extra credit on the test (see our webpage or Canvas); turn in your answer with original exam by Thursday.

HW hints needed for today?

Clickers: A 2.0 pF capacitor is in parallel with a 6.0 pF capacitor. If the 2.0 pF capacitor carries a total charge of 1.0 nC, what is the charge on the 6.0 pF capacitor? a) also 1.0 nC b) nC c) 6.0 nC d) 3.0 nC e) Need more information to decide.

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.)

Clickers: A 4.0 μ F capacitor and a 2.0 μ F capacitor are connected in parallel, and the combination 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 μ F one d) the 2.0 μ F one e) I can't answer because I just earned a... LEVEL UP!!

Clickers: A 4.0 μ F capacitor and a 2.0 μ F capacitor are connected in series, and the combination 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 μ F one d) the 2.0 μ F 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 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.

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!)

Example: dielectric halfway inserted into a capacitor:

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).