## Thursday, November 12, 2015

### Classes for November 16

Happy Monday, everyone!

4 Chem-Phys:

Take a couple minutes to compare work and answers on the multi-loop circuit problems. When you are OK with those, watch three short videos, and take good notes on both:

i. How do Kirchhoff's rules lead to the resistor rules for series and parallel? Hopefully this will make sense and you'll see where the weird reciprocal rule comes from for parallel.
ii. How do Kirchhoff's rules change for capacitors, and what are the rules for adding capacitance in a circuit? Turns out we have the same two rules as for resistors, but they are swapped.
iii. What are capacitor (only) circuits and how do we solve them? Keep in mind the main thing we will be looking for with capacitors is how much charge is stored by each capacitor in the circuit. The approach is basically identical to finding currents in resistor circuits: Find total capacitance, find total charge with Q = CV (this is like Ohm's law for capacitors), then redraw the circuit as a series circuit - each capacitor in series has the same total charge on it, and then you can find the voltage for the parallel branches using V = Q/C.

Keep in mind that there is a new unit for capacitance, called the farad (F; named after Michael Faraday). One farad is defined as a device that can store 1 coulomb of charge by using a 1 volt battery to hold that charge on the capacitor. A 1 F capacitor is actually quite large...you will see units of microfarads, and even as small as picofarads, in real devices.

After watching these, break into groups and you can try the following. There are no hard copies of the problems, but you can pull up an online version of the book on the screen. To do this, do the following:
Go to the Mastering Physics site.
Username = ethsphysics; PW = ethsphysics1
You can type in 812 in the page box at the top to get to the problems below.

HW Set for Tuesday: Chapter 24, Exercises #15, 16, 17, 21 on page 812, and 63 on page 815.

AP Physics C:

Take a few minutes and talk through the homework problems together. Are these torque problems making any sense to you?

Watch a video on equilibrium with rotations involved. Take good notes, and realize there is one new condition to equilibrium: not only do we balance forces in each dimension, but now we need to balance torques if there is an axis of rotation.

HW Set for Tuesday: Ch 11 #13, 19 (page 7 of packet)
Ch. 11 Either #46 or #70 (page 8)
Torque brain teasers (page 9 of packet); reach class consensus
If you have any time left over, definitely feel free to work on your lab. We are looking for the lab report on Wednesday.

I will see all of you Tuesday!