HW4 hint

Check it.

UPDATE: on number 4, I get currents of 188.67A (live battery), 168.56 (starter), and 20.11A (dead battery). On number 1, I get 0.128 ohms.

UPDATE 2: numbers above corrected.

Exam- & Grade-related items

First: I am dropping quiz 4. This is for your own good :-) It will not be entered in the grade book, but it did give you a preview of 4 of the 25 exam questions. The average was going to be such that most of you would have ended up dropping it anyway. More generally, this is why I dislike multiple-choice quizzes, and should stop giving them: with four questions, if one trips up most of you, the average is already going to be below 75% at best. I made them multiple choice this time just so they would be identical to the exam questions.

Anyway: the quiz served its purpose in my mind - you really nailed those four questions on the exam.

Second: the exam is graded, and your grades are on eLearning. There is also a new column which lists your current overall average grade. This does not include any dropped grades, and thus you may consider it a worst-case scenario at the moment.

Here are some more details on the exam grades:

The average was 75.3, standard deviation 13.5. This basically means it was not necessary to scale it, and it went better than you think (mostly). Here are a couple of histograms. The first is coarsely-binned at 10 points (roughly corresponding to A, B, C, etc), the second is more finely-grained at 4% increments (corresponding to +/- 1 question). (Click on pics for larger versions.) Looking at your average score per question is - for me anyway - far more insightful. This tells me which stuff I need to review a bit more before we get to the final.


We'll go over all the questions which averaged less than 70% or so in Friday's class.

Third: it is still far too early to freak out about grades. If you have been completing most of the homework and quizzes you are not in any trouble yet. Conversely: if you haven't done any homework ... now would be an awesome time to start. The average course grade right now is 85.4, a solid B.

Quizzes, homework, and labs will shore up one bad exam easily. Here are the class averages for various components:

Lab - 94.6
Quiz - 83.5
HW - 90.0
Exam 1 - 73.6

Clearly, the best plan is to do the homework and show up for the labs ... apparently these points are easier to get.

Fourth: Check your grades carefully to make sure nothing is missing. Sometimes, I forget to upload a homework score, etc.

Quiz 4

Here's the quiz you're taking right now. These questions will appear verbatim on this afternoon's exam.

HW 4

HW4 is ready. Due by the end of Friday. Five problems, but only three require any sort of calculation.

We'll go over most of the HW4 problems during Friday's lecture.

Wednesday's lab

You can find tomorrow's procedure here. Two documents: an introduction to the data acquisition hardware & software we'll use for the next several labs, and the procedure itself.

It should not take more than 60-90 minutes to do the actual lab. We'll devote about 30min at the start of the lab period to answering homework questions and discussing the exam.

Grades

I just uploaded more grades into eLearning. So far, we have:

Lab 1 (papers), Lab 2 (errors), Quizzes 1-3, Homework 1-2

If you are missing a homework 2 grade (or it just says zero, but you know you turned it in), that means I haven't gotten to yours yet. In particular, homework turned in to the TAs at the end of last lab session are not graded yet.

Quiz 3 from Fall 2007

Quiz 3 from Fall 2007 had an error in the solutions. This has also been fixed.

Please, do let me know if you find errors in any material, current or not. Even if it is not an error per se, but you think the explanation could be better, it is helpful feedback.

Quiz 3 and its solution ...

... are right here.

Due to a stray minus sign on the formula sheet, question 1 has been thrown out. I'll explain in class on Wednesday, but basically it means everyone gets credit for number 1.

Exam I details

Exam I is, as you know, scheduled for this coming Thursday, 16 July. Perhaps the most notable thing about the exam is that it is going to be held during the lab period. That's right: no lab on the exam day. Sadly, there will be a lecture Thursday morning as usual. No rest for the wicked, but as a small consolation, we will spend time reviewing for the exam during lecture.

Material
The exam will cover the following material:

• Electric forces & fields
• Electrical energy & capacitance
• Current & Resistance

Basically, it will cover everything that we have done through the end of Wednesday's lecture, with the notable exception of relativity.

Format
The exam will be multiple choice, and roughly 25 questions long. I suggest studying the end of chapter questions in the notes, as well the homework and quiz solutions from last semester (start digging in here). In the book, you are best off studying the example problems, and the "quick quizzes."

Roughly half the questions will be conceptual or qualitative (no calculations), and roughly half the questions will involve real, live quantitative answers. With units and everything.

Location
The exam will be in the lecture room, Gallalee 227, but at the usual lab time of 3:00. We will devote the first 15 mins or so of that period to answering last-minute questions, after which you will have about 90 minutes for the exam. I expect the exam to take you closer to 60 minutes, but don't want anyone to feel too rushed.

Needs
You will want a calculator for the exam. Badly.

You can bring in ONE standard 8.5x11 inch sheet of paper front and back (or two sheets front-only), containing any information you like. This could include pictures, formulas, example problems, or even the first few chapters of "The Count of Monte Cristo" in a very small font. Whatever you think might help. You can use both the front and back of your sheet, typed or written, crayon or pencil, your preference.

One interesting variant is to write from left to right with a red pen, and then turn the page 90 degrees and write over the top of that with a blue pen. If you can then lay your hands on a set of 3-D glasses ... presto, you've doubled your storage density.

The plan for Tues, roughly

Basically, we'll finish up electrical energy & capacitance.

• More details about the exam ...
  
• Review HW2 problems
• Review of electric potential - relation to PE and work; point charges
• Systems of charges - potential energy and stability of crystals
• Isolated conductors and equipotential surfaces
• Review of capacitor basics - adding in series & parallel; parallel plate capacitors
• Complex capacitor combinations - how to analyze
• Capacitors with stuff inside - dielectrics & dipoles in electric fields

HW 3 / HW 2 solutions

I know you're dying to start the next homework. Due by the end of Wednesday, we'll set up a couple of them in class tomorrow. There are no vectors involved in this problem set :-)

Also, here is a draft of the HW2 solutions. My solutions are much longer and more pedantic than yours need to be, so don't worry that the solutions are 9pgs long ... I tried to be explicit as possible, without skipping any steps. Also ... I have this obsession with exact answers involving many square roots, rather than approximate answers like 12.5, which lengthens things considerably. That's just how I roll.

Definitely read the solutions to the problems you missed, but at least skim the solutions to the problems you answered correctly - maybe you thought of a way I didn't, and vice versa.

Tomorrow's quiz / HW2 solutions

Tomorrow's quiz covers three main topics: (1) adding identical capacitors in series or parallel, (2) potential energy change of a charge moving from one voltage to another, (3) relationship between a constant E field and potential difference (say, when moving through a distance x). I will give the quiz at the end of lecture, so things may be a bit clearer by that point anyway.

Also, I have draft solutions of HW2 ready, and will post them some time before I sleep. Probably around 2am, i.e., after the deadline. Check back here in the early AM.

Numerical answers for HW2

1) Q = 0.957q (expressed in terms of radicals is better ...)
2a) flux through one side: 1.9e6 Nm^2/C
2b) flux in total 1.13e7 Nm^2/C
3) did this one in class; no numerical answer
4) larger charge is 12.5 times the smaller ones (again, exact answer preferred)
5) E = 4.4e5 N/C

Grades

By the way ... I'm going to experiment with using eLearning for the first time. Right now, you should be able to see your grades to date (specifically, for the first homework, quizzes 1 & 2, and the paper).

Not much else there for now, save my slides for Relativity and Electrostatics. I will probably try using the Tegrity business to record lectures this week. I had a demo of it last week, and was favorably impressed.

Mainly, this gives you an easy (?) way to see your current grades online. If you haven't used eLearning before, I'll try and give you a quick run-down on how to log in and check your grades Monday or Tuesday.

Lab for Mon 13 July 09

Monday's lab will be an investigation of capacitors. There is no need to read the procedure well in advance of the lab session, just form your lab group and begin reading it when you arrive at the lab session.

It isn't that we don't like you to be prepared - it's just that we haven't gotten to capacitors yet, so it may not make much sense for you to spend time on the procedure at this point.

That implies that tomorrow's plan for lecture is to get through electric potential and capacitors so you understand what the lab is all about. This means that I may reorder some of the material in tomorrow's lecture compared to the notes or textbook and pick it up on Tuesday. We'll see how it goes though ...

Regardless, either in lecture or lab, I will be going over some sample problems and giving out hints on HW2.

Quiz 2 and its solution

Enjoy.

Also, this.

Homework 1 solutions

Here you go. Let me know if you find any mistakes or require further clarification ...