The IDEAl solution for this week is the first problem of the revision edition of Quiz 11, finding the electrostatic potential energy of three charges at the vertices of an equilateral triangle. If you make quiz revisions, please use the version posted here (and in the program fileshare).

A quick remark on IDEAl solutions. When you write these, each equation should have a brief justification. By this I mean in terms of the physical situation – why a particular law applies, how the geometry of the problem leads to a particular formula, etc. The one thing that is not what I mean by “justification” is a description of the algebra you have done (or will do) in solving the problem. For instance, if I were doing a problem involving a mass vibrating on a spring that asked for the period, and you wrote the formula omega^2 = sqrt(k/m), you should justify it be writing something like “formula for angular frequency of mass on a spring” rather than “I can use this to plug into T=2 pi omega.” True, you may use that formula to eliminate an unknown quantity as you do the algebra, but the reason you can legitimately use the formula is not because it has a variable you want to eliminate, but because it is relevant to the physical situation.

A way of remembering this is to realize that when I look at your solution, the algebra steps you use will be right there in front of me. You have no reason to describe what you are about to do, because I can see what you actually did! So words outlining your algebra work are unnecessary busy work for you to write and for me to read. What I want you to do is focus on identifying how you know, based on the physics of a problem, what equations are applicable, rather than choosing equations based on what symbols appear in them.

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As rated by students elsewhere, Problem 10 was rated 4 for difficulty, 5 and 15 are both a 3, and 2, 11, 12, 13 and 17 were rated 2, with the rest deemed a 1.

System-recorded median time to complete problems rated 10 as the longest, then 17, then 5, 11, 12, 15, 16 and 17 (which were all the same)

Wednesday’s quiz is based on a lot of material. To narrow the scope a bit and reflect what we had time to cover in class, I do not plan to ask questions about the net capacitance of various combinations of capacitors connected in series and parallel. I also noticed that on the last couple of questions of the Week 14 homework there were parts that asked about material that was really covered in Ch. 24 (this week’s reading) – basically, any questions about current. I will not expect you to be able to answer questions regarding electric current on this week’s quiz. (The opposite will be true next week, of course!)

This coming week (week 5 of Winter quarter already?!) we will be checking your problem set and lab notebooks in all subjects.

• Please bring your problem set notebooks for chemistry, math and physics to the Math exam Monday morning.
• Chemistry lab notebooks will be collected at the end of chemistry lab Tuesday.
• Physics lab notebooks will be collected at the end of physics lab Tuesday.

As a reminder, spot checks of problem set and lab notebooks can occur at any time. The main reason for this announcement is so that we can expect you to have problem set notebooks with you Monday.

The entries you record in any of these notebooks are simply a record of the work you have done in the corresponding part of the program; “fixing” or “completing” notebooks is not extra work that you should do in response to an announced notebook check, but part of your ongoing learning process. In this spirit, you should not be making a special effort to polish your problem set notebooks for Monday; rather, you should be completing problem sets as usual, studying for your math exams, and turning in the notebooks simply as a record of the work you have already been doing in support of your learning, rather than as pieces of “busy work.”

Here is Quiz 10 for corrections. Since you are already doing #3 as for your IDEAl solution, I’ll evaluate that submission in lieu of re-writing it on any corrections you turn in.

Speaking of #3, Example 20.6 outlines how to solve the problem. The Evaluate step in the text is very compressed, and I want you to give a more detailed explanation of what is going on, in your own words and maybe pictures, than Wolfson offers. What I’m most interested in is your understanding. Also note that some elements of this problem are the same as in this week’s Math Lab (particularly the way one handles the element of charge dq). The main difference is that in Math Lab you found solutions for arbitrary locations in the plane of the ring, while this problem asks for E at a specific location on the axis of the ring, but in a different plane 15 cm away. You might think about what change you’d need to make to your Mathematica notebook to solve the fully 3-dimensional problem – you may be surprised at how small the change would be!

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Rather than taking workshop time, I thought I’d give a briefing on the longest/most challenging problems of the current week’s Mastering Physics assignment here. There are a lot of problems but only three that are rated higher in difficulty than 1 out of 5. Number 1 is rated a 4, and #7 and #16 are each rated a 2.

You might also be interested in the median time it takes students to do problems… the eight longest problems, in descending order, are #6, #15, #16, #1, #19, #18, #4 and #8.

Finally, I plan to collect physics lab notebooks at the end of next Tuesday’s lab period. I would also like to do some checks on problem set notebooks; since Krishna is already doing that with math problem set notebooks during the math exam, I think I’ll join him in checking physics problem set notebooks at the same time.

This is going to be the 3rd problem on this week’s quiz.

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Although some of this information is not immediately pertinent to the Energy Project posters, here are a few sites intended to help people create posters for scientific research presentations. (The main difference is that you are not presenting original research, which affects what some of the appropriate subsections should be.)

Designing Conference Posters

Better Posters (blog)

You’ll notice a wide variety of examples and even some conflicting advice (for instance, Colin Purrington is not a fan of the portrait layout for posters, while Better Posters features one as an example of good design.

In the end, you have significant latitude simply to do what makes sense for the work you are presenting. But these sites provide lots of ideas, thoughtful critiques, and resources that you may find helpful.

Though there was no physics quiz this week, there will still be an IDEAl solution problem due next Wednesday, Jan. 31, based on Chapter 8 material. This is Problem 7 from the Week 12 Mastering Physics assignment. This should look a lot easier after working Chapter 20 problems!

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Here are my solutions to the Week 12 homework on gravity

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