Key for the escape speed problem from Friday 5/10:

 

The attachment repeats the problem and has a brief and thorough key. Also, I had an arithmetic problem on the board and said one answer came out to square root of 46 m/s. The actual answer to that was the square root of 49.7 m/s. The physics and algebra were fine; just an arithmetic error. It's correct on the attachment.

 

And see the other posting about the Magic Mountain details. I handed out the F-602 form in class. The trip cost is $85. Please don't wait to tell me you're coming and get form and payment in. I'll be out of town on Thursday May 16 and Friday May 17, so tell me sooner than May 15.

Full Year Topic Outlines

 

As part of AP Review, topic inventories... You do them, right?

 

The attached ones (one document, two semesters) are thorough. See if they help.

Mechanics MC Tests Attached again

 

You have the keys for these for 84, 88, 93, and 98 over where I posted the bunch of E&M MC tests.

 

The pdf is the FULL booklet for the 2012 exam. You don't need to print out the whole booklet. You can scroll to the MC test and then find its key toward the end of the booklet. I've just been informed that this 2012 one exists for E&M as well. I've attached that here too. To be used sometime between 5/2 and 5/11 if anyone wants to.

Differential Equation Pattern Summary - Extreme shortcuts that are very useful for solutions that have exponential form, decay, and tau.

 

You've seen the same form 3 times, with the same patterns repeating over and over. It's time to take advantage of that. Attached. Very brief.

By April 24:

 

1) Work toward completing Unit VIII ASAP. (It won't all get done in two days.)

2) Open the attachment to this message. There see all 8 experimental scenarios, two of which you were graded on, in class on Monday, April 22. On paper, do the 6 you weren't assigned to do in class, and redo the ones you were assigned to do if you know you did them incorrectly. See below the list of potential weaknesses (coded A, B, C, and D) which could have a affected a person's performance on this. (I do believe there are only 4 possible ways that a weakness could impact a person.) On graded student papers, I wrote any applicable letters A, B, C, D, on the graded paper as a code to tell a student where he/she was weak in relation to those 4 areas (if applicable.)

3) After doing or redoing the 8 tasks, check the key.

4) After checking the key, link what's in the key to the central idea in Unit VIII named Lenz's Law. When all is said and done, a flawless answering of all 8 scenarios begins to amount to what one could call the lab-based discovery of Lenz's Law.

 

An important difference between the attachment and the papers used in class: To make the scenarios answerable at home, any ammeters in the diagrams for situations 1, 2, 3, and 8 now include the location of the red wire port and the black wire port, exactly as they were set up in class. This information was not included in the in-class paper to answer for credit, because you're supposed to know that you were expected to observe those red and black locations for yourself. It was not my responsibility to tell you where the black and the red of the ammeters were located. By working as an independent observer and from being reminded that you might need to get up and look at the apparatus to answer the question, it should be expected by you that your observations were part of the lab grade. Situations 1, 2, 3, and 8 are impossible to answer if one doesn't see how the red and black of the ammeter are connected to the secondary coil, and I was never going to say "Now don't forget to go see where the red and black are connected."  I did notice that some people who had those stations to answer did not seem go get up and look. (Maybe I'm wrong; maybe they could see the colors from across the room. Decide for yourself what was required for you to see, and know that everything was available with flawless matching between diagrams and apparatus. And everything worked correctly.) If a student neglected to see how the black and red ports of the ammeters were connected, this could signify weakness D that I list below.

 

So in the attachment, to make them worthwhile practice problems at home, I added the info on how the red and black ports were plugged in AND I added the actually way that ammeter needles deflected. This will enable you to pretend you were here seeing those with your eyes.

 

Weaknesses that could have affected performance on this specifically are lack of certainty with the following skills:

 

A. Basic vector directions, and distinguishing a vector's direction from the direction of that vector's change and factoring into this distinction whether the vector is strengthening or weakening.

B. Correctly associating the direction of a circular source of current and the direction of the B field that that source causes*.

C. Basic definitional knowledge of B's direction to begin with.

D. Ability to successfully observe the direction of current either through meter behavior or via magnetic force evidence.

 

*Example: If a person in one place said that CCW current causes an interior B field that's dot and in another place said that CCW current causes an interior B field that's x, then it might be a struggle for that paper to earn a B, if this assessment is supposed to measure basic Unit VII knowledge.

 

Clarification: In class, at 2:45, I understated the number of people eligible for A's when I listed off a series of names, and it's good for you to know why. I based that initial filtering of the papers on whether people always had deltaB opposite to B-induced in their final answers. (You'll find out why this matters.) I at first used this criterion to see who could get an A, because I know what final answers I'm looking for (through Lenz's Law.). But at the time, I also knew that I'd be looking more carefully and possibly broadening the A category, because since nobody had to know about Lenz's Law before coming into this, a person could have a minor mistake that causes one directional reversal, and that alone could be the source of why Lenz's Law appears violated in the final answer. As of 4/22, I'm not expecting that anyone would use Lenz's Law to check a final answer; I'm just saying that I WAS using it to give me a quick key at first. Now, my 4-lettered list of important strengths and weaknesses above is the more thorough grading tool, and that's what I used in the end. So here's an example:

 

A person was given scenarios 1 and 8 to analyze. (I recommend you open the attachment to follow along with this carefully.) It's important for you to know that I made sure that everybody got one scenario with a strengthening primary B field and one with a weakening primary B field. This student said "dot followed by x" in the first two entries in scenario 1's chart and "0 followed by dot" in the first two entries of scenario 8's chart. This means the person has no weakness in topic A above. (I also would have accepted "dot then dot" for the first two entries in scenario 8.) But the student had a final answer that violated Lenz's Law, so I knew there was an error somewhere, and where was it? So next I looked at his last two entries: I-induced compared to B-induced, and there I saw no problems whatsoever with skill B above - his clockwise current always went with x caused by it in the middle of the solenoid. Then I looked at skill C. The fact that the student said the original B fields caused by the primary sources were all in the proper direction told me there was no weakness with skill C. So the only possible weakness was in current-meter observation, and he got one of the two of these current-meter observations correct. So the violation of Lenz's Law in this person's final answer has been narrowed down to one bad meter read and only in one of the two scenarios. This paper is a 9/10. Papers with only one weakness are possibly eligible for an A, but it depends on how Unit VII-heavy that weakness was. For example, if the one weakness is skill B above, that invalidates the A-grade, because Skill B is a big one. (Not that this matters, but a large fraction of the class did not show signs of weakness B on 4/22.)

The attachments here aren't connected to what I said was due on Thursday, April 18. (That stuff is handled in a posting I was prompt and thorough with and completed by the night of April 16.)

 

One attachment here is connected to a topic that's past due, a topic upon which only one person has asked a question.

 

The other attachment is also connected to something past due. It's the full set of Unit VI solutions from problems at the end of the unit. No one should want to look at these thoroughly written solutions until he/she has tried each problem. There is a reason I've held them back until now. But once problems are authentically attempted by the student, the solutions are VERY useful for checking. If you look at a solution before truly trying the problem IN WRITING on your own, you destroy the problem as a study aid.

 

I've also attached the Unit VII solutions for near-future study checking. You'll have to decide on your own, by being reading-aware, when is the best time to start using them.

 

Skill with application of all topics in Units VI and VII is due upon arrival to class on Monday, April 22.

Study Guide Calendar:

In class on Friday April 12, I said to know the following topics:

 

The algebra of a charged particle in UCM in a magnetic field

The way a velocity selector works

How to analyze the torque caused by the magnetic force on a loop of current.

 

I said the goal is to know these things "by Tuesday", by which I meant April 16. This means questions on these things will be entertained on Tuesday, April 16. Don't assume that time will be given on other days for such questions. There will shortly be credit items (done in a fair way) that will depend on being caught up on things like this.

 

I handed out out two documents to help with two of the three things on the list: Velocity Selector and Torque caused by magnetism. Those handouts are attached.

 

The above specifics are all consistent with what I said recently when I said, "Complete Unit VI ASAP." The above three topics are the heart of Unit VI.

 

There will be more study timing specifics posted soon on April 15. No challenging grading items are planned for the class of Tuesday 4/16, and that fact has nothing to do with a person's incentive to stay caught up. The final is very soon, roughly two weeks away. So do all of Units VI and VII ASAP with urgency.

DC Circuit New Practice Problem

 

This falls in line with the specific skills that I said would be on the April 10 test. This practice document is especially good, because it links directly to the Experiment 5, which was graded and returned. People who didn't do Experiment 5 need not look at this attachment.

 

The Experiment 5 was designed to be more than a credit item. It's a useful resource. That is the reason I wanted people to speak with me directly on their Experiment 5 grading before leaving for break. And it's the reason I asked people to take care of speaking to me about their Experiment 5 grading ahead of time if they knew they would be absent on March 29 or both March 29 and 27. So it was already a useful resource, and I just thought of a way to make it better. This particular attachment adds to its usefulness, and it's an idea I just came up with today and am happy to pass on to the people who took full advantage of Experiment 5.

Absorb this Summary+ of the main idea of the lesson of class of 3/29/19. It was from Period 6 of the Friday before break, and a lot got done that day.

 

People already know they have a short circuit-related test on Wed. April 10. Therefore, this attachment is to be done before April 8. So there is a bit of break HW*, meaning this. People in class on March 29 will have gotten much of it done, because it's the same problem as the front board that day. Everyone needs to read the key after working the problem.

 

*In case you want my advice about how to handle vacation and doing schoolwork or not: I believe break should be break and school should be school. So I told the class that people shouldn't work themselves to exhaustion during time off. I hope people take big breaks. I am. But if I were a student, I would also want a review helper so I'm not overwhelmed when I return on April 8. The purpose of this attachment is to make it so that March 29 wasn't lost effort after a week has gone by. It focuses a person on quite a few important main ideas.

Aside from that, you can decide on your own how much circuit review, circuit study, AP exam review, or final exam review you want to do during the week of break. Remember, I've provided the full workbook Unit V, and it is very good with a lot of problems at the end.

 

And of course, if someone didn't get to my postings from the week of 3/25 through 3/29, those were things I intended to be done in class time and home time during the week of 3/25 through 3/29. So if that's the case, that person needs to catch up, and break would be a good time to do that.

An E field superposition calculator - works for 3 sources. If using two sources, make the third input zero.

 

Posting this has nothing to do with staying up to date in the circuit unit.

Three Vital Circuitry Answer-sets from the lesson of March 27, 2019. The answer to #1 I posted immediately after class on March 27, 2019. I fully expect questions about #1's answers from individuals. The answer to #2 was to have been tried in earnest DURING THE PERIOD on March 27, 2019, because I put the task up clearly on the board and gave plenty of time. Seeing its solution document (attached) is not crucial prior to March 29, but trying the task AT THE TIME I wrote it on the board on March 27 was crucial. Its answer is now attached (as described in item 2 below) for anyone who needs it, but it won't have value to those who didn't embrace the problem in class when I said to on Wed. 3/27. Items 3 and 4 on the list below are self-explanatory.

 

1. Answers to the circuit problem that had 70, 30, and 50 Ohm resistors and a capacitor. I7 was given as 0.075 A at a certain instant. At that instant, solve for all other currents and each component voltage.

 

I5 = 0.135 A

I3 = 0.06 A

V-50 = 6.75 V

V-30 = 1.8 V

V-70 = 5.25 V

V-Cap = 3.45 V

 

There is a solution document.

 

2. This was a set of mathematical responses to a scenario that was simply a capacitor in series with a single resistor, no junctions. This was an interactive lecture problem on the board. It's very important, and if you don't learn it from this, learn it from McGehee's Unit V - The Series RC Circuit. The ideal set of 5 correct responses for the way I wrote it on the board is explicit on an attached Word document. (Alert: as of 4:30 PM on 3/27, a power brownout took down my classroom internet, but it's been fixed.)

The ideal solution to the junction-free RC circuit is now attached. It's one page. Now that certain things are in equation form on this page, there is extreme danger that certain people will write down these particular equations in application to circuits for which these equations have not relevance. People who actually learn circuits and express Kirchoff's Laws as their habits (instead of memorizing these equations) are people who derive their own equations, and they will not have this problem. They understand that the process mattered and not the final solution formulas. People who write down formulas without building those formulas by themselves are effectively not students of the topic. The formulas are built by clear, fluent expression of K1 and K2.

 

3. Responses that got credit for the Little HW check are explained in the attached document. People who had the charge principle intact (from 2 and 3) and at least one of the voltage principles intact (from #1) earned at least a B. It's not a lot to ask that a person just be aware of those simple basics. People aware of them were able to check their answers and assure not just some right answers, but ALL right answers. That's how circuit solutions work.

 

4. One of your classmates asked for help with Challenge Problem 8 from the end of Unit IV. It involves two connected capacitors, and I asked questions about "What do you think is conserved for this change situation?" That would be important to stress. I will shortly post my way of solving that. You should try it on your own before I do that.

"Circuit Notes Solving Basics from ..." is the vital document here. It covers the main idea of the class of 3-24-19 and is extremely thorough. I added to it at 5:12 PM on Tuesday 3/26, so if you don't have the 5-page version, it's incomplete. This is for self-monitoring of lesson comprehension - Nothing new to do here. This is a resource for things that happened in class on 3/24.

 

Do NOT use DC Part 1, DC Part 2, DC Part 3, DC Part 4 first. That defeats the whole purpose. To find out why DC Part 1, DC Part 2, DC Part 3, DC Part 4 are posted here, you must first see the main idea content of "Circuit Notes Solving Basics from ...".

 

McGehee's Unit V is excellent for anyone who wants to use it. I strongly recommend it. Finish it by April 8.

 

Final answers to the capacitor-based problem in "Circuit Notes Solving Basics from ..." are:

 

I5 = 1.493 A

I7 = 0.648 A but its rightward not leftward

I3 = 0.845 A

 

Note: I had to fix an algebraic typo on "Circuit Notes Solving Basics from ...", and I did so and reposted at 5:54 PM on 3/26/19.

Due March 25:

 

The completed packet named Experiment 5 plus what additional pages are needed to handle full data sets and responses for the times of t = 0 and t = infinity.

 

R1 problem: at the power section at the end, where it says to find the power at R1, skip the power calculation at R1, because R1 was replaced with a capacitor.

 

Reminder: right click on your capacitor and make sure its capacitance value is 0.2 F.

 

Tip: Use the pause button to make time-dependent measuring easy.

 

Other file: Completion of a capacitor-only Practice problem that came out of a student question on March 21.

Correction needed on the document: "From Fundamentals to Circuit Tricks - Part 2"

 

On the last page under "Do the Algebra", it references "Fundamentals to Circuit Tricks" in two bullet points. Reminder: I told you in class that that is not a paper document; it was the board problems of the March 19 Lesson. In 2019, the capacitors were in a 5-to-1 ratio and the voltage across them was 12 V; you should remember this from class. The bullet points, however, report those events as if they were in a 2-to-1 ratio and the voltage across them was 9 V. The latter were the 2018 numbers. You should change it to the 2019 numbers in the Part 2 document to keep consistency, but the main idea isn't really changed either way.

 

Also, the second bullet point calls something the "'conceptual' document". By that, I just mean Fundamentals to Circuit Tricks, AKA the thing done in class on 3/19 that was not on paper.

 

See my other notice for what to be doing between 3/19 and 3/21.

 

What to be Doing Between 3/19 and 3/21:

 

Do your best with the notes called "Fundamentals to Circuit Tricks - Part 2" and "Introduction to Capacitor Network Solving". The latter of these isn't supposed to be immediately obvious, and I said, I would expect some questions. Also, it mentions some "as introduced in class" history, and that was from previous school years when I first wrote the document. But that doesn't prevent it from being followed.

 

I also pointed out that the alternative to the two note-sets that I just mentioned is to just learn it all as well as possible from McGehee's Unit IV workbook.

 

So either way is fine, but make it your job to be comfortable knowing how the battery voltage divides when two capacitors are in series with a battery and how the charge divides when two capacitors are in parallel with a battery.

If you completed all four examples of equipotential diagram analysis from the lesson of 3/19, you will actually have this covered.

 

Now, if that's covered, you can move on to the new file called C-EQ Breakdown. And if you do so, that would be working ahead. C-EQ Breakdown is attached, and the prior notes mention it.

 

In a couple places, the notes mention something called the PhET Site, where it's possible to make all these groovy circuit diagrams and press play to simulate nature. Anyone with internet can get to that site for free, and it's address is:

 

Those who can't function on a Central-Topic-Test  - "can't" isn't the right word. It's a choice those people are making and they made it over a week ago, probably two weeks ago, three in fact. They'll change the habits if they want, but if they don't, I won't be listening to their subjective opinion. I WOULD be listening to their physics questions, which they could come and ask anytime (as has always been the case), questions from the exercises, questions that are specific and informed from reading. In this case, those questions over the past three weeks should have had phrases like "where would my Gaussian surface be?", "How do I figure out the enclosed charge here", "which matter is the Gaussian surface?", "which structure in the solution requires volume?", "which structure in the solution requires surface area", "When is it useful to use the integral form to relate voltage to E?", "When is it useful to use the derivative form to relate V to E?", "Can I really trust situations when E is theoretically uniform..."  The way a class environment is supposed to look.

 

Attachments.

 

One attachment is the Key and Rubric (A correction was made to it at 5:10 PM Saturday 3/16).

 

The attachment with the long title (final form completed on March 18, 5:25 PM) has a meaning that's explained only if you read the entire key.

 

As of Saturday is a brand new one named Key 2. A person who is dysfunctional on tests needs to use Key 1 AND Key 2, if they care. Key 1 contains more preaching about the effects of not reading on time and not engaging with what I say to do, essentially not following the directions that I give over a three week period, and yes, there are people who need to hear that preaching.

 

And Key 1 is the only one that explains all the scoring. Key 2 is more brief, gets more directly to the answers without so many words in between.

 

Key 1 = Here is what the consequences of a bad job concentrating are

 

Key 2 = Here is what good concentration looks like

 

On Tuesday, you will see photocopied student sample keys, successful things written by the students of February and March 2019.

Capacitor Circuits and Circuit analysis in general begins Tuesday 3/19.

This means complete all the rest of Unit IV ASAP.

 

(I did explain the utility of Unit IV's first 6 pages before the midterm. A person who listened by my directions on that would have seen something on Page 5*. If they're seeing that page for the first time now, they'll realize they made a mistake.)

 

 

*Page 5's fill-in spots are precisely what I was referring to when, before the midterm I said, "we had solved those Page 1 through 6 problems in other places in class. All pages 1 through 6 do is repeat what we had already solved and they give a nice fill-in structure to engage in such a repeat/review for yourself."

 

So, before the test, did you re-review the E field, voltage, and capacitance derivation for the coaxial cylindrical shell capacitor? You had a cookie-cutter structure for you to do so, right there on Page 5. Did you look at it? Did you fill it in? Because I said all of this and I posted it? Well, some people did, and they earned a free 9 points from it. They did exactly what I said to do on the same exact cylindrical problem that I put on the midterm.

Test day was long since announced to be March 15. This message contains three items of specific practical information further down so read the entire thing whether you want to or not.

 

Paragraph 2: On March 13 the majority of the population of Period 6 should have arrived with urgent questions about practice problems THAT THEY HAD ALREADY TRIED OUTSIDE OF CLASS, efforts made before the March 13 class ever got underway. Why was it not the case that the majority of the class had such urgent questions from items already attempted.? (You know why.)

 

Paragraph 3: After hearing me rave some about that sad state of affairs, maybe some people then finally dug out problems to try during the period of March 13, NEVER HAVING LOOKED AT THOSE DUG-OUT ONES UNTIL PERIOD 6 was already underway. Such a strategy has about 10% of the study effectiveness of what I wrote as the ideal in Paragraph 2 above. Urgency too late.

 

Any individual who does things the good way (paragraph 2) could get high quality solving efficiency advice from me beyond what I've already put in my other teaching. But the only way I can share that efficiency advice is to be able to refer to what the student already tried before March 13 ever started.

 

There will be no modified scale or curve on the March 15 midterm. 85% and up will be an A. 70 to 84% will be a B and so on. Straight percentage arrived at from points earned divided by points available. This is because it won't be a timed test. People who know concepts and definitions will finish in a short amount of time and find it simple like they've been there before. People who don't would find the questions impossible no matter how much time they're given.

 

Two loose ends:

1) Someone asked me a detail question about calculating a potential value at a location near a spherical concentric shell system. It was from College Board 2007 problem. I told that person to figure out which limits to put in the integration in order to answer the question asked. I then said that the student should give a few minutes of thought and identify those limits of integration to finish the problem while I take care of some business. I said we could come back to it. Maybe that student figured it out, so maybe that person is fine. But that question about those limits of integration never got answered publicly in the Period 6 time, which matters if other people were listening in on that conversation. (And again, if you were listening in on that conversation and trying the problem, YOU need to be vocal and demanding and say you want its answer before you leave that day. I should not feel like I have to go post something on the internet to take care of that loose end when it could be that students speak up in class.)

But I'm not contradicting myself to say that I will post the answer to what those limits of integration are: this is specifically because I said to the individual who asked the question "You figure out what those limits need to be while I take care of some business for a couple minutes, and we'll reveal it later." "Revealing it later" so that I keep to my word to that individual has to be done on the internet in this case. I'll post it shortly under the title "Limits of Integration on work per charge Integral for FR2007".

 

2) I have provided 2 important documents for Experiment 3, which is due upon arrival on March 19. Those documents have the stated fact in them that the voltage between a and b was 15.0 V*. I was basing that on the way the equipment was set up. People informed me on 3/13 that the voltage between a and b was not 15.0 V. It was 12.5 V. This means I have to modify those two Experiment 3 documents and attach them here. People present on 3/13 were given the 4-page document called Experiment 3 2019, and that hard copy contained the erroneous 15.0 V value. It could easily be changed to 12.5 V by hand as opposed to printing out another copy. Carefully read the whole thing when making the change. Watch out for it in the graphical displays as well as the text.

 

*There were two other typos in addition on the 4-page hard copy of Experiment 3 2019: In one place where I meant to write "15 V" it accidentally says "5 V". But since it's supposed to be 12.5 V, that point is now moot. Also, the 4-page hard copy lists the wrong due dates for the assignment. In class, and here, I've been clear that the due date is 3/19. The electronic version of the document attached to this message does not have these errors.

All the questions I expected to hear asked on March 13 but didn't:

 

1) Questions about specific problems from "Gauss's Law Mastery Practice Document". I had recently reminded people to use this. All students have had this document for weeks.

2) Questions about specific problems 14' and 14'', the short additions to "Gauss's Law Mastery Practice Document" that I handed out March 11. Didn't try them by March 13? Sorry, too late; you should have tried them. Way harder to try them between March 13 and March 15.

3) Questions about specific items on the file titled "Self-Testing on Gauss's Law" that I handed out on March 11.

4) Questions about specific items from the 2009 AP exam problem I gave out a while ago.

5) Questions about specific items from the file Circular Symmetry in 2D: V(r) and E(r) Experiment 2019 that I posted some days ago, especially Objectives 2, 3, 4, and 5.

 

Need I go on? This is what I mean when I say that I can tell that there are people not looking at what I give them. Nobody learns this by being passive. I told people how to stay caught up in specific ways over the past several weeks. To gain skill requires using the applications I give and doing it close to when I give them. A person is supposed to do them urgently when I give them and then ask meaningful specific questions promptly. To do otherwise is what I'm talking about when I say people choose to fall behind.

 

The solutions attached here won't be that helpful to the people who let themselves fall behind. It'll be too late in the game. Those people will look at the workbook too late and spend way too much time on the peripheral things with little vision for the most important things that are central. How is a person supposed to have perspective, to have vision: that was supposed to come from having been done with Units I and II long ago when I said so and to have been spending much time on my applications such as in the list above and also on the problems at the ends of any workbook units (I through III).

 

This solutions attachment-set has a specific purpose. It's not for the behind people. It's for people who do things the right way, for the non-crammers who test themselves while things are happening, who've been practicing for weeks. Those people will by now have tried the workbook problems to which these solutions refer. Those people will do quick glances at the printed solutions and get nice confirmation of the good things that they've already been doing. They will not need to labor over these solution documents. NOBODY has time to labor over these solution documents and nobody who is caught up needs to labor over them. They are for quick glances. Do most of the practicing by writing solutions yourself, and do NOT use these printed solutions as a crutch to work backwards from. Working backwards from them is in many cases worse than not doing the problems at all.

How about this?

 

"Mr. Warren, when I tried the 2012 AP problem #1 that you mentioned on Monday March 11 (thank you, by the way), when they asked me to solve for the charge amount, Qi + Qo, what is the best place for me to draw my Gaussian Surface to answer such a question?"

 

Why should I not be hearing questions like that from all students on March 13?

 

Midterm, March 15. Coming prepared March 13 was a good idea.

Midterm Friday 3/15: Students who've missed classes are expected to be caught up and ready, because I've been putting the content online.

 

Files for finishing the unit, as of Monday 3/11:

Attached is the key for the Homework score that I graded on Monday 3/11

Attached is the setup for Experiment 3. Title is Circular Symmetry in 2D

There will soon be postings about what you are to do for Experiment 3. But they depend on the document attached here so start with this. The file attached here is the only one that will contain the data set that all people will use.

Attached is Unit IV itself. I explained clearly and specifically in class what certain Pages of Unit IV are used for. And anyone paying attention would see that its Pages 1 through 6 completely parallel practice applications that I already stressed in class. No other pages of Unit IV are required before the midterm. And all anyone needs to do is use Pages 1 through 6 as a resource and find that they provide useful review structure for what's already been covered.

 

There is more: Do you go to this page and test yourself on many applications:

 

I found the 2012 Problem. You may do it and grade yourself. I found it was a bit weak. It only asked for the sum of the two shells' charges. Do more. Find the charge of the inner shell. And find the charge of the outer shell. As separate quantities.

Answer to Unit IV Intro Problem 2 that was on the board in class on Thursday 3/7/19:

 

The charge needed for the voltage to be 100,000 V is 3.12 microCoulombs.

 

Think: is this right? Capacitor 2 was the concentric cylinders. It tends to deliver more charge than Capacitor 1, which was the 12 cm radius sphere. You can feel it. But Capacitor 1's answer was 4 microCoulombs. These two charge answers contradict the experience of how these two shockers feel.

 

There is an answer: you will learn that the 3.12 microCoulomb answer has be multiplied by 5.5 due to something called the dialectric effect. 5.5 is the dialectric constant of glass. There was glass between the two metal cylinders. The more accurate answer for the charge of Capacitor 2 is 17.2 microCoulombs.

Prior to Thursday 3/7

 

Check if you're caught up. One of the big problems of Tuesday's class had an answer of 70.7 megavolts. If you didn't get that value in at least one place, you did it wrong. If you got it right but the numbers were slow and tedious, you can do better. Attached is a one-page document to prove that metric value calculations can be quick, thorough, and not sloppy. It contains all the answers to the extended problem of 3/5, quickly and efficiently done.

 

Unit III attached. Motor through it ASAP. The idea is to go slowly and thoughtfully in Unit II and then quick Unit III. I've already covered Unit III"s content. It's mostly that E = -dV/dr. The 3/15 midterm covers both units thoroughly. Everyone is supposed to be well done with Unit II by now and using MY practice items to test the ability to apply Unit II's high level stuff.

Answer to a student's question about modeling Gauss's Law for the huge rectangular slab of charge.

 

This was asked on Wed. 2/27. Answering it is working ahead a bit. The attachment here addresses much of what the student asked, but some conversation in class will probably be needed (mostly because 3-D diagramming is not in the document.)

Important Timing Information to Know before Friday March 1:

 

First off, if you have seen me make a substitution as a part of teaching (like "Q*(D/L)") into a term on the board and seen me act like it's common knowledge to know such a substitution, IT IS NOT. I have been speaking in class under the assumption that you have used my cylinder notes (which is now two separate 6-page packets, Parts 1 and 2). If you have seen me write some algebraic term and thought, "I don't understand why he's writing that", know that the slower, more methodical explanation that you seek is there in those packets.

 

The following timing information has one big picture goal: to have the skill of high quality application of Gauss's Law by the end of the week. The purpose of using Gauss's Law is to be able to derive any E field as a function of position coordinate near a charged source. For example, it could be: For two concentric spherical shells, with the inner shell (with radius a) containing charge 5Q and outer shell (with radius b) containing charge -3Q, what is the E field function of radius for all radius values that lie between a and b. So with that goal in mind, here was my strongly suggested order of tasks, pretty much as I stated it in class on Wednesday 2/27:

 

1) Complete both of my Cylinder Note sets and all of workbook Unit II as soon as possible. By doing that, you'll be used to the step-by-step process of using Gauss's Law to get an E field function.

2) Pretend I were checking for HW-based knowledge with a quick document called HW4. This was to be answered in 5 minutes or less or else it's a total waste of time. To think it takes longer means that one lacks the language to recognize anything that's simple, and that's what it means to not do the reading. People who were absent on Wed. 2/27, open up the "HW4" attachment and do it in 5 minutes or less.

3) People who were in class on Wed. 2/27, you spent 5 minutes answering "HW4". Check your answer to that now with its attached key.

4) Hear my Experiment 3 story which talks about a Voltage measuring activity to make these other activities less abstract. But it's very hard to process this task when at home. (The equipment and application was shown in class on 2/27) so if you're at home, skip to task 5.

5) Test yourself by applying your Unit II knowledge to the document I gave called "A thorough Run-through..." But only answer for the E field items. Hold off on the items that ask for potential (unless you already think those are simple.) Also, I only gave Page 1 of this document as hard copy during the Wed. 2/27 class. The attached version has more pages with solution discussion. You should keep those pages hidden as you try to answer all of Page 1 on your own with no help.

Note: where this document says something about a thing being "grounded" that refers to issues involving the potential, V, but not the E field.

6) Review the idea that E = - dV/dr, or it might be E = - dV/dy. It's a spatial derivative where the position coordinate can be different letters.

7) Then take a crack at the items that ask for potential in the "Thorough Run-through..." document. But don't fret if the answers still come out wrong. Did you forget that integrals have constants of integration? This is where "grounding" comes in. Grounding is the place in space where you choose to make V = 0 when you do your integral.

8) After all of the above, you test yourself with the paper called Gauss's Law Mastery Practice.

 

For people who were absent on Wednesday 2/27, the papers associated with the above things have been attached or were handed out prior or were posted on the internet prior.

 

Note: The single-page hard copy version of the "Thorough run-through..." document called the huge rectangular slab a "wedge". I've changed its name to a slab.

 

And finally, a student asked me specifically about setting up Gauss's Law when the source is a huge flat rectangular slab of charge. I ran out of time before I could do it. I plan to post something to answer that question.

Support for Unit II Gauss's Law:

 

Cylinder Notes Part 2

 

(I will also be giving these as hard copy Wednesday, but anyone who is making good progress with Gauss's Law should print their own ASAP.)

Study Schedule Update: Now through 2/25

 

First, I think it very important to look at the one-page summary I attached here to drive home my main idea from the end of class on Thursday 2/21.

 

It's all about Gauss's Law now. Make an effort to complete Unit II, most importantly the second half of it, ASAP. If you're not thinking hard about the following things, you're doing it wrong:

 

Gaussian Surfaces

Flux

Enclosed Charge

 

After you've gone through Gauss's Law in Unit II of the workbook, use the packet I gave today "Unit II Rod of Charge Notes". That packet is also attached.

 

If you're putting mental effort into the meaning of Gauss's Law (which includes active diagramming), then things will go well regardless of any success or failure in solving things in the short term. In fact, read multiple sources on it. Read your textbook. Gauss's Law.

A couple more E field Superposition (vector addition) examples you can use to test yourselves.

 

In the 2018 one, the idea of relating the E field to the numbers of volts you see (which is the topic of the second problem in the document) is beyond the superposition idea and would not be considered a signal of being behind if one couldn't figure out how to do that part, as of 2/21/19.

Reminders and Facts for February 19:

 

1) Hand in your Electrical Oscillator write-up when you arrive on 2/19. 15 points. See the note below about a scoring detail in Experiment 1. In write-up 2, each of the three energy results asked for has to be directly determined. You may not make conservation assumptions to link the three to each other when calculating them. (You use those conservation law linkages to check the accuracy of your answers after you calculate each of the three independently by direct methods. K-max is based on the square of v, and I showed you how to get each of the two different U values.)

 

2) Unit II of the Electricity Workbook is attached. I'm also going to have hard copies of it to give to everyone in class on 2/19.

 

3) The big topic to learn well from now through 2/21 is something that in class I called "Principle of Superposition". I need to define that better, because the E&M workbook never calls it that. So if you look at the attachment ("E Field Superposition Example 2017), you'll see a pretty good illustration. It basically is the process of adding together multiple electric field vectors together at a point in space. The final result is called total E field at that point. It's important, because charge systems usually involve multiple charged items creating electric field in their vicinity.

 

Note about a Scoring Detail in Experiment 1:

When you were asked to evaluate the maximum kinetic energy of the pendulum, the straightforward thing to do is to calculate 1/2 times I times the square of angular velocity. When I said one could evaluate the difference in PE between start and Equilibrium, I stated that in class AS A WAY OF CHECKING YOUR RESULT. The whole spirit of my conversation was to get results in more than one way and to cross-check.

 

Now, if a person defends a method that is the loss in PE and expresses that as the result for the maximum K, there is nothing wrong with that. If that person then sets that Joule value equal to 1/2 times I times the square of angular velocity to isolate for dTheta/dt later, there is not quite inaccuracy there either. However, I don't consider that the absolute best way to get dTheta/dt, because it does rely on one more assumption than the method of simply expressing Theta(t) and taking its derivative. Furthermore, if one takes Theta(t)'s derivative, squares it, multiplies by 1/2 times I, then that person has a direct K-max calculation. That person can then verify that that K-max value is approximately equal to the loss in PE - in other words, verifying the maximum kinetic energy by two different methods.

 

If a person only defends the loss in PE, and subsequently has dTheta/dt entirely depending on that Joule value, instead of getting the dTheta/dt in a direct way (and also getting the K-max in two ways), then I used the word "presuming" for that in my grading comments. Such a paper can certainly be a high A*, but I do not consider it 100%, because there is still a slightly better way to defend results. A person who earned a 95% instead of 100% might have been wondering why they did not earn 100%. In some cases, it is because of the presuming that I just described. Please ask me in class about this if you need to.

 

*A 95% in my AP class corresponds to a 96.67% in classes that use the 70-80-90 grade scale.

 

Furthermore, the document "Rotational Energy Intelligence" covered all of this and was handed out in class for people to fill in. I have re-attached it here.

 

Also attached is a typed version of Unit I, which looks a little nicer than your handwritten one. If you already have the handwritten one printed, there is no compelling need to use paper again to print the typed one.

Experiment 2 Simulation

Due upon arrival on Tuesday 2/12:

 

(Update: 10:30 PM Sunday 2/10. Hints document that gives away most of the HW task is now attached. It's called "Experiment 2 Pre-HW solution notes".)

 

First you have to find your assigned parameters customized to your name in the attached spreadsheet.

 

Based on your assigned parameters, you have to mathematically predict two charge values, called Q and q. They have to be chosen such that the period of oscillation of the interior charge is some number of seconds that can be easily measured in Interactive Physics. So that obviously means a period of at least 10 seconds or so but not many minutes. Your Q and q value must be customized to you and based on the attached spreadsheet where the specific lengths and mass have been assigned individually to each student.

 

You will need an image of the problem. Use Problem 8 from the attached Unit I. Problem 8 is found at the end of the unit in the problem section. I showed it in class. In the Problem 8 image, there is a distance called b. In class, I called this distance d. Same thing. I'm calling it d from now on. But in the Excel spreadsheet, it is still called "b". b = d.

 

So predict the period for the numbers assigned to you in the spreadsheet. Make up charges Q and q in order to do so. Come to class ready to report the Q and q that you chose that will make the period measurable on Interactive Physics. The Q and q you commit to have to be written before you arrive to class.

 

For someone who paid attention in class, that all is enough information to do this. I covered it. And now here, it's covered in writing in an overkill sort of way. The word document that more than covers things is called "Experiment 2 Pre-HW solution notes".

Due Tuesday: In writing, work on all the new things (AKA pieces of language) that are in this Pendulum Write-up, and do so before Tuesday, 2/5. Don't guess about how to respond to the assigned items in the assignment sheet. Don't assume you know; don't guess; read the entirety of the attached document here, "How not to screw up the Pendulum write-up" and actively apply it to the write-up assignment questions. And do so before Tuesday. For all of it. Know everything that's required of you before Tuesday, from start of assignment to finish. I'll answer any questions that day. I won't answer any questions on Thursday. On Thursday 2/7, the write-up will be handed in upon arrival. There will be stiff late penalties for late labwork. I'll elaborate upon that in class. None of that needs to be a problem, because people can deal with things now. The attached document is helpful and thorough.

 

Other news: The HW1 check I did today on Pages 1 through 6 of the packet: 76.2% of the population that did all pages of the packet easily answered the in-class question. One person who was incomplete with the 6 pages of the packet answered the in-class question correctly and simply.

 

You should have solved the Knife-Edge Thick White Hoop Pendulum's period prediction; It's predicted period is 0.58 s.

 

If you'd like to clue into it now, its inner radius was 3.85 cm, and its outer radius was 4.45 cm. Predict its theoretical period. A problem very similar to this is at the bottom of Page 10 of the SHM workbook packet.

The F=Ma competition exam is happening at 3 PM on 2/23. The same one I mentioned in class a week and a half ago.

 

We still have about 15 slots. If you are interested, come and tell me at my classroom tomorrow. First come, first served. A little semester 2 extra credit is available.