AP-1 Waves & Sound HW #2 problem 15

A rock band plays at a(n) 85 dB sound level. (a) How many times greater is the sound pressure from another rock band playing at 100 dB? etc.

So, this is your “challenge problem” on this assignment.  As such I will not help you complete it, but I will help you get started.

If you don’t know what to do with the logarithmic equation, this may help a bit:

Let x = your variable and y = a number.

Log 10 x = y

Raise both sides as exponents of ten (the log base)

10 Log 10 x = 10 y

Raising the log expression to be the exponent of the log base effectively undoes the logarithmic operator, leaving behind the result

x = 10 y

You can solve the equation with any log base in exactly the same way.


AP Study Resource (All classes)

We had just gotten access to some AP study resources through the library’s Gale databases.  Since I have not yet seen them, I cannot guarantee the quality of the practice exams, they do seem like a good study option.  Use the following link to create your personal account and track your progress:

At the top hover over “High School Tools” –> Advanced Placement Exams (AP)

Then select the class you want.  If you are looking at physics, be sure you select AP-1 or AP-C respectively.  Other classes like AP-US History are available.


AP Circuits HW 2, problems 2 & 3, an Quiz KEY

I think you will find these much easier than first appearance.   For number 3, just be careful with your signs.  I have included the solutions for the currents below for a random sample problem.

AP Circuits HW 2, problem 2

AP Circuits HW 2, problem 3

Notes on problem 3:

Many of you have asked about the negative current.  Here you go.

So, about the currents…as I explained it is something of a guessing game.  I think that I misconstrued a question in class today about negative currents.  Is it OK to have a negative current?  Yes! As long as your algebra proves the validity of your solution.  A negative indicates that you chose the wrong direction for one or two branches of your loop rules.  If your algebra does NOT work out, then you have to go back and redo your direction in your loops.

So, to my example problem.  First, there are a huge range of values available on that problem so yours may be VERY different.  In fact, I just recycled the problem twice on my teacher view (I can do that with my version of the assignment) and the values of battery 1 and 2 flip flopped…something that would change the way I did my loops.  Before I get into details here is something to remember: You have two parallel branches with batteries and one with a resistor.  If it helps you model it against my work, you can rotate your circuit by 180 degrees out of the plane of the paper until your larger voltage is on the left like mine was.  As long as you keep the right resistors attached to the right batteries in the right order you have the same circuit!

1) I used the inside loops rather than the outside since that would give the the current through the central 4 ohm resistor in both equations…easier algebra.

2) I choose the direction of loop 1 to be consistent with my 5V battery

3) I choose the direction of loop 2 to be consistent with the larger voltage, in this case the 5V battery.  Basically when 5V meets 3V in the opposite direction, you net 2V in the direction of the 5V source.  Essentially the bigger battery wins, right?  (underlying big picture question: why?  More unbalanced charge at the terminals of course).  So, you are correct that loops appear to conflict in the center 4 ohm resistor but…

4) Look at equation 2–notice how the signs of the voltage losses and gains compliment the direction established in equation 1.  Consistency is the key!  The middle term in equation 1 (-I1(4)) indicates a loss through the center 4 ohm resistor.  Look at equation 2.  The middle term is positive reflecting a “backflow” of current in the opposite direction that I established in equation 1.  Again, it doesn’t matter which is correct.  Consistency wins.  Your math will tell you the direction (positive or negative).

5) Equation 3 represents the node rule for the junction at the top of the circuit, again using the larger battery for the positive direction.  Again, consistency.

I trust you can follow the algebra?  Let me know if you need assistance with that.  It is no bother.  I should ask you to check me as I actually did this problem last year and just re-posted the work today, so by all means scrutinize my algebra.  It is very easy to switch a sign when you are dealing with systems of equations, as you surely appreciate.

Now, back to my original point.  Is it OK that I got a negative value for I2?  You bet!  Why?  When I check it against equation 3 it returns a true solution.  What does it mean?  It means that the direction of current flow I established in I2 (to the right) is wrong.  Turns out this current should be flowing to the left.  Does it matter?  No.  The value​ of the current is correct.  Remember also that I2 represents the current in the 5 ohm resistor.  Why should it be to the left?  Well, the math really answers the question but lets go back to the problem.  Set aside the need for consistency of signs in your algebra and just study the figure.  We have two batteries working “against each other” in the outside loop, but complicated by the fact that both are connected by the central branch with the 4 ohm resistor.  Both batteries are similar in voltage (one is not way bigger than the other), so imagine a positive charge (conventional current) in the wires.  If you are by the first 4 ohm resistor you dont want to go toward that battery.  If you are by the 5 ohm resistor you don’t want to go towards that battery.  Imagine both batteries pushing out positive charge…where does it go?  Towards the middle and down the central branch, getting sucked away to the negative terminals on both sides.  Now does the negative answer for I2 make sense?

So why didn’t I do that in the first place?  Consistency.  I made a guess, that turned out to be wrong, but because I was consistent in establishing direction on all of my work, it still returned an algebraically true solution that also makes good physical, conceptual sense for the circuit in the problem.

Below please find the key to the back side of the recent quiz (that I handed back today).  I did not grade this problem since you were pressed for time.  Please be sure to review prior to your exam.

AP Circuit properties quiz back side KEY


AP-1: Schedule updates and Waves test info

This information will be distributed independently to students via text & twitter for those subscribed to Mr. Boehringer’s communication services.

In compliance with Mr. Wright’s option and due to the weather event Thursday, March 5, we will postpone our exam over waves and SHM until after Spring Break.  This move will ensure that A-day and B-day students are assessed on an equitable level.  I hate to give you a test immediately after a break, even in an AP class, but we simply have no choice under the circumstances.  All students should expect to take their test covering waves and SHM when we return from break, exceptions being given only for those who have had extended absences due to illness and family emergency this past week of March 2-6.  We will review during the first 20-30 minutes of class, and take the test during the last 50 minutes of class.  Students should come to class with questions for review, and are encouraged to go back over completed homework, class notes, and vocabulary in preparation for the exam.

Please recall that you have a study guide available here.

A-Day students will test on Monday, March 16.  B-Day students will test on Tuesday, March 17.  The test will still count on the 3rd nine-weeks average.  Students who miss school on their test day may receive an “incomplete” for their 3rd nine-weeks average and this may impact their eligibility status for UIL event participation.  Tutorials the mornings of March 16 and 17 are reserved for students needing to complete make up work from the 3rd nine weeks. Students with questions in preparation for the test are free to email me in the interim and are encouraged to utilize the resources already provided to them.  Normal tutorials will resume later in the week.

Schedule of upcoming classes:

Friday, March 6: A day classes will begin learning about Sound waves and their characteristics.

Mon, 3/16 & Tues. 3/17: Tests over waves and simple harmonic motion for A- and B-day respectively.

Wed (A) /Thurs (B): Continue study of sound.  An updated syllabus is now available here:

AP-Physics-1-syllabus-Waves-A-day-REVISED with test after break

AP-Physics-1-syllabus-Waves-B-day-REVISED with test after break 


Since I have your attention…A note about the upcoming AP Exam:

Students are encouraged to obtain a review guide for the AP exam if they have not done so.  Unfortunately as this is a new course from the College Board, I cannot recommend any specific book.  Here are the details.

1) Buy a review guide that is for AP Physics 1 (or AP-1 and AP-2).  Do not buy a guide for AP Physics C or, the now defunct AP Physics B.

2) Princeton Review  Often a favorite of mine in the past, the book is quite well produced, with great problems and explanations, but it seems to have some issues in that it has topical material that is not supposed to be on your test (AP Physics 1).  This is probably because it is a brand new course and nobody has seen an exam yet, so the authors are likely hedging their bets.  This book will be a good help even if it doesn’t match the exam completely.  I own this book.

3) 5 steps to a 5  Used to be the worst physics prep book on the market, in my opinion, but this one is very well reviewed, for the most part.  However, even good reviews cite a lack of example problems and insufficient explanation on some problems and topics. I have purchased this book, but not yet received it.  This book could be a good help.

4) Barrons  At first glance it appears that Barrons did very little to change their old Physics B book into a new review guide for the new courses, except perhaps reordering the topics.  I have purchased this book, but not yet received it and will provide more detail later.  I do not recommend purchasing the book at this time.

5) AP Physics 1 Essentials  Do NOT buy this book as an AP exam review.  The title is misleading and even the author seems to aknowledge this in response to a negative review.  The book could be a good companion to the course, but likely falls short as a “AP review guide.”  A new 2nd edition just came out that may be a better study aid for the AP exam.  I have ordered this new book and will update my review after I have seen it.


AP-C: Circuits HW 1, number 12

AP Circuits HW 1 question 12


AP Physics 1: Study outline for Waves & SHM exam

I have decided that we have enough material in this unit to justify this exam as a full test grade, rather than the half-grade that I indicated prior to now.  I believe this exam is fairly easy, but please do not neglect the earlier study of SHM.  Here is a study  outline to help you out. I’m also re-posting my tutorial schedule for next week.  Please make sure that you have all of your work made up.  I will be putting in zeroes in skyward soon for missed quizzes and other items.  Check yo’self, don’t wreck yo’self.  Yo. (Just pretend I said that in my usual deadpan style.  Now laugh.  There.  How’s it feel to let loose with a good chortle?)

Study Guide–Waves-SHM test (part 1)

Tutoring Schedule n10


All classes: Calendar and Tutoring Updates

AP Physics 1 syllabus-Waves-A day–REVISEDagain

AP Physics 1 syllabus-Waves-B day–REVISEDagain


Tutoring Schedule n10

And now, introducing the reason for my absence…the very first baby boehr: Raynes Alice



Baby Boehr--2-20-2015


Prosper High School UIL Invitational Results

Thank you for attending the PHS 2015 UIL Invitational Academic Tournament.  We hope that you will join us next year.

  • Academic Champions “Sweepstakes”: McKinney High School
  • 2nd Place Academic Team: Liberty High School
  • Top Journalism Team: McKinney High School
  • Top Speech and Debate Team: Gainesville High School

All Academic Contests:
PHS Invitational 2015 Score reporting sheet ALL contests

Speech & Debate Results can be found on the Joy of Tournaments website.

We will do our very best to get outstanding medals and test materials to your respective campuses if they were not picked up.  If you wish to stop by the school to pick up materials we would appreciate it.  Please let me know your preferences.


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