Solutions to Some AP Chapt 15 Porblems

Here is a slightly readable PDF.

I haven't checked the arithmetic carefully but the approach is what's important.

APChapt15Solns07.PDF

Solutions to Electrostatics Worksheet 2

The moment you've been waiting for! Well maybe not, but here are the solutions for AP Unit 10 WS2


SolnsElectrostaticsWS2.doc

AP Wave Retest Info

AP students who had a raw score below 82 on the last wave test are eligible for a retest on Wednesday outside of class hours, provided they attend a mandatory review session either Tuesday after school or Wednesday at 7AM. The maximum grade you will be credited with will be the equivalent of an 82 (strong B-). You must also bring in corrections to the original test to qualify.

Note on Standing Wave Nodes

Many of you working on the standing wave lab confused the condition for an antinode and odd half periods with the conditions for a node. Note that there is usually no difference in effect between odd and even WHOLE periods. Odd HALF periods are needed for ANTInodes if a phase shift occurs on reflection. [SEE the Notes published on Feb 12 on this blog]

For a point on a string or similar medium to be a standing wave node, waves arriving from the left and from the right must be out of phase at all times. A fixed end produces a half cycle wave shift in a reflected wave. If a wave passes through a node say going left travels to a fixed end and gets reflected by the fixed end and then travels back that is to the right ( now shifted by half a cycle) to that node, it will arrive at the node out of phase with waves traveling to the left if it takes ANY INTEGER (even or odd) number of periods to make the round trip from node to end and back to node.

This is consistent with the fact that there are an integer number of loops between any node and any fixed end and each loop is half a wavelength. Therefore the round trip will include twice the number of loops as the one way trip from node to the end and therefore an even number of loops. The number of loops is equal to half as many wavelengths but half of an even number is always an integer. Therefore the round trip is an integer number of wavelengths and takes an integer number of periods. [ Remember wavelength = speed x period so a distance of N wavelengths takes a time of N periods].

AP Free Response Package

As you know, your free response package is due Monday. In order to facillitate reviewing the material, please prepare your solutions on separate clean sheets as requested in the instructions you received. Then, transfer the key parts of your solution to the problem sheets. This way we can go over the problems even though you have submitted your assignment on the separate sheets. The district's email system seems to be having a nervous breakdown, but you can reach me by submitting comments to this blog.

Enjoy the rest of your break.

Dr. Cherdack

Honors Break Assignment, Retest Data

Here are two worksheets for your assignment over the break. They are due Monday. You must make a serious attempt to meet with your new group at least once. Each student must write a paragraph about what you did at the meeting. I hope it includes some work on physics. The Unit Number on these is 10 not the Unit 9 we are currently on because they are from my AP class.

Also, for those who got a raw score of 67 or less on the wave test, you can take a retest on Tuesday, provided you come to a mandatory study session Monday after school or Tuesday before school by 7 AM. If you do not know your score you can email me.

Unit10Worksheet1.doc

Unit10Worksheet2.doc

AP Assignment due Fri 2/16

AP, as promised, here is your assignment for tomorrow Friday 2/16

Unit10Worksheet2.doc

Extra Credit Wave Problem

As promised, here is an extra credit porblem on waves. Its tough, but it will provide another perspective on waves as traveling oscillations. It will also give you some glimmer of differential calculus.

ExtraCreditProblemonW.doc

Simple Harmonic Motion Lab Sample

I have posted a sample of the Simple Harmonic Motion Lab. Please refer to this to for correcting your lab report since it will show you what was expected. This is much more efficient than writing the same comments a few dozen times. Note that I used a stronger spring and some different masses to get my data, so use this sample as a guide and don't expect your results to match it numerically.

SampleSimpleHarmMotLab.doc

Corrected Solutions for Homework

You can use this link to the corrected solutions. In addition to problem 5 I have corrected the phase shifts for questions on reflection to pi from pi/2. I meant 1/2 cycle, which of course is pi NOT pi/2.

HmwrkonOscillsWavessolns.doc

Correction 2

It was Molly Kirk who found and reported the error to my solution to problem 5. My apologies to her.

Notes on Standing Waves

Do NOT confuse crests or troughs with nodes. Maximum crests and minimum troughs have the same amplitude, just different signs, and occur at antinodes. Nodes ( not antinodes) are where there is no change because there is always destructive interference. A wave crest passing through a point going to the left will come back reflected from a fixed end as a trough going to the right. If the point is to be an antinode this reflected wave must return to the point an odd number of half periods later so that another, unreflected (i.e.”original”) wave going to the left will also be a trough, so that you will have constructive interference between the two waves.

For a place to be a node other than at the end of the string, it must have continuous destructive interference. That means that the wave reflected with a phase change must arrive an integer number of periods after the original wave passes through, or 2 x distance from node to end = integer number of wavelengths. The distance between a node and any antinode is always an odd number of quarter wavelengths. The distance between any two nodes is an integer number of half wavelengths. The distance between two antinodes is an integer number of half wavelengths. Draw a sketch and you will see all this.

New Corrections to Lab and Homework Prob 5

Standing Wave Lab has error pointed out by Laura Goldberg. The relationship is between lambda and sqrt of [FT/(M/L)] NOT square of [FT/(M/L)].

Molly Hahn dropped me a note about correcting the solution to number 5 . She pointed out that the square root of (6.4 E10/2.5E3) is 5059 m/s and this is the correct answer for the wave speed. Therefore the correct answer for the wavelength is 10.2 m and for the wave speed in the second material is 7133 m/s. Sorry, I calculated the orignal speed for Y =6.4E11 N/m^2 for some unknown reason.

Solutions to Oscillator and Wave Homework

Here are preliminary solutions to the assignment posted Saturday. I haven't checked them yet so be careful. Try to do the assignemnts without seeing the solutions first. For AP students, only those you solve on your own count for extra credit. Your on your honor.

HmwrkonOscillsWavessolns.doc