More on Waves: 4 Items

1) As noted earlier several of you have decided that wavespeed increases if frequency does. Once again, this is not true and the equation lambda f = v is very misleading and should be written as lambda = cT or lambda = c/f. This properly indicates that the two independent quantities c and f both affect lambda. Wavespeed is given by sqrt(FT/(M/L)) for transverse waves and sqrt(Y/density) for longitudinal waves. See notes on these.

2)There is a velocity affected by the frequency but pay close attention to the following: By now you should have noticed that I never use v for the wavespeed, I use c. The wavespeed is not the velocity of a physical particle. No matter is moving at speed c. Matter moves at a velocity which is determined by the frequency and the amplitude because the matter is oscillating. For example, in a horizontally traveling transverse wave, where y = A sin ( omega t - kx), the matter is going up and down, with a velocity = omega A cos (omega t -kx). This physical velocity IS NOT the wavespeed. Omega here is 2pi f as usual and is also 2pi c/lambda. Once again, the frequency, f, and therefore the omega depend on the oscillation causing the wave. For example the frequency with which you moved your hand from side to side in the lab. Note that wavespeed is constant for the wave traveling in a medium, while the particle, or matter, velocity varies harmonically , as you would expect in a harmonic oscillation.

3) Nearly all of you got the first part of the very last question wrong ( item 8 at the end of the lab). See my notes so see the correct way to add them and BE MORE CAREFUL. Not everything is the easiest possible.

4) In order to answer a question, you must read it it. If they ask about phase answer about phase, not amplitude, not energy, not your favorite flavor. PHASE!!!! Waves can be represented as say y = A sin (omega t -kx) omega t -kx is th angle, the argument of sin , the PHASE, i.e what part of a cycle the wave is in.

Waves again

I warned you several times and still many of you act as if f lambda = c means that increasing f increases c. This is not true, increasing f merely reduces lambda. This is why I urge you to think of the relationship as lambda = cT = c/f.

Another way to misuse this equation is to suppose that if the wave speed changes as you enter a new medium the frequency must as well. It is the wavelength that changes as the speed changes. The number of pulses per second doesn't change when you cross over. Every you pulse you send from one medium reaches the other and the number of pulses you send per second equals the number of pulses per second that reach the other end of the first medium and the beginning of the new medium. [Unless it is moving towards or away from you, a complication we will deal with next week.]

AP Labs

1) Waves: There are few ideas that are getting confused here. In the traveling wave (slinky) lab you created the wave at whatever frequency you chose to shake the end of the slinky. This frequency was imposed by you and had nothing to do with the properties of the spring. The wave speed, c, was determined by the mass per unit length and the tension and that's all[ except for friction with the floor which tends to slow things a bit, especially for waves that are weak in movement any way]. So frequency and wavespeed are independent.

Do not confuse this externally imposed frequency with the resonant frequency of an oscillator given by 2 pi omega. The frequency in the wave lab was imposed by you, not by the materials.

For the questions referring to two differing media ( like that Latin , huh?) note that except for the floor friction, which was substantial for the coil ( heavy spring), the tension in the two media had to be equal. You can't pull harder on one end then the other unless the springs were accelerating down the hallway. If they were, I missed it.

2) Standing waves. A standing wave is a different situation. This is where a single pulse, or small imposed oscillation of a frequency which is related to the wavespeed, causes large oscillations because waves reflected from both ends reinforce each other. In this case, say for two fixed ends L meters apart, the harmonic frequency IS related to wavespeed by f = c/2L or 2 c/2L or 3 c/2L etc. [This is because the wave must satisfy the condition that wavelength = 2 L/integer so that both ends are nodes and nodes are an integer number of half wavelengths apart. This is the same as requiring that reflections from the two ends cause the right kind of interference at nodes ( destructive) and antinodes (constructive).] Since in this case f = integer c/2L, a higher wavespeed and higher frequency are associated. If you try to impose a frequency other than a harmonic you will not get a standing wave pattern.

Wednesday's Attendance

While I would like to express my appreciation to those who came to class in the stands today, I can't help but be somewhat disappointed by the 20% absentee rate. I thought you folks were made of sterner stuff.

Wednesday

Ugly rumors of a snow day tomorrow are being spread. Since I know that none of you can survive a day without physics class, we will meet at regular class hours tomorrow. If necessary, we will meet in the stands on the south side of the turf (yes I know that's the visitor side, but it will be warmer on the south side, after all it's closer to the equator, isn't it?) Dress warmly and bring your waterproof notebooks and put plenty of antifreeze in your pens.

Also bring some snacks for the sled dogs, they can get pretty nasty when they are hungry.

PS If am not there DON'T wait for me.

Group Reports

Both AP and Honors should have their first study group meeting reports for this quarter in by Friday. Date (not "Sunday"and time; subject (not just "homework" or "problem sheet" tell me what topics you covered); attendees; and place are required. One per person.

Fire Drills

FYI:

we will be having the following two fire drills on Wednesday, February 17th:

1. End of period 5 @ 11:20 a.m.
2. End of period 8 @ 1:35 p.m.

Note the times and don't expect to remember to remind you.

LABS

See post of December 1 re late labs.

AP problem 2 Unit 9 WS 2

Apologies, linear mass density is .0085kg/m not /m^3 and answer is in N not Hz.

Honors problem

A plasma has a current density of 8x10^5 amperes/m^2 in a field of 4T. However the field is not perpendicular to the current, it makes a 9.5 degree angle( 1/6 radian) to the current. What is the force on a cubic meter of plasma from the magnetic field?

Honors and AP

The media center printers are being overloaded. Please refrain from printing large downloads on teh media center printers.

Extra Help Schedule

AP: Tues AM, Wed Fri PM

Hon: Tue Thu PM

Initial emphasis is repairing midyear exam damage

Other times as available

Honors Shedule Unit 8 Magnetics

Here it is [note on worksheet that 105 is 10^5]. Its not as bad as it looks because you have read much of this stuff once or twice already.

http://h1.ripway.com/DrCherdack/HnrUnit8Magneticsch10.doc

http://h1.ripway.com/DrCherdack/Notes%20on%20magneticsR8.doc

http://h1.ripway.com/DrCherdack/TokamakFields.doc

http://h1.ripway.com/DrCherdack/Magnetics%20Worksheet%20M.doc

http://h1.ripway.com/DrCherdack/MagneticsWorksheetM2.doc

http://h1.ripway.com/DrCherdack/Rotational%20Motion%20in%20Magn.doc

http://h1.ripway.com/DrCherdack/NotesMagFldsandTransprtPrt1%20.doc

AP Urgent Exam Registration

If you plant to take the exam read this Message from guidance:

This is just a reminder that AP Registration begins today, and runs through 11:30pm on Friday, February 19. Your students all received a letter at home with details and instructions for using Community Pass; that letter and all other links and information can be found on the Guidance web site, on the AP page (http://www2.bernardsboe.com/Guidance/Advanced_Placement_Testing.aspx).