The PHY 124 Fall 2010 Lab 5 pretest is now posted for you to work on and submit before the beginning of your Lab 5 section next week. You will find it in the Assignments content area in Blackboard for your PHY 124 Lab section. As explained in the course syllabus, each Lab pretest is worth 35 points. As you will see, the Lab 5 pretest consists of 2 questions. The first is worth 14 points, and the second is worth 21 points, making the total possible score 35 points. To do the pretest you will need to study carefully the Lab 5 manual and to re-familiarize yourself with MapleTA syntax, which is used in both questions. Go to
http://www.ic.sunysb.edu/class/phy122ps/labs/dokuwiki/doku.php?id=phy124off:phy124_main_page
and read the instructions there on the "main page". Please also click on and review the
-- "Instructions" link (you've seen this before)
-- "Uncertainty, Error and Graphs" link (you've seen this before, and it's important for both questions in the Lab 5 pretest)
and, finally, the
-- "PHY 124 Lab 5 - AC Circuits " link.
The "red" links are not yet active, on purpose, but as the course develops they will be made active.
As you already know, the lab pretests are prepared in the "Maple TA" software environment. You may work on each assignment as many times as you wish, but the link in the Assignments section of Blackboard for your PHY 124 section will become inactive at the starting time of your lab section. Therefore, you should not wait until just before your lab section to begin work on it. If you read the Lab Manual carefully and, there is no reason for you not to earn all 35 points for the pretest. Make sure you read carefully the instructions for each pretest problem. After each problem click on "Next" until finishing the last one, Question 2; then make sure you click on "Grade" in the Maple TA software environment. After you get your grade, you may click on "View Details" to see more about how you did. Finally, click on "Quit and Save".
Good luck!
Prof. Koch
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27 comments:
In Pre-lab Question #2: I am having trouble figuring out the equation for the relative uncertainty in the resonance frequency. I figured out the equation for the relative uncertainty in L*C, but how does that relate the the relative uncertainty in the resonance frequency?
The resonance frequency f_0 depends on L and C via a formula you're asked to submit. Since L is uncertain by 10%, and C is uncertain by 10%, the resonance frequency is also uncertain. Mathematically how the fractional uncertainty in f_0 depends on the 10% fractional uncertainty L and the 10% fractional uncertainty in C is what you have to derive. The referenced equations from "Uncertainty, Errors and Graphs" are what you need to use. You have to apply those equations to this case. Problem 2 "steps" you through this.
I understand how I am suppose to plug in the numbers once I have the equation, but I am still having trouble finding the equation for Deltaf/f. I have tried using the equations from the handout, but I am still not able to come up with the equation. This is the second to last part of problem #2.
I am having the same problem. I can come up with the formulas for the other error equations, but the equation for f is way more complicated than the one for the time constant. I'm not even sure how to approach this one.
Referring to UEG: (E.7) gives you the expression for the fractional error in S when S is either AxB or A/B. Take L for A and C for B; take LC for S. Then (E.7) tells you how to find the factional error in LC. Now f_0: it's a function of LC, in particular an inverse square root. The constant factor in the equation for f_0 drops out when you calculate the fractional error in f_0. (E.8) tells you how to find the fractional error when there is an uncertain quantity, here called A, that is raised to some exponent. Identify A with the product LC. From above you know what the relative uncertainty is for the product LC. So in (E.8) you identify DeltaA/A with Delta(LC)/LC which you know. You also know what n is from the inverse square root; n = -1/2. The minus sign is of no concern because in (E.8) you need only the magnitude of n. You identify S with the symbol f_0. This gives you all the steps. I'm not going to quote the final result here.
The directions given were a little unclear. Your explanation helped a lot. Thank you!
I know I have the correct equation at this point, because I am able to plug in the numbers and get the correct answer for the last part of problem #2, but it is still not accepting my answer for the equation. In looking at the problem there is no Equals Sign, like there is for all the other parts of the problem, could this be the reason it is not accepting my answer?
instead of just putting in deltaLC/LC use the entire equation that you figured out above for deltaLC/LC. I was very confused about this for awhile.
I also have a question about calculating F0 for question 2. I have the correct equation but when i use the numbers 2.5E-2 henries and 10E-7F i get 1006.58 hertz for the resonant frequency but it says its wrong. does the resistance come into play somewhere?
Anonymous at 3:45 pm: 10E-7 F for the capacitor?!? My lab manual shows 10-7 F. So does the Lab 5 pretest.
I always have a problem with the "equation" entering. It looks perfect to me in the "preview".
Looks exactly like the equation I used to get the correct answer...
I really don't like these questions :( It tests me "ascii art" ability instead of my understanding. IMHO
So...anyone want to care a common webassign glitch that may be responsible for the CORRECT equation not being accepted? Too many (), not enough ()? ^2 should be inside or outside and then nested again?
:D thanks
correction maple T.A. not webassign.
Does anyone know how to calculate absolute uncertainty, For question number I took the 10% of R, which was a 100 and then i took the 10% of C, and I got 1.0e-8, and then i put that into equation 8 and got 100 , but the answer is still wrong, please help!
And, for number 2, please help with the last two parts. I tried everything. Thanks.Bye.
Finally, my txt equation was accepted in one of them. I used that as a "template" and cut-n-paste it into the other equation questions (adjusting the letters as needed).
I moved those () around so many times (and the preview looked exactly the same for all of them)..
..in the end, I don't know what was different.
I heart maple t.a.
Dear Anonymous:
I guess you heart maple T.A. also :)
I'm having a hard time understanding which equation you are using.
October 18, 2010 12:53 AM
Anonymous Wrote:
" and then i put that into equation 8 and got 100"
10% means that error/value = .1
because 10/100 = .1
.1^2 + .1^2 = .02
When you multiply or Divide you would use equation (E.7)
As for Question #2...
The professor already posted a quite clear answer @
October 17, 2010 11:52 AM
To SKOR220: Equation 6 is the formula change in S=(((A)^2+(B)^2))^(1/2) )
(E.6), sorry i meant equation 6.
I put 10% of R= 100 and 10% of 1.0e-7 + 1.0E-8. And, then i used equation 6:(100)^2+(1.0E-8)^2 and then i took the square root and got a 100.
I know the professor was clear for question 2, but i am kind of slow, so can you please explain it step by step, I would appreciate it. It's fine, if you don't want to, I will consider it my bad luck. Thanks.
I know that the equation for resonant frequency is f(initial)=
1/(2*pi*sqrt(LC)) but for the question that asks you for the numerical answer where L = 2.5 x 10-2H, C = 10-7 F, and R=100, I keep getting 3184 hz as my answer when I plug values into the equation and its not right....wat am i doing wrong? :( Helppp!
anonymous wrote:
To SKOR220: Equation 6 is the formula change in S=(((A)^2+(B)^2))^(1/2))(E.6),
Okay, but that equation isn't for "us". We find the Time Constant by multiplying, yes?
Alright, so let's use E.7 instead of E.6.
Anonymous wrote:
I keep getting 3184 hz
Skor answers:
i plugged the numbers into my calculator and i get a slightly different answer...
try plugging in sections and not the entire equation at once.
Hey Chris, I did do it separately and the answer is stil in the 3183.9 ish area!
Ok,
I am not understanding the relative uncertainty for questions 1 and 2. I have no idea what to input, because when I put in sqrt(DeltaL/L)^2+ (DeltaC/C)^2)) its wrong. Please can someone explain in english instead of in math terms, its so hard to visualize anything like this on a blog.
Anonymous: you wrote
sqrt(DeltaL/L)^2+ (DeltaC/C)^2)) . This has a syntax error of one more right ) than left ( .
Everyone: As I wrote earlier, "UEG" (E.7) shows what do do to calculate the relative uncertainty DeltaS/S for a quantity S that is of the form AxB or A/B given relative uncertainties DeltA/A and DeltaB/B. This suffices for Problem 1 because A is R and B is C, and time constant is S. Problem 2 goes one step further. Now you have an inverse square root function (of L and C). I explained earlier how to handle this.
Anonymous said...
Hey Chris, I did do it separately and the answer is stil in the 3183.9 ish area!
Chriss Says..
When I plug in the numbers I get
3183.09
I wonder what it is that you're doing differently..
For question 2. part 6, I can't seem to figure it out. I have inputted abs(n)(Deltaf0)/(f0) this is not working. Prior explanations on this blog concerning this question have not been sufficiently helpful!
Thanks!
professor Koch, with all due respect, you merely restated what was in the lab manual, not helpful if you ask me.
Anonymous said...
when I put in sqrt(DeltaL/L)^2+ (DeltaC/C)^2)) its wrong
Chriss Says..
I have a terrible time with these myself. One thing I *know* for certain, is that (DeltaL) has to be nested.
For my own sanity, I use sqrt[stuff in here] brackets I think they're called, for the STUFF inside the square HOUSE.
Instead of sqrt(stuff)..which ends up looking like sqrt((stuff)operation(stuff(otherstufff)))
see how it's easy to loose track?
sqrt[stuff(things)+garbage(things)]
now i can see clearly that the stuff under the sqr is under the square.
That way I don't confuse it for something else that I nested.
Another thing, there's a magnifying lense icon next to the answer box. If you click it; you'll see a "preview" of what the equation looks like and it will tell you if there are errors.
For myself, I think I had a hard time with where to put the ^2.
Anonymous said...
professor Koch, with all due respect, you merely restated what was in the lab manual, not helpful if you ask me.
October 18, 2010 12:51 PM
Dear Anonymous..Let me not be the first to tell you, with all due respect..
This is one of those lovely instances where your incompetence has no fault beyond your own.
Thanks :)
**this has been another helpful comment. Please now return to your regularly scheduled programs.
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