Virtual Office Hours: Questions and Answers
Please send us your questions with "Phys2020" in the subject line of your email! They may be about anything - this week's homework assignment, the course content, course administration, or whatever. If appropriate, we will email an answer to you directly. Or, in many cases (where we think the question might be of general interest), we will instead post an anonymized version of the question+answer on this page instead (we'll chop your name off the posting).
Please email us your questions to steven.pollock (at) colorado.edu, or Andreas.Becker(at)colorado.edu.
Final exam questions
1. I am wondering if there is an example picture anywhere (couldn't find one in the book, your lecture notes, or on the Phet) that is of a diverging lens with the object between the focal point and the lens. I can't figure out where the image would be.....can you help me out?
Check out the link in the virtual office hours (see below!)
http://www.physics.uoguelph.ca/applets/Intro_physics/kisalev/java/dlens/index.html
See if that helps
2. Will we be needing to calculate angular magnification or just know the concepts of it? This is the material from ch. 25 that we went over, and is also covered on pg 17 in your lecture notes for the Ch. 23 section.
We never did hw problems on this, so no, I won't zoom in on it. Regular magnification ("transverse magnification", m= hi/ho) WILL be on the exam though, since we worked with that on several homework problems.
An article about a study technique from a psychologist of learning
Sim for Diverging Lens:
Hello Prof Pollock, I found this phet-like simulation online for diverging lenses! It really helps clarify how a diverging lens works.
http://www.physics.uoguelph.ca/applets/Intro_physics/kisalev/java/dlens/index.html
And some other sims:
2 slit: http://www.walter-fendt.de/ph14e/doubleslit.htm
single slit: http://www.walter-fendt.de/ph14e/singleslit.htm
ripple tank:http://www.falstad.com/ripple/
superposition:
http://webphysics.davidson.edu/Applets/superposition/default.html
Rainbows:http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=44
or http://www.atoptics.co.uk/rainbows/orders.htm
From a student: Since we are doing optics I think you will appreciate these cool mirrors designed by a mathematician. For example, there is a "nonreversing" mirror, and one that doesnt distort perspective, and some other cool ones.
http://www.newscientist.com/gallery/dn16585-amazing-mirrors/1
________________
Answer key error on CULEarn
Dear Dr. Pollock, I wanted to inform you that I believe there is an error in the CULearn answer key for CAPA #9. On question #11, you have included (delta t) in the equation used to solve the problem. The problem asks only for the magnitude of the change in the magnetic flux and NOT for the change in EMF; thus, (delta t) is not involved in this question.
You are absoluately right, I have just changed the solutions. We are so used to questions asking about EMF, it just became automatic. Thanks!!
Steve
_____________________
From a student: (Pretty amusing/interesting, about whether cows respond to the Earth's magnetic field) NPR article about cows and magnets
(And another: Here is one link to the cow story. I heard it on NPR, they talked about the study done by the German guys and this article mentions them and gives some other links. Pretty crazy!
http://www.physorg.com/news156445879.html
From a student: Very cool article about nickel particles in a special magnetic field.
http://blog.wired.com/wiredscience/2009/03/snakes.html?npu=1&mbid=yhp
Thanks - pretty interesting indeed! (I was amused by the quote at the end of the article)
And while you're there, there's a nice link to their "top 10 amazing physics videos":
http://blog.wired.com/wiredscience/2008/09/top-10-amazing.html#previouspost
Exam 2 - extra office hours
Hi Dr. Pollock,
I hope you enjoyed your conference. I was wondering if you are going to be holding an exam review tomorrow night, as you did before the last exam. Let me know,
Yup, I plan to be there (G1B30) - see you there! (It's not a prepared exam review, simply an extra evening office hour, 5 PM)
SP
Exam 2 question: crib sheets
I was wondering if for the cheat sheet we are allowed to have for the test, if we can use the back of the one we had from Exam 1, that way we do not have to do a whole new one? I feel like I remember you saying in lecture that we are allowed to do that, but I just wanted to double check! Thank you very much!
Yes, you can just use the back of the old one for new stuff. From the syllabus:
You may bring a single side of 8.5 in. x 11 in. paper for the first exam, with your own handwritten notes. We will let you add one more side for each exam (i.e. one side for Midterm1, two sides for Midterm2 etc.) Calculators with scientific notation are allowed and sometimes needed. (Of course, no phones, internet access, "electronic crib sheets", etc!)
Interesting use for magnets:http://www.reuters.com/article/oddlyEnoughNews/idUSTRE51O5RV20090225
People confused about units
Prof. Pollock - Here is the kWhr calculation failure I just mentioned in class:
http://www.youtube.com/watch?v=lCJ3Oz5JVKs
Theramin:
Don't worry, by happy:
http://www.youtube.com/watch?v=e6bSRcRAhnc
Joke from a student (can you do better?! I'll post 'em here)
Why did the guy driving the train get hit by lightning?
Well, he was a good conductor.
Exam coverage:
I am just curious up to what section in Chapter 17 is the test going to cover?
Officially, I said through 17.5
But I also said in class that "equipotential lines" won't explicitly be on the exam, since we haven't had much practice with those yet. (Also, we haven't talked about the "eV" unit yet, that's tomorrow in class, so that also obviously wouldn't be on the exam.)
Basically, any Ch. 17 material that was emphasized in CAPA 3 would be fair game!
LONG ANSWER 1, 2nd to last question
When computing the E field at the center, you need to know the charge on both bells, but we only gave you the charge on the right bell. Since the left one is grounded, it does not NEED to stay Q=0 (it's connected to the earth so charges can flow to or from it), but for the purpose of this question, that would be the simplest assumption to make, and is what I would suggest you make!
CAPA #3, question #3:
Hello Prof. Pollock, im a little confused about #3 on this week's capa. To determine
electrical potential, dont we need to know where the potential is 0? And if a pt charge is along the bottom, but has an electric feild charge upward, would its electrical pot. be 0 or non-0??
Thanks!
Excellent question!
Yes, to determine potential, you need to know where V=0. In class Wed I mentioned (but only super quickly) that when you have a point charge, or a collection of point charges, we make a standard assumption/convention that V=0 off at infinity. That way, V(from any given Q) = k Q/R (where R is the distance from you to that charge. You can see from that formula that it has "built in" the fact that if R goes to infinity, then V goes to zero). That's what you should assume in Capa #3 (and really, CAPA should have been explicit about that assumption, it's just quite standard)
As for your second question, perhaps the answer above will help you? V(at a point) = the sum of V's at that point arising from ALL other charges. (Each of them contributing k Q/R) Of course, Q has a sign, so Voltage has a sign, and thus voltages can "add up" or they can "partly cancel". They are NOT vectors, they are numbers (they have no direction), but they DO have a sign, and thus can cancel...
Hope this helps - we'll be covering this more in lecture tomorrow too.
Cheers,
Steve
Questions about exam and solutions:
Hey Prof -
Will there be practice exams posted for this class?
Also will the clicker questions ever show the answers online or is it up to us to get them in class. Thanks
There were (or will be, if your lab is later this week) practice exam questions in this week's lab!
And yes, the clicker questions do have answers, have you spotted the links at the top of the concept test web page, where you can see the histograms from the lectures. In those, it also indicates which answer is correct.
Cheers,
Steve
_______________
Public Service announcement :-)
VOLUNTEER FAIR: February 2nd
Looking for ways to gain valuable experience while doing something you love?
Want to be more involved in your community?
Wondering how you can make a difference?
All this plus free lunch and a chance to win a free ipod?
Please stop by the
SPRING VOLUNTEER FAIR
Monday, February 2nd
11am to 3pm
UMC Glenn Miller Ballroom
CAPA SET #1, Question #2:
Hi Steven: I got an early start on capa and the only one that was a problem was #2. The question is not complete
Here's what the CAPA hint says (which appears if you get it wrong!) Let me know if you need any more help:
"This problem is phrased a little funny, but we're asking you to think about
what units (or "dimensions") can tell you when you're solving problems. If an
equation is "dimensionally correct", that means the units of the left side match
the units of the right side. (What would you think about an equation where that
was not the case?)"
For fun: check out some sites below. (If you find a link that you think is relevant for Phys 2020, let us know, we can add it here)
Many fun (and potentially useful) electromagnetism links: http://www.colorado.edu/physics/phys1120/phys1120_fa07/links/links.html
The CU "physics simulators" page: http://phet.colorado.edu
A crazy demo: http://drmegavolt.com/index.html
_____________________________________________________