Grades are in

Your exam 3 scores and final averages are now posted to eLearning. Your exam 2 scores have also been updated to reflect any gains from the extra credit problem. Exam 3 went very well, with a class average of nearly 90%, meaning there was no scaling.

Your final averages have taken into account dropping your lowest quiz, HW, and lab scores. Recall that exams are worth 20% each, HW is 20%, and labs and quizzes are worth 10% each.

Let me know if you see anything still missing or incorrect, or have questions about how your grades worked out. Just to head off a common question: I know some of you will find yourselves very close to a grade boundary, but at this point I can't really offer any additional extra credit. I will very carefully check borderline cases though.

Your final letter grades are also posted on myBama now.

UPDATE - your exam 2 score will only change if your extra credit score is higher than your lowest non-dropped question on exam 2, since the extra credit question replaces the lowest-non dropped question if it is higher. Thus, if your lowest non-dropped question on exam 2 was 7/10 and you got 7/10 on the extra credit, your exam score does not change.

More grade updates

UPDATE: I have added HW5 grades, and made a few corrections as you've notified me of missing grades (some of which I am still tracking down). The only thing missing now should be exam 3 and the exam 2 bonus ... all labs, quizzes, and HW should be there. If you notice anything missing let me know as soon as you can, and I'll track it down one way or another. Should have exam 3 grades and a final overall average posted by Sunday night I expect.

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Grades for everything except HW5, the bonus for Exam 2, and Exam 3 should now be on eLearning. Once Exam 3 is graded, I will post a final average as well. Recall that I will drop 1 quiz, 1 HW, and 1 lab.

Grade updates

A few more things on eLearning. The grades that are not ready yet (and hence do not show up on eLearning) are:

HW4, HW5
Q5, Q7
L6, L9

I expect most of those things to show up some time tomorrow. Final grades will be ready by Sun-Mon I hope.

All quizzes now have solutions

at least the ones from this summer, that is.

Summer 2009 final

the relevant problems (and a couple more) have solutions.

Final time & location

The final is tomorrow morning at 8am in room 203, i.e., the lecture hall.

Spring 2008 final

The questions relevant for tomorrow's exam now have answers, and some of them solutions. 

Relax. Read your textbook. That is all.

If you're not going to that, you can check out the formula sheet though.

Summer 08 final exam

has a partial solution up. All the optics problems have answers at least. 

Old finals

Once I finally get home, I will start posting answers to as many of the old final questions as I can. I have many more requests than I can probably do tonight, but I'll do what I can. Start looking for them around 7pm.

A few more example problems

on quantum/atomic physics. I should be able to post some more tomorrow evening, but these are highly relevant for the exam.

Exam 2 extra credit hints

So the problem is a tough-looking one, but I swear this is an incredibly useful circuit. The circuit I gave you is a filter, and one fair tactic is to google "RLC filters" and see if you can find a match. Inelegant, but effective. I will tell you it is a "bandpass" filter, but you'll probably spend just as much time trying to understand the pages you find as you would just working the thing out with a little help. I say that because you will run across a lot of electrical engineering pages which speak a different breed of math than you are used to, and the language barrier will be tough to overcome, not because this is a particularly tough circuit.

We know reactances add like resistances. The whole circuit has L and C in parallel, and that combination in series with a resistance. The total reactance is then $X_{tot} = X_R + X_{L||C}$, where $X_{L||C}$ is the reactance of the capacitor and inductor in parallel, which you'd find by adding the $L$ and $C$ reactances like you would parallel resistors. The resistor's reactance is just $X_R=R$. The current in the circuit is then the input voltage divided by the total reactance, $I=V_{in}/X_{tot}$.

What is the output voltage? You're measuring the output voltage across the $L$ and $C$ parallel combination, so it must be $V_{out}=IX_{L||C}$. Find the equivalent reactance of $L$ and $C$ in parallel, add the resistor's reactance, and you have the total reactance. That gets you the current, and the current times the equivalent reactance of $L$ and $C$ in parallel gives you the output. This will all be a function of frequency as well as $R$, $L$, and $C$.

Qualitatively, you can ignore the $L$ and $C$ one at a time. Without the $C$, it is a high-pass. Without the $L$, it is a low-pass. The whole circuit looks like a combination of the two - hates both high and low frequencies, but will like a middle range in between.

To sketch the plot, presume some values for $R$, $L$, and $C$. I would say perhaps $R\!=\!100\,\Omega$, $L\!=\!1\,$mH, and $C\!=\!1\,\mu$F. What is it good for? Any time you want to let a small range of frequencies through, but nothing else. Radio comes to mind.

HW5 solutions

can be found here. The exam problems on this material will (promise) be easier than the HW questions - the exam problems will probably be somewhere between these problems and the optics questions on your quizzes.

These HW problems were hard (mostly) intentionally - given that we had to cover the material rather quickly, I wanted you to wrestle with some harder problems in the hopes it would help the material sink in a bit better. Whether that works or not is debatable, but I think it makes sense at this point that at least reading the solutions for these harder problems will help you understand what's happening in simpler situations, even if you didn't quite solve them correctly on the HW.

Possible extra credit on exam 2

can be had by solving this problem. Partial credit will be generous, and there is no way that doing or not doing the problem can hurt your grade. I will happily provide hints if you ask relatively direct and specific questions.

Wednesday's lab

will be on atomic spectra. Specifically, hydrogen and mercury, with a chance of helium. 

Homework 5 hints

I'll start off with number 1 and keep adding to this throughout the evening.

1. The apparent depth is where the light appears to have come from under the water (according to the observer) if refraction were not present. Extrapolate the light back from the observer, ignore refraction, and when the extrapolated ray reaches the same horizontal position as the actual object, you're at the apparent depth. Draw a little picture, it will help.

a. The angle of incidence can be related to the angle the (real) ray in the water makes with the surface via Snell's law. Once you have this angle, since you know the real depth you can find the lateral (horizontal) position of the object with some trigonometry. Now worry about the extrapolated ray under the water. You know its angle with respect to the surface is the same as the ray above the water by construction. Using the lateral position of the object and this angle, you can find the apparent depth.

b. You can generalize your result from a if you are careful about setting the angle to zero, or you can realize this is the same as a spherical surface with an infinite radius. For a spherical surface you know $n_1/p+n_2/q=(n_2-n_1)/R$. If $R$ tends to infinity, this gives $n_1/p+n_2/q$. The apparent depth is $|q|$, the real depth $p$, and that's that.

2. You are given $p+q=L$, and the lens equation tells you $1/p + 1/q = 1/f$. What will determine the minimum value for L? If we want to focus an image, we need to have a real image, not a virtual one. For a convex lens, this will happen only when $p>f$. We also know that $q$ must be positive for the image to be real. Combining these two facts, we really only need to make sure that $q$ is real and positive in order to have a real image formed. If $q$ is negative, the image is virtual. If $q$ is imaginary, so is the image ...

What you need to do, then, is solve one equation for $q$ and put it in the other. The resulting expression for $q$ will be quadratic, the condition that the roots are real is that the discriminant is positive. Start by noting that $1/p + 1/q = 1/f$ can be rewritten as $1/f = (p+q)/pq$. Knowing $p+q=L$, change the numerator. Knowing that $p=q-L$ (since $p+q=L$), change the denominator. Solve for $q$ in terms of $p$ and $L$. Do that stuff I just mentioned above. A "4" should be involved.

3. The previous problem should have given you two possible solutions for $q$ that give the minimum $L$. Those solutions should be solutions to the quadratic equation, and look something like: $q\in\{q_1,q_2\}=\{x+y, x-y\}$ where $x$ and $y$ are quantities involving $L$ and $f$ (and likely a "4"). The "$y$" quantity is likely to look like $\sqrt{\ldots}$. Those are your two possible $q$ positions, which imply two possible corresponding $p$ positions  $p\in\{p_1,p_2\}=\{L-q_1,L-q_2\}$.

You know the magnification factor in each case in terms of the q's and p's. In the first case, $M_1=-q_1/p_1=a/h$, and in the second $M_2=-q_2/p_2=b/h$. Solve both of those for $h$, and multiply them to get an equation for $h^2$. In the resulting equation, you will have to show that $p_1p_2/q_1q_2=1$. Use your equations for $q$ above and multiply your two solutions together, cancellations will occur and it will end up simply. Do the same for the $p$'s, noting that $p_1=L-q_1$ and $p_2=L-q_2$. If you are careful in your algebra, the desired result will follow.

4. a. There is only one sort of spherical mirror that always gives virtual images. For any spherical mirror, the focal point is at a distance $R/2$. However: if the (virtual) image forms on the opposite side of the mirror as the image, what does that imply about the sign of the focal length? Remember the sign conventions!

b. You know $M=-q/p$, or $q=-Mp$. You also know $1/p+1/q=1/f$. Taking careful notice of the sign of $f$, you can plug in $q=-Mp$ and $|f|=R/2$ and solve this thing for $p$ in terms of $M$ and $R$, both of which are known. Your value for $p$ should come out to be positive.

Notes for today

I will follow this, more or less.

Stuck in Memphis

So. I write this from a dingy hotel in Memphis, thanks to Delta airlines. My flight back was cancelled, too late to get a different flight out or start driving. This has a number of important consequences:

1) The extra credit assignment is not ready, and will not be until Tuesday afternoon some time. I will let you know.

2) I am not going to make it right at 11am tomorrow. My best-case scenario will get me to UA in time to start class at 12:00, so let's start then. Better an hour than nothing. Should my travel plans go further awry, I will have someone leave a note on the door so you don't wait around for me ...

3) I am unlikely to be very responsive to email before noon tomorrow - I have been on minimal internet connections all weekend, and tonight is not great either. I will reply to what emails I can.

4) If you are wondering about what is going to be on the final, keep wondering until Tuesday night.

5) If you are wondering about an alternate final exam time and have a good pre-arranged reason to do so, find me after class tomorrow.

Today's lab

you will measure Planck's constant.

HW5

is out. Due Tuesday. We'll go over some of these in class on Tuesday if you get stuck.

Left my phone at home ...

so if you need to reach me today (Thurs), email is basically it.

Updated grades

Your exam 2 and homework 3 grades are now available on eLearning. Enjoy, and let me know if anything seems mistaken.

The average for exam 2 was 78.6 with a standard deviation of 14.3. The A/B/C/D/F breakdown was 15/11/10/10/6. I am happy with it overall. More intelligent comments after I've slept a bit.

Thursday's lab

you will do 2 short experiments. First, demonstrating the laws of reflection and refraction, and second, playing with lenses. Print both procedures, you should finish relatively quickly.

There will be a quiz in the lab, on refraction.

HW4 solutions

are here. I don't have a solution typed up for the last problem right now (and it is exceedingly unlikely to happen before the exam), but nothing like it will show up on the exam. Also, I did it all in class today ... if you missed that, you can ask one of your classmates.

Summer 2010 Exam 2

This handwritten solution is as good as it is going to get. The last problem I didn't solve since it isn't relevant to the upcoming exam, and time is running short ...

Summer 2009 exam 2

problem number 4 now has a solution. The original number quoted (without solution) was a factor 2 too high for part a. Part b was fine. 

Exam coverage

I will not ask questions about EM waves, on further reflection (<- you will see the pun after tomorrow's lecture. HA.) That leaves:

in the Knight book, Ch. 23.1-6, 24.1-7, 25.1-4. 

in the Serway book, 18.1-4, 19.3-9, 20.1-6, 21.1-3

or, by topic, dc circuits (multiple battery circuits especially), magnetic fields, induction, ac circuits (filters only, as in notes)

HW6 summer 2010

is relevant to your interests, and I just put up a solution. Must have missed it at the time.

Spring 08 HW6 is also updated to have more or less complete solutions as well.

Grades online

You can now view your grades online through eLearning ... I have grades for HW1-2, Q1-4, L1-4, and Exam 1 posted right now. I won't use eLearning for anything but posting your grades. I don't much like it in general, but it is convenient enough for this purpose.

If somehow you have escaped eLearning until now, consider yourself fortunate. You can access it by going to your Student tab on myBama, it should be a fairly prominent link. Once you're in, it should be easy enough for you to figure out how to find your grades. If you have any trouble, let me know. 

Monday's lab

will be about induced voltages and transformers. There will be a quiz after the lab.

Homework 4 is out

and you can find it here. Sorry for the delay - meant to have it done Friday, but helping my parents move this weekend, among other things, took more time than I planned. Note that it is not due until Tuesday now as a result of my delay.

And, yes, these are excellent problems to study for the upcoming exam. We'll go over most of them in class Mon & Tues.

Exam grades

Your first exam is graded, as of right this minute. The average was 83.6 with a standard deviation of 9.5, which implies you were more clever than expected (or I was more generous in partial credit, but let's be optimists here). In any case: well done, most of you will be happy.

Do show up tomorrow to get your graded exams, you'll also get back Quiz3 and Lab2, and possibly Quiz4 and Lab3.

See you in a few hours ... we'll go over the exam and then begin with induction and so forth. 

Tomorrow's lab

will be on simple transistor circuits. You will understand transistors, but be initially baffled by the protoboard of all things. It is just the way of the world, apparently, so don't feel bad about it.

Random exam information

Earlier a student asked by email what they should do to study, etc. Here's my response with the original questions paraphrased [in brackets].

1) [What to study/read] I think you'd probably be just as well off reading the notes - start with the chapter on electric forces and fields, and continue through dc circuits. Most of my problems will be close to the treatment in the notes. I think the notes are worth a quick read to make sure you get the ideas down, but the best thing is probably to work problems. Look at the old HW and exam problems and try to work through some of those, that will be closest to the real thing.


2) [What to focus on?] Some things to focus on in particular are:

• what is the E field from an arrangement of point charges, like a square?
• what is the potential energy of a system of charges, like a crystal (like the HW problem)?
• the equivalent capacitance of an arrangement of capacitors and the charge stored
• the equivalent resistance and the current through each resistor
• what is the power in resistors and how current relates to drift velocity and stuff (chapter on current)

3) [What to put on the formula sheet?] you'll have all the formulas you need on the test, so the best thing for the formula sheet is probably to have a few example problems and notes to yourself on when to use different formulas. A good example might be to have one of the crystal problems or equivalent resistor problems worked out so you'll know the steps if one of those shows up on the exam.


4) [When are you around?] I should be on campus by 10:15 and around my office, or at least I'll try to be. I will also try to leave a little time after class and end early so you can ask questions - I have a meeting at 1:15, but I can try to leave at least 12:30 or 12:45-1:15 free.

HW3 solutions

are out for your studying pleasure.

UPDATE: I corrected a dumb mistake in 3a - I forgot to multiply by 4 when I collected all of the q5 terms, so the answer should be 4-15*sqrt(2) times the energy of a single pair, or about -17.21 as pointed out in the comments. The link above has the corrected version.

UPDATE: Small mistake in 1b as well - the initial formula for acceleration is OK, but the final expression where I've plugged in the result for d was incorrect, as was the numerical answer. Fixed now. 

Quiz 4

and its solution are now posted.

Quiz 2 and 3 solutions

are here.

HW3 hints

1. First, remember that e=1.6x10^-19 when you plug in numbers for the charges. The alpha particle has a certain amount of kinetic energy, and the closest it can get to the gold nucleus is when it has spent all of its kinetic energy at the expense of electrical energy required. At closest approach, the electrical energy between the alpha particle and gold nucleus will be equal to the initial kinetic energy of the alpha particle. At that distance, acceleration is force per unit mass, and there is only one force present ...

2. Use Gauss' law - the potential due to a spherical grain at a distance r, its radius, would be the same as that of a point charge of the same magnitude. That gets the total charge, and you know the charge of one electron.

3. Sum up the energy of all unique pairs. See this example from a previous homework set to get started.

4. If S₁ is open, the battery is not connected to a closed circuit ... If S₁ is closed, and S₂ is open, C₃ does nothing, since it is connected on only one end. This leaves you with two capacitors in series ... find their equivalent, and then find the total charge. What do you know about the charge on capacitors in series?

5. Remember that series resistors split the total supply voltage while parallel resistors have the same voltage. Remember that power is current times voltage, and power determines brightness. Given the same battery voltage in each case and the same resistance R for each bulb, find the voltage and current for each bulb to get the power and then rank them.

Monday's lab

will be all about resistors and RC circuits. It will use the lab boxes like last time, hopefully things will be a bit easier the second time around.

Homework 2 solutions

are here. I'll add Quiz 2 & 3 solutions tonight or this weekend.

HW3 is out

you can find it here. Due Monday in the lab period.

Thursday's lab

is all about circuit elements and learning to wire things up. There are two files: an 'intro' one to walk you through how to use the test equipment, and a procedure for the actual lab. This one will again be a bit more inquiry-based, the main idea is really just to wire stuff up and see what happens ... and then explain why it happened.

[Yes, there will be a quiz, it will be on capacitors.]

Today's lab

was probably a bit hard for you. That is OK. There are really two reasons for this.

1. Scheduling. I chose to do relativity first, the other two classes I have to stay synchronized with lab-wise did not. My way is sort of a pain on the instructor (not following the textbook order for one), but I think the course as a whole ends up more coherent. Also, the PH106 section we're syncing with doesn't even cover relativity at all. As a result, for the first week or so the labs come a bit sooner than I would like. This will be resolved next week.

2. Realism. You walked into the lab knowing only a little bit about capacitors, and then were expected to figure out how they acted in combination - while also knowing next to nothing (on average) about the power supply and meter you were supposed to use. This probably meant you were confused at first, had to read and think a bit, and ask a lot of questions about the equipment. It is frustrating, but this is a much more realistic picture of how real experiments work - many unknowns, all you can do is be systematic and careful. It is more a barely-guided exploration, driven by your inquiring as to how things work, than baking a cake or assembling some Ikea furniture. There is never a complete recipe or instruction manual that will always work. We can't do all the labs like this - just throw you into the fire, as it were - but I think it is a good experience to just turn you loose with minimal preparation and see what you can figure out some times. We will do some 'cake baking' labs too, but doing only those gets boring.

That being said, tomorrow in lecture we'll discuss capacitors, meters, and circuits in detail, and I think in retrospect everything will make sense, and you'll have a better appreciation for what you did today.

Tomorrow's lab is a milder version of the same thing - we're going to turn you loose with various electrical components and see what you can make them do. It will be less difficult than what you did today, but will still involve an element of exploration that is really key to proper experimentation. 

Wednesday's lab

is all about capacitors. We will start to discuss capacitors in Wednesday's lecture before the lab, and use the lab period to verify some of the things we'll discuss.

To be perfectly honest, it is a fairly simple little lab that is as much intended to get you used to operating electrical equipment as it is to prove various things about capacitors. You won't really have to apply the uncertainty analysis you learned on Monday just yet either, but that will come back soon enough. In light of this, take Wednesday as an opportunity to do a relatively simple procedure carefully and really get the hang of connecting electrical components and wiring things up. You can also take advantage of the lab time to ask the TAs questions about homework or lecture material.

In Thursday's lab we will start to learn about circuits. It will also be relatively simple in principle, but the explanations for what you will observe will be more subtle. Paying attention on Wednesday and getting used to wiring things up will pay off on Thursday.

Solutions and such

I have solutions for quiz 1 and homework 1. You'll get your graded HW1 back tomorrow (Wed), I suspect the TAs will have the quizzes back to you shortly as well. 

Tutoring

We have one undergrad physics major who is game for doing some tutoring over the summer if anyone is interested. Send me an email, and I'll forward his contact information. Don't want to put his email & phone numbers online for obvious reasons ... he did some tutoring for a few PH105 students during the last summer semester, and it seemed to work out well for them.

HW2 is out

and you can find it here. Due at the end of Wednesday's lab period.

HW hints

For the first problem:

1. http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/polebarn.html
2. http://en.wikipedia.org/wiki/Ladder_paradox
3. http://www.xs4all.nl/~johanw/PhysFAQ/Relativity/SR/barn_pole.html

For the second problem, the length must be contracted according to the astronaut. The astronaut would report a velocity which is the contracted distance divided by the time he or she measured.

For the third problem, the proton sees a contracted length. It would observe a time consistent with this distance divided by the relative velocity, whereas in the laboratory we would observe a time which is the rest length of the tube divided by velocity.

For the last problem, the difference between dilated and proper time is 2s every 86400s. This must mean that the spaceship clock runs a factor (1+2/86400) slow.

Tomorrow's lab

will be on uncertainty analysis, a short experiment in counting cards. There will be a quiz during the lab period on relativity - very similar to problems I went over in lecture. It will be mostly about time dilation and length contraction.

Homework 1

is out. Due at the end of the lab period Monday (so you can ask me or the TAs questions in lab ...)

The first problem is tricky. Discussions of it are easy to find online, we will go over it in lecture on Monday a bit.

Slides from today

are here.

Monday we start with electrostatics, so that's your reading for the weekend. We'll be done with relativity for a little while, so things will get conceptually a lot easier (less confusing!) but mathematically a bit more difficult (in particular, vectors).

If you're feeling lost

don't fret it just yet. Special relativity is probably among the most conceptually difficult material we will cover all semester, and it involves relatively subtle arguments and reasoning. (Oddly enough, the math is straightforward.)

So, if you felt a bit lost during today's lecture, don't worry, it is not at all unusual. It will take time for the arguments and really the necessity of relativity to coalesce properly for you, it probably isn't going to happen just listening to me for 45 minutes. Read through the notes (really read, don't just scan for highlighted items), read through your textbook, and don't hesitate to ask me if there is something you're not getting.

It will still be weird even when you think you understand it. The weird won't go away, but the confusion will. There is a difference - weird can still be logically consistent or even a necessity to have logical consistency.

Slides for lecture 1 & 2

Here are some slides I'll be using for today's lecture, as well as part of tomorrow's.

Video of previous lectures

Back in the summer of 2008, I made videos of a good fraction of my PH102 lectures. Not everything is there, but you might find them useful as the semester wears on. You should also check out iTunes U, lots of nice lectures there, as well as an iPad/iPhone app.

Welcome to PH102!

At the bottom of this page, or here, you will find our course calendar, which you should review carefully. Note that if you click on any lab event, you will see a link to the relevant lab procedure. A separate table of the laboratory experiments can be found here. We will stick to our schedule rigidly, as our time is quite short over the summer semester. 

You may also want to look over the course syllabus and other information, which you can find here. The slides I'll present in lecture tomorrow will also show up online shortly, I will let you know separately in a follow-up post.

Finally, I have written some fairly lengthy notes for this course, and my lectures will follow these notes for the most part. For this week, we'll be covering relativity, which is Ch. 1 in my notes.