physics

Playing with Transformers & Building an AC to DC Converter

My one remaining IB Physics HL student was alone in class today, so I made a quick switch on plans to do one of the required activities in the syllabus: building a full-wave rectifier circuit. This was also an opportunity to play with transformers (no, not that kind of transformer) which I presented last class as a really useful application of electromagnetic induction:

I did almost nothing during the whole class period, aside from giving instructions and asking questions. With wires, resistors, and other circuit parts all over the place, this was not a well organized lab, but that doesn’t matter. Here’s what we did:

Measure the AC voltage and frequency coming out of the supply
Connect the supply to one of the transformer coils, and place the other coil inside. Measure the voltage and frequency on the secondary coil. Comment on the number of coils on the primary and secondary. Observe the effect of inserting an iron core into the transformer.
Predict what would happen if the two coils were switched, then measure to compare with prediction.
Build a half-wave rectifier circuit with a diode and resistor using the secondary coil as a supply. Measure the voltage wave form.
Build a full-wave rectifier circuit, and measure the voltage signal across a resistor. Check this out:

This yielded a nice ripply DC signal:
Connect a capacitor in parallel with the resistor.

As we had hoped, the ripples are almost gone!:

In the final step, I connected a 5V regulator to the output and showed that this would be a way to make a cell phone charger if we had the right USB connector.

This is quite possibly the most satisfying and successful ‘let’s build something’ lab I’ve done with students. It fit neatly within a class period, including the clean up time. Granted, things would be different if there were multiple groups going through the process, but I’ll just ignore that fact because it’s a Friday.

A New Year, A New Formula Sheet

I wrote earlier this year about my difficulties with equation reference sheets. Students fall into the habit of using them like a menu rather than a set of tools.

Yesterday I had the great experience of trying a revised approach. The idea is to rely more on memorization, but only so far as keeping close definitions that are crucial to understanding.

A student asked during a quiz for a formula for electric field. I said I wasn’t going to provide one, so I pushed further to find out why this student needed it.

The student asked for the relationship between electric field, charge, and force. I provided this:

The student subsequently nodded and told me what went where. I then stepped away, keeping the post-it note with me.

This was an application of the definition of electric field, not Coulomb’s law, and the student either knew this, or guessed. In either case, it was enough to enable the student to then solve the problem completely.

I don’t know yet what this means, but it seems like a step in the right direction. I wonder if providing a reference sheet with random elements erased might be enough of a skeleton to encourage the students to know how to fill in the blanks, but wouldn’t allow the sort of hunting that students often do in a problem solving situation.

It has been a nice start to the year, folks. Here’s to a great school year for all of you.

2014-2015 Year In Review: IB Physics SL/HL

This was my first year teaching IB Physics. The class consisted of a small group of SL students with one HL, and we met every other day according to the block schedule. I completed the first year of the sequence with the following topics, listed in order:

Semester 1

Unit 1 – Experimental Design, Uncertainty, Vectors (Topic 1)
Unit 2 – Kinematics & Projectile Motion (Topic 2.1)
Unit 3 – Newton’s Laws (Topic 2.2)
Unit 4 – Work, Energy, and Momentum (Topic 2.3)

Semester 2

Unit 5 – Circular Motion, Gravitation, and Orbits (Topics 6.1, 6.2)
Unit 6 – Waves and *Oscillation(Topic 4, AHL Topic 9, *AHL Engineering Option Topic B3.1,3.2)
Unit 7 – Thermal Physics (Topic 3, Engineering Option Topic B2)
Unit 8 – *Fluid Dynamics (Engineering Option Topic B3)

For the second semester of the course, there was at least one block every two weeks that was devoted to the HL student and the HL only content – the SL students worked on practice problems or other work they had for their IB classes during this time. Units 7 and 8 were concurrent, so the HL student had to work on both the thermodynamics content and the fluid dynamics content together. This was similar to how I did it previously while teaching the AP physics B curriculum.

One other fact that is relevant – none of my students are native speakers of English. More on this later.

What worked:

The growth students made during the year was significant. I saw students improve in their problem solving skills and their organization in the process of doing textbook style assessment problems.
I learned to be honest about the IB expectations for answering questions on assessments.In the beginning, I tried to shield students from questions that combined conceptual understanding, computation, and complex language, often choosing two out of the three of them for any one question that I either wrote or selected from a bank. My motivation was to isolate assessment of the physics content from assessment of the language. I wanted answers to these separate questions:
1. Does the student understand how the relevant physics applies here?
2. Does the student understand how to apply the formulas from the reference table to calculate what the question is asking for?
3. Can the student process the text of the question into a physics context?
4. Can the student effectively communicate an answer to the question?
On official IB assessment items, however, this graininess doesn’t exist. The students need to be able to do all of these to earn the points. When I saw a significant difference between how my students did on my assessments versus those from IB, I knew I need to change. I think I need to acknowledge that this was a good move.
Concise chunks of direct instruction followed by longer problem solving sessions during class worked extremely well. The students made sense of the concepts and thought about them more while they were working on problems, than when I was giving them new information or guiding them through it. That time spent stating the definitions was crucial. The students did not have a strong intuition for the concepts, and while I did student centered conceptual development of formulas and concepts whenever possible, these just didn’t end up being effective. It is very possible this is due to my own inexperience with the IB expectations, and my conversations with other teachers helped a lot to refine my balance of interactivity with an IB pace.
Students looked forward to performing lab experiments. I was really happy with the way this group of students got into finding relationships between variables in different situations. Part of this was the strong influence I’ve developed with the Modeling Instruction curriculum. As always, students love collecting data and getting their hands dirty because it’s much more interesting than solving problems.

What needs work:

My careless use of the reference sheet in teaching directly caused students to rely excessively upon it. I wrote about this previously, so check that post out for more information. In short: students used the reference sheet as a list of recipes as if they provided a straight line path to solutions to questions. It should be used as a toolbox, a reminder of what the relationships between variables are for various physics concepts. I changed this partly at the end of the year, asking students to describe to me what they wanted to look for on the sheet. If their answer was ‘an equation’, I interrogated further, or said you aren’t about to use the reference sheet for what it was designed to do. If their answer was that they couldn’t remember if pressure was directly or inversely related to temperature, I asked them what equation describes that relationship, and they were usually able to tell me.
Both of these are examples of how the reference sheet does more harm than good in my class. I fault myself here, not the IB, to be clear.
The language expectations of IB out of the gate are more of an obstacle than I expected at the beginning of the year. I previously wrote about my analysis of the language on IB physics exams. There tends to be a lot of verbal description in questions. Normally innocuous words get in the way of students simultaneously learning English and understanding assessment questions, and this makes all the difference. These questions are noticably more complex in their language use than that used on AP exams, though the physics content is not, in my opinion, more difficult. This is beyond physics vocabulary and question command terms, which students handled well.
Learning physics in the absence of others doesn’t work for most students. Even the stronger students made missteps working while alone that could have been avoided by being with other students. I modified my class to involve a lot more time working problems during class and pushed students to at least start the assigned homework problems while I was around to make the time outside of class more productive. Students typically can figure out math homework with the various resources available online, but this just isn’t the case for physics at this point. It is difficult for students to get good at physics without asking questions, getting help, and seeing the work of other students as it’s generated, and this was a major obstacle this semester.
Automaticity in physics (or any subject) shouldn’t be the goal, but experience with concepts should be. My students really didn’t get enough practice solving problems so that they could recognize one situation versus another. I don’t want students to memorize the conditions for energy being conserved, because a memorized fact doesn’t mean anything. I do want them to recognize a situation in which energy is conserved, however. I gave them a number of situations, some involving conservation, others not, and hoped to have them see the differences and, over time, develop an awareness of what makes the two situations different. This didn’t happen, partly because of the previous item about working physics problems alone, but also because they were too wrapped up in the mechanics of solving individual problems to do the big pciture thinking required for that intuition. Group discussions help on this, but this process is ultimately one that will happen on the individual level due to the nature of intuition. This will take some time to figure out.
Students hated the formal process of writing up any parts of the labs they performed. This was in spite of what I already said about the students’ positive desire to do experiments. The expressions of terror on the students’ faces when I told them what I wanted them to do with the experiment break my heart. I required them to do a write-up of just one of the criteria for the internal assessment, just so they could familiarize themselves with the expectations when we get to this next year. A big part of this fear is again related to the language issue. Another part of it is just inexperience with the reality of writing about the scientific process. This is another tough egg to crack.

There was limited interest in the rising junior class for physics, so we won’t be offering year one to the new class. This means that the only physics class I will have this year will be with the same group of students moving on to the second year of IB physics. One thing I will change for physics is a set of memorization standards, as mentioned in my post about standards based grading this year. Students struggled remembering quick concepts that made problem solving more difficult (e.g. “What is the relationship between kinetic energy and speed?”) so I’ll be holding students responsible for that in a more concrete way.

The issues that need work here are big ones, so I’ll need some more time to think about what else I will do to address them.

Circuits & Building Mental Models

I moved up my electric circuit unit this year for the senior physics class. Usually I put it after a full unit on waves, but after completing the waves unit with the IB students, I wasn’t so pumped to go through it again from the beginning.

I began by having students try to generate the largest voltage they could from a set of batteries, motors, solar panels, lemons, and some other fun gadgets. That was a great way to spend a full 90 minute block. The next class, we played around with the PhET circuit simulator as I described in a previous post. The goal was to get them to have some intuition about circuits before we actually got down to analyzing it. Our conversation focused on batteries generally contributing energy to the circuit, and other circuit elements using that energy, leaving nothing behind at the negative terminal of the starting battery. Our working definitions for voltage, current, and resistance came out of the need for describing what was happening in the circuit.

This started a pretty textbook version of the modeling process. I gave students some circuits, asked them to make a prediction for voltage/current, they made the predictions, and then tested them in the simulator. As needed, they made adjustments to their mental model to make it consistent across all examples.

What interested me most about the results here was that the students put together a pretty solid mental model that centered on the voltage divider concept. This came out of their other assertion that current is the same for resistors in series.

This led to the students tackling this problem on the second day of looking at circuits this way:

In my AP Physics sequence, this is something I don’t get to until Kirchoff’s rules, so I was impressed with how nonchalantly they reported their answers after only a minute or so of thinking about the circuit.

On day 3, we went through an approximation of this lesson that I described in a previous post titled Starting at the end. We didn’t get to the more complex circuits, but did get to the concept of parallel circuits.

On day 4, we spent a day getting our hands dirty building actual circuits, not with the simulator. The students had a good time piecing things together and seeing bulbs light up and make measurements with actual voltmeters and ammeters.

Today, on day 5, I was finally thinking I was going to teach them about equivalent resistance…but I hesitated. I was too scared that providing a formula would risk undermining all of the intuition they had developed.

The students worked through some Physics Bowl questions from a while back. Here’s one:

I noted down a student’s explanation of why the answer was 36 volts, and another student’s addition to explain why it had to be 42 volts:

It then I threw this one at students:
I set the battery voltage to be 10 volts.

If my students had followed the sequence of physics lessons from the 2005 me, this would have been a piece of cake because they would have had the formula. Instead, they went through a nice sequence of stating what they knew and didn’t know and making guesses. I suggested a spreadsheet as a way to keep track of those guesses and their reasoning in one place:

We went through the spreadsheet cell by cell and decided on formulas to put in. In the end, they figured out that the final two currents had to be the same.

I did some guessing and checking following their monitoring of the values, and eventually ended up with the 100 ohm resistor having a voltage drop of 9.923 volts.

Only at this point (which was five minutes before the end of class) did I apply an equivalent resistance formula:

It was a great moment to end on. My presentation of the equivalent resistance formula came out of a need, and for that reason, I was glad to provide it. I’m so happy I waited.

Speed of Sound Lab – Another Update

Two years ago, I wrote about how I took tuning forks out of the standard resonance tube lab for measuring the speed of sound.

My students all have phones and make a modest effort to keep them put away. I decided to get them to take them out for the lab today.

I found this free function generator app that generates clean sine, square, triangle, and sawtooth waves across a pretty good range. Above 1 kilohertz, harmonics are visible on a software frequency analyzer, so I didn’t have students go quite that high. The frequency can be set across this range by entering the frequency manually, or by using preset buttons on the app. By playing the waveform, plugging in earphones, and hanging them on top of the tube, finding the fundamental vibration frequency is pretty straight forward.

Collecting data in this lab has, in my experience, been a pretty slow process. Today though, my students were able to collect 15-20 frequency and height pairs in less than half an hour. I took all of their data and graphed it together. I’m pretty impressed with how consistently the data sits in a line:

The slope of the best fit of L vs. 1/(4f) forced through the origin is 320 m/s, which is probably the closest result to theoretical that I’ve ever gotten. The precision of the data is the big winner here. It was a simple task to ask students to cycle back through their range of frequencies and check that their new measurements meshed well with the old.

When can we neglect air resistance?

This was supposed to be the shortest part of a warm up activity. It turned into a long discussion that revealed a lot of student misunderstandings.

The question was about whether we could ignore air resistance on a textbook being thrown in the air. We spent most of our time discussing the differences and similarities between the three items here:

There were interesting comments about what factors influence the magnitude of air resistance. I was definitely leading the conversation, but it wasn’t until a student mentioned acceleration that anyone was able to precisely explain why one fell differently from another. We eventually settled on making a comparison between gravity force and air resistance force and calculating acceleration to see how close it was to the acceleration of gravity.

Projectile Motion with Python, Desmos, and Monte Carlo Simulation

I’ve written about my backwards approach to to projectile motion previously here, here, and here.

I had students solving the warm-up problem to that first lesson, which goes like this:

A student is at one end of a basketball court. He wants to throw a basketball into the hoop at the opposite end.

What information do you need to model this situation using the Geogebra model? Write down [______] = on your paper for any values you need to know to solve it using the model, and Mr. Weinberg will give you any information he has.
Find a possible model in Geogebra that works for solving this problem.
At what minimum speed he could throw the ball in order to get the ball into the hoop?

The students did what they usually do with the Geogebra projectile motion model and solved it with some interesting methods. One student lowered the hoop to the floor. Another started with a 45 degree angle, and then increased the speed successively until the ball made it into the hoop. Good stuff.

A student’s comment about making lots of guesses here got me thinking about finding solutions more algorithmically. I’ve been looking for new ways to play around with genetic algorithms and Monte Carlo methods since they are essentially guess and check procedures made productive by the power of the computer.

I wrote a Python program that does the following:

Get information about the initial characteristics of the projectile and the desired final location.
Make a large number of projectiles (guesses) with random values for angle and initial speed within a specified range.
Calculate the ending position of all of the projectiles. Sort them by how far they end up compared to the desired target.
Take the twenty projectiles with the least error, and use these values to define the initial values for a new, large number of projectiles.
Repeat until the error doesn’t change much between runs.
Report the projectile at the end with the least error.
Report the entire procedure a number of times to see how consistent the ‘best’ answer is.

I’ve posted the code for this here at Github.

As a final step, I have this program outputting commands to graph the resulting projectile paths on Desmos. Pasting the result into the console while a Desmos calculator open, makes a nice graph for each of the generated projectiles and their intersecting at the desired target:

This is also on a live Desmos page here.

This shows that there is a range of possible answers, which is something I told my physics class based on their own solutions to the problem. Having a way to show (rather than tell) is always the better option.

I also like that I can change the nature of the answers I get if I adjust the way answers are sorted. This line in the code chooses how the projectile guesses are sorted by minimizing error:

self.ordered = self.array.sort(key=lambda x: abs(x.error))

If I change this to instead sort by the sum of error and the initial speed of the projectile, I get answers that are much closer to each other, and to the minimum speed necessary to hit the target:

Fun stuff all around.

Analyzing IB Physics Exam Language Programmatically

I just gave my IB physics students an exam consisting entirely of IB questions. I’ve styled my questions after IB questions on other exams and on homework. I’ve also looked at (and assigned) plenty of example questions from IB textbooks.

Just before the exam, students came to me with some questions on vocabulary that had never come up before. It could be that they hadn’t looked at the problems as closely as they had before this exam. What struck me was that their questions were not on physics words. They were on regular English words that, used in a physics context, can have a very different meaning than otherwise. For these students that often use online translators to help in decoding problems, I suddenly saw this to be a bigger problem than I had previously imagined. An example: a student asked what it meant for an object to be ‘stationary’. This was easily explained, but the student shook her head and smiled because she had understood its other meaning. On the exam, I saw this same student making mistakes because she did not understand the word ‘negligible’, though we had talked about it before in the context of multiple ways to say that energy was conserved. Clearly, I need to do more, but I need more information about vocabulary.

It got me wondering – what non-content related vocabulary does occur frequently on IB exams to warrant exposing students to it in some form?

I decided to use a computational solution because I didn’t have time to go through multiple exams and circle words I thought students might not get. I wanted to know what words were most common across a number of recent exams.

Here’s what I did:

I opened both paper 1 and paper 2 from May 2014, 2013, 2012 (two time zones for each) as well as both papers from November 2013. I cut and pasted the entire text from each test into a text file – over 25,000 words.
I wrote a Python script using the pandas library to do the heavy lifting. It was my first time using it, so no haters please. You can check out the code here. The basic idea is that the pandas DataFrame object lets you count up the number of occurrences of each element in the list.
Part of this process was stripping out words that wouldn’t be useful data. I took out the 100 most common words in English from Wikipedia. I also removed some other exam specific words like instructions, names, and artifacts from cutting and pasting from a PDF file. Finally, I took out the command terms like ‘define’,’analyze’,’state’, and the like. This would leave the words I was looking for.
You can see the resulting data in this spreadsheet, the top 300 words sorted by frequency. On a quick run through, I marked the third column if a word was likely to appear in development of a topic. This list can then be sorted to identify words that might be worth including in my problem sets so that students have seen them before.

There are a number of words here that are mathematics terms. Luckily, I have most of these physics students for mathematics as well, so I’ll be able to make sure those aren’t surprises. The physics related words (such as energy, which appeared 177 times) will be practiced through doing homework problems. Students tend to learn the content-specific vocabulary without too much trouble, as they learn those words in context. I also encourage students to create glossaries in their notebooks to help them remember these terms.

The bigger question is what to do with those words that aren’t as common – a much more difficult one. My preliminary ideas:

Make sure that I use this vocabulary repeatedly in my own practice problems. Insist that students write out the equivalent word in their own language, once they understand the context that it is used in physics.
Introduce and use vocabulary in the prerequisite courses as well, and share these words with colleagues, whether they are teaching the IB courses or not.
Share these words with the ESOL teachers as a list of general words students need to know. These (I think) cut across at least math and science courses, but I’m pretty sure many of them apply to language and social studies as well.

I wish I had thought to do this earlier in the year, but I wouldn’t have had time to do this then, nor would I have thought it would be useful. As the semester draws to a close and I reflect, I’m finding that the free time I’ll have coming up to be really valuable moving forward.

I’m curious what you all think in the comments, folks. Help me out if you can.

Standards Based Grading(SBG) and The SUMPRODUCT Command

I could be very late to the party finding this out. If so, excuse my excitement.

I gave a multiple choice test for my IB Physics course last week. Since I am using standards based grading (SBG), I wanted a quick way to see how students did on each standard. I made a manually coded spreadsheet eight years or so ago to do this. It involved multiple columns comparing answers, multiple logical expressions, and then a final column that could be tallied for one standard. Multiply that by the total number of standards…you get the drill.

I was about to start piecing together an updated one together using that same exhausting methodology when I asked myself that same question that always gets me going: is there a better way?

Of course there is. There pretty much always is, folks.

For those of you that don’t know, the SUMPRODUCT command in Excel does exactly what I was looking for here. It allows you to add together quantities in one range that match a set of criteria in another. Check out the example below:

The column labeled ‘Response Code’ contains the formula ‘=1*(B6=E6)’, which tests to see if the answer is correct. I wanted to add together the cells in F6 to F25 that were correct (Response Code = 1) and had the same standard as the cell in H6. The command in cell I6 is ‘=SUMPRODUCT((F6:F25)*(E6:E25=H6))’. This command is equivalent to the sum F6*(E6=H6) + F7*(E7=H6)+F8*(E8=H6)+…and so on.

If I had known about this before, I would’ve been doing this in some way for all of my classes in some way since moving to standards based grading. I’ve evaluated students for SBG after unit exams in the past by looking at a student’s paper, and then one-by-one looking at questions related to each standard and evaluating them. The problem has been in communicating my rationale to students.

This doesn’t solve the problem for the really great problems that are combinations of different standards, but for students that need a bit more to go on, I think this is a nice tool that (now) doesn’t require much clerical work on my part. I gave a print out of this page (with column F hidden) to each student today.

Here is a sample spreadsheet with the formulas all built in so you can see how it works. Let me know what you think.
Exam Results Calculator

Revising my thinking: Force Tables

I’ve avoided force tables as a lab in the past. This is primarily because when I first started teaching physics and saw some collecting dust in the lab equipment room, the activities that were written for them seemed so formulaic that I was bored by them. I didn’t know then what I might do to make it more interesting.

In making an activity using them today, I actually played around with them a bit. They are a bit tricky to set up, but once you have the weights balanced, it’s oddly satisfying to see the ring in the center floating there:

The theme of my lesson planning is a search for this type of gold: how can we play with this?

I’ve done activities involving ‘find the unknown mass’ before, and the force table offered an efficient way in to doing this.

I asked students to figure out the mass of the weight circled in blue. I asked them to decide what information they needed to do so, and they requested the other two masses, which I provided.

Students worked quickly using their knowledge of forces and equations of equilibrium. They figured out pretty quickly that the angles between the threads were approximately equal, a fact I didn’t notice until I looked from above:

Their predicted answer of 290.9 grams was impressively close to the actual answer of 292.2 grams. We discussed that the assumption that the angles were the same might contribute for the error.

On the whole, this was a fun way to put to use a piece of equipment that I’ve kept out of my classroom for largely silly reasons. I think I’ll definitely add this to the playlist for future units on equilibrium.

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