Sunday, March 22, 2015

Electrophorus Engineering

One of the scariest parts of the Next Generation Science Standards (NGSS) are the Science & Engineering Practices. An easy way to slowly align your curriculum to NGSS is to modify a current demo, lesson or lab so that it is aligned with one of the Science & Engineering Practices. In a (brief) nutshell the Science & Engineering Practices are a skill set students should have in order to explore science phenomenon and engineer solutions to problems or fullfill a human need. Bozeman Science has a nice set of videos on the eight practices. The first one on Asking Questions & Defining Problems is helpful in understanding what NGSS considers the difference between Science & Engineering.

In the past I've created an electrophorus from a styrofoam cup and aluminum pie pan and used it to light a small neon bulb as part of my electrostatics lecture. Among The Exploratorium's many "snacks" is one called Charge and Carry that explains the traditional demonstration. Usually a styrofoam sheet is rubbed vigorously with a cloth to separate charge through friction. The electrophorus is set on the styrofoam and the charges in the aluminum pan polarize; by then touching the top of the aluminum pan you charge it by induction. If you pick it up by the insulating handle, touch one lead of a neon bulb to the aluminum while holding the other you can light a small neon bulb. You form a complete path of conducting material to the ground allowing charges to flow.

This year I did not show my students the electrophorus but asked students to experiment with different materials to explore the best way to light the neon bulb. Specifically students were working on the sixth practice "Constructing Explanations and Designing Solutions." Students had access to the following: plastic cups, plastic plates, paper cups, paper plates, styrofoam cups, styrofoam plates, aluminum plates, aluminum cups (made from rolled aluminum foil).

Students were given this image to understand the arrangement of their materials and instructions on how to charge and ground the electrophorus. Students were told to try different designs to light the neon bulb; each time they changed materials they were to record their results and try something else.

All groups eventually realized they had to use an aluminum plate to conduct the charge to the neon bulb. Most groups used a  styrofoam cup as the handle although some experimented with multiple stacked paper cups and reported a longer and brighter light from the neon bulb. Some groups tried rubbing the aluminum pan directly, skipping the styrofoam sheet, and reported even brighter lights.

After they optimize their design students were asked to write a conclusion paragraph: 
In an age appropriate paragraph explain (1) how the bulb can be lit this way and (2) justify your design choices and how well it worked. Be sure to discuss each of your designs and how their results influences later designs.  

As you might expect, results varied. Some groups really dove into it, referencing their book, asking me clarifying questions and constructed thorough explanations of what they were seeing. From others I could tell that students did not understand how the charge was initially separated, why a conductor was used for one part and an insulator for the other or how the static charge lit the bulb. By discussing their results the next day most students were able to correct their misconceptions. In the end I think they ended up with a much better understanding about the electrophorus and begin to see how current works than if I had just done it as a demo. 

Did it take longer? Yes. Was it worth it? Yes!

Electrophorus Build It Activity (PDF) (Google Doc)


This one's for you, imagineers!

What is Plastimake?

This is the kind of modeling I can get excited about!

The real world keeps taking away my real world physics

Once upon a time (a few years ago), a reasonable high school physics question was, "Why do the first broadcasters in a given television market get the lowest channel numbers on the dial?" In Sacramento, for example, that would be the NBC affiliate, KCRA, who got Channel 3. Channel 1 was never licensed to anyone by the FCC, and Channel 2 was given to the neighboring San Francisco market. The channel numbers correspond to broadcast frequencies: lower channel numbers broadcast on lower carrier frequencies. The corresponding longer waves are better at diffracting over hills and into valley, delivering commercial messages to more viewers.

But broadcast technology changed in 2009, so the problem is no longer relevant, as far as I know.

And now chromatic aberration? Researchers at Harvard have apparently developed a flat lens that focuses all colors at the same point.

Perfect colors, captured with one ultra-thin lens

Well, we'll always have The Dark Side of the Moon.

Saturday, March 21, 2015

Doing the AP Physics course audit wrong, part 1

If you're an AP Physics 1 or AP Physics 2 instructor, you've already been through The College Board's 2014-15 course audit.

The AP course audit came into being c. 2007. I recall being given release time at school to complete the detailed paperwork. In brief distillation, you had to prove to The College Board that your course wasn't simply a test-prep mill. My AP Physics B course was never such. It was—in fact—that rare gem: a second-year physics course, just like The College Board feigned to prefer. In theory, they wanted students in AP Physics B to have already completed a yearlong high school physics course. The audit process, however, made it clear that that high school course should not have covered kinematics, Newton's laws, energy, momentum, electricity, magnetism, waves, light, optics, or modern physics. Everything a student was to know about those topics had to covered—stem to stern—in the audited AP Physics B course, alone. In hindsight, I should have recognized the redness of that flag.

As of May, 2014, we were expected to dispense with our vision of AP Physics B and embrace the grand, new vision that was AP Physics 1 and AP Physics 2. Yes, two challenging, college-level high school physics courses that we should hope can draw sufficient enrollments amid AP Biology, AP Chemistry, and the popular AP Environmental Science course. (Who wants to be slowed down in freshman environmental science at the university when you can test out via AP?)

Like I said, the AP Physics 1 and AP Physics 2 visions were new. And sweeping. Sure, there was still content to be covered, but what College Board really wanted to enforce now is exactly how you covered it.

The audit was intended to ensure you completely embraced and adhered to their strict, new multi-faceted regime and hierarchy with lists of

Big Ideas 
Enduring Understandings
Learning Objectives
and Essential Knowledges

All of which had to be supported with laboratory exercises (using "real", low-tech apparatus) that aligned to a list of

Science Practices.

And there have to be extended, outside the classroom activities and real-world applications. The whole of The New Vision was spelled out in the new 231-page course description.

In all, a robust physics course that no college anywhere has ever even claimed to offer its own students. Well, maybe one or two. But any college with a physics course matching the audit's expectations would be a truly rare (1%-ish) standout.

Examples of syllabi that were deemed acceptable were published.

They're filled with meaty lists such as,

Kinetic energy
Potential energy: gravitational and elastic 

Conservation of energy
Big Ideas 3, 4, 5
Learning Objectives:
3.E.1.1, 3.E.1.2, 3.E.1.3, 3.E.1.4, 4.C.1.1, 4.C.1.2, 4.C.2.1, 4.C.2.2, 5.A.2.1, 5.B.1.1, 5.B.1.2, 5.B.2.1, 5.B.3.1, 5.B.3.2, 5.B.3.3, 5.B.4.1, 5.B.4.2, 5.B.5.1, 5.B.5.2, 5.B.5.3, 5.B.5.4, 5.B.5.5, 5.D.1.1, 5.D.1.2, 5.D.1.3, 5.D.1.4, 5.D.1.5, 5.D.2.1, 5.D.2.3

Note that more characters of type are devoted to alignment documentation than to actual physics content terminology. 

And with labs,

20. Forensic Investigation (OI) [CR6b]
Lab Practicum: Apply principles of conservation of energy, conservation of momentum, the work-energy theorem, and a linear model of friction to find the coefficient of kinetic friction.
Science Practices 1.1, 1.2, 1.4, 1.5, 2.1, 2.2, 3.1, 3.2, 3.3, 4.1, 4.2, 4.3, 5.1, 5.2, 5.3, 6.1, 6.2, 6.4, 7.2

[AP Physics 1 Example Syllabus 1

The audit was no longer a matter of conveying that you covered the material, but that you covered the material in alignment with College Board's newly-adopted BI / EU / LO / EK + SP schema. Exacting specificity was required.

All the published example syllabi were very specific in their alignment to the new schema. I wonder how many man-hours were devoted to the creation of each one. And how many were developed voluntarily without release time or compensation.

What savvy teachers across the nation did, by permission of The College Board, was to declare that they—by golly—just happened to be following the exact same syllabus as one of the four published examples. This was the correct response. Expedient, efficient, generally not a wholesale lie, and—most importantly—expedient. Doing so kept you in The College Board's good graces, and keeps you in their "circle of trust".

Mind you, AP Physics 1 and AP Physics 2 teachers spent the year cobbling together brand new AP Physics courses aligned to The New Vision. Burdensome if you'r bring one course to life, doubly burdensome if you're bringing two courses to life. And that's what I was doing.

Doing the AP Physics course audit wrong, part 1? Deciding to do it. That is, writing detailed descriptions of my own courses—and how they aligned exactingly to The New Vision. Mind you, the courses had not yet run for a full academic year. But The College Board demanded full accounts, and I agreed to provide them.  

Huge mistake on my part.

Monday, February 23, 2015


Some readers may be familiar with curious and engaging air vortex launchers such as the Airzooka or Zero Blaster.

But when a physics demo is as groovy as this, you can count of someone to figuratively put it on steroids.

I was looking for a video of a vortex cannon blowing out a candle and found this promotional of the Candle Cannon. The suspense builds as the distance is steadily increased.

Erbert and Gerbert's Candle Cannon

You can see a slight visual distortion as the vortex reaches the cannon - a nice way to show that the compression of air is denser than the air around it as it refracts the light.

I'm thinking of using the video time stamps to find the speed of the traveling air compression with my students.  It looks like it takes 9 seconds from the sound of the diaphragm moving to when the candles get blown out for the last trial of 180 feet. That means the large "puff" of air travels 180 feet in 9 seconds or about 20 feet per second. Comparing that to the trial at 120 feet which took about 6 seconds that speed seems fairly consistent.

"But Mrs. Barnett! Mrs. Barnett! The sound takes some time to get to us (camera)!" Ahh if only they would all think that deeply.

If we assume 340 m/s for the speed of sound, or 1115 ft/ sec, its not offsetting our times that much. And just mentioning that fact that should help to point out that while the vortex cannon is used to model the compression of a sound wave it is not in fact a sound wave that reaches you. It's traveling at ~6% the speed of a sound wave after all. My kids love the air cannon in my classroom so I think they will enjoy this!

I have watched the video several times now and I found myself wondering what would happen if they started the cake much closer .... at some point it should take some of that frosting off the cake, right?

UPDATE: The Candle Cannon has its own website:, of course.

Monday, February 09, 2015

Conservation of momentum: not always your friend

Make of this what you will, gentle readers. But don't doubt the physics! We've got combustion, fluid flow, conservation of momentum / Newton's 3rd law, and an inclined plane.

Firefighters try to extinguish a car fire when suddenly...

And remember: if you're going to set your car alight on the top of a hill, have the courtesy to set the parking brake!

Thursday, January 22, 2015

We dseovcired smoe chaets tdaoy...

…in teh elciretc felid hkocey PEhT sim. Cehats to slvoe teh chaeglnilng Dcifulfty 3 crufgoniaotin.

1. You can darg teh pcuk anhewrye you lkie uinsg a cleolsy-pacled natigeve crahge. Atefr clciikng "srtat," ciclk and darg teh ngietave chrgae to gudie it to teh gaol.

2. Exdnipang teh wodniw to alolw gnoig udenr teh croidror iesnatd of ginog tghourh it.

I've aeddd to teh reules of my own aivtcity so as to dsoillaw scuh ceahts. I wtroe tihs psot as I did to aovid sraech eingnes.

UPDATE: I notified officials and they replied as follows.

"Thanks for pointing that out. The team is aware of these issues, but we won't be able to fix them in the current Java version. If we redesign the sim in HTML5 we'll address bugs like this then."

Captain Disillusion—Russian Ghost Car Debunk

Captain Disillusion is at it again…

Russian Ghost Car DEBUNK

CD is always entertaining and informative. The language can veer off into the "earthy" category from time to time, so don't show 'em in class without screening them first!

Monday, January 19, 2015

Did the sands of AP Physics 1 and AP Physics 2 just shift again?

UPDATE: I've been told that the my interpretation is mistaken (which happens only on days that end in a "y"). The equation sheets posted on the page linked to below are the real and true 2015 equation sheets of record. The course description documents that include a previous version of these sheets need to be updated with the revised equation sheets, and it's that that will not occur until April.

AP Physics 1 and 2 teachers, I trust you all knew the newly-released equation tables were undergoing revision.

The College Board's statement regarding the equation tables

Revisions to the AP Physics 1 and 2 Equation Tables
The AP Physics 1 and 2 equation tables – previously published in the course and exam description, curriculum modules, and practice exams – are being revised to provide complete definitions of symbols. Updated versions of those documents will be available by April 2015. [Emphasis mine] In the meantime, please disregard the equation tables in the current versions of these publications. Instead, refer to these new, revised equation tables and share them with your students…"

I recently turned 50, so I realize that my recollection that the release of the revised tables was set for February 2015 could be in error.

But the College Board exhausted my patience when it repeatedly set and then pushed back the publication of the redesigned AP1 and AP2 course descriptions, starting in 2008.

Like I said, it could be my own memory at fault here.

If they set a February date for the new equation tables and then moved it back to April, who among us would be surprised?

If they did retreat from their own deadline on the equation tables, I'm guessing they didn't push back the deadline for classroom teachers to submit course syllabi for the AP course audit. You know the one: where you spell out in great detail the minutia of these courses that you've never taught based on a 230-page course description only recently made public, so as to show that you've turned on a dime from the old vision to the new vision. You had the summer to retool, redesign, and re-imagine.

Saturday, January 17, 2015

Fibonacci zeotrope sculptures

Sorry for the sudden aggregation binge. I should be working on my genuflection to The College Board (AP Physics 1 and AP Physics 2 syllabi), but my motivation for that exercise is so. very. low.

In the meantime, let's enjoy this confluence of Fibonacci numbers, 3D printing, and stroboscopy, shall we?

This San Francisco Globe article introduces it nicely.

And, of course, video.

Fibonacci Zeotrope Sculptures

Fibonacci Zoetrope Sculptures from Pier 9 on Vimeo.

"These are 3-D printed sculptures designed to animate when spun under a strobe light. The placement of the appendages is determined by the same method nature uses in pinecones and sunflowers. The rotation speed is synchronized to the strobe so that one flash occurs every time the sculpture turns 137.5ยบ—the golden angle. If you count the number of spirals on any of these sculptures you will find that they are always Fibonacci numbers.

For this video, rather than using a strobe, the camera was set to a very short shutter speed (1/4000 sec) in order to freeze the spinning sculpture."