Friday, October 12, 2018

Banked Turns

In AP Physics C students deal with forces in two dimensions, on inclines, acting in circles and more. Inevitably during the unit students will ask the following question in fear:
"Will we ever have to deal with turns on an incline?"

Oh yes kids, but lets build up slowly.

I created this worksheet that referred back to sample and back-of-chapter problems in the textbook. Students worked on increasingly difficult problems with a partner before we reviewed. Here are a key for the worksheet and a key for the textbook problems.

To introduce the concept I delved into the cabinets left to me by my predecessor and found the toys I needed. I raised the whole demo on a wood stand to raise it up. I found a double lane flat track (in yellow) and a banked Hot Wheels track (in orange). Amazingly the Hot Wheels track fit into the inner lane of the flat track. For my first period I used clay and ring stands to hold up straight track to feed cars into each lane. By the next period I added a third straight track to feed one car straight to the wood support, with no turn. By the last period I added color coded signs ...

When I demonstrated this for students I ask students what will happen to the car that goes on the "no turn" track. They correctly guess that it will continue straight so I ask why it doesn't turn. It may take a second but they realized, "There was no force to push it into a circle!"

So then we drop the second car down the "flat turn" track and the initial ramp is high enough that the car rides the edge of the track through the whole turn. I ask students which force kept the car going in a circle and they realize that it isn't the force of friction but the force from the wall in this case. We talked about how that wouldn't be good for "real cars" and that instead we prefer a force of friction to keep our cars going around a turn.

The final car goes down the banked turn and while it may briefly touch the wall it doesn't ride the wall like the flat turn. They notice the banked orange track is about as smooth as the flat yellow track so I wasn't sneaky and just adding friction. Then I asked them what force caused the car to go around a circle and they realize its a portion of the normal force. 

Here is a video of the three cars being sent down the ramps at the same time:


The radius and coefficient of friction for the two turns are not the same and I did not bother to match the masses of the Hot Wheels cars I grabbed. I did not introduce any quantities for students other than identifying which force was causing the motion. It helped for my students to see the differences in the types of turns and they loved being able to run the cars down the tracks themselves. For a quick set-up it did well to help show my students the differences for each type of turn they were going to work through.

Wednesday, September 12, 2018

#AAPTSM18: Alternatives to AP Physics 1 and AP Physics 2

At AAPT's Summer Meeting 2018, I attended session AD: High School, with considerable interest. After a series of College Board-friendly talks by AP Physics Redesign proponents, Mt. Olive High School's Brian Holton presented "AP Physics 1: A Seasoned Perspective".

Holton's talk was clearly not sanctioned by the good people of The College Board. But his expression of frustration and exasperation with AP1 resonated with me. Apparently my expression of frustration and exasperation resonated with him, too. (He cited my lament in his talk.) His critique was much more robust than mine was.

A small group of us had a combination perambulation, ventilation, brainstorm as we migrated to our next sessions.

We concurred that dropping AP Physics 1/2 from a school's curriculum constituted a marketing challenge for any school that would dare to try. We now advertise and market our schools on the basis of the breadth an scope of Advanced Placement offerings and performance.

AP courses are to be added to a school's course catalog; not removed. That other high school being visited by shopping 8th-graders and their parents is offering AP Physics, so your school must match.

One idea we tossed around was running a course that would prepare students for the SAT II Physics exam. Does anyone, anywhere run such a course? I'd love to hear from anyone teaching such a course. For now, it's just a thought. And The College Board still wins.

I know AP Physics C fans are happy with their exams. Abandoning AP1 and AP2 for an SAT II-based course is a different set of conversation. One might argue that outstanding performance on the SAT II Physics wouldn't get students out of any physics course at a college or university. I would hasten to add that outstanding performance on an AP Physics 1 or 2 exam doesn't necessarily exempt a student from intro physics courses at college, either.

Here's what the SAT II Physics exam covers. (A physics content-based assessment: how tantalizing!)

Mechanics 36%-42%
Kinematics, such as velocity, acceleration, motion in one dimension, and motion of projectiles
Dynamics, such as force, Newton’s laws, statics, and friction
Energy and momentum, such as potential and kinetic energy, work, power, impulse, and conservation laws
Circular motion, such as uniform circular motion and centripetal force
Simple harmonic motion, such as mass on a spring and the pendulum
Gravity, such as the law of gravitation, orbits, and Kepler’s laws

Electricity and magnetism 18%–24%
Electric fields, forces, and potentials, such as Coulomb’s law, induced charge, field and potential of groups of point charges, and charged particles in electric fields
Capacitance, such as parallel-plate capacitors and time-varying behavior in charging/ discharging
Circuit elements and DC circuits, such as resistors, light bulbs, series and parallel networks, Ohm’s law, and Joule’s law
Magnetism, such as permanent magnets, fields caused by currents, particles in magnetic fields, Faraday’s law, and Lenz’s law

Waves and optics 15%–19%
General wave properties, such as wave speed, frequency, wavelength, superposition, standing wave diffraction, and Doppler effect
Reflection and refraction, such as Snell’s law and changes in wavelength and speed
Ray optics, such as image formation using pinholes, mirrors, and lenses
Physical optics, such as single-slit diffraction, double-slit interference, polarization, and color

Heat and thermodynamics 6%–11%
Thermal properties, such as temperature, heat transfer, specific and latent heats, and thermal expansions
Laws of thermodynamics, such as first and second laws, internal energy, entropy, and heat engine efficiency

Modern physics 6%–11%
Quantum phenomena, such as photons and photoelectric effect
Atomic, such as the Rutherford and Bohr models, atomic energy levels, and atomic spectra
Nuclear and particle physics, such as radioactivity, nuclear reactions, and fundamental particles
Relativity, such as time dilation, length contraction, and mass-energy equivalence

Miscellaneous 4%–9%
General, such as history of physics and general questions that overlap several major topics
Analytical skills, such as graphical analysis, measurement, and math skills
Contemporary physics, such as astrophysics, superconductivity, and chaos theory

Friday, September 07, 2018

Making Invisible Waves Visible, Near Infrared Imaging - Part 2

With IR Cut Filter                                                                Without IR Cut Filter
In my previous post I showed many uses for a near infrared camera. To be clear, this is NOT a thermal infrared camera.  Thermal IR cameras image in the mid IR and mainly detect electromagnetic radiation due to the random thermal motion of the atoms and molecules in objects. A near IR camera isn't any different than a regular digital camera that creates images from visible light. It is a digital camera that has been modified to make images from the wavelengths that are just beyond the wavelength humans perceive as red. It is like being able to see an additional color that is invisible to the human eye. In this post I will describe how you can modify a digital camera to be a near IR camera.

Remote Control Using iPhone Selfie Camera
The fact that digital cameras are sensitive to IR is a detriment to good photography. The lens of the camera is not designed to focus IR, resulting in a fuzzy picture. The extra IR light can make exposure settings unreliable. To get around this, all digital cameras have a built in IR cut filter, but some IR still gets through. The cheaper the camera, the less effective the IR cut filter. You can demonstrate this by pointing a remote control at any digital camera. When a button on the remote is pressed, the IR LED on the remote can be seen flashing in the camera display. I first discovered this when my 3 year-old daughter was toddling toward me with a remote control as a took a video. I was startled to see it flashing in the viewfinder but not to my eye. She is now 25. The better the IR cut filter, the dimmer the flashing. New phones cut almost all of it, try using the lower quality selfie camera. The IR cut filter must be removed to make a near IR camera. This will void the warranty and possibly wreck the camera. That is why a computer webcam is a good choice for a near IR camera. They are cheap and easier to disassemble.

IR Cut Filter on a Webcam Lens
Webcams have several types of IR cut filters. The worst is a coating on the lens. This must be scratched off, that can degrade image quality. Second worse is a filter attached to the CCD chip. This must be carefully removed to avoid damaging the CCD. The best filters are part of the lens housing. These can usually be popped out without difficulty. The IR cut filters are dichroic, they look transparent straight on but usually pinkish from the side. They are worth saving to explore their characteristics with a spectrometer.

I have converted 5 different webcams over the years. Because I like to use a Mac, it was more difficult do find suitable webcams. Macs have come with built-in webcams for a long time, so few vendors make Mac compatible Webcams. The most recent near IR webcam I converted is the Logitech HD Laptop Webcam C615. It can be purchased for about $25. It was a little difficult to take apart until I found out I could peel off the flat plastic panels on the front to reveal the screws. The IR cut filter was glued on to the lens housing but easily broke off with a quick blow from a pencil. It worked well with my Macbook Pro and is my only HD near IR webcam. I made a video showing the conversion process for this webcam in case you want to give it a try.
My other converted webcams are older models but still available as of this writing. I tried the IceCam2 by Macally. It worked well with my Mac but unfortunately the IR cut filter was a lens coating. I was able to scratch it off with an X-Acto knife with acceptable results, but I wouldn’t recommend it. The Macally MegaCam had a removable IR cut filter and works well. The only downside is it had a limited range of focus. Next I tried a Logitech QuickCam Connect. Although there was a Mac driver when I made it, currently there isn't one. The IR cut filter was a small square piece of plastic that easily popped out of the housing. This camera worked well on a PC and my Mac laptop running Windows. I recommend this one for PC users. I then tried the Logitech QuickCam Chat Web Camera because there was a Mac driver available for it. Unfortunately it had an IR cut filter coated on the lens. I swapped the QuickCam Connect lens with this webcam. This is what I used until a Mac OS update caused that driver to stop working too!

Once the IR cut filter is removed, replace it with a filter that only passes IR radiation. I use a Wratten #87C filter. Edmund Optics sells one for $175. That is expensive but it would be enough to make many near IR cameras. The filter is large enough to be attached over the lens of the camera. This allows pictures to be taken with and without it for comparison. I chose to place it inside the lens housing of the webcam so it is more secure. I can then use an unmodified webcam for comparison pictures. You can find less expensive Wratten 87C filters on eBay for $30-$40.

Unexposed, Developed Color Film
A much less expensive filter can be made from developed, unexposed color film. The resulting dark negative works almost as well as a Wratten 87C, see spectra below. If you look through old boxes of color negatives at your grandmother's house you will probably find some developed, unexposed sections on the ends of negatives that you can use. They are the portions that look completely dark. This material will pass IR while blocking almost all visible. Some people recommend floppy disk material but it passes a lot of the visible spectrum. I don't recommend it for near IR webcams. However, looking directly through it gives you an approximate view of what things look like in near IR.
View of My Classroom Through the Material from a 3.5" Floppy Disk
The figure below shows the continuous spectrum of an incandescent light (red line). The other curves show this spectrum after passing through various filters. The sensitivity of the spectrometer was adjusted to show the spectra at approximately the same scale so the intensity values can't be compared. The purple line is the IR cut filter that was removed from a webcam. You can see why these must be removed as most light above the 650 nm wavelength is blocked. The orange curve is the material from a 3.5" floppy disk. Although it passes IR, it also allows a lot of red and orange light to pass too. The green curve is developed, unexposed color film. It works almost as well as the Wratten 87C (blue line), letting through only a small amount of visible light. If you use developed, unexposed film as a filter, try doubling it up for better results.
Continuous Spectrum Compared to Spectrum After Pass Through IR Cut, Floppy, Color Negative, and Wratten 87C Filters
You can use any software that displays a video preview of your webcam to capture images. On a Mac this is Photobooth if you have a compatible webcam. For PCs, the software that comes with the Logitech webcams works well. It has settings for low light levels, still and video capture, and some basic editing tools. There are many other choices for PC webcam software including ManyCam

A near IR webcam is very useful in the classroom but awkward for taking around town and country. Many people would have difficulty successfully taking apart a webcam AND putting it back together in working condition. I know I did. Another option that overcomes these obstacles is the Sony Nightshot line of cameras. They have a switch that slides the IR cut filter out of the optical path. They also have IR LEDs to illuminate objects so they can be seen in total darkness. I use a Sony MiniDV Handycam DCRHC40 that I purchased new in 2004. There are usually many inexpensive Sony Nightshot cameras for sale on eBay. To convert a Nightshot camera to a near IR camera all you need to do is place an IR pass filter over the lens. Either a Wratten 87C or one made from developed, unexposed color film works well. I made a card stock holder so I can quickly attach and remove the filter to take near IR/visible comparison pictures. I also cover the IR LEDs with electrical tape although they are useful for some applications.
My Sony Nightshot Camera With IR Pass Filter Installed and Tape Blocking IR LEDs
Sony Nightshot cameras have been manufactured since 1998. There was some media attention when people used them to see through certain fabrics when illuminated with a bright near IR source. Sony modified them so that they overexpose in bright conditions to prevent voyeurism. This does not affect their use as near IR cameras because the IR pass filter dims the image considerably. However, I sometimes have issues with overexposure in bright sunlight. There is a way to defeat this, set the Nightshot switch halfway between on and off. That is how I took the picture of the billiard balls at the top of this post.
Fabric in Visible Light                                                                  Same Fabric in Near IR
Another option would be to modify a digital camera like a webcam by removing the IR cut filter and replacing it with an IR pass filter. This is best done with an old digital camera that would not be missed if it refused to work after reassembling it. I tried this with an old Kodak digital camera and it did work for a little bit, then became e-waste. There are companies that will do this but their main purpose is to remove IR cut filters from DSLR cameras to make them better for astrophotography. If you are interested in pursuing this option, here is a clearinghouse of information and here is a good place to start.

I learned how to convert webcams to near-IR cameras by searching for DIY websites and videos online. This post is an amalgamation of what I have learned from this research and from using near IR cameras for many years. Here are a few websites that I found useful:

https://youtu.be/Xytt8YHhlOU

http://www.hoagieshouse.com/IR/

http://www.instructables.com/id/Infrared-IR-Webcam/

Wednesday, August 29, 2018

Making Invisible Waves Visible, Near Infrared Imaging - Part 1


Simone models her scary giraffe costume in visible and near IR
The human eye is not capable of detecting wavelengths much larger than 700 nm. However, digital cameras have the capability to detect near infrared radiation a little beyond 1000 nm in wavelength.  This is almost as large a range in wavelength as the entire visible spectrum! Digital cameras are able to detect this electromagnetic radiation and reveal it as images and movies. This allows many creative and novel ways to explore IR radiation in the classroom and beyond. For example, clothing looks different in near IR as the above Halloween costume reveals. In this post I will give examples of near IR images I have made. In Part 2 I will describe how to make your own inexpensive near IR camera.

There are many things that can be explored with a near IR camera. One of the more surprising things to observe is paper money. The back of $10, $20, and $100 bills have strips of near IR reflecting pigment that obscure the printing behind it.
The reverse side of a $20 bill in visible and near IR
I remember showing this to a laser physicist and he got so excited he went home and made his own near IR camera, and he worked every day at the National Ignition Facility at LLNL! The patterns are different allowing scanners to quickly and accurately differentiate between denominations as these images of other denominations show.
The reverse side of a $10 and $100 bill in near IR, notice almost invisible 100 on right
The counterfeiting measures on the front of the $100 bill look very different in near IR. The inkwell disappears revealing the Liberty Bell. This happens in visible if you tilt the bill. The fountain pen and phrases from the Declaration of Independence disappear and the security ribbon becomes very faint.
Obverse of $100 bill in visible and near IR
It is interesting to watch appliances like hotplates, toasters, hairdryers, and a range top burners warming up. These are all easily visible in near IR even when there is no red glow yet detectable to the eye. The range of brightness is much greater than seen in visible. Below is video of me touching a hotplate when it was just warm but glowing in near IR.
 
Near IR passes through black plastic bags. You can shine an IR LED through it or place IR emitters like this space heater behind it and see right through the bag.
Visible and near IR image of a black plastic bag draped in from of space heater
Fluorescent light sources are dimly visible, the warmer ends showing up brighter. Compact fluorescent lights show up brighter than standard fluorescent tubes. Reflected fluorescent light is mostly invisible. A room illuminated by only fluorescent light is dark.
My classroom with all the fluorescent lights on in visible and near IR on a foggy morning
Incandescent lights are very bright and can light up the whole room with near IR. The filament can be seen for a few seconds after they have been turned off. Different color LEDs are interesting. Red shows up brighter than other colors. The red LED has a peak wavelength in the visible but still emits light on either side of this peak, some in the near IR. Most of the near IR from the red and other LEDs is probably from thermal radiation. IR LEDs show up as intense lights. Remote controls have near IR LEDs and can be used as near IR flashlights to “see” objects in an otherwise dark room.
The red LEDs of my binary clock and shining a remote control on my face in the dark with a remote control shining on it
Smoke and dust particles scatter visible light better than near IR. As a result, near IR reveals distant objects that are obscured in visible light.
South San Fransisco Bay Area from Saratoga, distant mountains only seen in near IR include Mission Peak
There was a small wildfire in North Cascades National Park during our trip in July of 2018. I took several photos of the mountains in visible and near IR. This shot of Colonial Peak turned out the best. Notice the extra details visible in near IR and the smoke is less of a problem. The shadows are more stark, showing that near IR does not scatter as much as visible. Trees reflect near IR more than visible, showing up brighter.
Colonial Peak looms over Diablo Lake in North Cascades National Park
The Los Gatos High School pool looked very different in IR. Very little IR is coming from the bottom of the pool so the lane markings on the bottom can't be seen. The sky reflected from the surface looks dark making it look like a senior prank involving ink has been perpetrated. Other reflections off the pool are more visible like the flags and light poles in near IR.
Los Gatos High School pool in visible and near IR
There are many other things to check out, I won't show them all here. LCD monitors show up as a dull glow with nothing showing from the display. Some have a bright corner where the back light is located. Red lasers are absolutely dark unless you shine it directly into the camera. The resulting dim glow is probably thermal radiation. Different items of black clothing that looks identical in visible can have dramatically different shades in near IR. Plants tend to reflect a lot of near IR but this varies by species.

I hope I have piqued your interest in near IR imaging. Near IR cameras can be made by modifying a webcam. All you need to do is replace the IR cut filter with an IR pass filter. Another option is to purchase a Sony Nightshot camera and place a near IR pass filter over the lens. You can find used Nightshot cameras on eBay for less than $40. I will describe both of these options in detail in my next post. If you can't wait, there is a lot already posted on the Internet, search "DIY infrared webcam".

Friday, August 10, 2018

#AAPTSM18: Garbage can smoke rings

I will post as many gems as I can from the American Association of Physics Teachers Summer Meeting held in Washington, DC July 28-August 1, 2018. On Twitter, that's #AAPTSM18. The items may be new or classic; simple or complex. Here's one such gem.

Dave Maiullo shows us his smoke ring cannon, adapted to the constraints of the room. Like most things, ring vortices look even better in slow motion.

Maiullo Smoke Rings


Bonus: Maiullo Dragon Voice!

Tuesday, August 07, 2018

#AAPTSM18: Using physics to win a tug-of-war

I will post as many gems as I can from the American Association of Physics Teachers Summer Meeting held in Washington, DC July 28-August 1, 2018. On Twitter, that's #AAPTSM18. The items may be new or classic; simple or complex. Here's one such gem.

At the "30 Demos in 60 Minutes" session, Colorado State's Brian Jones—director of The Little Shop of Physics and author of College Physics by Knight, Jones, and Field—showed us his secret to winning a tug-of-war.

Brian Jones Tug-of-War


I love it: so simple!. Two concerns:

Those once-ubiquitous single-use plastic bags may be an endangered species as voters outlaw them out of concern for the growing plastic islands and gyres in the oceans. It should work equally well with the thicker multiple-use bags that are now coming online.

I wonder if this will work as well on tile as it does on carpet. It clearly works like a charm on carpet. I guess I'll find out soon enough how well it works on tile.

Sunday, August 05, 2018

#AAPTSM18: Google Science Journal

I will post as many gems as I can from the American Association of Physics Teachers Summer Meeting held in Washington, DC July 28-August 1, 2018. On Twitter, that's #AAPTSM18. The items may be new or classic; simple or complex. Here's one such gem.

It is not rare when I spend time with my colleagues at AAPT meetings that I feel like I missed a memo somewhere along the line. If my head is swelling with overconfidence, mixing with my very talented, informed, creative, and knowledgeable colleagues puts me back in my place.

Such was the case when someone casually mentioned the use of Google's Science Journal during AAPTSM18. What? I have a bunch of Google apps, but I hadn't heard of Science Journal. Why not? I don't know. Sometimes I'm just not paying attention.

Anyway, Science Journal accesses sensors built into phones (or as we older folks think of them: smart phones or cell phones) and manages the data for easy use by phone users, such as students. Take a look.

Science Journal

Science Journal, an initiative by Google #ScienceJournal

I don't allow phone use in my class. Strictly and absolutely. Unless I invite exceptions. Phones are never far from students' hands. Given a green light, the phones can be out and about in no time. I'm not sure if I'll find a use for this in class. But at least I know it's there. Hmmm... experiment-based homework?

#AAPTSM18: Color mixing with Ikea's Ledberg

I will post as many gems as I can from the American Association of Physics Teachers Summer Meeting held in Washington, DC July 28-August 1, 2018. On Twitter, that's #AAPTSM18. The items may be new or classic; simple or complex. Here's one such gem.

Rutgers University demonstration specialist, Demo a Day co-author, and That Physics Show star, Dave Maiullo, presented many demonstrations, as he does. With trademark Maiullo panache, of course. One of them was the use of Ikea's Ledberg color-changing LED light strip in conjunction with diffraction ("rainbow") glasses. The LEDs in the strip act as nice point-like sources, a controller allows for variation of the emitted color, and the diffraction gratings produce a nice spectrum that changes in correspondence with the LEDs.

Here's Dave's presentation to a group of physics teachers (Session CM: 30 Demos in 60 Minutes). [Note: this is operationally a family affair as many folks in the room know each other and the demonstrators; so the atmosphere is familiar and casual.] As an attendee, I did my best to capture as much of the session as I could. I did better with some demos than I did with others. My rainbow glasses were secured beyond my ability to deploy them while recording the demo. That didn't stop me from trying!



I managed to make an effort to hold the glasses in front of my big camera during the official demo show. A bit slapdash; I'm delighted it worked as well as it did to show the spectra of the LEDs off to the right.




Tuesday, July 31, 2018

Physics teacher conferees treated to a demo show par excellence

Images from the 2018 Summer Meeting Demo Show, July 31 at the Renaissance Hotel & Conference Center in Washington, DC. Show produced by AAPT's PIRA Team (ft. David Maiullo and Stan Micklavzina) in conjunction with the Bubble Magic of Tom Noddy.

2018 07 AAPTSM18 Demo Show
2018 07 AAPTSM18 Demo Show

Thursday, July 26, 2018

Vintage transformer

I've seen a meme going around recently that says,

"It's not hoarding if your sh!t is cool."

True. And this is the excuse I will give for keeping old vintage equipment that still demonstrates some physics. The same could be said for my father-in-law, a retired hospital mechanic that got to keep a lot of old equipment when it was replaced. He recently gave me a National Twin-Control Transformer, pictured below. It was used in the surgical wards to change the voltage of the lights (left dial) and cauterizing gun (right) from the 120 V wall outlet. I found a few more of these online for sale as interest pieces, luckily no one suggested actually plugging it in. There is an on/off dial at the top and what appear to be banana-type lead ports underneath each control.


The art deco style seem to be from the 1920s or early 1930s. The plastic shell is Bakelite, the first synthetic plastic for which the American Chemical Society named it a National Historic Chemical Landmark. One of its most common applications were casings like this for electric devices because it was very electrically resistant.

I took the front of the case off so that I could see the very large transformers inside. My father-in-law had cut the cord internally, just in case any students decided to get too curious. The transformer wire gauge is quite large and the whole mechanism quite heavy. I took it out to show some students but I'm concerned about some of the wrap components so it will probably be left sealed up in between demos.

It would be interesting to encase the transformer opened up so students could see it but it was safe from curious hands. Its a reminder that what we teach them is actually used for something and hopefully they are able to recognize similar components.

Monday, July 23, 2018

Representation matters in Physics

Some of the hesitance that our students have about studying science is this misconception that you can only "do science" if you're "smart enough." Humanizing those that pursue science and their path to a STEM job helps more students consider doing the same.  A Twitter account called @RealScientists has a new curator each week to share their particular corner of science with the world. They have a whole week to talk about their specific job, the path they took to get there, publications, unique and funny memories, what they wish everyone knew about their field, etc. Some hashtags have gone viral to show the human side of scientists like #PregnantintheField with which female scientists share pictures of themselves with baby bumps while carrying out field research. The universal #BadStockPhotosOfMyJob hashtag gets really funny when scientists get a hold of it.

While most of us recognize that people come in all different shapes, sizes, colors and backgrounds outwardly acknowledging it is important in your classroom. In a time when many are feeling less welcome in our own spaces (work, home, country, etc) it is vital that we actively work to recognize marginalized groups in our fields and help to lift their voices.

On June 4th a new Twitter account @500QueerSci was launched with the purpose of bringing awareness of LGBTQ+ people working in STEM related jobs during the annual Pride Month. Their goal was to reach 500 personal stories and by mid July were at 630 biographies. People can submit their abbreviated biographies, pictures and social media links to be added to the catalog. The full list is found on their companion website 500QueerScientists.com.

The resource is amazing and as many have said when they first tweet their own featured biographies  they wish such a thing had been available when they were young. It serves as a reminder to young LGBTQ+ people interested in science that they are not alone and that can be life changing.

I recently went back to the website hoping to find a search bar to type "physics" into so that I could have a list of many LGBTQ+ people with backgrounds or jobs in the fields  of physics. I could not find one so I tweeted at @500QueerSci and asked:


They responded that such a feature was in the works but in the meantime offered to find such profiles for me. Below are the names and links they provided:

  1. Chanda Prescod-Weinstein
  2. Izzy Jayasinghe
  3. Giampiero Mancinelli
  4. Axiel Yael Birenbaum
  5. Nicole Ackerman
  6. Tzula Propp
  7. Shawn Cole-Woods
  8. Kelsey Collier
  9. Carlos Arguelles
  10. Aidan Robson
  11. Ana Barbara Rodriquez Cavalcante
  12. Beck Strauss
  13. Kerstin Nordstrom
  14. Tessa Carver
  15. Christopher Aubin
  16. Robert Newberry
  17. Jost Migenda
  18. Andrew Princep
  19. John Barentine
  20. Mackenzie Warren
  21. Katie Mack
  22. Ashley Spindler
  23. JJ Eldridge
  24. Stephen Lawrence
  25. Alysa Obertas
  26. Nick Geitner
  27. Andrew Welsh
  28. Georgia Squyres
So I have a list, now what? I intend to use it. I've seen several lessons and activities in the last year promoting learning about diverse modern scientists, not just the "dead white guys" we find in our textbooks. Depending on your school and community's comfort level you could do a few things with such a list; and the same could be done for scientists of color and women. 
  • Whenever a particular field comes up in class, say astrophysicist, feature an LGBTQ+ person or person of color instead of whoever is at the top of a Google search list. 
  • "Profile a scientist" activities are already usually limited to exclude the few that everyone already knows about like Einstein or Curie or Tesla. If you're going to set restrictions for who they can't research you may find yourself just producing a list from which students have to choose. And look at that, you have a class set (or almost) of physics related names right up there! How convenient!
  • You can present the list in its entirety, or the full website, for students to explore st their own pace. You can lead a discussion or offer reflection time for students to think about the featured scientists' paths through education and the hardships they may have endured for being LGBTQ+, why their visibility is important, etc. 
  • Make posters of the featured physics related persona biographies to display in your classroom or school all year.
  • Feature an LGBTQ+, woman and/or person of color that is in the STEM field each week in your classroom. 
There are lots more things you could do to help expose your students to a more diverse world of STEM, but that should be a start! I encourage you to read through the biographies above and more to learn more about our very diverse and large STEM community. If you have any other fabulous lessons to share please let me know!

Edit: I was able to add a few more resources I could not find the first time around thanks to recent AAPT plenary Frank Nochese:

The hashtag #ActualLivingScientists is used by all kinds of, well, actual living scientists, to share information about their work and why its cool. Several teachers print out some of these profiles to make displays in their classrooms or around

Heather Waterman @WatermanPhysics makes a daily doodle on her board about a scientists from an underrepresented group. I have a minor in art and I still don't know if I could draw a scene every day that was this good!

Wednesday, July 18, 2018

Be careful with your parabolic mirror


Let's say you you were into making solar ovens. Let's say that you decided a few years ago to make the best solar oven ever. Further, let's stipulate that you saw a nearly meter-diameter Direct TV antenna on the side of the road. An idea happened. You rushed to the local plastics store and bought highly reflective Mylar and glued it to the antenna.

Your solar oven was pretty amazing. While the hot spot wasn't super small, it was hot. Really hot. It can pasteurize a liter of water in 15 minutes.

And now you work at the Exploratorium and you think that you might bring it to work for grins. If you forget it in the back of the your Outback face up on a sunny day near the solstice, well, it can melt the molding in a fairly impressive way. I think I was lucky that my car didn't catch on fire.



You might be wondering how I could make such a mistake? I had a lot to carry into the Exploratorium, and the mirror wouldn't fit on the cart. I planned on coming back in a few minutes, but I got busy doing something else, and it slipped my mind. Coming back in the afternoon, I sat in the driver seat and looked into the rear view mirror.

Uh oh.



If you want to make your own parabolic mirror, you can find some excellent instructions here.

Marc "Zeke" Kossover

Sunday, July 08, 2018

It's Not Easy Explaining Green

I had heard that when an LED is submerged in liquid nitrogen (LN2) it changes color. After the PTSOS workshop last January I had some leftover LN2 and decided to see for myself. I replaced the incandescent bulb in a Mini-Maglight with a green LED and submerged it in a Dewar of LN2. This was the result:


I was pleased when the color change was very apparent as the audio track confirms. My expectation was that the green light would change to a lower energy wavelength like yellow. I had a little familiarity with how LEDs work and knew the energy of the emitted photons was based on the band gap energy. I thought a colder material would have a lower band gap energy. My observation confirmed what I would find out later to be an erroneous expectation. Fortunately, after I made the video I got out my Red Tide spectrometer and recorded the spectrum of the LED at room temperature (22 degrees C) and in LN2 (-196 degrees C). I took a quick look but didn't study it because it was a Friday and I had spent enough time fooling around, er, I mean experimenting, in my class room. I tweeted out the video and went home.

Still image of original Tweet, link to entire thread below

The next day the tweet was getting a lot of attention. Several people responded that the color should blue shift when submerged in LN2. Others sent me related links and I started reading up on the subject. These confirmed that the band gap energy should increase as the temperature decreases although one source indicated that it could go the other way for some materials. Most of these sources invoked quantum mechanics in their explanations, much of it went over my head. After reading many sources I found an explanation that would satisfy a high school student (or teacher!). But first they would need to understand how p-n junctions and LEDs work. If you want to wade through all the replies to my tweet, here is the link:
https://twitter.com/kilroi22/status/962145895805476864
https://twitter.com/kilroi22/status/962145895805476864
p-n junction diagram from TheNoise at English Wikipedia

A p-n junction is a piece of semiconductor divided into two regions. The p-type side (p for positive) has impurities added that cause the semiconductor to accept electrons. The n-type side (n for negative) impurities cause the semiconductor to give up electrons. At the junction between the two materials, the electrons from the n-type side are attracted by the p-type side and drift into it. This leaves the region on the n-type of the junction side lacking electrons and the region on the p-type side with extra electrons. A region lacking electrons can be described as having positive "holes" (see diagram above). Now the remaining electrons on the n-type side face a potential barrier from the electrons on the other side of the junction. An applied voltage can get them to flow to the p-type side if it is large enough to overcome the potential of the barrier. The energy required for an electron to overcome the potential is the "band gap energy". An LED is a semiconductor with a p-n junction. If enough voltage is applied to the LED, the electrons receive enough energy to cross the junction, then drop back down in energy, giving off light. This is analogous to an electron being excited to a higher energy level in an atom, then dropping back down, giving off a photon of light having a very specific energy or wavelength. An LED differs because there is a spread of possible photon energies that can be emitted, centered on what is most probable, the band gap energy. The color or wavelength of an LED is often, but as we will see, not always the same as the wavelength corresponding to the bang gap energy. The figure below shows the spectrum for a green LED at room temperature. The peak wavelength is 571 nanometers (nm). This is right on the boundary between what is perceived by the human eye as green or yellow. The overall green color that is seen is a result of all the photons coming from the LED. Since the human eye peaks in color sensitivity at about 555 nm, the green photons to the left of the peak have more of an effect on what is seen than those on the right.

Spectrum of green LED at room temperature using Ocean Optic's Red Tide Spectrometer

I naively thought that cooling down the LED would "shrink" the band gap energy, causing the most probable wavelength to be of lower energy. This would mean it shifts to toward the red end of the spectrum. When I saw the color change from green to yellow, it confirmed my expectation. After doing some research, I learned that my expectation was wrong and there should be a blue shift. This is because at lower temperatures, the electrons on the n-type side have a lower initial energy from the random thermal motion in the material. This makes it harder for them to overcome the band gap energy much like it is harder to jump over a ditch while standing still versus taking a running start. My video seemed to refute this explanation so I took a closer look at the spectrum of the LED at room temperature and in the LN2. (see figure below) The peak wavelength in LN2 did show a tiny blue shift. It went from 571 nm to 568 nm. The Red Tide spectrometer has a 1 nm resolution but I would say this essentially unchanged. The main difference between the two is the narrowing of LN2 spectrum. The room temperature spectrum has a larger fraction of green photons coming from it. The LN2 spectrum is depleted of the green photons, allowing yellow to dominate. There also are what appear to be some nitrogen absorption features in the LN2 spectrum.

Spectrum of green LED at room temperature and in LN2

I now had a new question, why does the spectrum narrow? The temperature change also affects the number of photons emitted. Being submerged in LN2 selectively suppresses emission of green photons over yellow. Why this occurs I am not sure but it is definitely happening. This experience left me with other questions. What happens with other color LEDs? Will an infrared LED blue shift enough to become visible? Is the amount of blue shift dependent on the energy band gap? What would happen to a laser in LN2? When can I get some more LN2 and have a chance to try this? My chance came during the Fusion/Astrophysics Teacher Research Academy workshops I conduct at Lawrence Livermore National Laboratory. At the end of the first day we had some extra time and I brought out a Dewar of LN2 and we proceeded to immerse various light sources in it. The red laser pointer did not appear to change color but it eventually stopped working. It did recover after warming back up. An infrared LED appeared to show a very dim red light but it was hard to tell through the bubbling LN2. The ICE LED strip that contained blue, green, yellow, orange, red, and infrared LEDs was more impressive. It survived a lengthy immersion allowing us to see the blue dim but stay the same color, the green turn to yellow, both the yellow and orange turn to green, the red get slightly red-orange, and no sign from the infrared. None of the video turned out well and the teachers gathered closely around the Dewar made measurements difficult. I saved some LN2 to try again later by myself.

I finally got a chance the week after the LLNL workshops ended. I used ring stands and clamps so I could immerse the LED strip and record video hands-free. This allowed me to carefully measure the spectra. Below is the video in three parts, the initial immersion, after the LED strip reached equilibrium, and a sped-up video of the LED strip warming back up after removing it.


Frame grabs from video showing LED at room temperature and submerged in LN2

The video shows the same color changes that we observed in the workshop. No sign of visible emission from the infrared LED was seen. The infrared blocking filter on my iPhone prevented any infrared emission from showing on the video. The infrared emission does show on the spectrum as seen below. It shows a blue shift when immersed in LN2. The peak emission shifted from 934 nm to 904 nm. It would need to shift to at least 750 nm to be visible to the human eye. Some infrared LEDs peak at 850 nm. It is possible that they would shift enough to be visible but I doubt it.

Spectrum of infrared LED at room temperature and in LN2
The blue LED also shows a blue shift of the peak emission from 445 nm to 422 nm. However, this is due to a change in the relative intensity of the two peaks in each spectrum as shown in the figure below. The room temperature spectrum has a peak wavelength of 445 nm but shows a secondary peak at 425. The LN2 peak is at 422 nm with a secondary at 445 nm. I think the relative change in peak intensity has a similar cause as the green LED appearing yellow. The lower temperature is suppressing emission of certain energy photons more than others. Note: The intensity values on the y-axis of the spectrum graphs are not relevant because they depend mostly on how I aligned the fiber optic cable collecting the light. The blue LED dimmed noticeably so its intensity should be lower if everything else was equal. The small peak at about 570 nm is coming from the green LED that is adjacent the blue.

Spectrum of blue LED at room temperature and in LN2
The remaining 4 spectra are shown below. They all show a significant blue shift as they should according to the references I consulted. This green spectrum is very similar to the one I showed and discussed earlier. They both show a very slight blue shift and a depletion of emission of green photons. The small peaks on the sides are from adjacent LEDs on the strip.

Spectrum of green LED at room temperature and in LN2
Spectrum of yellow LED at room temperature and in LN2
Spectrum of orange LED at room temperature and in LN2
Spectrum of red LED at room temperature and in LN2

The red LED spectrum confirms the slight change from red to red-orange that was visible to the eye and in the video. Below is a data table of the wavelengths of the peak emissions as measured by the Red Tide spectrometer and Logger Pro software.



Notice the peak wavelengths for the yellow and orange LEDs are almost the same for both room temperature and in LN2. This is evidence that the color seen by the eye is more dependent on the overall emission from the LED than on the peak. The yellow LED at room temperature is not as narrow as the orange LED but has a little bit more emission from the yellow and green where the human eye is more sensitive. In LN2 their spectra and are almost identical and their visual appearance is almost the same green color.

If you are curious about the effect of temperature on LEDs, I have copied my references below. If you know or learn something relevant to this topic, please leave it as a comment. I am sure the next time I get some LN2 I will have a list of new things to try.

https://ecee.colorado.edu/~bart/book/eband5.htm

https://wiki.brown.edu/confluence/display/PhysicsLabs/7A30.10+LED+in+Liquid+Nitrogen

https://physics.stackexchange.com/questions/80513/how-does-temperature-affect-a-semiconductor-band-gap

https://www.researchgate.net/post/How_are_the_wavelength_of_LEDs_dependent_on_temperature

https://www.reddit.com/r/askscience/comments/2qxazo/why_does_led_glow_brighter_in_liquid_nitrogen_but/

https://io9.gizmodo.com/watch-an-led-light-change-color-in-liquid-nitrogen-1574982405

https://rebrn.com/re/changing-the-color-of-an-led-by-changing-simply-cooling-it-in-li-2844214/

http://www.circuitstoday.com/understanding-the-pn-junction

https://www.osapublishing.org/josk/abstract.cfm?uri=josk-19-3-311

Tuesday, July 03, 2018

Cell Phone Airbag Challenge

Videos and pictures of this airbag device have been circling the web the last few days:

While not available for production yet it is getting a lot of publicity, and rightly so. It is an ingenious design that appears to be effective and reusable. It got me thinking about Dan Burns and my Crash Cushion project. Students are asked to design a crash cushion for either a smart cart or a cart with an accelerometer on it to crash into. They are challenged to decrease the impact force as much as possible, something we hope they realize is accomplished by increasing the impact time. 

I want to assign this as an emergency sub assignment for students that can be done theoretically, no cracked screens needed. So I looked to see what other designs might already be out there and found this 2013 parody by Honda:

It is in Japanese and does not have subtitles but the engineering process is still evident in the parody video. The end product is a giant case for your phone which of course makes the phone impractical. I see the Honda version as where my students my start in the process and then the new spring loaded German design as where they might end, with lots of R&D in between.

I plan to start the activity by asking students what is necessary for an automatically deployed air bag for a dropped cell phone. They could work in pairs or groups and discuss the basics of the design criteria for a device that protects the dropped phone from breaking. I expect students to think about drop proof cases they may have seen commercially available that have enforced corners. Once they have made a list of the design needs groups/ pairs could share their individual lists to come up with a whole class list.

After their criteria has been established I would like to show students the original Honda parody video above. The original Honda video has been removed from their YouTube channel but the video is available on a few news sites. Since I can't find one with subtitles I'm not 100% sure its clean for the classroom but its probably safe since it was originally published on the official Honda page. Even on silent students can watch the video and observe his design process; it could be considered an advantage that they have to rely on the visual only and can't regurgitate anything they hear the engineer say. They will probably laugh at the final design but it will serve as a starting point for the next stage.

Before watching the video, or after, students can be given the shorter article about the parody video  that summarizes it and includes stills from the video. Ask students to discuss if the Honda Case N meets all aspects of their design criteria. If their class list was missing something about the phone case being of a practical size they will probably want to add it now. This should lead to a discuss about additional criteria they might want to add.

At this point you can ask students to actually start brainstorming an air bag device on paper. This may include some conjecture and may not hold up to questioning:
"There will be this bag that shoots out here..."
         "How will it shoot out?"
"Ummmm some kind of compressed gas..."
         "Where will that come from?"
"Uhhhh..."

And to an extent that is completely okay. Students aren't going to be able to build a workable model like they do with the Crash Cushions project. This activity is not even necessarily focused on the ideas of impulse either but more on reasonable design criteria

I found this article about the spring loaded German design and made a pdf to share with students. The original video can be shared as well, although it is in German. I plan to ask students what is most important to that design and if it meets all of their design criteria. Students can discuss differences in their design and the German spring loaded design, which of their own design criteria it does not meet, etc.

The viral German spring loaded design is expected to go to Kickstarter soon to crowd fund enough capital to begin production. You could continue the activity with students by asking them which of their own designs they would help crowd fund (before showing them the German design). After they see the German spring loaded design you could ask students if they support it enough to fund it as well, hypothetically of course.

While I plan for this to be a substitute activity it does require the sub to be capable of playing online video clips if your students do not have one-to-one devices like Chromebooks. My subs are not usually capable of operating my projector nor are we a one-to-one school so I don't know how likely I will be to implement this in the next school year. I've summarized everything, including questions I would ask students in this teacher guide for the activity.

I would love to hear any one else's ideas for extending this activity below.