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:
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.










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?"

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.