Saturday, February 18, 2017

Is there gas in that light bulb?

I'm a fan of topics in thermal physics. Not thermodynamics as much as temperature, heat, and heat transfer. These topics have been largely abandoned by NGSS (HS-PS) and are not included in AP Physics 1. For the moment, I teach AP Physics 2, which has some concern for thermodynamics. That gives me reason enough to teach thermal physics as a precursor.

We cover that content in the fall. We are now into geometric optics. That's how it was that I came to have a 40-W incandescent bulb with a dimmer switch on a gooseneck lamp glowing on a classroom table. I was demonstrating the classic image-formation trope of "What will happen the to the image if half the lens is covered?" (It's right up there with "How would the results of this [mechanics] experiment be different if it were conducted on the moon?" among well-worn item-writing chestnuts.)

But we had a few minutes before the end of the period. So I offered a question: "How could you know whether or not there's gas in this bulb?" If I had planned this inquiry a bit better, I would have  begun with "Why is the filament enclosed in a glass bulb?" If incandescents weren't becoming increasingly rare, we could break the glass on one and see what happens when the filament is exposed to air.

Of course, the immediate solution posed by my thoughtful teenage (mostly male) scholars—after milliseconds of contemplative deliberation—was "break it!". You hardly need a question. The best and most immediate answer is going to be "break it!". It just is.

I asked how breaking the bulb would reveal the answer to the question. Forced into a corner of their own making, they suggested weighing the bulb before and after the break. A difference in weight would reveal the prior existence of the gas. I argued the difficulty of the logistics and the precision required.

To move them off the property destruction solution, I moved the goalposts. How could you know without breaking the bulb? And without any other instruments? Hemming and hawing ensued. The end of the period was approaching.

Is the glass strong enough to hold up under atmospheric pressure if it's evacuated? Maybe. Could there be a gas in there given the rapid burn-out that happens when the filament is exposed to air? Yes: inert gases, noble gases.

I touched the bottom of the bulb and reported that it was warm. I casually kept my fingers on the glass of the bulb. Didn't the transfer of heat from filament to bulb require a conducting medium? No, they insisted. The bulb could have been warmed by thermal radiation.

An inquisitive student got up and touched the top of the bulb. He did not keep his fingers on the bulb very long at all and complained about how hot is was. (I had adjusted the 40-watt bulb down to about 10 watts, so it wasn't as bad as it could have been. I also assured him any burns would heal in a few days.)

At that point, they got it. The bottom was warm but the top was hot. Gas in the bulb is heated by the filament, rises, and deposits heat on the top of the bulb. Convection!

The key to knowing there was gas in there was feeling the top and bottom of the bulb. Having the bulb in a horizontal orientation helps: you have a "top" and "bottom" made of the same glass. The FLIR One thermal camera image was an afterthought, and is not needed to develop a solution.

When the period was over and students were filing out of class, the inquisitive student told me that breaking the bulb could be a simple solution, as long as you broke the bulb underwater.

He had me there.

Sunday, February 12, 2017

Nothing's as cool as seeing the heat

Next week I begin my Thermodynamics unit which includes discussing the 0th, 1st and 2nd Laws of Thermodynamics. When I teach the First Law of Thermodynamics, we discuss how it is basically a restatement of the Conservation of Energy. A favorite demo of this is to use large ball bearings that get slammed together on either side of paper. They are often called "colliding spheres" and are a really simple way to show the heat lost in even a simple collision. When you slam the spheres on either side of the paper a small hole is burned into the paper. When I demonstrate this to students I have a volunteer hold a piece of paper straight up vertically and slam the spheres on either side of them several times. It takes students a moment to realize that holes have been made in the paper and then they notice the smell. Only a few holes in the paper is enough to fill the surrounding area with the smell of burning paper. I talk about how hot the paper must have gotten to literally burn at the contact point and that the thermal energy comes from conserving the energy from the initial collision. Dean Baird uses this as an exhibit in his student run Exploratorio, called "Fire Clap."

Even though it seems obvious to me that the burned hole is an example of thermal energy I wanted to show students the collision as viewed through a thermal imaging camera. I tried looking online but I could not finding any such video. I don't own a FLIR camera (yet) but the Exploratorium Museum of San Francisco does! I was there today to help with a Teacher Institute workshop and headed down to the FLIR exhibit with a set of the colliding spheres. Some other teachers and I got some videos:

Our first attempt showed that there was in fact a bit of heat around where the holes were made. You can see the color change around the edge of the hole over time:


While rearranging for another take we noticed that our hands left residual heat lines on the paper so we drew on the paper that way for awhile. Physics teachers are easily distracted by cool stuff. We found that my fingers didn't work well and when everyone held their hands up we saw why. My fingertips showed up black (cold) while everyone else's were white, the same color as the rest of their hands, apparently I have cold hands.


In this video you can see the experiment take place on the right and the projected FLIR video is on the left. Again the holes produced have a bright white that eventually fades to the color of the paper.


At this point we remembered that we were making holes and therefore we could "see" the heat signatures of things behind the holes. We oriented the paper so that a dark color was behind it so that we did not have contrast behind it. A well timed museum visitor passed behind and we can see that the color changes:


Another experiment commonly done with the colliding spheres is to slam them on either side of a piece of foil. This Educational Innovations post explains both aspects of the experiment. When we tried the foil we found that there was no heat seen through the FLIR camera. We could not heat the foil like we did the paper and see the residual lines from our hands.


According to Zeke Kossover of the Exploratorium it is due to the low emissivity of the foil. This FLIR article explains it a bit but basically the foil is so good at reflecting radiation (visible light and heat) that the FLIR camera does not accurately show its temperature. In the picture above the black rectangle on the right and the two spheres in my hand appear black which translates "cold" through the FLIR camera. They are in fact both room temperature or warmer as they have been held for a moment.

So now I have video to show my students that confirms, in more ways than one that thermal energy is produced when the two spheres are slammed together. There's nothing quite as cool as seeing the heat ... *bad dum ching*.

Tuesday, February 07, 2017

Scientist Valentines 2017

With one week to go before the Big Day, it seemed like a good time to repost this perennial favorite. (The image below is an embedded slide show: click left or right to see the other 23 valentines.)

Scientist Valentines

Scientist Valentines (Flickr album)

In addition to the extant set available on Flickr, I've modified the optional student assignment that I make available for those seeking extra credit. It might seem burdensomely detailed, but considerable thought and execution goes into making a good Scientist Valentine.

Scientist Valentines: The Next Generation

For previous Blog of Phyz posts regarding Scientist Valentines, follow this link:

Blog of Phyz Scientist Valentines.

And we keep links to the Flickr set and the label over in the column to the right.

Friday, February 03, 2017

Brainiac clips

Every year, for as long as I can remember, I've shown a clip from the British show Brainiac that makes a giant pendulum mirroring the in-class bowling ball demo. My downloaded copy is grainy and pixelated so I decided to try and find a better version. I downloaded one Brainiac episode (Season 1, episode 3) with the intention of editing it down to the 4 minutes or so that I wanted. I ended up watching the whole 40 minute episode and editing out six clips to use in my classroom. Not too shabby for some fun TV time.

Conservation of Energy and a giant pendulum:
Well explained and stands alone well.

Oil Slip & Slide:
Even really slippery surfaces have a coefficient of friction that slows down moving objects. You could have students estimate it using the values given in the clip.

LN2 filled water bottle:
Quick example of pressure, boiling and of course liguid nitrogen.

Does a duck's quack echo?
Sometimes students just won't believe you unless they see it for themselves. Or in this case hear it. 



Don't microwave a CD:
#ThingsThatShouldGoWithoutSaying

Playground G forces:
Brainiacs (the volunteers and staff that put on the science of the show) try to get the most G forces possible out of a playground merry-go-round. You could get more but they are limited by human power.

Iron in cereal:
This is an easy demo to do in the classroom but it does take some prep, the right cereal, etc. This is a super short clip that demonstrates it if you don't have the time.

Now I want to watch more of it. Besides the energy pendulum the only other clip I have seen prior to this was another all time favorite, "The Electric Fence." It is pretty much all the things you wish you could do in your classroom but couldn't:



Update: For an exhaustive video demo lesson on the Brainiacs: Electric Fence clip, see this old Blog of Phyz post:

Electric Fence Redux

Wednesday, February 01, 2017

How many magnetic poles?

Last weekend I presented at the Exploratorium's 4th Annual NGSS STEM Conference "Making Science Count: Integrating Math into an NGSS Classroom." I presented a few inverse and inverse square relationships participants explored using hands on experiments. One of them was to investigate the relationship between the strength of a magnetic field and the distance to the object.

Thanks to sponsors, participants were able to go home with their own "cow magnet" (if you don't know why they are called that read about Hardware Disease). While preparing for the workshop that morning senior scientist and staff physicist Paul Doherty cautioned me that while I would expect cow magnets to be dipoles they could be tripoles. After he check with magnetic viewing film it turned out they were quadpoles. And that can complicate an experiment.

Workshop participants either borrowed my Vernier Magnetic Field Sensor or used the magnetometer on the Physics Toolbox app during the workshop. When I was preparing for the workshop I found that this could be an inverse square or an inverse cubed relationship depending on the physical dimensions of the magnet. Given the orientation of these quadpole magnets if you rotated the cow magnet at all as it approached the sensor the polarity could change.
1. Asking questions (for science) and defining problems (for engineering)
Below is a video using a dipole donut magnet, a dipole cow magnet and a quadpole cow magnet that models what I would expect students to see.

I investigated further using my Vernier Magnetic Field Sensor once I got back to school. Below is a graph made by starting the sensor perpendicular to one end of the cow magnet and then moving up the length of the cow magnet to the other end. I put a pencil in between the cow magnet and the sensor to maintain the same distance between them.

On the left, the red line was made using a dipole cow magnet and the blue by the quadpole magnet. In this case they are similar and one might conclude that they are both dipoles. (Differences in slope are due to the sensor's speed.) On the right, the red line is the same, made with the dipole magnet. The orange and green were both made with the quadpole magnet. When the green line was made the magnet must not have had a pole directly facing the magnetic field sensor.

I also pointed the sensor at the end of the cow magnet and rolled it along the table, keeping the sensor from rolling and at the same distance away. I tried it twice with the quadpole magnet, creating the green and purple lines in the middle. This was harder to keep steady but you can see the polarity switch as the lines pass the time axis. Repeating the experiment with the dipole created the brown line at the top of the graph. For the dipole rolling it made no difference in the polarity strength or direction. 
So what can you do with a pesky quadpole cow magnet? Why, confuse your students of course! I plan to hand groups of students one dipole and one quadpole cow magnet and ask them to determine the number of poles on each. If they are lucky they will get a compass and/ or viewing film. Otherwise having two magnets should be interesting enough. If you're keeping track of the NGSS Science & Engineering Practices, such an investigation could lead to quite a few of them in one lesson:

2. Developing and using models [of thinking]
3. Planning and carrying out investigations
4. Analyzing and interpreting data
5. Using mathematics and computational thinking
6. Constructing explanations (for science) and designing solutions (for engineering)
7. Engaging in argument from evidence
8. Obtaining, evaluating, and communicating information


Sunday, January 29, 2017

Analyzing a simple Crash Cushion design

When my students make their crash cushions many of them create paper tubes. This can lead to a successful design or a horrible one. I saved one such simple design and tested it in three different orientations. Depending on the level of your students you could show them this to aid with their own design development.

Tuesday, January 24, 2017

Crash cushions cont.

Several years ago Dan Burns and I started discussing an engineering project for which students build a crash cushion to investigate momentum and impulse. Using only a few sheets of paper and hot glue student groups design crash cushions (similar to water barrels or guard rails on roadways) to lower the force experienced by a cart rolling down an incline that crashes into them.

Since then we have both completed the project with classes albeit differently. We also presented at the Summer 2016 AAPT meeting in Sacramento, all of the materials discussed there are here. I wanted to share the project here again, with the tweaks my partner Jon Brix and I have made to it since.

My colleague Matt Miller continued the project in Conceptual Physics this year although opted not to use the Vernier sensor that I had last year.  He opted for the resettable Drop N Tells I bought years ago instead as it is more visual for the younger students. He set up a ramp and used a lightweight impact car that had an additional <200 grams of mass added. Miller adjusted the ramp set-up until the 25, 15, 10 and 5-g sensors were consistently tripping. His students were challenged to design the crash barrier that did not trigger all the sensors. I believe he set the grading up this way:
C = triggering the 15, 10 and 5-g sensors
B = triggering the 10 and 5-g sensors
A = triggering only the 5-g sensor
Extra Credit earned for not triggering any sensors.

Dan uses a PASCO Smart Cart while I use Vernier sensors. The first year I tried this I used a low-g accelerometer because it was what I had. Through a Donors Choose grant I was able to purchase the higher 25-g accelerometer3-axis accelerometer and a Wireless Dynamic Sensor System (WDSS). The wired sensors require some coordination to prevent the cord from catching but are workable. I found that the wireless WDSS made for easier set-ups but would disconnect occasionally. Both the WDSS and the 3-axis sensors were almost too accurate and the graphs produced were difficult for students to interpret. I opted this year to use the single-axis 25-g accelerometer because even collecting 500 samples per second the peak accelerations were easier for students to determine.

In the past I've used a wood ramp and a big heavy dynamics cart that then travels along the flat lab bench into a wall. The transition from ramp to flat tabletop caused additional acceleration peaks so we opted to have the cart run directly into the wall from an incline. The heavy cart and steep ramp produced a high acceleration that exceeded the accelerometer's limits. We decreased the ramp angle and still occasionally "missed" the hit because the time of impact of the cart against the wall was so short. This year we opted to use a low-friction (not smart) PASCO cart and track from another colleague. The 120 cm long track was raised above the table by one textbook and pushed against the wall. A box of weights (over 30 lbs) was pushed against the higher end of the track to prevent it from moving. In initial tests the conservation of momentum caused the track to move quite a bit when the cart struck the end of the track.

This year student designs proved very successful. Because of the light cart and small incline students were able to reduce the acceleration of the cart at impact fairly easily. Usually the designs that "failed" did so because the cart passed underneath the crash cushion and still struck the wall. I had only one set-up in the classroom so groups took turns testing their barriers and collecting data. We stored a trial of the cart running into a book at the end of the ramp and then printed out graphs for each group with their trial on top of the control data. Here is an example of the data student's received with the control (green) and their trial (blue):
Students were to take measurements of their crash cushion before and after their collision although most cushions did not permanently deform. For some reason students were very pleased when their crash cushion suffers little damage; several cited the fact that it could be reused as a positive attribute. Students were also to calculate the Force with and without their crash cushion based on the mass of the cart.

Using this information students were to write a Claim, Evidence, Reasoning (CER) conclusion to answer the question: "Was your crash cushion effective?" After grading these conclusions we realized a few things:
1. Students did not agree on what made a crash cushion effective. Most students realized that decreasing the force, as shown on their graph as a decrease in acceleration, by increasing the time of impact made for a successful crash cushions. A few more realized that stopping the moving cart without letting it bounce back was also good. Yet many students considered their crash cushions ineffective if there was any acceleration, even if they reduced their force by more than 50% .
2. Students do not know what is fact vs. opinion. This must be going around recently. Students often stated opinions or qualitative observations in place of specific measurable data. "Our crash cushion was good because it stopped the cart slowly."
3. Some students did not understand the graph axis, significance of peaks, etc. Referring to the example above, some students incorrectly described the "time of impact" to be just over 2.5 seconds for the control trial. They did not understand or forgot the fact that the cart had to roll down the ramp before the impact.
4. When in doubt, students are prolific. I expected three, maybe 5, sentences from students yet often received a full page. While grading these conclusions I often crossed out over half of what was written because it was superfluous. They seemed to just keep writing and praying for partial credit.

Before handing back their conclusions I reviewed the CER format with students and showed them a few pictures of correctly written (short) examples of their peers. I showed them a few sample graphs from their trials and reviewed the significance of each peak. In the future this will be done the day after to give students a chance to correct their CER conclusions before turning them in.

Saturday, January 07, 2017

Where the Bolts Are

Exploratorium genius, Paul Doherty, recently shared this gem of a resource. It's a real-time visual representation of lightning all around the world.

LightningMaps.org


Christmas Convection: The Fiery Fairy

I might very well be the last high school physics teacher teaching the topics of temperature, thermal expansion, heat, and heat transfer. Don't worry, I'll eventually retire and that will be that. (As it is, I've relegated those delicious topics to AP Physics 2, and that course is subject to the whims of enrollment.)

In the meantime, here's a tidbit that I'll be adding this sparking gem of black humor to my convection curriculum.



In my assessment, the flying fairy is a sensitive sensor for air currents in a room. And the fireplace sets up a "negative pressure" by sending heated air up the chimney and drawing air in from the living room. The fairy successfully discovered this air current. Great Success!

The video above went viral a few years ago. And some clever folks produced a satirical follow-up: Fiery Fairy Funeral.

Monday, January 02, 2017

Normalize Science

Most people that know me realize I'm a "science nerd," probably more appropriately described as a "science enthusiast." This means I enjoy talking about, reading about and encouraging others to learn about science. Most science teachers I know are enthusiastic enough about their subjects that they randomly insert science into everyday conversations. Sometimes this is greeted with "Hey that's interesting! Thanks for sharing!" type comments. Other times our science related comments are met with awkward science and a change of subject. This happened several times over the holidays and I was struck by how unusual it seemed to be to "talk science."

At both the extended family Christmas Eve and our neighborhood New Years Eve party I shared our plans for a family trip to see the Great American Eclipse next August. It is common when this comes up to have to explain to an otherwise well educated adult what a solar eclipse is and how it happens. At each holiday party I asked kids not "How's school going?" but "What are you learning in science class this year?" When the horn a child was blowing at a party seemed too loud I broke out the decibel meter app on my phone and reminded a mechanic friend to wear hearing protection at work.

My active and conscious support of science for all also presents with gift exchanges. I try to buy science or educational gifts for our family and friends, which I sometimes have to explain after they open it. For instance, this year I gave an Airzooka, a favorite classroom demonstration, to a family member who had received mostly gift cards from the rest of the family. As a young teenage boy he is now "difficult to buy for" and "doesn't want toys." As this video explains the Airzooka pushes a pocket of air that can be felt across the room. As soon as I assembled it for him he began shooting air at others across the room, and soon adults of all ages were stealing it to play with it. They had never seen anything like it, all wanted to know where to buy it and wanted to hear how it worked. Of course I was happy to oblige them and explain the science of it and how I use it to model sound waves in class. Family and friends were surprised how fun the Airzooka and other gifts were since they knew they were science and education related. Somehow labeling them that way came with the assumption that they couldn't be fun or couldn't be for younger kids. We've also given a drinking bird, geodes to crack, a moon night light with lunar phases, a solar robot toy, and more.

Luckily, other friends and families asked what my kids' interests are or if I have any specific ideas for them. They received many science and educational toys this year and I'm happy to report they are loving them all so far. Their Nana gave them Code-A-Pillars which allows them to build a robot caterpillar by connecting segments with different instructions on each. It allows students to practice rudimentary programming and learn sequencing. GG (Great Grandma) got my six-year-old a special viewing planter that allows her to see the plant roots as they develop. And a good family friend obliged when I said she really wanted a toy metal detector and she happily pranced around the yard listening to it ping.

While many science teachers already talk with their friends and family about science or give the gift of science, I'd love to see such practiced by non-scientists as well. A common phrase these days is "We need to normalize [X, Y or Z]." One way to normalize science for all is to talk about it. When someone around you asks "Why did that happen?" or "I wonder if..." discuss it with them, even if you don't know the answer. Encourage the young ones around you to be interested in science, even if they "don't want to be scientists." People like to cook, even if they aren't chefs. They like to go for bike rides even if they don't have professional equipment. I would argue that science can be an interest or a hobby for everyone. All adults can dabble in science without having a degree in science and they can encourage every child around them to do the same. Coloring became a viral sensation and the hobby-du-jour; let's make science the thing to do this year.