Saturday, April 16, 2016

Moon shadow

As noted in a post in 2014, most of the United States will be able to see a total solar eclipse in August 2017. That is all kinds of cool and lots more information will be available as the date gets closer. This is an incredible video of the eclipse shadow, something I had never seen focused on before, for the 2012 Total Solar Eclipse in Australia.

The Sweeping Shadow - Total Solar Eclipse, Nov 14 2012,The Granite, FNQ, Australia. 

from Colin Legg on Vimeo.

Worst (electric) shock of my life


I was recently reminded of a bad shock I took once, in the name of science of course. One of my high school students had really enjoyed our film can Leyden jar. He enjoyed dabbling in science on the side (who doesn't?) and made me a massive Leyden jar out of a pickle jar and a terra cotta pot. It is over a foot tall and while we presumed it was functional I was a new enough teacher that I did not want to try it out. It was covered in orange duct tape with "DANGER" written all over it. In fact, I hid it whenever there were substitutes in my room so no one would even be tempted.

Years later that same student came back to visit me, while studying physics in college, and we reminisced about his project. With another physics teacher in the room we decided to test it. We charged it up with my Van de Graaff generator and I touched it. It hurt. A lot. In fact I felt the shock all the way from my hand to my shoulder on the same arm. It was larger than I had anticipated and it hurt for awhile afterwards.

Now I regularly allow the current from my Van de Graaff generator to flow through a long aluminum rod through me to the ground while demonstrating. I try to avoid the shock from my dissectable Leyden jar demo but I'll take it if necessary. The shock from this massive Leyden jar was worse. By a lot. Now at least I can tell my students how bad the shock was and use it as a starting point to our discussion about shocks and safety.

It was my understanding that if the shock had been large enough to continue across my heart it could have been a lot worse. I would like to measure the size of the "shock" off of it, and the smaller Leyden jars I have students make but never have. I would think the large one would surpass my analog ammeter and RAFT multimeters. 

I recently posed this question to the Exploratorium's Teacher Institute Listserv to see if anyone had any ideas about how to achieve this. They agreed the equipment available to me would almost certainly be fried by my massive Leyden jar. The level of equipment necessary was apparently expensive and usually only available at universities and research facilities. Public high school: poor, fewer cool toys. Got it. 
The other take-away was that measuring the "shock" was difficult because there were many things to measure.  I show my students this table when we begin our electrostatics unit and continue to reference it through our current electricity unit. While the level of shock is often determined by current, as with this table, you can also measure the energy, power, capacitance and even the distance it jumps. The shock is incredibly short lived, however, making measuring it difficult.

I wanted to share my story as a cautionary tale and hoped to add, "If you wanted to test how bad the shock from a demonstration might be you should measure the shock before by [detailed safe procedures]." 

Since there is no easy way to do so I'll just say it's not a good idea because it really hurt.

Don't look at the sun!

Anecdotal but an example of the burden of being a science teacher and knowing better. ;)

A neighbor boy was all excited to show me his magnifying glass, a little plastic thing he probably got in a goody bag. “Look Bree, if I look at the sun it burns a bit!”

You can probably imagine how loudly I shouted “Nooooo!” He was persistent and I had to actively block his eye and the magnifying glass from the sun for a while with my own shadow. I tried to direct him to looking at things on the ground with the magnifying glass. I had him hold the magnifying glass at arm’s length to view another neighbor. “He’s upside down!?”

He continued to play with it for quite a while, and my science-oriented heart swelled a bit, when he conducted some side experiments with a translucent cup on the ground.

“Look, the shadow is green like the cup! If I tilt the cup the shadow changes. If I move the magnifying glass the little sun changes.”

Out of respect for my friendship with his mother I did not teach him about the heat at the focus of the magnifying glass. Although when he briefly aimed it skyward again I was tempted.

The next time we got together I brought home some of my eclipse viewing glasses. I told all the neighbors (there are nine under the age of ten within a few houses) that they were the only safe way to view the sun. Their response was less than impressed, “Oh, it looks like an orange ball.” Then they tried to see if they could ride their bikes or walk or play tag while wearing them. Hey, everyday can’t be a victory.

Thursday, April 07, 2016

To CER with Love

We've all asked students to analyze their lab results. Sometimes we have constructed conclusion questions we hope will lead students to the big "aha!" realizations about their data. We may assume more advanced students can recognize connections on their own and just ask them to "write a conclusion." No matter our request, we are often met with students that fall drastically below those expectations, even while others exceed them.

While NGSSifying (sure, it's a word!) your curriculum one relatively easy step is to add Claim-Evidence-Reasoning Conclusions to your existing labs and activities. Often abbreviated as CER (or occasionally as ClEvR) it is a scaffolded method to help students organize the findings of their experiments. It is mentioned specifically in Appendix F Science & Experimentation Practices:

"An explanation includes a claim that relates how a variable or variables relate to another variable or a set of variables. A claim is often made in response to a question and in the process of answering the question, scientists often design investigations to generate data."

You have to start by asking them a question, an open ended one is even better! Perhaps the hardest break from their old habits is to wait until after their experiment to write a claim. It is not a hypothesis to be proved. Their claim is their answer to the question you posed based on their experiment. The next step is to describe their evidence that supports this claim without any explanation. This is difficult for students that are used to runningeverythingtogetherforthesakeoffinishingasfastaspossible. The third and final step is to explain the reasoning behind their evidence.

This is definitely something made more clear with an example. A photocopy of this chart was given to me at a NGSS training years ago and it is still my go-to to explain this method because there is an example for every discipline. When I show this to my students I usually the Earth Science one since they have less experience with that field of science.


Question
Claim
Evidence
Reasoning
Rebuttal
Earth Science
How was the Grand Canyon formed?
The Grand Canyon was mainly formed by water cutting into and eroding the soil.
The soil in the Grand Canyon is hard, cannot absorb water and has few plants to hold it in place. When it rains in the Grand Canyon it can rain very hard and cause flash floods. The flash floods come down the side of the Grand Canyon and into the Colorado River.
Water moving can cause erosion. Erosion is the movement of materials on the Earth’s surface. In terms of the Grand Canyon, the water moved the soil and rock from the sides of the Grand Canyon into the Colorado River where it was then washed away.
Some people may think the Grand Canyon was cause by a large earthquake, but the Grand Canyon is not near any tectonic plate boundaries. Furthermore, earthquakes in Colorado are rare and do not tend to be very large - largest earthquake on record had a magnitude of 6.6.


Adapted from “Supporting grade 5-8 Students in Constructing Explanations in Science” (CER) McNeill & Krajcik; Table 2.2 Examples of the Different Components for Scientific Explanations.

I've seen this taught down to the middle school level with graphic organizers representing each step to help guide students into adopting the process. You can find graphics organizers like these (left and right) from a variety of sources, just google "Claim Evidence Reasoning graphic organizers" and you will find lots more. There were even a few that took the shape of a hamburger ...

I've made a poster similar to the example on the right for my room and when a Claim-Evidence-Reasoning format fits our lab experiences I try to use it. So far, it has been about as hit-or-miss as the ol' "Just write a conclusion" format. I think the main reason for that is that I have not embraced it and taught the process repeatedly. Kids know when you half-heartedly attempt something and they reciprocate in kind.

The time I think the format was more successful was during my PVC Dart Gun Lab, a more structured format of the draft I previously posted. I wanted to informally assess the groups and their progress so I printed out quarter sheets with the different questions on them. I wrote the group member's names on them, which also made assigning the groups easier, and asked them to record their Claim-Evidence-Reasoning on it to return to me by the end of the period. It was a quick way to read each and give the groups feedback before they each turned in their own individual labs. I found quite a few that wrote claims as if they were hypothesis, a few more squished their evidence and reasoning together and quite a few more didn't actually offer any evidence despite having collected data. I like to think that this check in helped some students edit their individual labs but having since graded them, I know I didn't get through to them all.

Coulomb's worth of sand?

I teach Coulomb's Law to my regular Physics (mostly juniors and seniors) as well as my freshmen Conceptual Physics students during our Electrostatics unit.  The younger students struggle every year and because of their math level I agonize over including it in Conceptual Physics at all. The sheer size of the numbers involved confuse them and then the use of this "strange unit" called a Coulomb doesn't help. Every year I try to equate the use of the Coulomb as the unit of charge to a dozen eggs which helps some but not all.


A few years ago I decided that I wanted to make them a visual, something they could see that would help them understand the size of the Coulomb. I thought of the Mole cubes I had seen in Chemistry, they are about a cubic foot and proudly proclaim "1 Mole of" and list the masses of different gases. I could do the same thing and say I had a box with a Coulomb's worth of air in it but since they couldn't see the air this probably wouldn't help with their understanding. I wanted a Coulomb Cube, I just had to figure out how big to make it and what to put in it.

I tell my students this story as a way of introducing a Coulomb and they follow the same thought process I had when asked: "What is something fairly uniform and small that we could gather a Coulomb worth of?" Students usually volunteer answers: "Marbles! Rice! Sugar! Salt!" and then usually arrive just as I did at grains of sand.

When I first looked into this problem I found quite a few resources on the different sizes of grains of sand on math websites. Apparently it is a common estimation problem to figure out how many grains of sand there are on a beach, in a dessert or on the whole planet. Using the smallest average grain of sand CosmologyScience.com determined a cubic foot of sand would hold one billion grains of sand.

The Math Dude estimates "8,000,000,000 grains of sand per cubic meter." Since his are in metric, I'm going to discuss the calculation with his values. If we take a Coulomb to be 6.25x1018 charges ("grains of sand"), if we divide a Coulomb by the estimated number of grains of sand in a cubic meter we get:
Since I wanted to build a cube, (although at this point I already realized I was in trouble) I found the cube root of this amount of cubic meters to see how large my cube was going to be on a side.
So, I didn't build a Coulomb Cube, and realized that I had trouble grappling with such a large number as well. When I tell my students this story and tell them I would need a cube that was almost a cubic kilometer their jaws drop. While I don't have a physical manipulative to show them, discussing the thought problem at least illustrates the sheer volume of a Coulomb. Get it? *ba-dum-bum-CHING*

Monday, April 04, 2016

Ice Scream - a new ExploratoRio Science Snack

When broadcast television was something people watched, networks would pitch their "encore presentations with the line, "If you haven't seen it before, it's new to you!"

Somewhere in the running conveyor belt that is my Facebook feed, I recall seeing a video of a Kennedy half dollar coin being set into a cavity in a chunk of dry ice. The coin and ice produced vibrations and sound.

When two students ran me through the paces of their Bubble Suspension build (for our upcoming ExploratoRio2016 on Wednesday, April 13), they left some dry ice behind in my classroom sink.

So it's me, some sublimating dry ice in a sink, and no one else around. Recipe for serendipity if ever there was one.

After dousing the frozen carbon dioxide and blowing the subsequent cloud out of my sink a few times (that never gets old), I found a nearby electrophorus and pressed it into the sublimating mound.

What happened next is best relayed in the form of a video, which drops the action to 1/8th speed at about 20 seconds in.



So unexpected was the sound that I decided a new ExploratoRio Science Snack was in order. The title of "Ice Scream" wrote itself with little help from me.

The details (as much as I know them or foresee them) are in the recipe linked to below.

Ice Scream - ExploratoRio Science Snack.

Note: Adopting the look and feel of the original Exploratorium Science Snackbookq was not an accident.

Sunday, March 20, 2016

What's wrong with this photoelectric effect lesson?

Let me just say at the outset that this post is Reason I'm a Terrible Person #417.

I was scoping out the YouTubes in search of a nice video lesson on the photoelectric effect. Demonstrating the effect can be troublesome and finicky. So why not enjoy the fruits of someone else's labor?

I watched through Adam Micolich's lesson (below) and... found it wanting. Wanting to be used as a lesson in skepticism and critical thinking!

The story/script is nicely straightforward:
1. A negatively charged electroscope is discharged via exposure to ultraviolet light.
2. The UV light is causing the discharge: a glass plate blocks the UV and prevents the discharge.
3. The UV liberates electrons: a positively charged electroscope is not discharged by UV light.

Let's see how it goes. (The actual photoelectric demo begins at about 2:50.)



Each segment seemed to go according to plan and proved the aspect being investigated. The content as spoken is spot-on correct.

And I have no reason to suspect shenanigans. But in my judgment, the demonstration is flawed to the point it would be fraudulent if done intentionally.

Consider it a PhyzMaster Challenge: Can you find the flaws I found?

I offer this challenge in good faith as an exercise in physics-based critical thinking. These principles are ostensibly valued by Common Core, NGSS, and the reimagined AP Physics courses. I mean no disrespect to Mr. Micolich. He was good enough to produce the video lesson; I—someone who didn't bother to create a video lesson of the photoelectric effect—spent a Saturday night finding fault with it. So who is more deserving of harsh judgment? (That was rhetorical.)

Hints and specific allegations in the comments. I might remove these later if I assign a skeptical critique to my students.

Wednesday, March 09, 2016

Hewitt Drew It screencast index page updated

Conceptual Physics author and good friend, Paul Hewitt, recently spent a few years producing 147 physics screencasts. I was overwhelmed at the sheer volume of these gems, so I organized a set of links to the screencasts in a way that was helpful to me as a classroom teacher.

Since I originally launched my index page, several more screencasts were produced. So today, I updated the index page to reflect the new screencasts. Newly indexed screencasts include several concerning special relativity and a couple on general relativity, as well as "appendix" screencasts on sailing into the wind and another about the exponential function (it's more captivating than you might think).

Hewitt Drew It Screencasts

Monday, March 07, 2016

Take Physics: Course advertising–2016 edition

It's that time of year again. The counselors are headed out to register students for next year's classes. Sometime late in May, section counts will be determined and before school is out (fingers crossed), a master schedule will be unveiled.

If you want more students to enroll in physics next year, you might consider advertising your physics course offerings right now.

To that end, I post a series of fliers around campus. Over the years, it's actually become a few series of fliers. I'll post each series below as examples. Some you can probably use as is. One will absolutely not work for you.

In any case, you'll want to click each series to see what it looks like.

Series 1: Physics: You Don't Want to Miss It (The Original Series)

Series 2: Physics: You Don't Want to Miss It (French and Spanish)

I print Series 1 and 2 to fluorescent, coated (not dyed) "neon" paper. It's as optically bright as it is hard to come by. Here's the Amazon link I used last time I bought the stuff.

Series 3: Slogan meets Google Translate  [I'm confessing that I went too far with this series.]

Series 4: Dank Memes: Take Physics Edition

Series 5: The Groupie Shots (ft. Bill Nye, Neil deGrasse Tyson, and Adam Savage)

Dank Memes are fun to generate, but can easily go terribly wrong—use extreme caution if you intend to create your own. Many of the templates are fraught even when they're blank. Remember: Physics is easy; poorly executed attempts at comedy can cost you your job.

The Groupie Shots? You'll need to get your own. Mine will not work for you: sone kind of union contract issue, I think.

If you've got posters that you decorate your school with during course selection season, tell us about them in the comments.

Sunday, March 06, 2016

A variable "tuning fork" with volume

Tuning forks have one job: to vibrate with a single fundamental frequency. Simple tuning forks are often troubled with some harmonics, but the overtones are typically very small in amplitude. The fundamental frequency produced by each fork is determined by its composition and physical characteristics. Most are made of aluminum, and their tone is fixed by the length of their tines.

Variable tuning forks have weights attached to the tines; the location of the weights can be adjusted. The frequency of the fork depends on the location of the weights. Such forks are okay in a pinch, but they're a bit dull for my tastes and their Q factor leaves something to be desired.

The good folks at Next Generation Science (whose corporate name is harder to find via search engine results in the NGSS era), offered an electronic tuning fork (Sound Generator) that could produce tones at 400 Hz, 600 Hz, and 800 Hz.

But wouldn't it be nice to have a small-scale, full-spectrum variable tuning fork? Perhaps you already do.

There are frequency generators aplenty available for smartphones. (I have FreqGen, but that one seems to have vanished from the iTunes Store.) What's important is that you can adjust the frequency of the output tone down to the hertz. You can turn your smartphone into a variable tuning fork easily enough. But much like a tuning fork, the smartphone's sound doesn't carry out to much of a volume. The common solution is to set the tuning fork into a resonance box.

But for the smartphone, that's where a portable speaker comes in handy. I have a Bluetooth speaker that is capable of achieving considerable volume. Once the speaker has been paired with the phone, you can unleash whatever frequency you like at volumes that can annoy people and animals for many meters all around.

I don't recommend blasting out 16,000 Hz at full volume simply because it cripples teenagers while leaving you oblivious. That's just cruel. Every time I do it, my students hate me for several minutes.

Use this newfound tool for good, not evil. It could be leveraged in a "speed of sound" type lab. I'll share my primary use for it in another post.