Monday, March 27, 2017

Chladni Singing

Chladni plates are a beautiful and dynamic example of resonance in two dimensions. Originally a bow was used (as seen in this Harvard demo video) to create two dimensional standing waves in a flat piece of metal in different shapes. More common now is to use a frequency driver to control the frequency of the standing wave. Here is a video that goes through a wide range of frequencies:
Whatever the method, you can demonstrate that higher frequencies produce shorter wavelengths and thus smaller patterns on the plates. You can talk about the fact that the pattern on the plate seems to abruptly change as a resonant frequency is reached. Ask students if the locations where salt gathers are nodes or antinodes of those standing waves. There is a lot to learn from something that is also mesmerizing to watch. If you're not super comfortable with the explanation yourself try showing your students this Physics Girl video that has a good explanation and information on the next steps in modern science.

I thought I'd seen everything about Chladni plates (including this music video they appear in) but then I learned of someone who can sing to a Chladni plate! Meara O'Reilly works at the Exploratorium museum of San Francisco and has enough voice control to sing to Chladni plates to create different patterns. Her website devoted to this phenomenon is here and an example is below. While amplified, this is produced by the human voice and not a frequency generator. It is amazing.

Saturday, March 11, 2017

dB line-up

I tend to stand on a soap box during my sound unit and take the opportunity to talk about sound safety. [Other related Blog of Phyz posts: quiet fireworks and firework sound safety.] I discuss decibel scales and show students several graphics with common noises and their values. Some of my favorites are below:

But it is sometimes hard for students to understand all the principles at once. I want to discuss allowed time duration, compare sounds they can relate to, talk about decibel reducing measures, and more. I find myself flipping back and forth between graphics during lectures and still not getting it all together.

I found this infographic that details over 60 noises with their decibel level, marked OSHA standards and suggested exposure times. The graphic itself is too large to view all at once but I loved the format. Noises were divided up as occupational or non-occupational noises. I typed up all of the activities with their decibel levels into a word document and selected 40 that I thought provided a range of activities students would relate to. I created another document for printing with the sounds, and OSHA recommendations. I cut out each one, mounted them to thicker bright paper and we were ready to go.

In class I explained the decibel scale to students and showed them a few examples. Then I passed out one sound slip to each student and asked them to arrange themselves into order from 0 dB and up. After they were arranged students took turns reading their sound and decibel rating. As they were sounding off (*ba dum ching*)  I added in the cards describing the sound level (i.e. "comfortable 50 dB" or "Extremely Loud 100 dB") and the cards with suggested exposure times.

Students were surprised to learn the average decibel level of several common noise. They were shocked to see how many of them were past OSHA recommendations like "ear damage can occur here," or "unprotected noise exposure not allowed past this level." It gave them a chance to see all my talking points at once and they got to get up and move.

Space was an issue as 35 students shoulder to shoulder take up some space. I tried in my classroom for students to stand across the front of the room but they ended up in a weird convex arc and couldn't see or hear everyone else. I tried outside as pictured above but being right next to the street (on the other side of that fence) made it even harder to hear. In the future I would have arranged them in a circle around the room so that they could see everyone else. I would also hand students either a pictures of or the actual sound reducing devices where they would be needed to reduce the noises to a safe level. For example, students at 100 decibels would hold your basic foam ear plugs to show that their 20 dB reduction rating could lower those noises to a safe 80 dB. All in all I like the activity and look forward to refining it in the future.

Thursday, March 02, 2017

Dual purpose dimmer

At the 2017 Summer AAPT meeting in Sacramento Arbor Scientific had a booth in the exhibit hall showing off all their great demos. This Electricity & Magnetism Light Bulb demo caught my eye. It involves an incandescent light bulb with a long loose filament that can be turned on using AC or DC current. While on, a strong neodymium magnet is brought near the bulb with different results depending on the setting. When on DC the filament moves towards one pole of the magnet and away from the other. When on AC the filament vibrates when either pole of the magnet is brought close to it. I wanted one, but the reasonable price was still hard to manage for a once a year demo.

Last year I wanted to show my Conceptual Physics students a working dimmer switch and a simple light bulb. My retired father-in-law made me a self contained box with a dimmer switch and an incandescent bulb:
While shifting equipment around in my room I came across it again while my magnet cabinet was also open. As I recalled the Arbor Scientific demo I thought, "I wonder if...." and grabbed my biggest neodymium magnet. 

With the light bulb on dim so that the filament can be easily seen I moved the magnet close to the bulb. Since my bulb's filament wasn't as long as the one in Arbor Scientific's demo you don't see as much movement but it is vibrating! The second video is easier to see the motion. 

Now I know that my simple demo has a dual purpose and can be used in my current electricity unit and come back in my electromagnetism unit!

Saturday, February 25, 2017

Best Thermo assignment ever!

Made you click ;)

Then again, that is the point of this assignment. This year in my thermodynamics unit I ended up with two half days open (I couldn't rearrange them to make it a full day). I started making a list of topics related to thermodynamics applications seen in the real world that I don't usually cover. I wanted students to research these topics and share what they learned but was missing the buy-in. I found the topics interesting but knew looking at the list my students initially might not. They needed something to entice them ... something that would make them click on the topic if they were reading it on their phones. I needed some bait to hook them in ....

Click bait headlines!

[You may have seen that coming]

So I created this assignment (pdf or google doc) for groups to work on on the first of the half days. Students thought the click bait headline requirement was hilarious. Some topics required a bit more explanation to guide them in the right direction. For example, the second question about boiling water could go one of two ways. Some groups went the way I was thinking and found that most diseases and bacteria are killed around 80 degrees Celsius while water boils at 100 degrees Celsius. Raising the temperature an extra 20 degrees Celsius is a large amount of energy to waste. Some groups looked at pollutants and said that boiling water did not remove them and in fact can increase their concentration as water boils off.

Some topics, like the popcorn kernels, were high interest but lower level. Harvesting heat from roadways had more diverse answers depending on which of the possible collection methods they were more interested in. I also noticed that students tended to use more vocabulary from the unit in their explanations. The "Urban Heat Island" questions threw students at first; many thought it was merely a sum of body heat in such a densely populated area.

The second half day was spent sharing the responses. Groups read their headlines and summaries. This did  not take as long as I expected, so we watched some extraneous thermo videos. I think that the activity could be expanded to require more details like pros/ cons, next steps, potential costs of solutions, etc. This would not work for the popcorn question, but I'm sure with some time I could think of some more higher level topics.

A few examples of the results are below:

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:

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.