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April 2016

Physics 032 – Light is a particle?

A few weeks ago we proved light behaved as waves since it produces diffraction patterns after going through 2 slits.  Today we investigated the particle nature of light.  I told the students about the photoelectric effect and how it could not be explained with classical physics and led to the idea that the energy of light was quantized (comes in discreet packets).  We also talked about blackbody radiation and how Max Planck also came to the conclusion that the energy of light must come in little packets because classical physics failed to explain the blackbody radiation curves.  These videos explain some of this.

Trying to replicate the typical photoelectric effect experiment in one’s home is not terribly easy and I wasn’t sure I could do it. But after a quick search I found this video on youtube and there is a link to a pdf with instructions on building the demo, so we built this in class today.

Ours didn’t work as well since we didn’t have the nice thin xmas tinsel, but made our own out of regular aluminum foil.  It just didn’t move as easily as the tinsel. But we were able to see the foil strips loose their charge when we put the UV light on the coffee can.  I happened to have a UV light (black tube in the photo below)  for our fluorescent rock collection. Here’s a photo of one of our set ups.  So its wasn’t a ‘Wow!’ experiment but at least we could see the photoelectric effect in action without spending a fortune.IMG_2757.jpg

Big History 030 – Columbian Exchange

Today’s class focused on the Columbian Exchange – the exchange of goods, animals, plants and people between the New World and Old World. When the kids arrived I had papers spread out over the table from the Big History Project Columbian Exchange Snap Judgement (Unit 8) activity.  Each sheet of paper had one word one it and the kids had to decide if they thought it originated in the New World (Americas) or the Old World (Eurasia/Africa).  Some of the items, like llamas, turkeys and cocoa were pretty easy to identify as New World, but many of the students hadn’t heard of rubber trees so they weren’t sure where they were from (New World), and cinnamon (Old World)  and vanilla (New World) stumped most of them.

original-23769-1.jpgThen the students did another activity , the Columbian Exchange Trade Route Activity by Michele Luck that I bought off Teachers Pay Teachers.  It was almost $7 but it saved me a lot of work and I liked that it combined regular history type questions with some geography.  Bascially three stations are set up, Americas, Europe and Africa. There are cards at each location showing plants, animals, diseases, etc that originate in that location.  The kid have one sheet where they write down a few items for each trade route.  They also have a sheet of questions at each station.  To help them answer the questions there is a one page article for each location that contains most of the answers.  To answer the geography questions they used an atlas or iPads to look up ocean names, etc.

After everybody had done all three stations we watched these two videos on the Columbian Exchange.

Not too much ‘science’ in today’s class but I managed to sneak in some more geography and since we talked about plants and animals taking over an ecosystem, not to mention diseases brought to the New World, I suppose we did some biology/ecology.  Next week we move on to Unit 9 of the Big History Project.

 

Physics 031 – Probability

We’re approaching the chapters in the text on quantum mechanics so today we brushed up on probability.  I found this amazing paper online that included a full tutorial and activities to do with a class.  The name of the paper is “Laboratory -Tutorial activities for teaching probability.” by M.C. Wittmann, J.T. Morgan and R.E. Feely at the University of Maine.   The tutorial starts with flipping coins and looking at the probability of getting heads verus tails for 1 penny flipped 10 times, and then flipping three pennies at a time and IMG_3779comparing the different outcomes.  Then we moved on to rolling two dice and looking at the probability of their sum being a certain number. For example, there is only one way to get ‘snake eyes’, a sum of 2, or a sum of 12.  But there are quite a few rolls that add up to 7.  The students each rolled two dice 8 times and made a histogram of their results and then we added up the data for the whole class (photo on left) and saw how that compared more favorably to the  predicted probability histogram.  We discussed how a larger set of data more closely resembles the predicted curves.  Another important point that we discussed was that the probability of any particular roll, did not depend on what had happened on the previous roll.  Just like when you flip a coin 4 times and even if you got 4 heads in a row, the chance of getting a head on the 5th flip, is still 50%, its not any higher or lower just because you already got 4 heads.

Then we threw some physics into the problem.  I drew a cloud on the top of the marker board and single rain drop falling from it.  I divided the board below the drop into 3 separate sections, one above the other,  and asked the students, if I was to randomly take a photo of the raindrop, is it more likely to be in one section or another, or is it equally likely to be in any of the 3 sections.  Some students will think its equally likely to be found any of the 3 spaces since they are the same size, but some remembered that the raindrop will be accelerating and going faster and faster as it drops, therefore spending more TIME in the top section and the least amount of time in the bottom space.  So you are more likely to find the raindrop in the top region.

IMG_2709We then took this a step further and used the Video Physics app on an iPad to record the motion of a ball being thrown up in the air and coming back down.  Just like the raindrop, we divide the vertical space into three sections and find the probability of the ball being in the top, middle or bottom space.  Here is a photo with the ball locations already marked.  Students took these printouts and used a ruler to divide the space equally and then counted the number of frames the ball was in each section.  The probability of the ball being in the top 3rd of the throw was equal to 56%  (14 frames out of a possible 25).

IMG_3781.jpg

Finally, we took a look at the glider on the airtrack.  Springs were attached between each end of the cart and the ends of the airtrack so the cart would oscillate back and forth.  Again, we used the iPad to record the motion and this time we divided the region into 5 sections.  This histogram looks a bit different then the ones we’ve done so far since the cart is moving slower at the IMG_3782.jpgextremes of its motion as it turns around, its more likely to be found in region A and E, and less likely to be found in region C where its moving the most quickly.

What does this have to do with quantum mechanics? In quantum mechanics we don’t know exactly where things are, but we can determine the probability density of finding a particle, like an electron, at a particular location. Similarily,  today we found the likelihood of finding the cart in a particular location.

Now I’m going to go to the University of Maine’s website and see if they have more tutorials because this was a great set of activities.

Big History 029 – Latitude & Longitude

IMG_3775 As soon as the students arrived I put them to work building quadrants.  This way the glue had time to dry while they watched presentations.  There are a LOT of free patterns available on the internet for making quadrants.  The one I downloaded had little sights to glue in place but I prefer having a straw to sight through.  Some of the kids tied beads to the ends of the string instead of  a washer and that works fine, they just serve to weigh down the string.

While the glue was drying I gave a presentation on Prince Henry the Navigator and Portugal’s efforts to explore the African coastline. Unknown Again I used material from The Road to There, Mapmakers and Their Stories by Val Ross to update an old slideshow I had from when we did Story of the World classes.  51-2KkH5gyL._SX379_BO1,204,203,200_Another book I used was Around the World in a Hundred Years, from Henry the Navigator to Magellan by Jean Fritz.

I also found this nice slideshow on latitude and longitude by Mr. Kreeger. I switched out some of the images and deleted some of the slides but it was so nice not having to start from scratch, so thank you Mr. Kreeger.  We talked about the difference betweeen latitude and longitude, how latitude is found easily in the northern hemisphere by measuring the altitude of Polaris (the north star) and how longitude was a much more difficult thing to determine.  51lhFBbetmL._SX311_BO1,204,203,200_I talked about the Longitude Prize and John Harrison the carpenter who made a huge leap in accurate time keeping pieces on land and sea.  There is a very good book, Longitude,  on this subject by Dava Sobel and a movie that can be found on youtube.  I actually got the illustrated version of Longitude out of the library.

Here’s the full length movie Longitude, which I have not watched,

and a 30 minute program on Harrison and the longitude problem by BBC, this I have watched and its pretty good.

IMG_3767To practice using the quadrants I taped a pieced of paper high up on my wall with a ‘north star’ on it and the longitude of Tokyo written on it, but not the name of the city.  The kids were told to sit in a certain spot and measure the altitude of the ‘north star’, then find the coordinates on the inflatable globe, in effect finding their ‘location’.  They then found the coordinates for another location and put up ‘north stars’ of their own and the other students had to find the latitude then try to find the secret location on the globe.  This worked pretty  well and is an activity I found on The Universe at Your Fingertips DVD.  I think using the globe instead of a flat map made it more meaningful as well.IMG_3774

For homework I asked the students to try to find the north star (Polaris) tonight and see if they get the right latitude for where they live.

 

 

Physics 030 – Polarization

We did two labs involving polarization today.  Doc Schuster has a lot of nice videos on optics, here’s his “Intro to Polarization Filters” on youtube.

The first lab we did involved placing a linear polarizer in front of the HeNe laser and measuring the intensity of laser light passing through the polarizer as we rotated the polarizer.  I didn’t have a nice rotation mount, so I just printed out a 360 protractor on cardstock, cut a hole in the middle for the laser to pass thru and put a pointer on the polarizer.  IMG_2685A ‘high tech’ clothes pin was used to hold the polarizer onto the protractor.  This wasn’t the best set up but it worked well enough for the students to see that the intensity varied with the angle.  We used a Texas Instrument sensor (little red box in the photo)that communicates with an iPad via bluetooth to measure the light intensity.  Students found that bumping the setup in any way could seriously effect their results, but they got enough data to show that the intensity depends on the cosine squared of the angle.

The other experiment involved playing with a chunk of calcite.  Previously we had talked about the index of refraction of materials and how it depends on the wavelength of the light.  Different colors of light will move through materials at different speeds and therefore refract more or less, that’s how prisms work and why we have rainbows.  But the index of refraction can also depend on polarization in some materials, like calcite. IMG_2686 Just putting a piece of calcite down on a piece of paper and looking through it you get two images because the light coming from the paper is split into two polarization components with each taking a slightly different path through the crystal.  The students placed the calcite in front of a laser pointer and determined the polarization of the two exiting laser beams were indeed different by 90 degrees.  IMG_2679

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