Tag Archives: Physics

Ph'amazing

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Top 10 physics videos (which constitutes a full rebuttal to the “Top 10 Amazing Chemical Reactions” I had linked to earlier, and had only rebutted with a single physics video, which is #5 on the list.)

You’ll also note that two of the so-called chemical reactions are properly classified as being in the physics videos (Meissner effect and breathing helium/sulphur hexafluoride). Ha! Take that, chemistry. You’re down to a top 8! (Without even arguing that floating a boat of air of the sulphur hexafluoride is a physics effect as well)

(Note the flash photography during the musical tesla coil video. Gee, I wonder if that helps?)

via The Great Beyond, which adds two Feynman drumming videos to the list. Wait, that’s twelve! Twelve videos! (A, ha, ha, ha. I love to count science videos!)

Add Grad Student and Shake Well. Ingredients for TA-ing

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How to be a good TA over at Built on Facts.

Disclaimer: I never did recitations as a TA in grad school, though I did tutor students (for a whopping 8 bucks an hour). I had just gotten out of the navy, where I had logged somewhere around 2500-3000 classroom teaching hours, so it’s not like I needed to acquire any lecturing skills. I did labs, which involved only a few minutes of lecture time, and then a lot of Q&A. I didn’t want the repetition of six or so recitation sections, and I knew (from being a student and having done undergraduate TA-ing as well) that labs didn’t always go the full three hours. So, does any of my advice or criticism really apply?

But what do my students say in their confidential evaluations? My scores are always pretty high, but the single most common good thing they have to say about me is this:

He speaks English.

Yeah, I got that a lot, too, as a TA. Which just goes to point out that student evaluations more-or-less follow Sturgeon’s law. 90% of them are crap. The student’s judgments are not always objective, nor do they usually give constructive feedback. They can like or dislike you, and give evaluations accordingly, based on criteria other than teaching quality. And that’s what many of them are — statements of whether the student like you, rather than your effectiveness. I remember one teaching evaluation in which the student complained about how I blocked the board some of the time and he couldn’t read it. He sat near the front in the left-hand row (as viewed from the back of the classroom). I’m right-handed and bigger than a breadbox. It’s physically impossible for me to not block part of the board, and the part I will block will affect those on that side of the class a little more. Basic geometry, really. But it didn’t stop the student from whining about it mentioning it.

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Stress Makes Them Bi

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Birefringent. Not that there’s anything wrong with that.

In my earlier discussion of polarization, I promised some photos of stress-induced birefringence.

If you have a polarizing filter, you can use an LCD as a polarized light source and view birefringent materials using it as a backdrop. Make the screen as white as possible, and rotate the polarizer until it blocks the light. Then place a birefringent material in front of the screen and look through the polarizer. Cheap clear plastic often will have stress-induced birefringence.

Which is what I did, and is why the background is black. The first photo is the plastic box in which the polarizers were packaged

These next two are the side and top views of a styrene drawer from a small storage cabinet, placed on top of an empty CD spindle.


Drinking Games: Physics vs. Aesthetics

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Swissmiss asks, hey, what’s that in your drink?

Not recommended for those suffering from pica or ice-chewers who are absent-minded.

However, this is almost all marketing and little (ahem) solid physics.

Nordic Rock is mined from ancient Swedish pollution-free base rock. It is the purest way of cooling your drink – literally ‘on the rocks’. Stone does not melt, which means no unclean water in your glass. They are also reusable making them very eco-friendly. To use, simply place the stone ice cubes in the freezer for approximately one hour before use. For a normal glass, two or three Nordic Rocks will be fine. They give off their cold gradually and equally.

Not diluting the drink is a valid claim (technically chemistry, though). Cleanly or not, though — don’t you use clean water in ice cubes? Ewww.

“Eco-friendly?” How much water does it take to clean these, as compared to the amount of water in an ice cube?

“They give off their cold gradually and equally.” Reasonable claim, once you get past “cold” being a substance. But the advantage of water is that, because of the latent heat of fusion (334 J/g), your drink will stay at 0 ºC until the ice melts, while the drink with the stone cubes will warm up continually. And what’s the heat capacity? I’m not sure exactly what these are (don’t recall “base rock” being a designation), but I’ll assume they are similar to granite, whose heat capacity is about 2 J/cm3 K (helped by its higher density), similar to ice, while water is 4.18 J/cm3 K, and again there’s that huge amount of energy from the latent heat. Let’s say you have 10 cm3 of these stones at 0 ºC and an identical amount of ice. The stones will absorb just 400 or so J of energy in warming to 20ºC, while the ice will absorb 3000 J just in melting, and then 750 more in warming up. Thermodynamically, ice wins.

If you don’t like the dilution of ice cubes, you’re better off using water frozen inside of another container, as in the trick I offered a little while back.

Focus, People

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This month’s Physical Review Focus: Nanoparticles Stick a Perfect Landing

They found that for speeds less than 1.2 kilometers per second, the nanoparticle bounces off the surface like a basketball. But at higher speeds, some of the nanoparticle undergoes a phase transition to a compressed state called β-tin, where each atom bonds to six neighbors. This transition is surprising, Dumitrică says, because the collision energy is not high enough to induce a phase transition in a macroscopic object. However, the impact force is applied over a few square nanometers, so the pressure inside the nanoparticle is extremely large–around 200,000 atmospheres, which is more than enough to cause the phase transition.

The β-tin state only lasts a few picoseconds, though. As the nanoparticle begins to bounce back, there is a second phase transition to an amorphous, or disordered, state. The combination of the two phase transitions, plus some heat generation, takes up all of the kinetic energy, and the particle remains on the surface. After all of this action, “the recoil is too weak to beat the adhesion forces between the nanoparticle and the substrate,” Dumitrică says.

However

A silicon nanoparticle flying at 8 times the speed of sound can slam into a surface and stick, but it bounces off if colliding at half that speed.

The speed of sound in what, pray tell? I wish journalists would remember (learn?) that the speed of sound is not a constant of nature.

Bringing Home the Gold

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From Google Maps to Gold Medal

Kristin Armstrong, who won gold in the Women’s Individual Time Trial in Road Cycling, got a GPS track when she rode the Beijing Olympic course in December of 2007

After returning home to Boise, Idaho, I exported the GPS data to several different formats, one of which I was able to launch with Google Earth. I was then able to trace the entire course from the comfort of my home half a world away and find a similar route to train on back in Boise. This capability along with having the elevation profile proved invaluable in my preparation for my Gold Medal race.

GPS relies on precise time, provided by some colleagues of mine, and knowing where the satellites are relative to the earth, which is aided by some other colleagues of mine. Woohoo! We won gold!

Physics: Don't Wine About It

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You can use NMR to tell you if it’s still good, and now you can tell how old the bottles are by hitting them with an ion beam.

Nuclear Physicists Fight Wine Fraud

The beams, which are directed at the glass, not the wine, can distinguish how old the bottles are and where they might originate.
[…]
To prevent counterfeiters from filling authentic old bottles with ordinary wine, Williams intends to combine the ion beam test with another established method that checks for levels of a radioactive isotope, Caesium 137, in the wine itself.
This technique, however, is only effective in identifying wines made in the era of heavy atomic weapons testing in the latter half of the 20th century.

Mmmmm. Cs-137. I know that’s what gave last year’s Beaujolais Nouveau such a perky flavor.

Update: Jennifer Ouellette has a rather extensive post about wine fraud over at Cocktail Party Physics (though this is wine, and technically not a cocktail. But it’s some interesting history and more information. The New Yorker article she mentions is quite interesting)

Polarized, Non-politically

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I’ve inadvertently (and advertently) been doing some experimentation with polarized light lately. Liquid Crystal Displays (LCD) typically emit linearly polarized light, and most decent sunglasses act as polarizing filters. This can cause some problems, if you happen to have some gadget whose display is inconveniently set to emit light with horizontal polarization — since reflected light tends to become polarized parallel to the surface from which it reflects, sunglasses are made to filter that light. But it also makes it tough to read any LCD that is oriented to emit that polarization.

There are ways around this, though. I’ve noticed that my iPod screen (unlike my GPS receiver and watch) doesn’t go black at any orientation of my sunglasses, though I do get some shifting rainbows on the screen. Here are two orthogonal orientations of a linear polarizer:

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