Category Archives: Physics

How to Think Like Approximately 1 Physicist

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Think Like a Physicist

Physicists and estimation.

Students (the vast majority of whom are engineers and chemists) invariably look at me like I’ve sprouted an extra head when I do dimensional analysis tricks, though, and whenever I assign a problem asking for an estimate, I’m all but guaranteed to get answers reported to all the digits that a calculator can muster, which misses the point.

But I’ve also had this happen even with other faculty from science and engineering departments. I’ve had several meetings where I’ve done some back-of-the-envelope toy model to check the plausibility of something or another, and get baffled stares from everybody else. Or arguments about how the round numbers I used weren’t exact (“But we don’t have 600 students in the first-year class. There are only 587 of them…”) It was a real shock the first time that happened, because I’ve always thought of that as a general science trick, but I’m coming around to the idea that it’s really more of a physicist trick. And maybe, if you’re looking for an explanation of what it means to think like a physicist, specifically, that might be the place to look.

I recall the first time I experienced this, in a physics class in college. The professor gave an answer to a question to within a factor of 2 faster than anyone with a calculator got to the more precise answer, and he explained that in a lot of (informal) cases, a factor of 2 or even order of magnitude would be sufficient — able to rule out possibilities or make a plausibility argument, or even check that you haven’t fat-fingered an answer on your calculator and gotten an obviously wrong answer. He was right, and I’v used the technique quite a bit. Later, in the navy, I heard this estimation technique called “radcon math” — the radiation control folks on a ship/sub care mainly about the order of magnitude of a radiation dose when first assessing a situation, because that tells you the level of urgency should you need to cordon off/evacuate an area. So it’s not just physicists, per se, but it’s plausible estimation is more prevalent in disciplines that do more computation.

Doing It Right

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Unlike some projects that I kvetch about where “design” is merely art, and nobody has run the numbers, here’s a project that can actually work.

Body-heat powered flashlight takes teen to Google Science Fair

Makosinski did some calculations to see if the amount of energy produced by warmth from a person’s hand was theoretically sufficient to power an LED bright enough to use in a flashlight, and she found it was more than enough.

Boom! That’s how you do it!

It’s not going to work (or work well) in hot weather because it needs a temperature difference, but it’s great for cooler climes and times.

At 171 Å, It's No Red Rubber Ball

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[T]hree years of the sun at a pace of two images per day.
SDO’s Atmospheric Imaging Assembly (AIA) captures a shot of the sun every 12 seconds in 10 different wavelengths. The images shown here are based on a wavelength of 171 Angstroms, which is in the extreme ultraviolet range and shows solar material at around 600,000 Kelvin. In this wavelength it is easy to see the sun’s 25-day rotation as well as how solar activity has increased over three years.

During the course of the video, the sun subtly increases and decreases in apparent size. This is because the distance between the SDO spacecraft and the sun varies over time. The image is, however, remarkably consistent and stable despite the fact that SDO orbits the Earth at 6,876 miles per hour and the Earth orbits the sun at 67,062 miles per hour.

Siphoning Beads

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The “cornering ability” of the beads seems to depend on the tension. Makes for an interesting effect.

Don't Believe the Financial News

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This Mysterious Ancient Egyptian Statue Has Started Moving On Its Own

“I noticed one day that it had turned around,” museum curator Campbell Price told the Manchester Evening News. “I thought it was strange because it is in a case and I am the only one who has a key.

“I put it back, but then the next day it had moved again,” Price said. “We set up a time-lapse video and, although the naked eye can’t see it, you can clearly see it rotate.”

Yes, you can see it rotate — during the time that people are walking past it, and not at night, when the place is empty. It’s settling from the vibrations it experiences.

Protip: pay attention to the actual scientists’ explanations, and not the wacky notion that the statue is trying to show off its backside. Also, if you offer the possibility that it’s magnetic, you should know that the idea is easily tested.

I Have Figured Out Your Evil Plan

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As part of the money saving in the sequester, DoD facilities have been told their thermostats will be raised a high as 80ºF (26.7ºC). We’ve avoided going that high thus far, because we’re going to do some mitigation like installing window tinting to reduce the overall heat load (plus there’s the leverage of operational equipment that can’t be allowed to overheat, but I don’t know if that card is being played and we humans are peripheral beneficiaries)

The rumor is that the chill water in the heat exchanger is going to be set at a much higher temperature than before, at around 14ºC, which is slightly below the typical dewpoint for the summer air. This saves money because condensing water out of the air takes a lot of energy. Warm, wet air (27ºC, ~50% relative humidity) will have upwards of 11 grams of water per kg of air, and air at a comfortable 22ºC and 30% RH has but 5 grams. The heat of vaporization is around 2.5 kJ/gram of water, while for air the specific heat is around 1 kJ/kg-ºC, so cooling from 27ºC down to 22 removes about a third of the energy as compared to condensing 6g of water, so if you can basically eliminate the water removal by only cooling to the dewpoint, you will save quite a bit of money. (They aren’t completely avoiding the removal of water, but the new system scales it back by quite a bit). For the recirculated air, they are just removing whatever we add from our evaporating perspiration and spilled coffee.

Unfortunately, 1 kg of completely saturated air at 14ºC still has 10 grams of water in it, and that represents 60% RH at 22ºC, which is really sticky. I’ll bet the mold will love it, though.