Category Archives: Physics

If You Love Them, Set Them Free

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Free charges in conductors

There’s a great discussion in Griffiths book on electromagnetism (typical book used for junior/senior course for physics majors) about what happens to free charges inside a conductor. He discusses how they will always find their way to the surface, distributing themselves to cancel the field inside the conductor. If they don’t do that, there will be a residual field to push more charges around. This happens until the field is totally cancelled. In a footnote he mentions how this surface effect only happens in 3D. In lower dimensions, like charges on a plate, say, the charges don’t necessarily go to the boundary.

Some neat simulations of both 3-D and 2-D cases.

Because Graham is on Holiday and Chun Yao's Dead

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Despite the fact that this is satire and thus includes some level of exaggeration, it does do a rather nice job of highlighting some of the frustration of science outreach. Not everything can be condensed down into an information mcnugget or two, and there are subjects where you really need a decent background in physics to understand what’s going on, even at a superficial level. That frustration comes to a head when you meet someone who thinks that advanced science should be understandable to them even without any background schooling, and those same folks seem to be the ones who hold you at fault when they don’t understand. (They also seem to conclude that it’s wrong if it doesn’t make sense to them, but that’s another level of frustration)

h/t to Schrödinger’s hat

Royal Observatory’s Astronomy Photo Winners

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Super Space: Royal Observatory’s Astronomy Photo Winners

On Sept. 8, the Royal Observatory Greenwich, home of Greenwich Mean Time and the Prime Meridian, announced winners from the third annual contest, which drew more than 700 entries. Prizes went to participants from four main categories — Deep Space, Our Solar System, Earth and Space, and Young Astronomy Photographer of the Year — as well as three special awards.

Fulfilling Their Expectations

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The Preconception Eliciting Tennis Ball

While holding the ball near the ceiling, I ask, “When the ball is at its peak, what is its velocity?” (They confidently say “zero!”)

I now expose their preconception by immediately asking, “What is its acceleration?” (The answers are split between “9.8 m/s/s” and “zero!” depending on the class) I keep the ball near the ceiling and ask one of the students who enthusiastically answered “zero!”, “If its acceleration is zero and its velocity is zero, what would happen to the ball?” After some thought, the student realizes that the ball wouldn’t fall. I then release the ball and it sticks to the ceiling.

via @rjallain

Only His Hairdresser Knows for Sure

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Did Einstein discover E = mc^2?

One thing that bothers me about the “somebody had mc^2 and only had the proportionality constant wrong” arguments is that it ignores one very important point: whatever you come up with, the units have to be that of energy. So it’s not like the Far Side cartoon where Einstein has written E=mc^3, E=mc^7 and various other powers and the cleaning lady says “Everything’s squared away. Yessir, squaaaaaared away!” (and Albert has a wonderful look on his face). Funny, for a cartoon, but in reality you need a speed squared term to go along with mass in order to get units of energy. E=mc^7 doesn’t have units of energy. E=mc^2 does. So, really, showing what the proportionality factor is is really the majority of the battle.

Throw Me a Frikkin' Bone Here, People

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BMW tests ‘1,000 times brighter’ laser headlights

The intensity of laser light poses no possible risks to humans, animals or wildlife when used in car lighting, BMW says reassuringly. The automaker says that’s because the laser light is first converted for use in road traffic, a bright, white that “is very pleasant to the eye.”

Maybe your eyes, but we’re not sure about everyone else’s. Did we mention laser headlights are 1,000 brighter than LEDs. Oh yeah. We did.

Yes, you did. Only that’s not what the BMW press release said. What they do say is

[L]aser lighting can produce a near-parallel beam with an intensity a thousand times greater than that of conventional LEDs. In vehicle headlights, these characteristics can be used to implement entirely new functions. Also, the high inherent efficiency of laser lighting means that laser headlights have less than half the energy consumption of LED headlights. Simply put, laser headlights save fuel.

So the observation is that lasers are 1000 times brighter than LEDs, which gets contorted into the claim that the headlights are 1000 times brighter. But they won’t be using as many lasers as LEDs — the goal is to save on energy use. The bottom line here is that laser diodes are more efficient at generating light than LEDs. You would lose that energy savings if for some reason you simply turn the brightness up to eleven.

A single statistic will make this clear: whereas LED lighting generates only around 100 lumens (a photometric unit of light output) per watt, laser lighting generates approximately 170 lumens.

For comparison, in generating white light the best you can do is about 250 lm/W. Presumably you would be using the same technology (phosphor or something else) to generate the white light, so you have to be more efficient at generating the photons.

The way you do this is by making more efficient use of the light. Laser diodes are LEDs with mirrors at both ends of the material, fashioned from cleaving the material — you get reflections whenever you pass from one medium to another with a different index of refraction. You can enhance or suppress this with the appropriate coating; high-power lasers will have a good reflector on one end of the lasing cavity and an anti-reflection coating on the other. Without the mirrors the light from the electrons spontaneously dropping down from one energy band to another can go in all directions. The mirrors allow for stimulated emission, and that will give you gain for photons that can reflect off the mirror and make another pass (or more) through the material. This means you are wasting a much smaller fraction of the photons to spontaneous emission. If you do something to the facets and interrupt the ability to lase (and I’ve seen this), the device reverts to just being an LED. One shortcoming is that laser diodes generally have a shorter lifetime than LEDs — they are static-sensitive and the coatings age — so for this to be viable there has to be the expectation that the lasers will last for several years.