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seen Jan 26 '12 at 20:37

Sep
14
comment Why is the energy density of gasoline so high?
If you "only care about" energy density per unit mass, then the densest chemical fuel will be hydrogen (assuming you get your oxygen "for free"). The hydrogen reaction with oxygen isn't especially energetic, but you win because the hydrogen is light. It's not as convenient to store as gasoline, but c'est la vie. If you don't insist on chemical fuel, you can do many orders of magnitude better with nuclear power. This is because nuclear energy scales are MeV, while chemical energy scales are eV. And if you're willing to get crazy, the energy density champion is an antimatter-matter reaction.
Sep
13
comment Why is the energy density of gasoline so high?
Re: "we only cared about energy density", are you talking about energy density per volume or per mass?
Sep
2
comment How do we know that C14 decay is exponential and not linear?
@Charles E. Grant: I certainly agree: if all you wanted to do was demonstrate that radioactive decay is exponential in nature, other isotopes would be a much better choice. But because the lifetime of C-14 is so important (carbon dating, etc.), folks have gone to considerable effort to nail down the lifetime. And since the original questioner seemed fixated on C14 for some reason, I figured I'd put in a reference to data that showed the decay of C14, and tell the story in terms of that isotope.
Aug
31
comment Do stationary states with higher energy necessarily have higher position-momentum uncertainty?
Could you clarify what you mean by "consider potentials with only a single local minimum, and increasing potential to the right of it and decreasing to the left."? I think that's a different situation than what Ron meant by "monotonically increasing". Because if the minimum considered is truly only a local minimum, and the potential decreases continuously to a lower V on the left, you don't have discrete energy levels.
Aug
29
comment Air vs Glass refraction coefficient
Yes. Practically impossible.
Aug
25
comment Question about Rayleigh scattering
Just as a followup to Ted Bunn's excellent answer, that tiny difference in the energy of the photon (and the energy of the scattering atom) is the basis of laser cooling. And - to note how laser cooling can be practical with such a small energy change - I'd note that his answer (the "recoil energy") is correct for an atom that is initially stationary. For a moving atom, you can get a much bigger change.
Aug
25
comment What color a transparent object reflect?
Until someone invents a perfect anti-reflection coating, EVERY object that refracts light will reflect light (except if you're exactly at the Brewster angle). It's not an all-or-nothing phenomena where it's total internal reflection or nothing.
Aug
23
comment Spin up, spin down and superposition
Talk us through your jump from how making the measurement affects the He state. If you make the measurement, what are the possible outcomes and their probabilities, and what are the outcomes for He in each of those cases? If you don't make the measurement, what are the outcomes for He and their probabilities. Also, in your last sentence, do you really mean "subluminally"? Transmission of information at subluminal speeds is A-OK in physics, and happens literally all the time.
Aug
17
comment Physics of Focusing a Laser
An excellent free resource for answering a variety of simple gaussian-beam-focussing problems, as well as a lot of other optics stuff, is the Melles Griot Optics guide (now the CVI/Melles-Griot optics guide) which is currently available here: cvimellesgriot.com/Company/CoolTips.aspx Reading this should answer your answerable questions.
Aug
16
comment Why does the weighing balance restore when tilted and released
I'm not gonna write up an answer, since there seem to already be a couple of correct ones, but I'll do my best to put it more succinctly. If the center of mass is above the rotation point, horizontal is an unstable equilibrium point. This is how you build a teeter-totter (or seesaw). If the center of mass is below the rotation point, horizontal is a stable equilibrium point. That's how you build a mass balance. For small mass imbalances, the deviation from horizontal is proportional to the imbalance.
Aug
16
comment Practical physics: where does this current come from?
Vilx: the fact that you can feel the current flowing through you may contradict my guess. It's true that you can't have voltage without the ability to supply SOME current, but the capacitive coupling here would give rise to a voltage source with very high internal impedance. Hence, if you hook it up to a high-impedance voltmeter you'll see a big voltage. Another test would be to hook up an ammeter between the lines, to see how much current it can source. But please make sure the ammeter has fuse protection on it - otherwise if there is a true short somewhere, you'll blow stuff out.
Aug
15
comment Why do Bell tests give perfect correlations?
I'd guess your confusion is coming from the term Sz. There are 3 different Sz's to consider here: The Sz of electron 1 (Sz1), the Sz of electron 2 (Sz2), and the Sz of electron 1 + electron 2 (Sztot). In the system you're talking about, the system is prepared in an eigenstate of Sztot=0, ensuring that a measurement of (Sz1 + Sz2) will always be 0. However, the system is not in an eigenstate of Sz1 (thus its outcome is uncertain) nor in an eigenstate of Sz2 (thus its outcome is uncertain).
Aug
11
comment Practical physics: where does this current come from?
Your title says current, but your question says you're not measuring a current, you're measuring a voltage. I would guess that - if the "ground" wire is floating as you say - John McVirgo is correct: it's just some capacitive pickup between that wire and one of the others. The reason you can see a voltage is because the impedance of your voltmeter is bigger than (or comparable to) to the impedance of the capacitive coupling. If you were instead to measure current, you should see extremely little, because the impedance of the capacitive coupling is very large.
Aug
11
comment “Regular” 20-sided die, vs “life counter” 20-sided die. Same probabilities?
Ben: I would revise your definition of "fair die" to say that for a random unknown initial condition, all outcomes are equally probable. But die rolling doesn't start from a random unknown condition - the die thrower can look at the die. And (as explained in the article I link in my answer), even for an "unskilled" thrower, the die result will be influenced by the initial condition, in a way that can be exploited more for the "life counter" die. We're talking minute effects, of course, but they are there.
Aug
9
comment 'Observer in double slit experiment' publications available?
Judging by your comments to the other questions, what I think you're trying to ask is: "Are there any papers published in peer reviewed journals demonstrating the destruction of the interference pattern in a double-slit experiment when an observer measures which slit the particle passes through?" If that's your question (when phrased that way, nobody needs to watch the YouTube clip) I try to answer it below.
Aug
8
comment Why would colder air disperse condensation on a car windshield?
Just to expand of David's answer: one of the most effective ways to remove the condensation is to run the air conditioner and the heater at the same time. The air conditioner reduces the humidity of the air inside your car (due to the water in your car's air condensing on its cold coils), the hot air will raise the temperature of the window. On many cars, the "defrost mode" will employ this technique.
Aug
8
comment Physics Equations for Grad School / Physics GRE Prep
Just to echo Jen's comment: get a copy of a past test (available from ETS), and take it under test-like conditions. There's no better way to see what kind of things are on the actual test than to see what kind of things were on an actual test. And there's also probably no better predictor of how you'll do on it.
Aug
2
comment Does tea stay hotter with the milk in it?
@Georg: I was around, but that question looks significantly harder than this one. I agree with your analysis of that problem: that evaporation will be the dominant effect. But it's hard for me to ascertain which of stirring vs. spoon in & out would be most effective at enhancing the cooling. Depending on how vigorous one was, I could see either one winning.
Jul
30
comment What are the properties of two bodies for their collision to be elastic?
Perhaps I should have rewritten the question to something of the sort "How could one minimize inelasticity in the collision of two macroscopic objects?" But given the manner in which the original question was phrased, it was not clear to me that the original poster understood collisions even at the level I was explaining.
Jul
27
comment heater in a perfectly insulated box
I agree with the answer to within its assumptions (which match the problem) but the assumptions are impossible. If the there is a mechanism for heat input, there is a mechanism for heat output. If the heating mechanism is shining a laser beam through a hole in the box, then the system can radiate back out through that hole. If the heater is a resistor connected to wires which run outside the box, the electrical conductivity of the wires provides a path for heat to leave. If the there is a mechanism for heat input, there is always a mechanism for heat output.