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seen Jan 26 '12 at 20:37

Aug
11
answered Capacitance of two cocentric spheres, contradicting results
Aug
11
revised “Regular” 20-sided die, vs “life counter” 20-sided die. Same probabilities?
deleted 5 characters in body
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
10
answered “Regular” 20-sided die, vs “life counter” 20-sided die. Same probabilities?
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
9
answered 'Observer in double slit experiment' publications available?
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.
Aug
2
answered Does tea stay hotter with the milk in it?
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
30
awarded  Revival
Jul
30
answered What are the properties of two bodies for their collision to be elastic?
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.
Jul
25
comment Why can we treat quantum scattering problems as time-independent?
I stand by the answer, and I disagree with your description of the method I describe: it's just a decomposition of the localized wavepacket into normal modes, which is something you're obviously familiar with. But after reading your answer, I'm giving up on the argument; I'm unlikely to convince someone who thinks an appropriate response to a undergraduate-level question about a time-independent potential is an explanation involving Green's functions and a time-dependent Hamiltonian.
Jul
24
comment Why can we treat quantum scattering problems as time-independent?
@Marek: Please reread the question. Reread the first sentence of the question. The guy is asking a question about what he remembers from his "undergraduate quantum mechanics class". In that same sentence, he is explicitly asking about "a static potential". Your objections are completely irrelevant for the question asked.
Jul
22
comment The Ozma Problem
Ben Crowell hit the nail on the head. If you want the aliens to figure out right/left via a fundamental physics experiment (i.e. a measurement of something we didn't prepare), then beta decay is probably the easiest way. But since your problem posits that we're communicating by radio waves, we can just send them circularly polarized radio waves, and tell them to measure how the electric field rotates in time with respect to the axis of propagation.
Jul
22
answered Why can we treat quantum scattering problems as time-independent?
Jun
10
comment How does spring constant change with resistivity changes
For the elasticity, it's very minor, but look up "electron degeneracy pressure". For the other, more important parts, see Ch 22 of Ashcroft and Mermin. For the conductivity, it depends very strongly on what kind of material you're talking about. Look up any book on semiconductors.
Jun
8
answered How does spring constant change with resistivity changes