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bio website sjbyrnes.com
location Massachusetts
age
visits member for 4 years, 1 month
seen 2 hours ago

Jul
25
comment Is Gauss' law valid for time-dependent electric fields?
Any textbook about quantum field theory should have a proof that Maxwell's equations + Lorentz Force Law are the classical limit of QED. (Although sometimes it's left as a homework exercise.)
Jul
19
comment Why are band maxima / minima often (always?) at high-symmetry points?
@DanielSank - if you make that comment into an answer I'll give you the bounty...
Jul
15
comment Can the energy/power required to mix two fluids together be calculated?
Definitely requires a full numerical model of the shape of the container, shape and speed of the paddle, etc. Just knowing the work is not going to help.
Jul
15
comment Why are band maxima / minima often (always?) at high-symmetry points?
Let me rephrase. Let's say the symmetry group has order 12 for example. In principle there can be 1 band maximum, or 2 or 3 or 4 or 6 or 12 maxima (related by symmetry). My question is: Why is it seemingly very rare for 12 maxima to occur, but very common for 1 (or 2 or 3)? After all, if the maximum occurred at a random point in the BZ, there would be 12 maxima with 100% probability.
Jul
8
comment energy efficiency - dryer vs hang+dehumidifer
Indeed, when it is hot outside, a well-ventilated basement has higher RH than the outdoors: When the outside air enters the basement, the walls (heat-sunk to the soil) cool the air.
Jul
5
comment Is this a good explanation of electron mobility to the layman?
"Electricity moves faster" is sorta nonsensical. Maybe you mean "more electrical current flows"?
Jul
3
comment Could anyone help explain this current voltage graph for an LED in liquid nitrogen?
I agree, but it's not a complete answer: You still need to explain why heating the LED might lower the voltage drop.
Jun
24
comment Does optical fiber preserve the angles of incoming light?
My description of how endoscopes work was terribly incorrect, it deserved to be downvoted. But I just deleted that part.
Jun
13
comment What is meant by shot noise?
"The number of photons hitting a photosensitive screen follows a possion distribution..." is a perfect description of shot noise. I'm not sure there's much else to say, you answered your own question.
May
31
comment Faraday's paradox
(It is no coincidence that my opinion agrees with the wikipedia article's; I helped write it that way!)
May
4
comment Do We Need Maxwell's Equations Since They Fail to Account for An Experimental Fact at Least in One Occasion?
Thanks, I corrected the text.
Apr
13
comment By what maximum amount can we concentrate sunlight?
The real limit is that you see the sun in every direction, not just the upper hemisphere of directions. Mirrors do exist! The upper hemisphere alone should only give you 25000x, iirc. You must have made an error but I'm not sure what.
Apr
6
comment How to calculate liquid pressure regardless of the gravity?
Two questions: (1) Do you know n (number of air molecules)? (2) Is the balloon made of a stretchy elastic material like rubber? (If so, the rubber provides almost all the pressure and the water outside can be ignored!)
Mar
23
comment Can a nuclear bomb be used as the power source for a laser beam
See en.wikipedia.org/wiki/Nuclear_pumped_laser but I don't know any details
Feb
16
comment What is thermodynamic equilibrium?
@EnochArden - The laws of thermodynamics are not axioms handed down by God. They are consequences of statistical mechanics, just as all macroscopic laws of physics can be derived from microscopic starting points. See any statistical mechanics textbook for detailed derivations.
Feb
16
comment What is thermodynamic equilibrium?
@EnochArden - The velocity distribution of a gas is macroscopically measurable - look at doppler shifts of spectral lines for example - as is whether light satisfies a blackbody distribution etc. I take the Boltzmann distribution as a starting point: P~exp(-E/kT). There's a letter "T" in that formula, it's a parameter, and everybody refers to this parameter as "temperature". That's all I mean by "temperature", there's nothing circular there.
Feb
13
comment Confusing Chemical potential of mixtures
I don't think there is any sensible or useful way to define chemical potential for a liquid mixture. So I would say it has no chemical potential. Lots of things do not have a chemical potential: The color orange, beauty, etc. :-D Chemical potential is a useful concept to the extent that something maintains its identity in different environments, and is usually (or always) conserved, loosely speaking. So it's a very useful concept for atoms or molecules, but not really for a liquid mixture, since the composition can easily and smoothly change.
Feb
12
comment Confusing Chemical potential of mixtures
What is "chemical potential of the total fluid"? I suspect that there is no such thing.
Feb
4
comment Probability of fluorescence: matching of binding energy and incoming radiation energy?
The binding energies are given by the "K edge", "L edge", etc.
Jan
10
comment Local EPR-experiments with photons in vacuum?
Indeed, the line from A to B is something called a "null interval" which is totally entirely completely unrelated to the concept of "empty intervals" of real numbers in high-school math. These two things are just as unrelated as the "intervals" in music theory and in aerobics. That's why I still think you should mentally rename it a "null shminterval".