8,594 reputation
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bio website sjbyrnes.com
location Massachusetts
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visits member for 4 years, 3 months
seen 51 mins ago

Aug
25
comment Silver vs Aluminum electrodes in silicon solar cells?
You need an ohmic contact to n-Si, and an ohmic contact to p-Si. Can aluminum be both? I thought it could only be one of those (I forget which).
Aug
19
comment Are there measurable quantities which directly depends on the Fermi velocity?
As soon as the Fermi level is away from the dirac point, vF has an obvious importance--density of states in particular. I guess I'm saying, it's easy to say in theory "I assume Fermi level is exactly at the dirac point", but it's not only impossible in practice, but stops being true as soon as you do something as simple as apply a voltage, even in theory.
Aug
19
comment Are there measurable quantities which directly depends on the Fermi velocity?
(1) Away from absolute zero the states above and below the fermi surface are all relevant; (2) Even with no (intentional) gating, the fermi level will not be EXACTLY at the dirac point in practice; (3) Even if it is exactly at the dirac point somewhere, it won't be exactly at the dirac point elsewhere, because of inhomogeneities like random dangling bonds in the substrate; (4) If you apply a voltage than the fermi level is tilted and cannot be at the dirac point everywhere. So again, every electronic and optoelectronic property is affected.
Aug
4
comment Do all the conservation laws of Physics take no time to propagate?
In my EPR pair example, there is a more energetic particle and a less energetic particle, but it is not determined which is which. If you measure the energy of one, you can immediately infer the energy of the other. But this does not require or imply that either information or energy is moving faster than light. Why would it? See physics.stackexchange.com/a/3163/3811 for more details, for example.
Aug
3
comment Do all the conservation laws of Physics take no time to propagate?
Yes, you can have two distant entangled particles (called an EPR pair), where one is more energetic and one is less energetic, but it is not determined which is which until you measure them. You can look online or in an intro QM textbook for an explanation of why information does not flow faster than light when you measure an EPR pair. This information-flow issue is commonly discussed. Well, energy also does not flow faster than light when you measure an EPR pair ... and it's for the exact same reason.
Aug
3
comment Do all the conservation laws of Physics take no time to propagate?
There is no such thing as just "continuity", there are a wide variety of "continuity equations" describing many different quantities. Energy and momentum are conserved in quantum mechanics, and yes, those conservation laws are written as continuity equations in quantum mechanics too. There is no conflict between entanglement (which is ubiquitous in QM) and these continuity equations. I'm not sure why you think there would be. Can you explain what you have in mind?
Aug
2
comment Is this diagram, concerning Young's double slits, inaccurate?
I added another couple paragraphs...
Jul
31
comment Why is SiO2 solid while CO2 is a gas?
CO2 is based on double-bonds, O=C=O. So the heart of the question is: "Why can carbon easily form strong double (and even triple) bonds, while silicon is barely capable of forming weak double bonds?"
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.