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bio website sjbyrnes.com
location Massachusetts
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visits member for 3 years, 4 months
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Dec
3
comment Why isn't temperature measured in units of energy?
See also my answer to a related question: physics.stackexchange.com/questions/17551/units-and-nature/…
Dec
2
answered Ejected Electrons with 0 KE?
Nov
23
comment How is Gauss' Law (integral form) arrived at from Coulomb's Law, and how is the differential form arrived at from that?
While it's healthy to know these derivations, you should keep in mind that Gauss's law is more general than Coulomb's law. Coulomb's law is only true if the charges are stationary, there are no changing magnetic fields, etc. But Gauss's law is true under all circumstances. So Gauss's law is more than just a consequence of Coulomb's law.
Nov
20
answered Using the monte carlo method to compute the magnetic field of a curent carrying loop
Nov
13
comment What is the approximate electrical conductivity $\sigma$ of graphene in S/m or S/cm?
If I assume the few-layer graphene in the Novoselov paper is actually 3 layer graphene (just a guess), then the graphene is 1nm thick, and with a strong gate its sheet resistance is 200 ohm/square. If we pretended that you could make 1nm thick copper without changing the resistivity, its sheet resistance would be 20 ohm/square. So I guess you could say that copper is 10X more conductive than graphene, at least in this particular paper. But it seems to me that this is comparing apples and oranges. [Note: This comment replaces a previous version I just deleted where I misread the graph scale.]
Nov
8
comment When I connect in series two floating capacitors, one charged and the other not charged, does current flow?
There is a rule: The charges on the two plates of a capacitor must be equal and opposite, and no (net) charge can be anywhere except the plate of a capacitor. When stray capacitance is very small -- usually the case -- this rule is very strictly enforced. No charge can flow from 2 to 3 without violating the rule: Either the charge on 3 will NOT be equal and opposite the charge on 4, or else charge will appear on the open lead connected to 4.
Nov
5
comment What software programs are used to draw physics diagrams, and what are their relative merits?
It is only fair to mention that matplotlib is extremely similar to MATLAB: Similar user interface, similar capabilities, similar default styles, etc. One advantage of Matplotlib over matlab is that you get to use python, which is a better programming language than matlab. Another is that matplotlib is free, while matlab is prohibitively expensive unless you get it through work or school or pirated. On the other hand, MATLAB has better help files, easier installation, and a bigger scientific user base [but I hope those things will eventually change.]
Nov
5
comment What common materials can effectively block infrared radiation?
It is worth noting that even when the reflection is not quite 100%, that doesn't mean light can pass through the aluminum foil. The rest gets absorbed, not transmitted. (Unless the aluminum is under ~50 nanometers thick, for visible and near-IR!)
Oct
26
answered Reflectance vs. Thin Metal film Thickness Graph
Oct
12
comment Dependance of temperature on color of metals
How do you know that the heating of cars in sunlight is dominated by the windows?? This is not obvious to me. Also, different paints may have more or less radiative cooling - especially important at night. There again, it's not obvious that the windows are more important than the paint. Personally, I wonder whether Klein's comment is actually true.
Oct
5
comment Does Nantenna (nano antenna) violates 2nd Law of Thermodynamics?
I would bet a large sum of money that the overwhelming majority of professional physicists would agree with my assessment that the device cannot possibly work as advertised in the application in question, because if it worked then it would violate the Second Law. However, I have not actually done a survey!
Oct
5
comment Does Nantenna (nano antenna) violates 2nd Law of Thermodynamics?
I am the one who put those words into the wikipedia article. What I wanted to say was: "Obviously this application would violate the second law of thermodynamics." But I ended up writing that more understated and diplomatic sentence which you quote, because of wikipedia's referencing rules. Indeed, if you have a room at temperature T, and a nantenna device at the same temperature T, it cannot charge a battery. This is a textbook example of the Second Law.
Oct
3
comment Liquid crystal shutter with >90% transmission?
Agree. The 50% limit is sacrosanct as long as there is unpolarized light to start, and at least one polarizer in the system. If I had to guess, the "90%" quote is actually shorthand for "90% of the theoretical limit" -- i.e. 45%. Especially if it was a brief offhand remark, it may have been misunderstood!
Sep
21
revised Why is torque not measured in Joules?
clarify wording in third paragraph
Sep
21
answered Why is torque not measured in Joules?
Sep
9
comment Does anyone know the difference and relation between $k\cdot p$ method and tight binding (TB) method?
en.wikipedia.org/wiki/K%C2%B7p_perturbation_theory
Sep
8
comment Conservation Laws in Photon Parametric Down-Conversion
I understand your question. But you don't understand what phase-matching is or how it works. I suggest you look it up!! The experimenter can choose all three momenta, and then set up the experiment to achieve it ... Most importantly, when you purchase a nonlinear crystal, you get to tell the company what type of crystal you want, and what angle they should cut the faces relative to the crystal axis or axes. This helps determine at what angles the phase-matched light comes out.
Sep
7
comment Conservation Laws in Photon Parametric Down-Conversion
Phase matching is NOT really related to conservation of momentum. With phase-matching, the waves add in phase to become very strong. Without phase-matching, the waves add with random phase offsets to give a small (but nonzero) total. The fact that nonlinearly-generated light goes primarily in phase-matched directions is analogous to the fact that light bouncing off a diffraction grating goes primarily in specific directions.
Sep
7
comment Conservation Laws in Photon Parametric Down-Conversion
You, the experimenter, are free to decide whether they are parallel or not. You decide based on whatever makes the experiment work best. If you want it to be parallel, you make it phase-matched for parallel waves. If you want it to be non-parallel, you make it phase-matched for non-parallel waves.
Sep
7
answered Conservation Laws in Photon Parametric Down-Conversion