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bio website sjbyrnes.com
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visits member for 3 years, 4 months
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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
Sep
3
comment How can the Hall effect ever show positive charge carriers?
@Rojo: Good question! An electric field affects all the electrons in the valence band, both the top of the band (where the action happens) and the bottom of the band (which is always full). The field pushes the electrons near the top of the band in one direction, and pushes the electrons near the bottom (where the band curves the opposite way) in the opposite direction. In a FULL band, the average motion is zero. (This is related to the fact that the band is a periodic function of k-space, so it can only curve upward in some places if it curves downward in other places.)
Aug
18
answered Electric field outside a capacitor
Aug
18
comment What is the maximum surface charge density of aluminum?
@Yrogirg, you're saying "Maximum charge is the point where the potential energy of an electron monotonically decreases as it goes from inside the metal to the vacuum". OK, with this definition, I think the maximum charge is some large and finite value. But I don't think this criterion corresponds to what thrusty was asking about. It is, after all, pretty irrelevant in the real world. A bit below this threshold, the charge spits out almost just as fast as it does a bit above the threshold, due to thermal fluctuations and quantum tunneling.
Aug
17
comment What is the maximum surface charge density of aluminum?
Yes ... you need an extra 1500 volts to pull off a 1s electron from aluminum compared to a valence electron at the same spot. (I got that number from x-ray data.) So, as voltage increases, you will pull valence electrons out of a thicker and thicker depth from the surface, until the only remaining valence electrons are so far inside that there is a 1500V drop between the remaining valence electrons and the surface atoms. Then as the voltage increases even more, you will simultaneously pull off valence electrons from the inside and core electrons from the outside.
Aug
15
answered What is the maximum surface charge density of aluminum?