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bio website akhmeteli.org
location Houston, TX
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visits member for 3 years, 6 months
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1d
comment Dielectric constant of water
@Floris: No disagreement was implied in my comment:-) And thank you for the interesting info.
1d
comment Dielectric constant of water
@Floris: Actually, measurement of water conductivity is the leading method of water purity measurement, AFAIK:-) And ultrapure water's conductivity very strongly depends on the temperature iccontrols.com/files/4-2.pdf
Jul
4
answered Why does transmission probability decrease, increase, then decrease again?
Jul
3
comment Charge present at the centroid of equilateral triangle
@Mixcels: Maybe you can under some exotic definitions, but not in my book: in reality, there will be no periodic motion, the charge will get out of plane in no time due to instability.
Jul
2
comment Why doesn't atmospheric pressure crush thick walled structures?
@user2820052: Thank you. It can be done, so someone will do that one day.
Jul
1
answered If I had supermanlike body would it really be possible for me to overcome gravitation without touching the ground?
Jul
1
comment Why doesn't atmospheric pressure crush thick walled structures?
@user2820052: Again, thank you for your interest. Sandwich structures are indeed successfully used under compression - see hexcel.com/Resources/DataSheets/Brochure-Data-Sheets/… , p. 18. I only tried some technology required for layered shell vacuum balloons. The development (together with my coauthor and contractors) achieved its goals, but was expensive. I am going to proceed further, but lack both time and money. Sorry, cannot share with you the confidential details of the development.
Jun
28
comment Is possible to synthesize diamond at relatively lower pressures (6 to 7 GPa vs 12 GPa)?
@abduoman: As I said, it depends on the application; however, it is not obvious that HPTP is better than CVD for the application you mentioned. High pressure typically requires expensive equipment.
Jun
28
answered Electric field scalar quantiy or vector quantity
Jun
28
comment Is possible to synthesize diamond at relatively lower pressures (6 to 7 GPa vs 12 GPa)?
@abduoman: The "winner" depends on the application, e.g., "About a decade ago Chemical Vapor Deposition (CVD) diamonds began to appear in the gem diamond market. The CVD method can produce diamonds in a lower pressure environment than HPHT, using a gas such as methane and a substrate to grow diamonds. Diamond composition can be controlled to create a single crystal form allowing the production of very clean colorless diamonds over one carat in size much more economically then HPHT."kitco.com/ind/Zimnisky/…
Jun
28
comment Is possible to synthesize diamond at relatively lower pressures (6 to 7 GPa vs 12 GPa)?
@abduoman: 50-150 micron per hour (pnas.org/content/99/20/12523.full ) does not look "very very slow" to me.
Jun
27
answered Is possible to synthesize diamond at relatively lower pressures (6 to 7 GPa vs 12 GPa)?
Jun
24
revised How to measure the diameter of a thin wire?
Addition
Jun
24
answered How to measure the diameter of a thin wire?
Jun
20
comment Why doesn't atmospheric pressure crush thick walled structures?
@user2820052: There isn't (if I understood you correctly) - the vector sum of external forces vanishes.
Jun
20
comment Is it possible to make a low dense object sink in a fluid of higher density by cutting the object in particular shapes?
@MikeDunlavey: Not necessarily, but yes, this is an example of what I described.
Jun
20
answered Is it possible to make a low dense object sink in a fluid of higher density by cutting the object in particular shapes?
Jun
20
comment Why doesn't atmospheric pressure crush thick walled structures?
@user2820052: (cont. 4) Otherwise the pipe will collapse. (I forgot to say that $R$ is the external radius (of the sphere or of the pipe)). However, this requirement for material strength is not enough. You don't just want the half-pipe to be in equilibrium, you want it to be in stable equilibrium, otherwise it will fail through buckling (see, e.g., youtube.com/watch?v=Zz95_VvTxZM ). There is an expression for critical external pressure for a cylinder, for higher pressures the cylinder will buckle. But in general, vessel design is quite complex:-)
Jun
20
comment Why doesn't atmospheric pressure crush thick walled structures?
@user2820052: (cont. 3) The same approach can be used for the pipe. You consider a half-pipe and find the pressure force acting on it from one side, it will equal $2R L P_{atm}$. On the other hand, the tangential stresses acting on this half-pipe produce a force of $2\sigma t L$, where $\sigma$ is the compressive stress in the pipe, $t$ is the thickness of the pipe, $L$ is the length of the pipe. These two forces are equal, as the half-pipe must be in equilibrium. This is how you can calculate the tangential stress in the pipe. Of course, this stress must be lower than the material strength.
Jun
20
comment Why doesn't atmospheric pressure crush thick walled structures?
@user2820052: (cont. 2) However, it is easy to calculate the pressure force acting on the hemisphere from the right (on the flat part of the hemisphere), as the pressure force acts there in the same direction everywhere, so this force equals $\pi R^2 P_{atm}$. Again, the pressure force acting from the left has the same magnitude, as the air hemisphere is in equilibrium in the atmosphere. On the other hand, the force acting on a hemisphere of air from the left is equal to the force acting on a (metal) hemispherical shell from the left. That is why my result is half of yours.