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I am an electrical engineer who is easily distracted by physics.


Jun
26
awarded  Nice Answer
Jun
25
revised Is there a limit to the resolving power of a mirror telescope?
correct the example and radius instead of surface area
Jun
25
revised Would wearing clothing that is black on the inside and white on the outside keep you cooler?
added 217 characters in body
Jun
25
revised Do gravitational waves slow down as they pass through matter?
clarify
May
24
revised How loud is the thermal motion of air molecules?
added 22 characters in body
May
24
revised How loud is the thermal motion of air molecules?
edited tags
May
24
revised How loud is the thermal motion of air molecules?
added 193 characters in body
May
24
answered How loud is the thermal motion of air molecules?
May
23
comment Is it possible to create an audible sound source in mid air by intersecting ultrasonic sound beams?
One thing that no one seems to be mentioning is that this depends on nonlinear effects in the air. Mixing (summing) two ultrasonic waves would not produce audible sounds in a linear medium, they would just mix. The way this supposedly works is that the constructive interference at the point of intersection raises the pressure so high that the air behaves non-linearly, and the intermodulation distortion produces audible waves at that point. I don't know if these new waves would continue in the same direction as the original waves or spread outward spherically from their creation point.
May
22
revised How loud is the thermal motion of air molecules?
oh it's well-known in water!
May
22
comment How loud is the thermal motion of air molecules?
Maybe this is more well-known in the context of underwater acoustics? Wikipedia says "At very high frequencies, above 100 kHz, thermal noise of water molecules begins to dominate. The thermal noise spectral level at 100 kHz is 25 dB re 1 μPa²/Hz. The spectral density of thermal noise increases by 20 dB per decade (approximately 6 dB per octave)." citing R. H. Mellen, The Thermal-Noise Limit in the Detection of Underwater Acoustic Signals, J. Acoust. Soc. Am. 24, 478-480 (1952). So it has a violet spectrum?
May
22
comment How loud is the thermal motion of air molecules?
And if 0 dB SPL is really the limit, as an effect of thermal motion, how can anechoic chambers be measured at −9 or −12 dB SPL?
May
22
comment How loud is the thermal motion of air molecules?
"The minimum acoustic pressure audible to the young human ear judged to be in good health, and unsullied by too much exposure to excessively loud music, is approximately 20×10<sup>−6</sup> Pa [0 dB SPL] ... The minimum audible level occurs at about 4,000 Hz and is a physical limit imposed by molecular motion. Lower sound pressure levels would be swamped by thermal noise due to molecular motion in air." who.int/occupational_health/publications/noise1.pdf If this is true, I'd like an explanation of why.
May
16
comment Flow of electrons in a circuit
All the electrons in the entire loop have to move at the same time. Think of to like a bike chain. If you move any one link, it pushes and pulls and causes the rest of the chain to move, too.
May
16
comment Can electrons coincidentally flow along a circuit to cause current?
In a sense, the thermal motion of the atoms in the light bulb are already giving off light all the time even when it's turned off. It's not visible light, though.
May
14
comment total noise power of a resistor (all frequencies)
The thermal radiation from the resistor will be the same whether shorted or not, right? It will only depend on temperature? So does shorting it into a loop actually change anything? (Wolfram Alpha says it's 4 * Stefan-Boltzmann constant in 1 dimension)
May
14
comment What’s the relationship between thermal radiation and Johnson thermal noise?
If you short-circuit a resistor, the power dissipated in itself by its own thermal noise is ${v_n}^2/r = 4 k_\mathrm{B} T \Delta f$, which depends only on temperature and measurement bandwidth? But the amount of radiation emitted from the resistor due to its temperature will be the same whether shorted or open?
May
13
awarded  Popular Question
May
7
revised How loud is the thermal motion of air molecules?
no one knows this?? does this help?
May
4
comment How loud is the thermal motion of air molecules?
@oberron: Good point about the frequency weighting. I guess the best answer would specify the shape of the spectrum, along with 22-22k flat RMS measurement and A-weighted RMS measurement.