1,847 reputation
627
bio website
location New York, United States
age
visits member for 3 years, 8 months
seen yesterday

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


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.
May
4
comment How loud is the thermal motion of air molecules?
@hdhondt: Why would it be above the audible range? Thermal noise in other contexts is white up to a very high frequency, and then drops off.
May
3
awarded  Nice Question
May
1
revised How loud is the thermal motion of air molecules?
added 23 characters in body
May
1
comment How loud is the thermal motion of air molecules?
@hdhondt It is 100% about sound. Is there something I can change to make it not about microwave radiation?
Apr
30
comment Is it possible to create an audible sound source in mid air by intersecting ultrasonic sound beams?
@BMS: No, ultrasound is nonlinearly converted into sound at the intersection of the beams, and then spreads outward in all directions from that point, like any other sound source.
Apr
30
answered Is work done by sound wave on air particles?
Apr
30
comment How loud is the thermal motion of air molecules?
@oberron That sounds vaguely correct? Sound pressure is measured in Pa = N/m², so I think it is the same regardless of the actual surface area used to measure it? It's rms, but if the instantaneous pressure is Gaussian-distribution, then rms = standard deviation.
Apr
30
revised How loud is the thermal motion of air molecules?
added 274 characters in body
Apr
30
comment How loud is the thermal motion of air molecules?
Why would it need to be sensitive to 5 GHz? Is the spectrum of the noise not white?
Apr
29
revised How loud is the thermal motion of air molecules?
added 61 characters in body
Apr
29
comment How loud is the thermal motion of air molecules?
@tpg2114 "pressure isn't defined in any way other than a continuum (average) property" Then sound doesn't exist, since it always averages out to atmospheric pressure.
Apr
29
comment How loud is the thermal motion of air molecules?
@garyp: 0 dB is supposedly based on human threshold of hearing, not on the air. There's an anechoic chamber that measures -9 dB SPL, so that's not the limit.