For Questions That Don't Merit Their Own Thread

[Green Baron]

Yes, speed of sound changes, but in the low percentage range for the altitudes we can move in. But as you said not the frequency. That will change with the medium, as you said in Helium for example. Maybe temperature and moisture can have a somatic effects on the human voice (cold :-)).

Differences in pitch of a human voice aren't really noticeable with altitude. Even a radio call from Mt. Everest top doesn't sound (much) different in pitch, though it'll be hard and harsh from the cold. Sound producing cords in the throat don't swing faster in lower pressure.

Edited April 7, 2017 by Green Baron

[p1t1o]

1 hour ago, K^2 said:

No, this is absolutely wrong. Once sound is produced, its frequency will stay the same. The wavelength will change, certainly, but frequency has to stay the same. If you received more oscillations than were produced, where did additional ones come from? The future? Unless you are dealing with relativistic red/blue shifts that actually have to do with time behaving in a funny way, frequencies never, ever change. Be it light or sound.

I think your explanation of voices in helium is correct, but you can absolutely change the frequency of sound or light, eg: the Doppler Effect, and you dont have to invoke any timey-wimey stuff [for EM radiation] either as the observer, emitter and the photons are all in the same frame of reference.

[p1t1o]

3 minutes ago, Green Baron said:

I would think that in our case (voice spoken in a microphone, transmitted to someone else, heard through a helmet) the doppler effect is negligible 'cause it would affect the near lightspeed propagation of the radio wave.

Well that is collapsing an EM doppler shift into an audio signal, and then trying to detect it with the human ear, yes I doubt that is possible!

I was just resisting the idea that "frequencies never, ever change"

[sevenperforce]

4 hours ago, K^2 said:

No, this is absolutely wrong. Once sound is produced, its frequency will stay the same. The wavelength will change, certainly, but frequency has to stay the same. If you received more oscillations than were produced, where did additional ones come from? The future? Unless you are dealing with relativistic red/blue shifts that actually have to do with time behaving in a funny way, frequencies never, ever change. Be it light or sound.

Ah, yes, you're absolutely right. My mistake. I was thinking of a wavelength shift and I had photons on my brain, so I was still associating wavelength change with frequency change.

[K^2]

19 hours ago, p1t1o said:

but you can absolutely change the frequency of sound or light, eg: the Doppler Effect

Good call on Doppler Effect. My mind was locked on a static picture, where distances don't change.

[0111narwhalz]

Do interferometers care about the resolving power of their component telescopes?

[wumpus]

On 4/7/2017 at 9:08 AM, p1t1o said:

Well that is collapsing an EM doppler shift into an audio signal, and then trying to detect it with the human ear, yes I doubt that is possible!

I was just resisting the idea that "frequencies never, ever change"

For sound, any reason I can't remember frequency changing rapidly thanks to a wind?  I mean, the medium of propagation moves all the time (although rarely faster than 1/20th of the speed of sound).  If you can hear the the doppler effect of a train whistle, shouldn't you hear some weird variations due to a 100kph wind gust?  Maybe the variations are completely overwhelmed by the wind noise, but I'm surprised I can't recall *ever* hearing such a shift.

[0111narwhalz]

1 minute ago, wumpus said:

For sound, any reason I can't remember frequency changing rapidly thanks to a wind?  I mean, the medium of propagation moves all the time (although rarely faster than 1/20th of the speed of sound).  If you can hear the the doppler effect of a train whistle, shouldn't you hear some weird variations due to a 100kph wind gust?  Maybe the variations are completely overwhelmed by the wind noise, but I'm surprised I can't recall *ever* hearing such a shift.

Possibly because both you and the source are being shifted the same amount in opposite directions?

[wumpus]

Just now, 0111narwhalz said:

Possibly because both you and the source are being shifted the same amount in opposite directions?

Makes sense (unless you are on a balloon and thus hear the doppler shift due to speeding along with the wind).  Most of the cases that might not be true would be thunder (plenty of weather between you and the thunder to have different wind speeds) where you wouldn't be expected to know the exact frequency of the source.  I wonder how far you have to be from a source so that the wind gusts are at different speeds and how often a sound travels that far with a known frequency, [church] bells or other civic bell-ringing?

[K^2]

12 hours ago, 0111narwhalz said:

Do interferometers care about the resolving power of their component telescopes?

Not particularly. The main reason to have large aperture on individual telescopes is to collect more light. If you are trying to look at a really distant object, inverse square law really bites you. And with interferometer, it tends to be much harder to have steady tracking, which is what something like Hubble would use to take a long exposure picture. To reduce exposure time to something reasonable, you need rather large telescopes, which is equivalent to having telescopes with large resolving power, even if you aren't actually using that aspect of it.

You can always think of an interferometer telescope as one giant telescope whose entire lens is covered by something opaque except in a few places where you have your component telescopes. As you probably know, covering parts, even significant portions of the lens on a telescope doesn't affect resolving power, but does reduce amount of light collected, requiring longer exposure time.

This probably won't be as big of a problem when we start building space interferometers. An interferometer satellite in space should, in theory, be able to track just as well as a smaller space telescope, allowing for very long exposure images of very distant objects, finally letting us capture images of various nearby exoplanets.

[Spaceception]

So... 9.375 x 10³³ Joules

What's the total energy compared to

The dino asteroid

Earth's rotational energy (I think it's much higher, not 100% on that)

And the world's nuclear arsenal.

I've tried to figure it out myself, but I can't.

Edited April 26, 2017 by Spaceception

[peadar1987]

Rotational kinetic energy of the earth = 2.138×10²⁹ J

Orbital kinetic energy of the earth = 9*10³¹ J

Chicxulub impactor = 4.2*10²³ J (all from wiki)

World nuclear arsenal = 6400Megatons = 2.7*10¹⁹ J (from here. Not sure of the accuracy of the source, but it gives an idea of magnitude)

[Spaceception]

7 hours ago, peadar1987 said:

Rotational kinetic energy of the earth = 2.138×10²⁹ J

Orbital kinetic energy of the earth = 9*10³¹ J

Chicxulub impactor = 4.2*10²³ J (all from wiki)

World nuclear arsenal = 6400Megatons = 2.7*10¹⁹ J (from here. Not sure of the accuracy of the source, but it gives an idea of magnitude)

 

 

Wow... are my numbers wrong, or are they higher than the Earth's rotational energy?

Btw, for context, that's an averaged mass Asteroid at 20% the speed of light.

Edited April 26, 2017 by Spaceception

[sevenperforce]

34 minutes ago, Spaceception said:

8 hours ago, peadar1987 said:

Rotational kinetic energy of the earth = 2.138×10²⁹ J

Orbital kinetic energy of the earth = 9*10³¹ J

Chicxulub impactor = 4.2*10²³ J (all from wiki)

World nuclear arsenal = 6400Megatons = 2.7*10¹⁹ J (from here. Not sure of the accuracy of the source, but it gives an idea of magnitude)

Wow... are my numbers wrong, or are they higher than the Earth's rotational energy?

Btw, for context, that's an averaged mass Asteroid at 20% the speed of light.

That's a big asteroid -- I'm getting 5e18 kg, which is roughly the mass of Hyperion, Saturn's ninth-largest moon. Were you calculating Newtonian kinetic energy or relativistic kinetic energy?

[Spaceception]

2 minutes ago, sevenperforce said:

That's a big asteroid -- I'm getting 5e18 kg, which is roughly the mass of Hyperion, Saturn's ninth-largest moon. Were you calculating Newtonian kinetic energy or relativistic kinetic energy?

 

I used this calculator (http://www.csgnetwork.com/kineticenergycalc.html), and, it's Newtonian I believe.

[sevenperforce]

7 minutes ago, Spaceception said:

I used this calculator (http://www.csgnetwork.com/kineticenergycalc.html), and, it's Newtonian I believe.

Yeah, I checked; it's Newtonian. Of course, the Lorentz factor at c/5 is only 1.021 so it's not a big difference.

If you're interested in getting a ballpark estimate of what a particular amount of energy compares to, you might find this article to be really, really helpful. For example, your 9.375e33 J is 3.5x the orbital kinetic energy of Earth, or about 78% of the total annual energy output of the Sun.

But like I said before, your "average asteroid" is a little bit on the large size. It's kind of hard to say what an "average" asteroid is; asteroids can include anything from little fragments less than 10 meters across all the way up to minor planets like Ceres. Something closer to 1e14 kg is probably a better guess. A 1e14 kg asteroid at 20% the speed of light packs 1.9e29 Joules, roughly equivalent to the rotational kinetic energy of Earth and about a tenth of what you'd need to blow up Mercury.

[Spaceception]

Just now, sevenperforce said:

Yeah, I checked; it's Newtonian. Of course, the Lorentz factor at c/5 is only 1.021 so it's not a big difference.

If you're interested in getting a ballpark estimate of what a particular amount of energy compares to, you might find this article to be really, really helpful. For example, your 9.375e33 J is 3.5x the orbital kinetic energy of Earth, or about 78% of the total annual energy output of the Sun.

But like I said before, your "average asteroid" is a little bit on the large size. It's kind of hard to say what an "average" asteroid is; asteroids can include anything from little fragments less than 10 meters across all the way up to minor planets like Ceres. Something closer to 1e14 kg is probably a better guess. A 1e14 kg asteroid at 20% the speed of light packs 1.9e29 Joules, roughly equivalent to the rotational kinetic energy of Earth and about a tenth of what you'd need to blow up Mercury.

 

Alright, thanks, I got that 'average' from this site: http://physics.ucr.edu/~wudka/Physics7/Notes_www/node19.html

 

Guess it was wrong, anyway time to vaporize some aliens

And a tenth of what you'd need to blow up mercury? I'd hate to see what it does to Ceres...

[sevenperforce]

9 minutes ago, Spaceception said:

And a tenth of what you'd need to blow up mercury? I'd hate to see what it does to Ceres...

The gravitational binding energy of Ceres is 7.5e25 Joules, so a 1e14 kg asteroid traveling at c/5 would obliterate it about 2,500 times over.

[Spaceception]

Just now, sevenperforce said:

The gravitational binding energy of Ceres is 7.5e25 Joules, so a 1e14 kg asteroid traveling at c/5 would obliterate it about 2,500 times over.

Well, that's one way to get ET's attention...

[0111narwhalz]

What unit would you use to rate heat pumps in terms of heat transfer per mass? WK/kg? It's not just W/kg, because passive (and hence active) energy transfer depends on the gradient.

What is the current ballpark answer for small consumer-grade cooling systems? Single-room-level.

Also, what is the nominal skin temperature of the average human?

[p1t1o]

1 hour ago, 0111narwhalz said:

What unit would you use to rate heat pumps in terms of heat transfer per mass? WK/kg? It's not just W/kg, because passive (and hence active) energy transfer depends on the gradient.

What is the current ballpark answer for small consumer-grade cooling systems? Single-room-level.

Also, what is the nominal skin temperature of the average human?

How can you have heat transfer per mass? Mass of what? I think heat pumps are just rated in Watts.

I was just reading an XKCD "what-if" the other day and apparently your average fridge moves heat out of itself at a rate of approx 100-150W.

Skin temp of an average human? Quick google shows that it varies depending on environment and some other factors, but its generally slightly less than core temp, within a few degrees, so around 34-36C?

Anecdotally, a good approximation is that an average human radiates about 100W of heat, at rest.

So. How many people do you need to freeze? Or are you building a matrix-style human-body power station?

 

[peadar1987]

1 hour ago, 0111narwhalz said:

What unit would you use to rate heat pumps in terms of heat transfer per mass? WK/kg? It's not just W/kg, because passive (and hence active) energy transfer depends on the gradient.

What is the current ballpark answer for small consumer-grade cooling systems? Single-room-level.

Also, what is the nominal skin temperature of the average human?

Are you talking about mass flow rate of the working fluid, or mass flow rate of thermal fluid (water for a water source, air for an air source, not really relevant for a ground source)?

Normally you just talk about J/kg, which gives the amount of power per kg/s of flow rate (whichever one you prefer, although the thermal fluid is the more usual one). The output temperature is normally fixed for a given operation, so you don't need to take it into account as a variable when comparing systems for the same application.

If you were concerned about comparing cycles with different outlet temperatures, you could compare them based on Second Law Efficiency. This has a few different definitions, but the most simple is just the Coefficient of Performance of the cycle divided by the Carnot Coefficient of Performance, which is given by COP_Carnot=T_cold/T_hot-T_cold. This will just be a percentage, and standard values are roughly 50%, as far as I have seen.

[0111narwhalz]

The reason there's a mass there is for the mass of the heat pump. Per kilogram of apparatus, how much heat can be pumped up a gradient of a given temperature?

 

 

[p1t1o]

23 minutes ago, 0111narwhalz said:

The reason there's a mass there is for the mass of the heat pump. Per kilogram of apparatus, how much heat can be pumped up a gradient of a given temperature?

Well any heat pump will have a performance measured in Watts, and a mass, so W/kg? Though it will only be a constant on a per-model basis I'd imagine. Because you cant add just any mass of heat pump, you can only add "another heat pump" or modify your existing one, the value of W/kg will change constantly, so I'd just stick with Watts.

I presume there will be a thermodynamically-limited max value possible to attain for W/kg, based on the properties of various materials, but that would be pretty difficult to calculate, and being a theoretical maxima, not particularly useful.

[0111narwhalz]

But if one were to build a new heat pump, one would be able to build it to a certain mass and/or power. Your argument appears to invalidate rocket engines' N/kg because you can't stick a quarter of an engine on a ship. Besides, if you put enough units on, their specific power becomes a useful quantity.

I just need a ballpark answer.