Science desk
< September 9	<< Aug \| September \| Oct >>	September 11 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

September 10

What's the explanation for "Larger diameter axons have a higher conduction velocity"?

I read on khanacademy site "Larger diameter axons have a higher conduction velocity, which means they are able to send signals faster. This is because there is less resistance facing the ion flow. We have a lot of ions flooding into the axon, so the more space they have to travel, the more likely they will be able to keep going in the right direction.". The reason that this site brings it isn't understandable to me, because according to my logic many times the smaller diameter the faster fluid velocity. I have two examples for my claim: 1. Hypertension of the blood circulation. 2. A simple water pipe, as the diameter is smaller the flow is stronger and faster. So this clame that "Larger diameter axons have a higher conduction velocity, which means they are able to send signals faster" isn't understandable to me. What's the correct explanation for that? 93.126.116.89 (talk) 01:57, 10 September 2018 (UTC)[reply]

I don't know anything about axons, but in your water pipe example, the larger diameter pipe transports far larger volumes of water per unit time than the smaller diameter pipe, assuming other parameters like head pressure are held constant. A sudden reduction in pipe diameter (i.e. a nozzle) may result in a sudden increase in the flow velocity, but over longer pipe lengths that effect is opposed by increased drag against the sides of the pipe (due to the square cube law) which reduces velocity. 202.155.85.18 (talk) 02:36, 10 September 2018 (UTC)[reply]

We explain part of this at Length constant, but there's a lovely presentation at [[1]] that goes further. The length constant is pretty easily understood as the amount of the current that goes down the neuron versus the amount that escapes; the other relevant concept (from the link) is that "Each time an ion channel needs to open to recharge the action potential, this delays the propagation of the action potential by ~1 ms." Hence the evolutionary tendency either to make bigger cables (decreasing r_i) or else to insulate them (increasing r_m). Wnt (talk) 03:07, 10 September 2018 (UTC)[reply]

Electricity = a flow of electrons

Would it be correct or accurate if I'll defined "electricity" as 'a flow of electrons'? (I know what the article here says, but I'm asking specifically about this definition). 93.126.116.89 (talk) 03:45, 10 September 2018 (UTC)[reply]

No it really doesn't work like that. 173.228.123.166 (talk) 04:29, 10 September 2018 (UTC)[reply]

A flow of protons can also be an electric current carrier. But if the electrons and protons flow together they cancel each other out! Graeme Bartlett (talk) 07:39, 10 September 2018 (UTC)[reply]

A flow of positrons can also be a current carrier, as can a flow of ionic species within a solution...but for a relatively lay understanding of electricity, it's fine to describe it as a flow of electrons as that describes almost all of the electrical phenomena familiar to the average punter. 202.155.85.18 (talk) 07:44, 10 September 2018 (UTC)[reply]

Inside an ordinary metal conductor it is correct to describe an Electric current as a flow of electrons. But Electricity (see article) embraces both this and many other phenomena involving Electric charge, and with magnetism constitutes the phenomenon of Electromagnetism. DroneB (talk) 11:23, 10 September 2018 (UTC)[reply]

"A flow of charge" would be better. A flow of electrons is certainly electricity and it's the common one especially in metals, but so would be a flow of any charge carrier, including positive charges. Andy Dingley (talk) 11:42, 10 September 2018 (UTC)[reply]
"Flow of electric charge" is really the best definition, there are all sorts of electricity that do not involve flow of electrons, besides the ones already notes, P-type semiconductors have electricity that flows in the form of positive charge "holes" in the semiconductor. Of course, the actual Wikipedia article on electricity would have answered the question for the OP sufficiently, so I'm not sure why he didn't look there. According to that article "Electricity is the set of physical phenomena associated with the presence and motion of electric charge." That seems like a perfectly fine definition. --Jayron32 12:01, 10 September 2018 (UTC)[reply]
- The OP actually went out of his way to note that he has looked at the actual Wikipedia article on electricity. -165.234.252.11 (talk) 18:54, 14 September 2018 (UTC)[reply]

Fire has to have heat, fuel and oxygen?

I understood from the article fire that fire needs three things in order to exist: heat, fuel and oxygen. I have gotten some questions on it: 1) Is it always necessary to have oxygen? (I'm asking it because I saw this video on Youtube that says: "to create fire you need 3 things: some fuel, and oxidizing agent normally oxygen, and enough heat to raise the fuel to what known as ignition point.". It seems from his things that there are other oxidizing things that can cause to it. What are the other oxidizing materials for example that can cause fire? 2) Does any electricity take place while having combustion? (in other words there is a relation between the light that we see in an incandescent light bulb that generated by electricity to a combustion of a wood for example?) I'm looking for a relationship between electricity to any kind of combustion. Can I claim that any burning involves electricity characters? When people in ancient times took two stones or woods and rubbed them in each other it caused to ignition as I understand due to electricity in those things. 93.126.116.89 (talk) 19:24, 10 September 2018 (UTC)[reply]

Consider, for example, the act of striking a match. The match will flare, but if there is no oxygen present it will not burn. 86.133.58.126 (talk) 19:28, 10 September 2018 (UTC)[reply]

We discussed the theory behind fire - and we specifically discussed whether matches can burn in anoxic environments - almost one year ago today. Nimur (talk) 01:39, 11 September 2018 (UTC)[reply]

So that's why this sounded so familiar... Yep. See my comments from last time. shoy (reactions) 18:33, 11 September 2018 (UTC)[reply]

Light bulbs used to have a filament in a vacuum. The light produced is not a burning fire, it's simply that the passage of electricity through the filament heats it up, causing it to glow. 86.133.58.126 (talk) 19:34, 10 September 2018 (UTC)[reply]

They still do. ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:29, 10 September 2018 (UTC)[reply]

No, mostly they don't. --76.69.47.228 (talk) 03:01, 11 September 2018 (UTC)[reply]

Interesting. What percentage is "most"? ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:06, 11 September 2018 (UTC)[reply]

Where I live, it's becoming difficult to find filament light bulbs. Supermarkets are hardly stocking them anymore. The country has moved strongly toward LED lamps. Akld guy (talk) 03:27, 11 September 2018 (UTC)[reply]

Assuming you mean ordinary household light bulbs, I don't believe this is much of the reason. I believe our article 76 linked to is correct, and incandescent bulbs of that sort were inert gas fill for a long time. (Halogen also started to become more common although there were always small light bulbs, inside a larger covering if designed as a replacement for ordinary A60/E26 bulbs.) What vacuum light bulbs still exist are specialist or very old ones. Our article mentions smaller lamps but AFAIK even torch light bulbs are usually inert gas. Xenon if they're fancy. I'm not sure about incandescent bulbs used in Christmas lights however. Nil Einne (talk) 10:01, 11 September 2018 (UTC)[reply]

"Fire" is not a scientifically rigorous defined concept. "Heat, fuel, and oxygen" is a good rule for most of the fires directly affecting humans - it's, IIRC, a rule that is taught to help people to avoid dangerous accidents with fire. But you can burn many things that burn in an oxygen atmosphere in a fluorine atmosphere (usually to much more spectacular effect) - it's just that fluorine atmospheres on earth are rare outside of laboratories. A famous combination for rocket fuels is Hydrazine plus nitric acid - what comes out the back of the rocket is certainly flames, but do you want to call this fire? --Stephan Schulz (talk) 20:54, 10 September 2018 (UTC)[reply]

Quoting myself from last year - "Flame, in the conventional sense, requires the three parts of the fire triangle..." In a "normal" fire, that would be your fuel (maybe some wood), and oxygen from the air, and ignition by a spark. In an abnormal fire, you might have a strange fuel - like amorphous boron or tungsten carbide; you might have a strange oxidizer - like nitrous oxide - and the heat of friction caused by boring a tungsten-carbide drill into a gas-pocket, resulting in an ignited BLEVE - a fiery hazardous material explosion. You can even get fire with no oxygen at all: instead of oxygen, you can burn with fluorine or chlorine - which is certainly not a safe thing to do at home!

As Stephan Schulz correctly points out, the term "fire", as we use it in day-to-day speech, is not really clearly defined. If we want a technically-accurate definition that meets our intuitive and normal use of the word, we might say that fire is any exothermic oxidation reaction that releases gases that are hot enough to incandesce.

From a practical perspective, we might defer to the definitions used by the experts: say, the National Fire Protection Association, a fire safety advocacy group in the United States. In their extremely thorough glossary of technical terms, they define "fire products": flame, heat, smoke, and gas; among the hundreds of definitions they provide for entities relating to fire, they also have some half-dozen definitions specifically for the word "fire," with excellent citations for each. For example, in one case, they define: "a rapid oxidation process, which is a chemical reaction resulting in the evolution of light and heat in varying intensities." In another usage, they define: "any instance of destructive and uncontrolled burning, including explosions."

The specialist - whether they're a fire-fighter or a physicist - might use the word "fire" with many different meanings in different contexts. The relationship between flame and electricity is also quite complicated: electricity can cause certain types of fire, and can be caused by certain types of fire (in the form of gases that get ionized by the heat of the flame). In the most common cases that apply to the study of fire, we would consider electricity to be a source of heat and/or spark for an otherwise conventional chemical flame - so you'd still need fuel (like a wooden material) and oxidizer (like air in the room). Here's one of the frightening famous Christmas Tree Fire Safety video from NFPA. Electric wires that are frayed or overheating can cause a huge conflagration in a real hurry, "with flashover occurring in less than one minute." When you reach flashover, your fire is so exothermic that it no longer spreads chemically, but actually ignites distant objects entirely via the emission of infrared radiation. Most humans don't really have a great appreciation of this fact - a fire that's about three inches in radius is controllable and extinguishable, but once it gets a little bit bigger than that - the physics gets weird, and the practical consequences get really bad.

Nimur (talk) 01:57, 11 September 2018 (UTC)[reply]

[is there] a relation between the light that we see in an incandescent light bulb that generated by electricity to a combustion of a wood for example? As explained above, light bulbs work by the path electricity -> heat (see Joule effect) -> radiation (see Blackbody radiation). For fires, it depends; there is an appreciable amount of blackbody radiation from soot particles etc., but in many cases it is due to intermediary reaction species undergoing chemiluminescence (that is why you can see blue flames in Bunsen burners, even though the flame temperature there is lower than 2000K yet blackbody radiation is blue for about 6500-7000K).

I also questionthe assumption that flintstone fires involve electricity. According to Flint#To ignite fire or gunpowder, which unfortunately has poor sourcing on that point (the only ref is [2] which I would not imagine to be very accurate scientifically), "sparks" are created by exposing reactive iron or iron sulfide to the atmosphere; we are talking about this spark, not that spark. Tigraan^{Click here to contact me} 08:27, 11 September 2018 (UTC)[reply]

A well-designed fuel-burning lamp from the Victorian age onwards is brighter and whiter than black-body radiation, but this doesn't usually involve chemiluminescence, it uses a gas mantle and candoluminescence. This is a device for holding tiny quantities of rare-earth elements so that they can be heated by the flame. They then (like chemiluminescence) produce colours of light which are bluer than the black-body colours, but they do it by physics and spectroscopy (and aren't consumed) rather than by a chemical reaction. Andy Dingley (talk) 09:49, 11 September 2018 (UTC)[reply]

There is a difference between oxidation (in the chemical sense) and fire. You can also have fires which don't involve any oxygen. But we live on a planet with a lot of free oxygen in the atmosphere, so that's by far the most common oxidiser, and so we tend to assume that it's the only one.

Look at some of the chemistry for liquid propellant rockets. These use a rapid combustion (i.e. a fire, and an oxidation) to release energy as mechanical thrust. They have to bring their own oxidisers with them, just because they need so much of it and it would be difficult to supply enough of it by scooping it from the atmosphere, not just because that also allows them to operate in a vacuum and so reach space. Many different sets of chemistry have been used to make rocket fuels, and there are several used for the oxidiser too. See liquid rocket propellant or read Ignition!. ISBN 0813599199. (neither of these wiki articles are very good, but Ignition! is excellent and newly reprinted). A popular oxidiser is liquid oxygen, because it's relatively cheap and safe and it works well. Unfortunately it has to be kept very cold to liquefy it, so it can't be stored in the rocket. So the military use nitric acid, or nitrogen tetroxide instead. There are also exotic chemicals like chlorine or fluorine compounds - these will certainly give "a fire" just like oxygen, if you did an experiment in an atmosphere rich in them. Andy Dingley (talk) 09:43, 11 September 2018 (UTC)[reply]

As far as a connection between the chemical reactions that we call "fire" and electricity, the two halves of the reaction mixture can be segregated such that in order for the oxidation reaction to proceed, electrons must cross through a conductive material. This allows us to extract work in the form of electrical energy. Such an arrangement is known as a fuel cell, the most common of which is based on the reaction of hydrogen combustion, but they can also be designed around the combustion of the regular hydrocarbons we more commonly use as fuels. What this illustrates is that there is an electrical interaction taking place in all combustion reactions (and, in fact in all chemical reactions) involving changes in the electronic structures of the atoms, ions and molecules in question, but without particular contrived situations being deliberately created to take advantage of the electron flows, the relationship to electricity is trivial. 139.194.67.236 (talk) 10:22, 11 September 2018 (UTC)[reply]

What parameters do you need to describe an audible sound?

Frequency and amplitude of the wave, duration, intensity? Is that all? --Doroletho (talk) 21:04, 10 September 2018 (UTC)[reply]

Wikipedia has some good information at Sound pressure and Decibel#Uses. Dolphin (t) 21:46, 10 September 2018 (UTC)[reply]

Depends what you mean by "sound". Describing or synthesizing the sound of a musical instrument would take many more parameters (in the old days you'd define a sound envelope by attack, sustain, release, and decay, at different frequencies and amplitudes. Most sounds would have a complex timbre or mixture of frequencies and still be perceived as one sound. - - Nunh-huh 22:22, 10 September 2018 (UTC)[reply]

Basically, I had human speech in mind. But I imagine music is also complex and well-researched, and there must be good references about this. --Doroletho (talk) 23:59, 10 September 2018 (UTC)[reply]

Free books! Julius Smith, a professor at the Stanford Center for Computer Research in Music and Acoustics, has made all of his textbooks available at no cost. He's a world expert on the practical mathematics to represent music and sound.

The starting point for a representation of a sound is a full waveform recording, which may be digitized; and from there, it takes a lot of math and science to compress the sound in a manner which is perceptually similar to the original waveform using as few parameters as possible. A good audio compression scheme can achieve thousand-to-one compression, or better, in ideal circumstances - in other words, you can describe a sound using just a few parameters. The more parameters you use, the more accurately you can re-create the original sound with minimum distortion; phrased another way, the more parameters you use, the more different types of sounds you can accurately and uniquely describe.

Nimur (talk) 04:40, 11 September 2018 (UTC)[reply]

...Regarding human speech: here's a link right to the chapter on the Vocoder, a review of one historically-important effort to parameterize human speech at Bell Labs. These books are great - they're thorough but simple enough to understand, and Julius cites more references cited than you'll probably read. Nimur (talk) 13:52, 11 September 2018 (UTC)[reply]

Properties of sound are often simplified to concepts like sound intensity (amplitude), pitch (fundamental frequency), and Timbre (related to waveform shapes). There are also psychological effects of sound, which include things like loudness. --Jayron 32 13:57, 11 September 2018 (UTC)[reply]