Wikipedia:Reference desk/Archives/Science/2016 October 14

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October 14

How long would electrons take to stop accelerating if homes used DC?

Drift velocity not field propagation velocity. Is this why AC has less resistive losses? (the electrons not getting that "fast" in 1/200th 1/100th or 1/240th 1/120th of a second) Sagittarian Milky Way (talk) 00:54, 14 October 2016 (UTC)

No, the average time between collisions is orders of magnitude shorter than the switching time of AC current. Dragons flight (talk) 01:27, 14 October 2016 (UTC)

So since the voltage hardly changes in the short time between the first and second collision after the current's on does that mean both collisions are about same speed, at least for the average electron? (luck at dodging atom centers and quantum mechanics and all that) Sagittarian Milky Way (talk) 02:09, 14 October 2016 (UTC)

As I understand it, the advantage of AC for long-distance transmission is that you can use a higher voltage; therefore you don't need as high a current to transmit the same power. (Power = voltage times current, but resistive power loss = resistance times the square of current).

So you may ask, why can you use a higher voltage? Well, theoretically, you could use just as high a voltage for DC transmission, but then what are you going to do when you want to power a house? Stepping down DC voltage is complicated and costly. For AC it's easy; you just use a transformer. --Trovatore (talk) 01:34, 14 October 2016 (UTC)

Ah, I did not know your penultimate sentence. I always guessed AC was to keep drift velocities down and they didn't exceed 60Hz to avoid needing gears for the generator. Sagittarian Milky Way (talk) 02:27, 14 October 2016 (UTC)

Also some further info can be found in HVDC converter. Vespine (talk) 04:11, 14 October 2016 (UTC)

See also War of Currents for the history of AC vs DC mains electricity. Tevildo (talk) 07:10, 14 October 2016 (UTC)

• Is this why AC has less resistive losses? No. Shameless self-promotion of a section I wrote: Electric_power_transmission#Advantage_of_high-voltage_power_transmission

If you are interested in those questions, you might also want to give a look at the skin effect, which is the reason why power cables everywhere have the same diameter (well, High-voltage cables are larger, but the extra section is not made of copper). Tigraan^{Click here to contact me} 07:40, 14 October 2016 (UTC)

The Drift velocity of electrons in wires is typically 1570 km/s (related to the Fermi energy concept in quantum mechanics). Please compare the values in bold as they relate to the OP's question. As derived in detail in the linked article[1]:

- 1A DC flowing in a small copper wire changes the electron velocity by 0.000 023 m/s. It's probable that the majority of electrons in a switch when it left the factory never move out of it throughout the life of the switch.

- Going from DC to AC means the change direction reverses 120 (US) or 100 (Europe) a second. Here the original electrons certainly never leave the switch or indeed any metallic conductors.

This may make it clear that electrons never "stop accelerating" and our use of them for electric power distribution has almost negligible effect on their individual paths.

Virtually all electronic equipment in the home, including the computer or phone you are using, uses DC supplies. Homes could be (have been) supplied by DC mains but AC distribution is universally preferred because of its advantages of low-cost voltage conversion by transformers, the possibility to balance generators and loads in a network by phase control, and other minor advantages in avoiding corrosion at connections between dissimilar metals, arc quenching and avoiding permanent magnetization.

The saving in cable weight by using a high voltage (low current) for long-distance distribution applies to both DC and AC. However AC conductors need more insulation to handle 41% higher peak voltage than DC for the same power level. The choice of AC frequency is a compromise between contradictory requirements.

Lower frequency	Higher frequency
More reliable rotating generators and converters	Smaller transformers for a given power  :-)
Negligible skin effect	Skin effect reduces effective cross section of conductors
Negligible power factor loss	Power factor effects require correction and over-dimensioning of generators and network components to handle extra reactive current
Negligible impact of cable capacitance	Cable capacitance hinders long-distance power transmission
Tolerable audio hum from transformers or interference from mains wiring to audio equipment	Increasingly noticeable noise from transformers (see Magnetostriction) and potential for man-made EMI

The main results of standardization are:

• Home distribution in Europe and Asia: 50 Hz
• Home distribution in USA: 60Hz
• Aircraft on-board power distribution: 400 Hz (for low weight)
• New long-distance high-power lines: DC (as new HVDC conversion equipment becomes economical). AllBestFaith (talk) 17:42, 14 October 2016 (UTC)

Magnetostriction, incidentally. Worth a redirect as a likely misspelling? Tevildo (talk) 18:28, 14 October 2016 (UTC)

Thank you for the correction. AllBestFaith (talk) 19:36, 14 October 2016 (UTC)

@AllBestFaith: Oh crap, nevermind. I should have read what you wrote... Wnt (talk) 20:44, 14 October 2016 (UTC)

• Add to the above table the fact that low-frequency AC produces visible flicker with some types of lighting. Some people can see flicker even with 50-60 Hz, I believe, but with the lower frequencies hat used to be used in some places it was more common. --69.159.61.230 (talk) 06:42, 16 October 2016 (UTC)

That is true if the power waveform is sinusoidal but not necessarily so if a trapezoidal waveform were chosen. AllBestFaith (talk) 13:52, 16 October 2016 (UTC)

• I think the common misconception as "electricity is the flow of electrons" is what is at heart of the misunderstandings here. Such misunderstandings come from MANY common misunderstanding of electrons as little, discretely identifiable little hard balls of negative change, bouncing around like ping pong balls. I'm not sure that level of misunderstanding is capable of being rectified in this scope of this discussion, so let's ignore it, and use the "little balls" model (wrong as it is) to show that even if we were to assume it correct, electricity is still not the movement of electrons. Electricity is the flow of energy through an electric field. Not a single one of the electrons actually has to carry the energy individually from one place to another. Even if we think of electrons (quite wrongly, but lets keep it for now) as little rigid balls; those little rigid balls don't travel down a wire as though they were marbles being rolled down a tube. Direct current is NOT all of the balls rolling down the tube in one direction, and alternating current is NOT all the little balls shaking back and forth, though some simplified models try to show it that way. Instead we should think about electrical energy being passed through a medium in the same manner that energy gets passed though the stationary ball in the roquet move in the game of croquet. See this video if you're unfamiliar. Notice how the red ball doesn't go anywhere and yet is able to transmit a bunch of energy to the blue ball. Electricity is like that: the electrons don't go anywhere in either AC or DC. They just "pass the energy" down the chain. Of course, that's still a very wrong way to think of electrons and electricity, but it's right enough to get us to reach the correct conclusion that electrons don't move. --Jayron32 18:49, 14 October 2016 (UTC)

  I think you're overstating this a bit, Jayron. Direct current in particular does involve flow of charge, and you can't have flow of charge without movement of charge carriers. If you take a line carrying one ampere, and pick an arbitrary point, one second later, there will be approximately six quintillion electrons to the right of that point, that were to the left of the point when you started. (That's a net figure; there could be electrons to the left of the point that were to the right when you started, but they count negatively.) --Trovatore (talk) 22:26, 14 October 2016 (UTC)

  Yes, you're probably right there; but the issue is that when we consider electrons as isolatable, individual particles with a clear identity, it introduces all sorts of problems with how physics works. Your statement also isn't strictly correct; it would imply a "hole" with six quintillion less electrons to the left of that point. Instead, what we know is that the electrons to the left of that point have lower potential energy than the ones to the right of that point. The problem is, you can't say that any specific 6 quintillion electrons actually moved; like if you painted them red and followed them with a little microscope. Electrons are not individually identifiable. To say that some arbitrary number of electrons moved implies you could say which ones moved and which ones didn't. It's not even a limit of our measuring capabilities, like we know that they are moving, but we just can't build a device capable of tracking them. We literally couldn't track them because the physics says we can't, per Heisenberg and all that jazz. Certain aspects of electron behavior imply movement and motion and mobility, so we have concepts like electron spin and electron mobility; the mathematical predictions of such concepts bears out fine, but considering electrons as discrete balls that can spin and travel around freely creates problems as well; if electrons were discrete balls, then we wouldn't have the contradictions of the double slit experiment and the like. --Jayron32 00:09, 15 October 2016 (UTC)

  Well, so we need to keep two aspects of the question separate, those being the classical picture, and the quantum weirdness.
  For the classical picture, the point about the "hole" is addressed by the fact that what we really have is a circuit. It's like a big hula hoop with sand inside, the sand being the electrons. For AC, the sand is sloshing back and forth, and energy is transmitted via the sloshing. For DC, the sand is moving in a consistent direction around the hoop, and that is how it transmits energy. Complications being that the energy is not transmitted via the kinetic energy of the particles, but rather via their interaction with the EM field; and that the particles are individually moving much faster than their collective drift.
  Now when you get to quantum weirdness, what you can do is think of each of the possible classical descriptions as being a single Feynman diagram, and then you take the path integral over all of them. --Trovatore (talk) 00:22, 15 October 2016 (UTC)

  Yes, but the problem is that "classical picture" doesn't represent what happens, because the classical picture makes too many inaccurate predictions about the behavior of electrons. Electrons as grains of sand we could track if we wanted to, but just can't be bothered to is still not the same thing as saying that electrons are not actually discretely identifiable particles. If one is taking a VERY long birds-eye view of electricity, the classical description works fine as a lie-to-children for broad behavior like Ohm's law. Once we start getting down to the small picture, however, and looking at what is going on with those electrons, the classical model is woefully inadequate. --Jayron32 00:48, 15 October 2016 (UTC)

  It's inadequate, but not on the question of whether electrons transmit energy by moving. They do transmit energy by moving. --Trovatore (talk) 01:03, 15 October 2016 (UTC)

  Which electrons moved? That's the problem. Saying "the electrons move" leads to the question "which ones moved and which ones stayed put?" Which is why we need the reminder that we can't identify individual electrons as such, and then say electrons move. We can say both "charge moves down the wire" and we can say "electrons carry the negative charge". The problem comes from the chain of thinking that says "this electron here moves over there..." as though that were a question that were even answerable. I agree with you that I was way too overzealous in speaking in absolutely, but the problem is that we really need to unask the question. QM causes such confusing leaps of logic, because our brains operate in a classical world. The same problem exists with considering electron spin. Electrons clearly behave as though they have spin; and yet if electrons are true point particles, then we have the geometric paradox that a point can't spin. Being able to work in quantum mechanics requires one to carry physical contradictions happily in one's brain. I guess I would have done best here to listen to David Mermin. "Shut up and calculate". --Jayron32 01:36, 15 October 2016 (UTC)

  We can do a little better than shutting up and calculating: the wavefunction of all the electrons in the system will have an observable momentum (directed around the circuit appropriately) even if the electron probability density is everywhere constant (in time). I'm not sure, however, if you can say, in a quantum-mechanically-correct way, what the drift speed of the electrons is (and thus how many are moving). --Tardis (talk) 15:21, 16 October 2016 (UTC)

Going back to the question "how long", this is determined by the inductance of the conductors. The current will not stop instantaneously, instead it will have some tendency to keep flowing into whatever load is on the wires. Our article will be hard to understand, but what you would have with a powerline, is a pair of parallel wires. You can expect that the current will stop on the order of several nanoseconds. If your try to stop it too fast, a high voltage will appear across what ever you try to stop it with, so for example flicking a switch or pulling out a plug may make a spark. Graeme Bartlett (talk) 22:53, 14 October 2016 (UTC)

This is correct and is a consequence of the fact that utility power transmission lines are not impedance matched transmission lines. An ideal matched line has uniformly distributed inductance and capacitance that together give it a characteristic impedance. It can successfully deliver a wideband waveform such as an abrupt on/off pulse to a load that has the same impedance. Near-ideal matched lines and loads are used throughout electronic circuits that handle high frequencies. The exactness of their match can be measured by TDR technique and expressed in SWR values at frequncies of interest. Matched power feeding is essential for, say, a radio Transmitter antenna but would be very impractical for utility power distribution. AllBestFaith (talk) 12:57, 15 October 2016 (UTC)

If we treat this as a high school level exercise where we pretend that Ohm's law is valid on any time scale (it only starts to be valid for time scales that are a bit larger than the typical collision time), we can argue as follows. Ohm's law says that the relation between the current and the drop in voltage $\Delta V$ across a piece of wire of resistance R is $\Delta V=IR$. The resistance R can be written as $R=\rho L/A$ where $\rho$ is the resistivity of the wire, L the length and A the cross-section. The current can be written as $I=-Anev$ where $n$ is the free electron number density, v the (average) velocity, and e the magnitude of the electron charge. We can then write the electric field E in the wire, given as the voltage drop divided by the length, as:

$E=-\rho nev$

The force exerted on an electron due to the applied voltage is thus given by:

$F=-eE=\rho ne^{2}v$

This is for a steady state situation where the electrons are moving at some constant average velocity. This means that the total force exerted in the electrons is zero, therefore there exists a friction force that is equal in magnitude but opposite in sign as the force due to the applied voltage. This friction force is thus given by:

$F_{f}=-\rho ne^{2}v$

it is caused by collisions of the electron with lattice vibrations and other electrons. If we suddenly cut the voltage, the friction force will still be there, slowing down the current. Newton's second law yields:

${\frac {dv}{dt}}=-{\frac {\rho ne^{2}}{m}}v$

where m is the electron mass. The current will thus decay on a time scale of

$\tau ={\frac {m}{\rho ne^{2}}}$

This is of the order of $10^{-14}s$ for copper, which is similar to the collision time. So, it will take somewhat longer than this time scale for the current to dissipate. Count Iblis (talk) 01:06, 17 October 2016 (UTC)

The Hydraulic analogy is sometimes used to help students grasp electronic processes (by letting fluid pumps / pipes / pressure / fluid flow / fluid volume / pipe constriction / rubber diaphragm stand in for electric generators / wires / voltage / current / charge / resistor / capacitor respectively) in an intuitive way but is clumsy when it represents an inductor. The exercise above interestingly gives a result that implies that the inertia of the electron mass gives every current flow in wire an inductor-like behaviour. The article Electric current explains the rôle of electrons as the charge carriers in metals. The suggested model of solid unmoving croquet balls is incompatible with modern understanding of the free-moving delocalized Valence electrons throughout the metal, that distinguish it from an insulator that has only bound electrons. AllBestFaith (talk) 18:59, 17 October 2016 (UTC)

The science behind how "Metanium" cream works

Please could you explain to me how the ingredients in "Metanium" cream actually treat nappy rash. According to this website the cream contains;

Titanium dioxide 20.0% w/w
Titanium peroxide 5.0% w/w
Titanium salicylate 3.0% w/w
Dimethicone 350
Light liquid paraffin
Tincture of benzoin
White soft paraffin

I am only interested in the scientific understanding of how the ingredients in this cream treat nappy rash. I believe this question is acceptable on the Reference Desk and does not contravene the medical advice prohibition because I am not asking for any diagnosis or treatment plan. To make absolutely clear, I do not have nappy rash and I am not asking this question in relation to any specific case of nappy rash that has ever existed. This is purely a scientific question.

ありがとう — Preceding unsigned comment added by 2A01:430:D:0:2CC:B0FF:FE9B:CC73 (talk) 12:57, 14 October 2016 (UTC)

Irritant diaper dermatitis#Treatments has some information. DMacks (talk) 13:08, 14 October 2016 (UTC)

"A review of the pathophysiology, prevention and treatment of irritant diaper dermatitis" [2] - seems to be a very comprehensive and relatively recent review, including titanium dioxide ointment treatments like the one you mentioned, mechanisms, and many further refs. It is unfortunately paywalled, I can provide a copy to interested readers upon request. Here [3] is a less comprehensive but freely accessible overview of topical agents in neonates. SemanticMantis (talk) 14:20, 14 October 2016 (UTC)

We currently redirect titanium peroxide to titanium oxide, but this seems invalid, as it apparently contains a true peroxide group. Actually, the combination of this peroxide + the organic chemicals in this mixture leaves me a little bit surprised it doesn't go boom... Wnt (talk) 15:57, 14 October 2016 (UTC)

10% benzoyl peroxide hasn't blown me up yet. The article says it can be explosive in pure form, but apparently diluting and mixing it with a base makes it safe, or else it presumably wouldn't be sold in every drugstore. I can definitely vouch for its bleaching effect though! Even a little bit of residue is great at bleaching all your towels. --47.138.165.200 (talk) 09:52, 15 October 2016 (UTC)

What's larger, follicle cell or parafollicle cell?

it's not clear to me if the follicle itself is cell or something which composed of many epithelial cells 93.126.88.30 (talk) 19:27, 14 October 2016 (UTC)

A follicle can be a single cell, a group of cells, or even a subpart of a cell. A follicle just means "little pocket", and the term can refer to any number of structures, its basically a description of a shape, not of a specific type of structure or functional part. --Jayron32 19:58, 14 October 2016 (UTC)