Wikipedia:Reference desk/Archives/Science/2021 April 22

= April 22 =

Earthquakes and magnetic field
Could it be argued that earthquakes is a certain tradeoff for having a magnetic field generated by geodynamo? That is, if the theoretical lack of earthquakes across entire Earth entails fixed, static tectonic plates, would this also mean the lack of magnetic field due to absence of plate-moving factors in the Earth's core? This is, of course, based on the assumption that magnetic field is vital for living organisms, otherwise it could be neglected. Thanks. 212.180.235.46 (talk) 15:17, 22 April 2021 (UTC)
 * Some points to consider:
 * one might have an active core generating a magnetic field without this resulting in surface tectonic activity;
 * a planetary magnetic field, once generated, might to a degree persist in the rocks even after the core activity ceased;
 * lack of a (strong enough) magnetic field would expose the atmosphere and surface to the effects of cosmic rays and solar wind that would likely have a detrimental effect on the existence and development of living organisms, both directly and through the results of atmospheric stripping;
 * lack or cessation of tectonic activity to recycle absorbed water and atmospheric gases back into the hydrosphere and atmosphere via volcanoes might result (particularly when combined with atmospheric stripping) in a dry and near-airless surface, such as appears to have happened in the case of Mars. {The poster formerly known as 87.81.230.195} 2.219.35.136 (talk) 15:50, 22 April 2021 (UTC)


 * As far as we know, the magnetic field is not vital for most living organisms. If the core froze solid, earthquakes and the magnetic field would probably both cease. The term "tradeoff" is usually reserved for design decisions, which I think does not apply here. --Lambiam 17:35, 22 April 2021 (UTC)


 * A magnetic field helps, but doesn't seem vital to me. The Earth's escape velocity is high enough that even without magnetic field the stripping of the atmosphere by solar wind would be pretty slow and most of the protection against cosmic rays is done by the atmosphere. It has a column density of 10 tonnes per square metre, equivalent to a lead wall over a metre thick. Not many cosmic rays are going to pass through that. On the other hand, plate tectonics might be vital to cycle nutrients trapped in sediments on the sea floor back into the biosphere. PiusImpavidus (talk) 09:04, 23 April 2021 (UTC)

Doubt on current flow
From hereI learn that electric current flows positive to negative charge, does it mean current goes back to battery and recharges it? Rizosome (talk) 16:20, 22 April 2021 (UTC)


 * See Electric current. Also see NASA's The Direction Assigned to Electric Currents and IOP's Electric charge and current - a short history --Guy Macon (talk) 16:51, 22 April 2021 (UTC)


 * Further, look at Electrochemical cell and Galvanic cell. Electrons aren't flowing out of a battery and into a battery. They are flowing around a circuit, all of the way around, and keep flowing as long as their is electrochemical force to keep driving the flow. That electromotive force is coming from the battery placed into the circuit. So, electrons flowing through the whole circuit, including the battery, do not recharge said battery, they are being "pushed" by the battery. That electrochemical force is driven by redox reactions in the battery (galvanic cell article shows this well). You have a redox reaction that starts under conditions far from equilibrium (i.e. most of what is getting oxidized is starting in its reduced form, and most of what is getting reduced starts in its oxidized form). The electrochemical "force" is the force of the system trying to reach equilibrium. As the reaction takes place to get to equilibrium, whatever side is being oxidized is "pushing" electrons to the side that is getting reduced, and we take advantage of this by placing a wire and all of our other electrical components between them. As the redox reaction approached equilibrium, the electrochemical force decreases, which we observe as the battery being "drained." Eventually, you reach equilibrium, and the battery is dead. The math for this can be found at Nernst equation. Since what is pushing those electrons is the reaction to bringing the system towards equilibrium, the electrons going back to the cathode doesn't recharge anything, as it is going to the cathode to reduce it. The only way to recharge the battery is to somehow re-oxidize the cathode and re-reduce the anode. --OuroborosCobra (talk) 17:35, 22 April 2021 (UTC)

Electrons aren't flowing out of a battery and into a battery. They are flowing around a circuit, all of the way around, and keep flowing as long as their is electrochemical force to keep driving the flow. You are saying electrons are not flowing and again flowing, so much contradiction in your edit. Rizosome (talk) 06:26, 23 April 2021 (UTC)


 * It sounds to me that you believe the SAME electrons are whizzing around the "circuit" like cars at the Indy 500. This is not the case. The positive and negative side of the battery itself are isolated/insulated from each other. The potential difference is what drives the electrons from the positive side of the battery through the circuit and to the negative side. 41.165.67.114 (talk) 07:03, 23 April 2021 (UTC)


 * In order to recharge, the current has to be forced to go in the opposite direction. The chemistry in the battery would have to permit recharging. Else you might get an exploding cell in your battery recharger. Graeme Bartlett (talk) 07:43, 23 April 2021 (UTC)


 * To make a physical analogy: one can drive machinery by the force of water flowing from a higher to a lower level, but to use that same water again, you'd have to use another energy source to pump the water back up to the higher level. This is actually done by pumped storage hydroelectric stations to take advantage of fluctuations in the cost of electricity (you pump the water up at night when electricity is cheaper because of lower demand, and use it to generate electricity at peak demand).
 * In an electric cell (or battery of cells), the negative and positive sides are like the higher and lower levels of water, but they are separated by their different electrical potentials (caused by there being more electrons on the negative side as a result of chemical properties) rather than by the water's different gravitational potentials (caused by height differences). {The poster formerly known as 87.81.230.195} 2.219.35.136 (talk) 09:24, 23 April 2021 (UTC)


 * A LOT of the misconceptions forming the basis of the confusion in this thread can be answered by this video, that YouTube channel also has other REALLY good videos on electrodynamics, and it is presented on a level that the average school-aged child could understand them; they are wonderfully presented at that sweet spot in being entirely accurate and well presented so a lay-person without the deep mathematical background to understand the underlying models can still "get" what is being described. Please watch them.  Also, insofar as it happens, the speed at which electrons flow through a circuit is called their Drift velocity, and it is probably a LOT slower than you're picturing if you carry the notion that they are "whizzing" around through the circuit like a racecar around a track.  It's more like being stuck in a traffic jam at the height of rush hour; individual electrons (insofar as it makes sense to think of the system as individual electrons moving around like objects, which it probably doesn't, but people like the model, so let's go with it) move really slowly.  They give a great example in that article all worked out; for a 1 amp current (which is REALLY high, 1 ampere will stop your heart if you take a jolt that big) and a 2 mm-thick wire (which again, is really thick), the drift velocity is 0.023 millimeters per second, lower currents and thinner wires that you might find in consumer electronics, will result in smaller speeds.  To put that number into perspective, if we treat that as the speed of a single electron moving through the wire (which I've said, has its own problems, but lets suspend those for now) then that 1 electron will move 1.38 millimeters in a minute, 82.8 millimeters in an hour (8.28 centimeters, or about the width (shorter dimension) of your cell phone) and in a day will have traveled a little under 2 meters.  At the currents and wire dimensions found in just about any consumer electronics device, essentially none of the electrons that leave a battery will return to that battery over the entire life of that battery.  Again, they're not whizzing, they're more moseying.  Still, the actual way that electrons provide the energy to power devices does not depend on them each individually travelling a circuit, it depends on them transferring energy through the electric field; this process is covered quite well in the video I have now cited twice.  Please watch it.  -- Jayron 32 11:51, 23 April 2021 (UTC)
 * That's a really good analysis of how little "actual motion" there is, even though there can be a large amount of energy propagated over a long distance. I think your guesstimate of what people might find in their lives is a bit off. A 2 mm conductor isn't that thick...it's about 12–14 AWG, which typical copper wiring in most residential and commerical installations in the US (even flexible lamp cords are over 1 mm). A current of 1 A is found in the cord for a lamp that has two 60 watt lightbulbs in the US. Also, is something off in your "lower currents and thinner wires that you might find in consumer electronics, will result in smaller speeds" note? Per drift velocity, speed is inversely proportional to conductor area. DMacks (talk) 14:25, 23 April 2021 (UTC)
 * The OP is talking about battery-powered devices; I tailored by analysis to one of those. House current of course works very differently, and in alternating current, net drift velocity is actually zero; and yet an AC circuit can power things just fine; the video I linked explains how.  -- Jayron 32 16:52, 23 April 2021 (UTC)
 * Regarding battery current, a typical USB charger provides about 1 or 2 amps when charging an Apple device. A good-quality USB cable has about 3/4 mm power conductors, whereas some power connectors inside a computer are upwards of 1 mm (but yeah, not 2ish). DMacks (talk)

17:48, 23 April 2021 (UTC)

At positive charge, voltage is acting as force pushing the electrons. So at negative side of the battery, what is happening? Rizosome (talk) 03:04, 24 April 2021 (UTC)


 * It's the other way around. People defined electric current in such a way that it runs from the positive side to the negative side before they discovered that, in a metal wire at least, the current is carried by negatively charged electrons moving from the negative side to the positive side. So the negative side pushes the electrons out and the positive side pulls them in. Inside the battery the loop is completed and current runs from the negative side to the positive side, powered by chemical reactions. Here, there are positively charged ions moving from the negative side to the positive side (although in some batteries there may be negatively charged ions moving in the opposite direction; it depends on the chemicals used in the battery). At the ends of the battery, there are redox reactions involving the electrons travelling though the wire and the ions travelling through the electrolyte.
 * From the point of view of classical electrodynamics it doesn't matter what the electrons do. Charge, current, voltage, electric and magnetic fields are concepts we can work with without invoking the concept of electrons. I prefer not to talk about electrons unless it's about the chemical reactions happening in the battery (usually we can consider it a black box voltage source with some electrical impedance) or when dealing with vacuum tubes or semiconductors. PiusImpavidus (talk) 10:37, 24 April 2021 (UTC)