How do they "know"? - Page 1
Forum section: electronics / beginners

author: AdamPanic
author_rank: Newbie
timestamp: July 21, 2014, 05:45:33 pm
content: Image a simple circuit battery negative end connected to bulb via wire. Bulb connected to positive side of battery.Bingo the bulb lights up.How does the electrons / hole "know" that circuit is complete, and thus start to flow?Do they "know" instantly or is there a delay?If we use the analogy of a water pipe, then there is a delay as the pipes fill with water.Or is there a better way to think about the problem.

author: retrolefty
author_rank: Super Contributor
timestamp: July 21, 2014, 05:57:21 pm
content: QuoteInstantly, at or near the speed of light.

author: Prime73
author_rank: Regular Contributor
timestamp: July 21, 2014, 06:00:04 pm
content: http://en.wikipedia.org/wiki/Electricity

author: pickle9000
author_rank: Super Contributor
timestamp: July 21, 2014, 06:00:38 pm
content: Quote from: AdamPanic on July 21, 2014, 05:45:33 pmThe pipe is already full of water, just not moving.

author: aroby
author_rank: Regular Contributor
timestamp: July 21, 2014, 06:12:56 pm
content: The electrons themselves move very slowly (google electron drift). Since the conductor is full of electrons, the charge (current) flows immediately.

author: BravoV
author_rank: Super Contributor
timestamp: July 21, 2014, 06:14:25 pm
content: Quote from: AdamPanic on July 21, 2014, 05:45:33 pmQuote from: aroby on July 21, 2014, 06:12:56 pm+1The other analogy, think of a big pipe "already filled" and "full" with steel balls, no gaps between those balls at all, densely filled, thats how to imagine the electrons at every conductor.An electricity flow is a pressure at one end of the pipe, say you hammer or continously push the balls at one end, at the other end of the pipe opening will "almost instantly" gets the pressure out, thats how electricity current moves. The actual balls move (electron drift as aroby stated) from one end to the other end are quite slow actually. If I'm not mistaken, slower than a walking ant.

author: IanB
author_rank: Super Contributor
timestamp: July 21, 2014, 07:59:30 pm
content: Quote from: AdamPanic on July 21, 2014, 05:45:33 pmThe short answer is that the electrons do not "know" instantly. It takes time for them to find out.Consider the water pipe. Even if the pipe started out full of water the flow would not appear at the other end quite instantly. If you start pushing the water at one end of the pipe the water will not move at the other end until the pressure wave reaches it. This pressure wave travels through the water at the speed of sound, and even though the speed of sound is very fast, there will be a definite delay before the water at the far end starts to move.Similarly with the electric circuit. In the circuit the pressure "wave" that travels round the circuit is the electric field, and until the electric field arrives at any point the current will not start to flow. The electric field travels through wires with a propagation speed somewhat close to the speed of light, so for short wires this is hard to measure. But in high speed circuits operating at GHz frequencies it really does matter and the lengths of wires and circuit traces have to be carefully adjusted so that all the signals arrive at the right time.Here is a very elegant video to demonstrate how the far end does not immediately "know" what happened at the near end:http://youtu.be/uiyMuHuCFo4

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 21, 2014, 08:00:03 pm
content: Yes! There is a delay. But the world is even richer than that.First off, because there is a potential between the wires, there is a slight difference in the surface density of electrons.Explaining that:Surface electrons exist because of the rules of electrostatic induction:1. Metals exclude static electric fields. If there were an electric field, it would displace electrons, which since they can move freely in metals, would draw current, which would subsequently short out the field (Ohm's law, in fact!). (Likewise, drawing a continuous current through a wire creates an electric field in it, albeit an extremely weak one, atomically speaking.)2. A charged object gives off an electric field; since a metal has no field inside, this field has to start from the surface. Which means there must be either a slight abundance or absence of electrons on the surface of the metal. In practical terms, the depth is, say, a few atoms' worth.Explaining that even more in depth (ha);Remember, further, that since electrons are spongy, foggy quantum mechanical waves, it's certainly not true that the electrons in question (or the lack thereof) are on the absolute surface; indeed, they *are* the surface, which extends into space maybe an atom's worth past the boundary you might call the surface, and also an atom (or a few) into the mass as well. Skittering about at random (thermal motion), around the surface (surface waves) and into and out of the bulk (phonon scattering). Because of charge conservation, an electron leaving the surface will sooner or later be replaced by another, and so on, so that's fine. And in any given area, there will sometimes be more or less, but it all averages out. And besides, since all electrons are identical, there's no way to identify and track just one, you can only measure the ensemble behavior (as they say in statistical mechanics).Returning to the circuit;The instant you connect the light bulb -- on the time scale of fractions of a nanosecond, assuming an ideal switch -- those surface electrons near the switch are drained into the bulb. Not very much, a surface doesn't hold much charge, maybe a few femtofarads worth. But that lack of charge causes nearby electrons to move over to fill the gap, and so on. The electrons move to restore electric energy to that part of the wire -- electric field energy. The motion of electrons likewise creates a magnetic field between the wires -- magnetic field energy. The fields, together, necessarily united and inseparable, are an electromagnetic wave, of electric potential on the wires, and magnetic field between them, propagating down the wire, starting from the lightbulb.You get a wave, and not just an instantaneous action, because the very act of current flow causes a magnetic field, which tends to oppose the current flow (effectively, what we call inductance); and the magnetic field induces electric field (which opposes the change in voltage), and so on. The opposition occurs over both time and space, and the product of these is the wave velocity. The wave is constrained along the wires (a waveguide of sorts, but more commonly called atransmission line), so it only propagates along the direction of the wires, traveling near the speed of light, forwards or backwards. It's not quite the speed of light (which is the velocity of a wave in vacuum), due to the effects of the wires, geometry, and nearby materials (insulation, etc.). For the case of loose, open wires, the velocity will be > 0.95c I think (c = speed of light). A tightly twisted pair, with typical insulation on the wires, might be 0.8c or so; a fully enclosed coaxial cable with solid PE dielectric usually runs 0.67c.At any given point along the wires, the voltage and current are affected as the wave passes. At the instant the switch closes, only the switch end "knows" anything has happened: the voltage goes to about zero, and current rises slightly.Explanation:1. I'm assuming the lightbulb is a pretty low resistance, which isn't all that unrealistic. The actual amount of voltage change isn't important, just that it occurs; 0V is a simple enough approximation.2. The current rise is well defined -- the ratio of voltage change to current change is called the characteristic impedance of the transmission line, and is determined purely by the ratio of electric and magnetic influences. The wave hasn't hit the battery yet, so it doesn't "know" how much power the lightbulb wants to draw. At this early stage, only the transmission line is involved.As the wave travels, more and more of the transmission line gets discharged to 0V. When the wave reaches the battery (assuming an ideal battery), the electric field change drops to zero -- it's shorted out by the battery, an ideal voltage source -- but the current is still flowing, the magnetic energy is still present. The energy in the wave doubles up on itself, as shorting out the electric field causes the magnetic field to double, which in turn launches the wave in the opposite direction, in the opposite phase: now as the wave flows back, voltage on the line rises to about 2 x Vbatt, and the added current sums to about double now.When the return wave hits the lightbulb, the voltage again gets shorted out by the low resistance, and reflects, negative going (maybe a few volts now, rather than 0V), back to the battery, and so on. The higher current causes a little more voltage drop across the lamp, which offsets the re-return wave, so that it is of smaller magnitude than the first time. (Starting to get the idea?) As the wave bounces back and forth, eventually, the light bulb's voltage comes up to the same voltage as the battery. If you draw a curve over the bounces, it looks like an exponential decay, with a time constant of L/R, where L is the equivalent inductance of the transmission line and R is the total circuit resistance.(If we did this example with a battery, a switch, and some wires that are open circuit, the exact same thing happens -- the open-circuit case isno less exciting, even though nothing ends up glowing -- the difference being, the reflection from an open circuit is in-phase, which means the wave reflects back, "telling" the battery instead that "I don't need more current flow" -- and, the time constant would be capacitive R*C instead. You can tell if a transmission line will effective behave inductively or capacitively based on whether the load impedance is lesser or greater than the characteristic impedance, respectively.)In practice, a hard, sharp wavefront will not be produced in the first place. Mechanical switchesareactually amazingly "sharp" -- for decades, the fastest pulse generator on the market used mercury-wetted reed relays for sub-nanosecond performance -- at pretty low repeat frequencies, of course! If you actually measured the microscopic events in the above sequence, you would probably still be able to observe transitions of similar order -- meaning, rather than being a hard shock wave, the wave front is more like a gradually swelling tsunami, so that over a short length of wire, it just looks like a level change, but over a sufficiently long time or length scale (1 nm ~= 20cm / 8 in), yeah, it still looks like a rolling wave front and everything. That's just perspective -- the underlying physics is always there, even if it's not as graphic as a crisp breaking wave.But it gets worse. See, a real lamp is not an ideal short, nor an ideal resistor -- same goes for the battery. Loose wires don't make a good transmission line, and real transmission lines are lossy anyway. In reality, a sloppy setup -- not particularly intended to observe this phenomenon or anything -- will still experience the "waves sloshing back and forth" action, but high frequency losses (attenuation or radiation) and dispersion (a frequency-dependent variation in propagation velocity -- effectively, the reason prisms split white light into rainbows) cause the wave front to round off or disperse into ripples, and the non-idealities of the components cause various bumps and losses as well. (Also, over an extremely long time scale, the light bulb... lights up. We're talking nanoseconds here, whereas it will take milliseconds for the filament to begin glowing. An LED, on the other hand, also lights in nanoseconds, more or less as fast as the voltage rises across it. But an LED makes an even worse example because it's very nonlinear...) Because of all these 'rounding' effects, the best you might actually observe is the exponential ramp, perhaps peppered with some bounces and squigglies due to a variety of possible transmission and resonance modes in a random setup. In that case, you could fairly reasonably just approximate the wire as equivalent inductance, and leave it at that (since, it's pretty well true by observation at that point!).Oh, one final loose end (among many, I'm sure, but..): surely current does not always flow on the outside of the conductor?! If so, what's the point of using solid wire, or wire at all?Answer: This is a subtle aspect, which is driven by that magnetic field created around the wires. The trick is this: if metals oppose internal electric fields -- and I mean for any frequency other than DC, then, they necessarily oppose magnetic fields as well: Faraday's law of induction. If metals were ideal conductors, this would be true: in fact, type I superconductors do indeed exclude internal magnetic and electric fields, and the effect extends all the way to DC. But real conductors don't exclude internal magnetic fields perfectly.The magnetic field at the surface of the wire (due to the bulk current flow through that wire) induces a voltage in the surface (induction), which draws current (an eddy current) due to the material's resistivity. This current opposes the magnetic field imperfectly, so that the next layers down see less and less magnetic field, until at the center (of an infinitely large wire), there is none. This is another exponential decay against time and distance, so the effect varies with frequency and relative size. The effect with our transient is that, once all the wave phenomena has settled (in the 10s to 100s of ns time scale), current begins effective "soaking in" to the wire: at first flowing only on the surface, but gradually deeper and deeper. For normal sized wire, after a millisecond or so, it's pretty much steady state, uniform from surface to center....And I bet you were told that the lightbulb simply lights up when the switch is thrown! Those lying teachers, eh?Tim

author: ejeffrey
author_rank: Super Contributor
timestamp: July 21, 2014, 08:34:46 pm
content: Quote from: pickle9000 on July 21, 2014, 06:00:38 pmThis. I mean, there are more complicated and detailed explanations (including those above) that provide more information, but this is the essential point: the wires are already full of electrons, and do not need to "fill up". If you imagine a water pipe in a loop with a pump and a valve and you throw the valve open, the water starts flowing almost immediately. Almost, but not quite instantaneous. In the water pipe example, the 'start moving' signal propagates as a pressure wave at the speed of sound. In a wire, it propagates as a electromagnetic wave at the speed of light.

author: Rerouter
author_rank: Super Contributor
timestamp: July 21, 2014, 08:37:13 pm
content: T3sl4co1l:

author: 22swg
author_rank: Frequent Contributor
timestamp: July 21, 2014, 09:05:46 pm
content: Put your foot on the pipe you have a resistor... fill a tank up you have a capacitor ... not sure about inductance..

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 21, 2014, 09:12:02 pm
content: Quote from: AdamPanic on July 21, 2014, 05:45:33 pm"they" dont know. they just follow the "rule". similarly when a person get hanged, if the chair is pulled, whether he like it or not, he's moving down. this "rule" that makes universe "non-random"Quote from: AdamPanic on July 21, 2014, 05:45:33 pmthere is a delay. with no delay, or absolute 0s movement we call that singularity.Quote from: AdamPanic on July 21, 2014, 05:45:33 pm2 electrons speak to each other....e1: why human are not moving?e2: no they are moving, its just too damn slow.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 21, 2014, 11:32:49 pm
content: Quote from: 22swg on July 21, 2014, 09:05:46 pmYou can't really get a magnetic inductance from a hydraulic system, but the momentum of the water itself acts like a kinetic inductance. Turn off a valve quickly when you have a lot of moving water behind it, and you get a huge pressure spike. That is the thunk you hear when you turn off a spigot. Real systems have short segments of pipe filled with air to cushion this pulse, otherwise it would break things. The difference is that because there is no 'magnetic field' equivalent, you can't bend a tube into a coil to get more inductance, and you can't make a transformer, although you can get a similar effect from using the water pressure to drive a motor, and then have that drive a mechanical pump on another circuit.Really though you shouldn't get too caught up in the hydraulic analogy. It is an useful for getting a basic intuitive understanding of what is happening in simple circuits, but it is only an approximation. Electrical circuits have electric and magnetic fields that exist outside the wire, which give rise to behavior that has no analog in the hydraulic system, while hydraulics have a number of non-ideal behaviors that don't normally exist in electrical systems such as leaks, air bubbles, viscosity, and turbulence. In other words, the hydraulic analogy is an analogy: not a literal equivalence!

author: Dave
author_rank: Super Contributor
timestamp: July 21, 2014, 11:57:13 pm
content: You can imagine inductance like a turbine attached to a flywheel. When the water current suddenly wants to change, the inertia of the flywheel causes pressure that counteracts that.

author: IanB
author_rank: Super Contributor
timestamp: July 22, 2014, 12:42:13 am
content: Quote from: ejeffrey on July 21, 2014, 11:32:49 pmActually, you really could bend a tube into a coil to get more inductance (what you would actually be doing is compacting a long tube into a smaller space, and a longer tube would give greater inertia). But this would be "self-inductance" of course. It doesn't help with the transformer equivalent.QuoteOne should not underestimate the power of abstraction and generalization! Analogies really do help with conceptual understanding, because even if the physics is different the mathematics often falls into the same patterns. It is also really good to learn how and when to neglect unimportant details in favor of an idealized model, and how to recognize when the details become important and have to be put back in.Richard Feynman touched on this kind of analogy in one of his introductory physics lectures:http://www.feynmanlectures.caltech.edu/I_23.html

author: AdamPanic
author_rank: Newbie
timestamp: July 22, 2014, 10:06:54 am
content: Quote from: T3sl4co1l on July 21, 2014, 08:00:03 pmOnly joking, That explanation helps at lot.So in theory you could measure that "wave" if you had fast enough and sensitive enough oscilloscope ?Now I can see why Debouncing your switches is useful. It minimizes the affect of the transition spikes induced by the closing / opening of the switch contacts.Why leads me to ask the question.Do transition spikes occur in solid state (no mechanical parts) relays? Or in transistor based switches?

author: Dave
author_rank: Super Contributor
timestamp: July 22, 2014, 10:35:53 am
content: Quote from: AdamPanic on July 22, 2014, 10:06:54 amYes. You can see effects of inductance ineverycircuit, not just ones with mechanical switches.

author: amyk
author_rank: Super Contributor
timestamp: July 22, 2014, 12:12:57 pm
content: Quote from: AdamPanic on July 22, 2014, 10:06:54 am

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: July 22, 2014, 12:46:04 pm
content: Draw / suction on the line

author: 22swg
author_rank: Frequent Contributor
timestamp: July 22, 2014, 01:23:43 pm
content: A water transformer ?

author: c4757p
author_rank: Super Contributor
timestamp: July 22, 2014, 01:59:59 pm
content: Not quite, electrical transformers don't work at DC. That does.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 22, 2014, 05:24:54 pm
content: Quote from: AdamPanic on July 22, 2014, 10:06:54 amEhh, two additional things there actually:- The transmission line effects take place on the order of the electrical length of the wiring, nothing more. Roughly 8-10 inches per nanosecond. Even with high quality reflections, it won't take much more than 5 times this to be in pretty much the ordinary "lumped equivalent" domain (i.e., treating the line as a simple RLC circuit). Which is equivalent to the rule of thumb, if it's less than 1/20th of a wavelength, it looks more like a piece of wire (with RLC effects) than a transmission line.- Capacitances and inductances charge in the nanosecond to microsecond range, except for very bulky parts. Say you're switching a load with a relay, and it has an 0.1uF bypass cap on it. That cap charges after the transmission line activity, through the equivalent inductance of the line and relay and such. You can use regular RLC analysis to determine how much the voltage will overshoot.Hint: if sqrt(L/C) > R, it will have a lot of overshoot.Where:L is the equivalent wiring inductance (for example from the 0.1u cap, through the relay and wiring, into the nearest ideal voltage source: another cap >0.1u; and back again),C is the total loop capacitance (capacitors in series go as C1*C2 / (C1 + C2), though you don't have to worry if one of them is very large -- try it),R is the ESR and DCR of the caps and wiring in that loop.And, you can still look at this as a sort of transmission line effect, but because you don't know what a capacitor is inside, other than a lump of capacitey stuff, it's not very useful.Very large inductors, and bulk capacitors, may take longer to charge. For example, a 4700uF 25V electrolytic with 0.1 ohm ESR takes no less than 1.5 milliseconds to reach 99% charge. (You're not likely to see overshoot when charging such a massive capacitor, because even if you have 1uH of stray inductance, which is a lot for wiring, sqrt(1/4700) ~= 0.014 << 1, so it's well damped and looks more like an RC charging slope. But the peak current is large: that cute spark you see when shorting the capacitor is dumping e.g. 16VDC / 0.1 ohm = 160A peak!)Anyway, back to the relay. Mechanical contacts bounce -- for reasons very similar to transmission lines, actually -- transmission line effects occur anywhere you have wave equations, so you get E&M, acoustic waves in any physical medium, gravity waves on the water -- or on a layer of cool/dry air in the atmosphere, or the bending of a beam (like the switch arm in a relay). Specifically, when the coil starts pulling the arm, it applies the force off-center, which starts the arm swinging, but not all at once, it flexes a bit. It takes time for the flexion to reach the end, and bounces off, and basically wobbles as it comes down. So there's an initial bounce already provided. When the contact hits, it smacks into the surface, which sends an even sharper wave down the arm, which bounces off the ends, and when it returns, the contacts open and slap closed again. This can happen several times as the waves die out (and also the re-slapping launches more waves of smaller amplitude, so it's a very nonlinear process -- you can do a similar thing with varactor or step-recovery diodes, where, rather than letting the waves wander and dissipate, they're used to excite a microwave frequency resonator, achieving frequency multiplication without using any amplifiers). There's also the non-ideality of the contact surfaces: while the contact arm is wobbling around, it's twisting too, which presents different parts of the contact surface, which are usually kinda round, but especially after some wear and pitting, that too.And just like the capacitor charging case... if only they provided enough damping resistance (the mechanical equivalent is friction, viscosity, etc.; the idealized component is called a dashpot), you could actually have a relay that doesn't bounce, the contacts just go "bump" and that's it. I'd be surprised if a manufacturer ever actually guaranteed that, but...Tim