REALLY why is current same through a series circuit? - Page 1
Forum section: electronics / general

author: FenderBender
author_rank: Super Contributor
timestamp: January 23, 2013, 01:27:34 am
content: I'm going a bit crazy. If you thought electronics is hard, well trying to figure out the physics behind it is another thing. I apologize in advance for not being a very competent physicist.We all know that in a circuit consisting of a battery, some connecting wires, and a resistor has the same current throughout the circuit. It makes sense logically, but if you get down to the nitty gritty, you notice that as electrons move through a circuit, they loose energy, and though charge stays constant, the electrons loose voltage because V = J(energy)/ C(charge).If we are talking about electron flow, (from - to +), then when the electrons get near the positive terminal of a battery, the voltage between the terminal and the electrons is very small, (close to 0). So if the voltage at that point in the circuit is close to 0, if we were to look at Ohm's law, it would say that I = VR, so if V approached 0, then I would also approach 0. But that is certainly incorrect, since we know the current is the same throughout the circuit.I've read up a bit and I've heard the term gradient of voltage used. Sorry to bug you guys again for a physics question, but I'm trying to solidify the under-workings a bit.Thank you.

author: tom66
author_rank: Super Contributor
timestamp: January 23, 2013, 01:43:26 am
content: Just because a water pipe may loose pressure along the way does not mean that the rate or amount of water flowing changes through it.Indeed, you can think of it as a charge model: the same charge must exit the battery per second, as must enter it again (of course, minus lost energy.) If there was a difference, where are these mystery electrons either coming from or going to?

author: FenderBender
author_rank: Super Contributor
timestamp: January 23, 2013, 01:48:15 am
content: Right. Well it makes sense, conservation of charge at least.Just wondering why the reduction in energy on the electrons would not correspond to a reduction in rate. I know it's not physically possible. And your analogy makes sense as do many others.Is the rate the same because the electrons are all pushing on each other? Not like that makes too much sense for my caveman mind anyway.

author: fbs
author_rank: Newbie
timestamp: January 23, 2013, 01:55:35 am
content: Quote from: FenderBender on January 23, 2013, 01:27:34 amI = V/R!V is small, but R is also small. So the V/R ratio stays just the same.

author: FenderBender
author_rank: Super Contributor
timestamp: January 23, 2013, 02:18:37 am
content: Wow mind just was blown.All comes back to ohm's law now doesn't it.Thanks. Sometimes you guys just make me feel dumb. Haha.

author: raymohi
author_rank: Newbie
timestamp: January 23, 2013, 02:24:19 am
content: It helps to remember that current is a rate of Coulombs per second. Just in my mind.-Another physics student

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 23, 2013, 03:14:40 am
content: With an AC circuit, the current will not be the same at different points in the circuit! Further away from the source, some current goes into cyclically charging and discharging the lines. It's negligible at low frequencies, but it can be very significant at RF.

author: FenderBender
author_rank: Super Contributor
timestamp: January 23, 2013, 04:22:54 am
content: Tricky tricky

author: AndyC_772
author_rank: Super Contributor
timestamp: January 23, 2013, 07:06:57 am
content: Quote from: NiHaoMike on January 23, 2013, 03:14:40 amSome energy may well be dissipated this way, but not charge. To suggest that current is not conserved is to say that charge is also not conserved, and that means electrons being either created or destroyed. This doesn't happen.In the rf case, the problem is in creating a simple series circuit to analyse in the first place, because the small parasitic elements become significant and must be included in the circuit analysis.

author: Kremmen
author_rank: Super Contributor
timestamp: January 23, 2013, 08:39:31 am
content: Quote from: FenderBender on January 23, 2013, 01:27:34 amThink of it this way: In a battery there is a chemical redox (reduction/oxygenation) reaction taking place. These reactions can be fearfully complex but the end result is always the same: a reduction reaction in one plate is absorbing electrons into bonds in the molecules of the plate substance. In the opposite plate, an oxygenation reaction is taking place, liberating electrons into the plate. The plate, being metallic i.e. conductive, permits the electrons to move.Now there is an imbalance: an excess of electrons in the negative plate and a deficiency of electrons in the positive one. The mutual repulsion between similar sign charges distributes those charges in the plates so as to minimize the potential energy of the whole system. If you then connect the 2 plates with a conductor (i.e. substance that also allows electrons to move semi-freely between the molecules) this repulsion between similar charges / attraction between opposite charges will cause electrons to flow from the negatively charged plate to the conductor (in order to minimize the potential energy in the new situation). All along the conductor the similar charges behave similarly, causing the effect to ripple almost instantaneously to the other end of said conductor. That in turn is connected to the opposite plate of the battery, where the electrons meet the reducing reaction and are absorbed into the plates. So it is like the analogy with marbles that someone quoted: fill a hose with marbles, push the last one and the first one pops out from the other end. Same force - electromagnetic repulsion.You can replace the battery with a generator as we do in the power grid and the result will be the same. In the first case the potential imbalance is created by a chemical reaction, in the second by pumping action with a magnetic field.Now, on a macroscopic level this is put into Ohm's law and Kirchoff's current law - that the sum of currents entering and leaving a network node always equals zero.The resistance of a circuit, and resulting power loss are explained straightforward by transformation of the potential energy in the electric field into heat. That potential energy in turn is created by the chemical reaction in a battery or by mechanical energy input to a generator.So, an electron charge ("energy") of course is constant. The "pressure" or electric field, i.e. electron concentration in a (DC) circuit varies based on the properties of the material and affects the field strength (voltage) measurable at each point along the circuit. In AC circuits new rules come into play in the form of magnetic fields in inductors and electric field concentrations in capacitors. Combinations of these effects create the time-varying effects you see in AC circuits. Since those effects are the consequence of an electron "feeling" the magnetic and electric fields, the same phenomena are present also in DC circuits although to a far lesser degree. But that also explains the concept of distributed inductances and capacitances.Hope it makes some sense.

author: JuKu
author_rank: Frequent Contributor
timestamp: January 23, 2013, 04:56:49 pm
content: Quote from: FenderBender on January 23, 2013, 01:27:34 amYou forgot that at that point R approaches 0, too.

author: Gall
author_rank: Frequent Contributor
timestamp: January 23, 2013, 05:01:13 pm
content: The answer is simple: generally the current is not the same.When the electrical process gets stationary, there are no more changes of potential in any single point of the circuit. This means that every point has its elecron income and loss equal. We call it "current is the same".In AC processes if the frequency is low enough we could count each moment as being stationary. This is true for most circuits up to 10s of MHz and sometimes even in GHz range. The above consideration still makes sense here.99.9% of all electrical circuits fall into one of these two categories. The rest should be calculated carefully taking speed of light and EMI losses into account. In 0.1% cases there is no such thing as Kirchhoff's circuit laws anymore.

author: G7PSK
author_rank: Super Contributor
timestamp: January 23, 2013, 05:25:38 pm
content: The current in a series circuit has to be equal at all points because it has to comply to the lowest denominator, Imagine a crowd of people all jostling to get to the other end of a que but there are doors in the way some will allow several people through at a time but one door only allows one person to pass at a time, the crowd can only go at the rate or speed allowed by the single door or highest resistance to the flow.

author: Kremmen
author_rank: Super Contributor
timestamp: January 23, 2013, 05:27:37 pm
content: Just 1 comment to your last sentence; I fail to see where Kirchhoff's laws would not apply. Please don't confuse a _point_ and a _region_. If there is an electric charge density gradient over a region, then equalization currents may very well flow in that region and the sum may not be zero. But for points the rule always applies.

author: Smokey
author_rank: Super Contributor
timestamp: January 23, 2013, 11:39:00 pm
content: This discussion reminds me of this video. I'm surprised no one posted it yet. I would love to take classes from this guy. Too bad he's mostly retired I guess.

author: Unknown
author_rank: Unknown
timestamp: January 23, 2013, 11:52:38 pm
content: The closest you might get to taking a class from Walter Lewin:MIT 8.02x on edX

author: Smokey
author_rank: Super Contributor
timestamp: January 23, 2013, 11:59:11 pm
content: Ya, but that's like reading the Feynman lectures. Still awesome, but not as awesome as being there. Talk about happening to be in the right place at the right time. Those were some lucky undergrads.~edit~Sorry to get a little offtrack, but that makes me think about what happened to all those Caltech physics students that got to see the lectures live since he only did it for one class of students (2 years). I wonder how many of them ended up doing something completely unrelated to physics or the sciences?

author: IanB
author_rank: Super Contributor
timestamp: January 24, 2013, 03:58:40 am
content: Quote from: AndyC_772 on January 23, 2013, 07:06:57 amCharge is conserved of course, but it is not correct to say that current is conserved. Conservation of charge is a general law, but conservation of current is not so. You must remember the general balance equation,input - output = accumulationIn flow terms, this becomesinput flow - output flow = rate of accumulationRelating this to current specifically, we haveinput current - output current = rate of accumulation of chargeOnly when the system is (or can be assumed to be) in steady state can we say that the current everywhere in a series circuit is the same. For most practical purposes this is the case, but we must not assume it is a physical law. It is only an assumption that usually holds.(Kirchhoff's current law does not contradict this. It applies to balances around a point, and since a point has no volume there is no possibility of accumulation. In all real world examples the conducting medium has volume and KCL is an idealization that is not exactly satisfied--but the error may be negligible, just as we may ignore relativistic errors when applying Newton's laws of motion to terrestrial engineering problems.)

author: IanB
author_rank: Super Contributor
timestamp: January 24, 2013, 04:06:37 am
content: Quote from: G7PSK on January 23, 2013, 05:25:38 pmThere is a very simple real world example where the current is not equal at all points, and that is the Van de Graaff generator. When the generator is running there is a current of several microamps flowing into (or out of) the accumulation sphere at the top, but this is a dead end flow. The current flowing into the sphere is being accumulated there and is not flowing out again. There is a jam where the people cannot get out of the door and they are all piling up in the lobby! Eventually the pressure builds up beyond breaking point and all the people burst out of the door in a large mass--this is what happens when a spark leaves the dome of the generator and finds its way back to earth.So the Van de Graaff generator is a series circuit where the current is not equal at all points (although it is on average). The current flows up into the dome from the ground electrode, but for considerable periods no current flows out of the dome back to ground. Only when the voltage reaches the breakdown voltage and a spark happens is there a momentary surge of current back to ground to balance things out.

author: Kremmen
author_rank: Super Contributor
timestamp: January 24, 2013, 06:39:33 am
content: Ahh, maybe this is verging on sophistry, but before discharge the van de Graaf is not a _circuit_. Rather it is an artificially induced charge density gradient - the motor and belt acting as the charge pumps. Exactly the same thing happens in a "normal" generator based on magnetic induction, should it feed a storage ball. The voltage would presumably be much lower but the same thing in principle. So, while Kirchhoff does not directly apply due to the (un)equalization current, i think it would be fair to say that the current is more or less equal along the generator. The charge storage ball of course is a macroscopic object and when the charge distributes in it, the moving charges (i.e. current) of course separate over a region. So yes, if you want to concentrate on that part then of course you can select a subregion and say that the current is not equal to the total charging current. But the sum total entering the whole ball is. That's where it breaks down since as i noted, this is not a true closed circuit. So obviously the charges cannot continue to the opposite pole of the source.

author: FenderBender
author_rank: Super Contributor
timestamp: January 24, 2013, 01:21:41 pm
content: What doesn't make sense to me is how an electron loses the same amounont of energy going through a 1ohm resistor as it loses going throgh a 1Mohm resistor. I know that a resistor only limits the rate of flow of electrons, but surely an electron would bounce around more in a 1Mohm resistor than it does in a 1ohm resistor. So if it bounces around more, wouldn't it lose more energy? Where is my concept faulted here? How can the energy it takes to go between two points the same even if there is different resistances. Again I know that resistance only limits the rate of flow but I'm still uncertain?

author: FenderBender
author_rank: Super Contributor
timestamp: January 24, 2013, 01:24:23 pm
content: The way I'm thinking abput it is that the electron is doing the same amount of work, but the rate of that work is limted to the value of the resistor. So the resistor WOULD affect power since power is rate/time.Am I wrong?

author: G7PSK
author_rank: Super Contributor
timestamp: January 24, 2013, 03:29:04 pm
content: Quote from: IanB on January 24, 2013, 04:06:37 amWith the Van De Graf surely we are no longer talking about a purely resistive circuit, Capacitance has been introduced, the capacitance between the two spheres on the one hand the top of the Van De Graf and the other the one we call Earth. There fore it takes time for a charge to accumulate to a sufficient point that there is a dielectric breakdown. In a straight resistive circuit the current has to be equal at all points (the PD might alter) conservation of energy will tell you that the current has to be the same

author: IanB
author_rank: Super Contributor
timestamp: January 24, 2013, 04:06:14 pm
content: Quote from: FenderBender on January 24, 2013, 01:21:41 pmBefore we discuss whether this is right or wrong, I'm curious, what caused you to think this? Did you read it somewhere, did someone tell it to you, or did you conclude it from other facts? You are stating something as a fact--"an electron loses the same amount of energy going through a 1 ohm resistor as it loses going throgh a 1 Mohm resistor"--and asking why it is, when your supposed fact may not be a fact at all.

author: Kremmen
author_rank: Super Contributor
timestamp: January 24, 2013, 04:07:13 pm
content: Quote from: FenderBender on January 24, 2013, 01:21:41 pmandQuote from: FenderBender on January 24, 2013, 01:24:23 pmThink of it this way: the electron does not do any work - it is only the medium by which work is done. It is the source of electrons that is doing the work. E.g. a battery converts chemical energy into electric field by liberating electrons in a redox reaction. Or a generator that converts mechanical work into an electric field by pumping the electrons. It is this field that pushes the current through the circuit. The electrons stay as they are, not gaining or losing energy (let's ignore kinetic energy here).

author: IanB
author_rank: Super Contributor
timestamp: January 24, 2013, 04:09:57 pm
content: Quote from: FenderBender on January 24, 2013, 01:24:23 pmPower has the dimensions of energy/time.If we pass 1 amp through a 1 ohm resistor the power dissipated in the resistor is 1 watt. If we pass 1 milliamp through a 1 megohm resistor, the power dissipated in that resistor is also 1 watt (the same). Does this affect your analysis?

author: tom66
author_rank: Super Contributor
timestamp: January 24, 2013, 04:52:23 pm
content: One thing that really confuses me about electrons is how the volt-energy is stored on them. For example, a volt is one joule per coulomb; where is this joule fitting on the electron? My physics teacher related it to a sort of "height", in that there's no such thing as a "height joule" on your body when you jump off a ledge, but you still experience it, but that still doesn't quite explain it.

author: Kremmen
author_rank: Super Contributor
timestamp: January 24, 2013, 05:32:17 pm
content: Voltage is the potential energy of the electric charge. In fact that is exactly what the term J/C indicates. Joule is the unit of energy, Coulomb the unit of electric charge. If you push a charge of X coulombs in 2 "containers" (i.e. capacitors), the "pressure" in the smaller container is going to be higher. Pressure = potential energy = joules. So the joules for a given charge are higher in the snmaller container. This is exactly what you see in a real capacitor.

author: IanB
author_rank: Super Contributor
timestamp: January 24, 2013, 05:47:29 pm
content: Quote from: tom66 on January 24, 2013, 04:52:23 pmWe may say that electrons have kinetic energy and/or potential energy. Kinetic energy only applies when electrons are moving fast, such as in a particle accelerator. Electrons in a wire basically have zero kinetic energy. Changes in potential energy occur when electrons move through an electric field (electric fields are voltage gradients). For example, if an electron moves through an electric field such that its potential changes by 1 volt, then it has gained a potential energy of 1 electron-volt (1 eV).Now the electron-volt is a unit of energy that applies to a single electron, so it is very tiny. If instead we moved 1 coulomb of electrons through a potential difference of 1 volt, then the collective increase in potential energy would be 1 joule. (There is a very large number of electrons in a coulomb--approximately 6.2 x 1018.) So we see that 1 coulomb x 1 volt = 1 joule, therefore by rearrangement 1 volt = 1 joule / 1 coulomb.

author: G7PSK
author_rank: Super Contributor
timestamp: January 24, 2013, 06:23:19 pm
content: An electron looses as much energy in the rest of the circuit as it does in a resistance whatever that resistance is or rather it creates as much heat, it is just that the heat is more concentrated at the high resistance point.

author: ee851
author_rank: Regular Contributor
timestamp: January 24, 2013, 07:31:36 pm
content: Quote from: tom66 on January 24, 2013, 04:52:23 pmGood analogy!Sort of like this: the cannonball that sits on the street running downhill is only a threat to the children playing soccer on the street 100 m below because of where it sits. It is a very big threat atop that hill. However, if we put the cannonball into a truck and move it to the bottom of the hill, and place it on the street there, it is no longer a threat. No gravitational potential energy at the bottom of the hill.Same with the electron. It is the electric field that imposes potential energy on the electron. Place the electron at ground and its electrostatic potential energy is nothing.

author: IanB
author_rank: Super Contributor
timestamp: January 24, 2013, 07:46:20 pm
content: Quote from: tom66 on January 24, 2013, 04:52:23 pmThere really is a "height joule" when you jump off a ledge. When you raise a mass (you) through a gravitational field (you go up to the top of a building), that mass gains potential energy. If you jump off the building that potential energy is converted to kinetic energy as you fall, and when you hit the ground that kinetic energy is dispersed as uncoordinated motion of your body parts eventually converted to heat (you go splat).The height joules are expressed as gravitational potential energy, given by the formulaE=MghwhereEis the energy in joules,Mis the mass in kg,gis the gravitational acceleration (9.81 m/s2), andhis the change in height in meters.

author: FenderBender
author_rank: Super Contributor
timestamp: January 25, 2013, 12:26:11 am
content: Quote from: Kremmen on January 24, 2013, 04:07:13 pmWell I've only taken one year of physics in my life and we spent about a month on electricity so some of this stuff is quite mind-boggling...and frustrating.You say that the battery is the source of electrons. Aren't the electrons already in the wire?So there's practically no energy on the electron itself, but rather voltage is the energy that the source must provide to cause a charge to move from point A to point B. Now, if a resistor produces heat energy, how is the energy source (the battery) actually doing work on the resistor if it's not by electrons themselves releasing energy? I read that after an electron passes through a resistor, it looses potential energy and that it has to be re-energized at the energy source to continue on its path. I guess that's wrong. (What's right?)Still having trouble with the resistance thing. If you have 1ohm and 1V across it and you have 1kOhm and 1V across it, the charge obviously moves much more slowly through the 1kOhm. And if Work = Charge x Voltage, then if both stay constant the work has to be the same for both. I guess this is like mechanical physics were two people can do the same thing but if one takes longer than the other, he has less power. But just to make sure, is it not true that the same amount of work would be done in both instances? The real difference would be in power?...because the electrons are moving more quickly through the 1ohm resistor than the 1kOhm resistor, but the energy it takes to move that electron from A to B is the same (still not sure why this is trueexactly). If the electron is getting from point A to B more quickly then the current is greater. and thus power is greater?This is all stuff that I'm pretty much formulating in my head. It's been driving me nuts all day today. I had a calculus midterm today and while I was supposed to be differentiating some inverse trig functions, my mind was all discdomboblulated trying to get my mind wrapped around electrons in a wire.I think I'm just going to quit on this soon. You guys are probably like "ugh why can't this idiot kid just get it already??". Haha. Sorry for being the village idiot here.

author: tom66
author_rank: Super Contributor
timestamp: January 25, 2013, 12:44:50 am
content: But what comprises this electric field?Is it made of something? If so, how does it presumably travel? In much the same way as the higgs is what gives way to mass and gravity, how does electric field develop and exist?

author: FenderBender
author_rank: Super Contributor
timestamp: January 25, 2013, 12:56:38 am
content: I'm not sure. I was taught that EF travel in "lines" though I'm presuming there are pretty much an infinite amount of lines. But I'm uncertain how the battery is actually creating the force to propel the electrons around the circuit.

author: FenderBender
author_rank: Super Contributor
timestamp: January 25, 2013, 01:06:24 am
content: This is from a book called "Basic Electricity". It is quoted on the Wikipedia page for EMF:In a device without internal resistance, if an electric charge Q passes through that device, and gains an energy W, the net emf for that device is the energy gained per unit charge, or W/Q. Like other measures of energy per charge, emf has SI units of volts, equivalent to joules per coulomb.This sentence says that the charge (electron) gains energy? Is this false?

author: IanB
author_rank: Super Contributor
timestamp: January 25, 2013, 01:37:36 am
content: Quote from: FenderBender on January 25, 2013, 12:26:11 amOK, let's take this one thing.If we have 1 ohm and 1 volt across it, the current flowing is 1 amp. Now for each electron that flows through the resistor, its potential has changed by 1 volt, so that electron has done 1 electron-volt of work. But 1 amp is 6.2 x 1018electrons per second, so the total power is the number of electrons times the work done by each electron.Next, consider the 1 k resistor. This also has 1 volt across it, so the work done per electron is exactly the same at 1 eV. However, this time there is only 0.001 amps of current flowing through the resistor, so the number of electrons per second is 1000 times less. The total power is correspondingly 1000 times less as well.

author: FenderBender
author_rank: Super Contributor
timestamp: January 25, 2013, 01:39:53 am
content: Quote from: IanB on January 25, 2013, 01:37:36 amYes that makes enough sense for me.

author: IanB
author_rank: Super Contributor
timestamp: January 25, 2013, 01:44:38 am
content: Quote from: FenderBender on January 25, 2013, 12:26:11 amYou can think of a battery and a circuit as like a roller coaster. The track is the circuit and the coasters are the electrons.Now the battery takes the electrons from one side and moves them through the battery to the other side, raising their potential (voltage) in the process. This is like the roller coaster leaving the platform and being winched up to the top of the ramp, raising its gravitational potential (height) in that process.When the coaster now coasts down from the highest point using all that stored up energy, it is like electrons flowing round the circuit. The electrons are giving up their energy as they flow round the circuit just like the coaster.Eventually the coaster gets back to the starting point having completed its circuit. The electrons arrive back at the battery just the same, ready for another go around. The coaster is back at the starting height, and the electrons are back at the starting voltage.So one way to think of a battery is like a winch, or like a pump, that moves the electrons up from a lower voltage at one terminal to a higher voltage at the other terminal.

author: ee851
author_rank: Regular Contributor
timestamp: January 25, 2013, 01:52:54 am
content: I like the roller coaster analogy, IanB. That is good.

author: FenderBender
author_rank: Super Contributor
timestamp: January 25, 2013, 01:56:36 am
content: Hmm now I understand on a physics standpoint exactly what you mean. E = mgh makes plenty of sense.But for instance, I found this page:http://amasci.com/miscon/eleca.htmlIf you scroll down to the heading: "ELECTRIC CURRENT IS A FLOW OF ENERGY? Wrong." and"ELECTRIC ENERGY IS CARRIED BY INDIVIDUAL ELECTRONS? Wrong."...the author says, and confidently so, that a particle cannot carry energy itself. So by this notion, an electron could not possible loose energy because it cannot actually have energy? He says that energy move in a circuit by electromagnetic waves. When charge flows, all of the electrons flow (think of the marbles in a pipe analogy), so this energy which is in waves (which I don't full understand) is separate from the particles themselves.His writing there is very informative. It is well written and if I can make sense of it, I think I will be a happy sailor. However, some of it is still abstract.What can anyone deduce from this?

author: FenderBender
author_rank: Super Contributor
timestamp: January 25, 2013, 02:19:33 am
content: And for some more brain-twisting, unless you know all of this:http://amasci.com/miscon/voltage.htmland this:http://amasci.com/elect/vwatt1.htmlI feel extremely stupid now...

author: IanB
author_rank: Super Contributor
timestamp: January 25, 2013, 02:54:48 am
content: Quote from: FenderBender on January 25, 2013, 01:56:36 amBe careful with that web site. It seems like the author is not trying to educate or inform, but rather is trying to demonstrate superiority over others and make people feel stupid.What would you feel if someone wrote a web site saying things like this?THE SUN RISES IN THE EAST, TRAVELS ACROSS THE SKY AND SETS IN THE WEST?Wrong.Of course the Sun doesn't actually travel across the sky, it stays in the same place while the Earth turns on its axis. But so what?What we have to understand about physics is it attempts to describe how weobservethe world to work. There are many ways to describe observations of things, some being more sophisticated than others.So for instance, electrons don't really exist as particles, they exist as some complicated de-localized probability field (or something) described by quantum mechanics. But so what? If we assume they are particles many things work out fine, so why not go with that as long as it suits the purpose at hand?When you read that web site you have to be really careful. Unless you are already a professor of physics you will end up more confused than before you started. I strongly recommend you give it a miss and seek out more useful sources of information instead.Technically, the guy may be accurate. But is he helpful? That's what you have to ask yourself.

author: FenderBender
author_rank: Super Contributor
timestamp: January 25, 2013, 03:18:45 am
content: In the scheme of general electronics, none of this really matters too much I think.But to be honest, I do like his website. Does it make complete sense, of course not...but neither does the idea of a charge or energy or a wave for that matter. We, as you say, can make analogies or describe it as something its not, so that the human mind can make sense of it. But..I've been reading through the website and it actually is starting to make a little bit more sense for me. Maybe I'll be confused shortly. I don't know.Whether this kind of understanding is helpful...well I'm not so sure, but I do agree with him that even after teaching electricity in schools, what understanding does a student actually have of it? Very little. Is it important that he does? I don't know. But I think things would be less magic if it was better understood.Ehh who knows.

author: AlfBaz
author_rank: Super Contributor
timestamp: January 25, 2013, 03:22:59 am
content: Quote from: FenderBender on January 25, 2013, 01:56:36 amI don't know about not having energy and as IanB said it's all a matter of perspectiveIf an electron is moving and it has mass (9.1E-31kg) then E=mv^2/2 should surely see it having some energy

author: FenderBender
author_rank: Super Contributor
timestamp: January 25, 2013, 04:44:48 am
content: Yes indeed it does have energy, but I do not believe that it has enough energy to make much impact. Could be wrong.

author: FenderBender
author_rank: Super Contributor
timestamp: January 25, 2013, 05:20:19 am
content: Sorry to spam here:http://science.uniserve.edu.au/school/curric/stage6/phys/stw2002/sefton.pdfEven though my entire idea of electricity is being pummeled with a jackhammer right now, I'm finding this a pretty interesting read. Energy still makes no sense...

author: PA4TIM
author_rank: Super Contributor
timestamp: January 25, 2013, 07:29:33 am
content: http://www.g3ynh.info/zdocs/AC_theory/AC_theory.pdfVery good stuff, it also starts with telling about the "wrong" ie to much dimplified concepts teached at school. But to get all physic laws right for AC and DC things are a bit different.Resistance and conductnce is also in fact just the opposite of what we think.

author: Kremmen
author_rank: Super Contributor
timestamp: January 25, 2013, 07:48:40 am
content: Quote from: FenderBender on January 25, 2013, 05:20:19 amObjection #5 in the linked text is exactly my point. The electrons don't do the work, they are only the medium by which work is done. The actual work happens in the source, whatever that might be.Think of the water analogy. If you want to get a fountain of X meters/feet/fathoms from your garden hose, are you concerned about the energy levels of the water? No, it is the _pump_ that must have the required oomph to push the water. Water (molecules) do not do any work. It is only the medium by which the pump does its work. The water is not changed in any way in the process.The important thing is to not confuse the subatomic particle effects (like the charge of a single electron, or its motion) and macroscopic mass effects (like a circuit voltage or potential between specific nodes, or current) with each other.Explanation in chapter 2.4 of the linked text exactly corresponds to what i have tried to explain in fewer words. Obviously, i think that is the way it should be visualized.

author: Kremmen
author_rank: Super Contributor
timestamp: January 25, 2013, 08:06:53 am
content: Quote from: FenderBender on January 25, 2013, 01:06:24 amYes, the answer to your question is: false. But read carefully: "...the net emf for thatdevice...". EMF or ElectroMagneticMotor Force is a macroscopic concept - voltage - of thesystem. It describes a mass effect of a huge number of charge carriers that undergo some kind of process altering the charge concentration. The tighter you pack electrons, the more "pressure" i.e. voltage you get. For example, gas pressure is a macroscopic effect describing the average motion of a huge number of molecules. The more gas you pump in a container, the more molecules hit the wall per unit time, resulting in more force i.e. pressure. Pressure is a macroscopic quantity describing the average behaviour of a huge number of individual collisions. The more electrons you pump into a wire, the higher the density of excess charge carriers, each emanating their individual electric field. The voltage is a macroscopic quantity describing the mass effect of all those charges (or the potential energy of the excess charge).Edit: typos

author: FenderBender
author_rank: Super Contributor
timestamp: January 25, 2013, 01:22:22 pm
content: Good explanation. Quite fascinating. Who knows how more complex systems work involving more components and in all sorts of arrangements. I think you'd go crazy.This thread got 1 star? Didn't think seaarching for truth was a bad thing...

author: TerraHertz
author_rank: Super Contributor
timestamp: January 25, 2013, 03:02:18 pm
content: "REALLY why is current same through a series circuit?"A: It isn't, in any case but the steady state.The example given of the VanDeGraff generator is just a case in which the 'rule breaking' is going on slowly enough to see with your eyes.In ALL cases, any perturbations propagate through a circuit at a rate determined by the physical characteristics of the materials (or spaces) involved. Always at less than the free space speed of light - and for a lot of electronics, that's quite slow. A few examples:* A coaxial cable, with a V step impulse applied to one end. A wavefront of changing E field (between core and outer conductor) together with an associated surge of charge carriers (electrons) travels down the co-ax at about 1nS per foot. Which is the energy carrier? It's BOTH the electrons and E field together.* A circuit consisting of battery, switch and inductor (with real wire, non-zero resistance). Circuit stable with switch closed and current flowing, then the switch opens. What happens? The state change can't instantaneously convey to the whole circuit. It propagates down the wires away from the switch, voltages spiking at the switch as more of the loop inductance comes into play. Energy was stored in the steady state magnetic field, and some of that is being dumped into plasma effects at the switch contacts and some of it into a V field impulse - that will propagate away as an EM wavefront.* Two conductive planes separated by a dielectric. (Say a square foot of double sided copper PCB, copper stripped from near the board edges.) At one corner a spark gap between the planes, and a high voltage supply charging the planes via high value resistors. Eventually the spark gap breaks down, effectively shorting the planes together at that corner. What happens is that a discharge wavefront expands across the board in a roughly circular arc (maybe some distortions at the edges.) It's much like the step impulse in a coax cable, except 2D not 1D. There will be reflections from opposite edges.In reality everything electrical is dynamic and relativistic, and physical circuits are just ways of concentrating relativistic interactions of fields and charge carriers to usefully small spaces.You should try understanding what goes on with current flows through low density plasmas. Now *that* stuff is counter-intuitive. Even the steady state behavior is mind-bending. For instancehttp://en.wikipedia.org/wiki/Double_layer_(plasma)But remember, a plasma is just a funny sort of conductor. So it's a good way to remind yourself that wires aren't actually all that simple either. Look up 'electron drift velocity' (in copper.) Surprising huh? And yet a 'signal' travels at not much less than C. The difference is because the 'signal' isn't the electrons moving. It's the combined E and B fields around and in the wire.

author: IanB
author_rank: Super Contributor
timestamp: January 25, 2013, 03:49:18 pm
content: Quote from: Kremmen on January 25, 2013, 07:48:40 amWhere I get uncomfortable with this picture is that it departs from the normal engineering treatment of this system.Imagine a hydraulic circuit where we have the pump at one end as you have, a length of piping, and instead of the fountain we have a turbine driving a generator. When the water leaves the turbine it returns back down another length of pipe to the generator. There is therefore a closed loop of pipe filled with water analogous to an electric circuit with wires.When we do an engineering analysis of this system we consider the water to be a motive fluid that carries energy and we do an energy balance around the loop. The water gains energy as it has its pressure raised through the pump and it gives up energy as it flows through the turbine and turns the output shaft. The water also loses some energy due to friction losses in the pipes between the pump and the turbine. (The friction losses are rather like resistance in a wire.)Now how do we account for the working of this system? If we turn on the pump, the turbine at the other end starts turning very quickly, much quicker than the "drift velocity" of the water in the piping. This is because we have created a pressure field around the circuit and a pressure gradient in each part of the system. The pressure field is how the energy is conveyed from the pump to the turbine, but this does not contradict the statement that the hydraulic fluid is how the energy is conveyed from the pump to the turbine. The two concepts are bound together and each is one aspect of the other. It happens that the pressure field results from the water molecules in the fluid being compressed closer together by the pump, and the increased water molecule density in the pipe leads to increased pressure. The increased pressure leads to water molecules tending to flow from regions of higher pressure to regions of lower pressure.In engineering terms we definitely say the water does work when it flows through the turbine and gives up energy in the process. There is no problem at all with the concept that the water "carries energy" around the circuit (we give the energy carried by the fluid a special name, enthalpy, and we can do energy balances around different parts of the system using the flowing enthalpy of the fluid).So in summary, I disagree if people say that electrons don't carry energy around an electric circuit. This is simply one kind of model and one way of looking at the system. It can successfully explain some electrical phenomena and therefore it is not a wrong model, just an incomplete one.

author: PA4TIM
author_rank: Super Contributor
timestamp: January 25, 2013, 04:04:12 pm
content: Terraherz, nice explanation. I think you shall like the link I for the knight pdf: AC theory I gave above. He is on the same wavelenght ;-)He starts to tell what "schoolbook simplifications or just not right " theory we should forget first. Then all the theory ( proven by a lot of very clear explained math) and sometimes extended or corrected electronic laws so to make them true from DC to daylight. For non-native english it can be a bit difficult, he uses a very acedemic language. ( so I have to read some things a few times but i like it very much)

author: ee851
author_rank: Regular Contributor
timestamp: January 25, 2013, 04:20:40 pm
content: IanB, I have two problems with your water pump/water generator analogy:(1) It would be simpler if you replace the pump with an on/off water valve in a pipe at ground level. This pipe is the discharge pipe of a water tank that sits 20m above ground level. (Good model for a source of gravitational potential energy.) Of course, then you would need a discharge basin to catch the high-speed water flowing out of the generator and another smaller pump to put the water back into the tank.(2) Air is compressible, but water is not. Water is incompressible.With these two modifications, your analogy would be good.

author: Kremmen
author_rank: Super Contributor
timestamp: January 25, 2013, 04:40:14 pm
content: Quote from: IanB on January 25, 2013, 03:49:18 pmYes but then i was not attempting an engineering treatment. While i agree with everything you say, and specifically agree that normally the water / electric current is considered to carry energy, that is just the way to generalize in a useful way in the realm of the macroscopic. I do that myself on a daily basis and what i vaguely still remember from my thermodynamics studies, that was indeed the method of treatment there as well.My point was and is that the elementary units comprising the system do not themselves change in the process. The discussion was about electrons losing energy and that is definitely wrong. At least as far as the charge is concerned - they of course may have kinetic energy, possibly potential energy in a gravity gradient etc etc that they may gain and lose. But the charge that drives electric phenomena is constant and that was my whole point. Detailed analogies with water and especially steam, the favorite element of thermodynamicists are a bit risky since with steam many of the concepts such as entropy and enthalpy are intimately associated with the motion of the elementary units, i.e. water molecules. With electrons the physical motion in a solid state circuit is so slow that it can almost safely be ignored at least as far as kinetic energy is concerned.On second reading of your reply i have to say that i didn't really spot where you differ with what i wrote. Maybe i was unclear but to me it looks like we are more or less saying the same thing. I just put my emphasis on the level of the elementary units.

author: ee851
author_rank: Regular Contributor
timestamp: January 25, 2013, 05:07:51 pm
content: FenderBender, here are my thoughts on Wm.J. Beaty's "misconceptions":1 All electric currents are flows of electrons. Wrong.Electric current can flow in any material that contains charged particles. In metals,the charged particles are electrons. Metals have a sea of loose electrons that are free to flow as soon as the slightest electric field is applied. However, some materials have only atoms or molecules (cations or anions) with net electric charges--no loose electrons. Electricity can still flow in these materials. And it does. It just flows slower, and takes slightly more time to get going.2 "Electricity" is made of electrons, not protons. Nope.What I said above. This is because in some materials, the only loose particles with net electric charges are cations. Also, some materials have both loose anions and loose cations. The anions drift in one direction; the cations drift the opposite direction. Because anions and cations have opposite electric charges, both these drifting particles add up algebraically to a NET electric current IN THE SAME DIRECTION.3 Electrons are a kind of energy particle. Wrong.Electrons are just one of the three constituents of matter, along with protons and neutrons. They only have energy only by dint of an electric or magnetic field and only with respect tosome other entity, be it macroscopic or subatomic.4 "Electricity" carries zero mass because electrons have little mass. No.This can only be understood with a working knowledge of chemistry. When we speak of chemistry and chemical reactions, we speak of electrons. Common, lightweight molecules like water and aluminum and copper and lithium only interact with one another by exchanging electrons.Sort of like guys playing poker. They only shift the cards from one hand to another. Same with electrons. There are ***other*** things they could exchange, but only at a much greater cost. It would end up like in the Western where a guy pulls a revolver at a poker game and a gunfight erupts because some drunk brought up an unpleasant fact every other guy had gone there to forget.Electrons are lightweight compared to other charge carriers--protons and cations. The later can carry current, but except in the case of a gunfight, only carry current in materials inwhich they are the dominant loose charge-carrying particles.Another thing to think about: almost all chemical reactions take place in materials in the liquid phase. Not solid. Not gas. Not plasma. Why?5 Positive charge is really just a loss of electrons. Wrong.There are loose protons in the solar wind. Each one carries the same charge as an electron, but of opposite sign. Same in a plasma.6 Positive charge cannot flow. Totally wrong.See above.7 To create "static" charge, we move the electrons. Not always.Again, chemistry. You may see some material in which the dominant charge-carrying particle is NOT an electron. These materials exist on earth. They are, in fact, common. The dominant charge-carrying particle can be a proton, or an anion, or a cation. All heavierand harder to move than the electron, so they purposefully aren't used by electrical engineers.Common solid conducting metals (Fe, Al, Cu, Ag, Au, etc.) share an extremely large number of loose electrons (called an "electron sea"). (When I say "loose" I mean not strongly held in place by nuclei.) This makes them ideal materials to conduct electric current and hold their shape for years. Thus their popularity with those who work with electricity.

author: IanB
author_rank: Super Contributor
timestamp: January 25, 2013, 06:21:06 pm
content: Quote from: ee851 on January 25, 2013, 04:20:40 pmBut I constructed my analogy very carefully with a closed circuit and water going round and round. The water molecules in the piping are the analog of the electrons in the wiring. If I changed the model as you describe I think the analogy would be less good.QuoteWe only say that water is "incompressible" as a simplification. Water does compress in response to pressure changes of course, only less so than air and following a slightly different model.

author: IanB
author_rank: Super Contributor
timestamp: January 25, 2013, 06:34:12 pm
content: Quote from: Kremmen on January 25, 2013, 04:40:14 pmIn the search for understanding I think that analogies can often be very helpful, especially for those trying to get their head around complex subjects.In that spirit I tried to draw some very careful comparisons between my water circuit and electricity. For example I mentioned that the water might flow very slowly round the circuit, much more slowly than the response speed of the devices like the pump and turbine. The "pressure waves" in the circuit can go much faster than the speed of the water molecules just like the "voltage waves" can go much faster than the electrons that convey them. (The pressure waves in the system travel at the speed of sound in the medium, and that speed changes according to the characteristics of the medium. It is much faster than the speed that the water molecules are flowing.)In the water analogy pressure is created by pushing the water molecules closer together so that the (electrostatic!) repulsive forces between the molecules try to push them apart again. Pressure goes up when the water molecule density increases, just as the voltage goes up when the electron (charge) density increases.And so on, and so on.For me at least, the analogy is very compelling. Maybe not for others.C'est la vie.

author: IanB
author_rank: Super Contributor
timestamp: January 25, 2013, 06:43:50 pm
content: Quote from: Kremmen on January 25, 2013, 04:40:14 pmOne specific observation about this is that potential energy can take many forms.Electrons (or the system) may gain and lose potential energy in a circuit, only it takes the form of electric potential. Charged particles moving through an electric field experience changes in electric potential, which causes them to gain or lose potential energy (which is none other than voltage). Voltage is the potential of a charge to do work, just as pressure is the potential of a fluid to do work.

author: Kremmen
author_rank: Super Contributor
timestamp: January 25, 2013, 07:47:43 pm
content: Certainly. No argument at all. If you think about it on the microscopic level, that potential (voltage) is the sum of the electron charges in the region observed. The source must push them into higher concentration against their mutual repulsion.Regarding the water analogy, again i don't really see where we disagree (provided you even meant that we did). I could have made the same points you did. Again, my original point was that the particles (electron, water molecule) are not changed and that surely holds even if waves of pressure/electric repulsion pass through the mass.I think we are already going on a bit so maybe it is enough to note that basically we seem to be talking about the same thing. I accept all your points and mostly tried to make the same ones on the one specific issue.

author: IanB
author_rank: Super Contributor
timestamp: January 25, 2013, 07:58:46 pm
content: Quote from: Kremmen on January 25, 2013, 07:47:43 pmSure, no problemI'm not disagreeing with you or what you said, rather I am disagreeing with those websites that sometimes seem to say "Everything you ever learned in school is wrong!".Physics is not as simple as that and I don't find that approach entirely helpful or constructive. A lot can be learned by students by considering different kinds of models of particular systems and understanding how far those models are applicable and when they are inadequate. It's not about right and wrong, it's about suitability and applicability of a given model to a given problem domain.

author: robrenz
author_rank: Super Contributor
timestamp: January 25, 2013, 08:01:31 pm
content: How about describing how water hammer occurs and how it relates to pressure gradients and inertial effects of the water traveling in a piping system. There must be some correlation to something like an inductive spike in a circuit. I am not knowledgeable or articulate enough to do it but some of you certainly are. That might give another metal picture that makes all this discussion more clear to some of us (like me)This is an excellent thread

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: January 25, 2013, 08:40:26 pm
content: Hay , I want to play .Something I came up with yrs. ago , to know / learn about something .Imagine a point in 3 dimensional space , place a sphere around that point and call it the earth .We need accumulated experiences to view with , that to some degree make up your position on the sphere is part of what makes up you perspective , or that we add perspective = an individuals placement on that sphere .An ideal of mine is to perceive as many others perspectives as possible .The point being is that to better understand the original point in space .Now to the electrical part of the thread .Most of my experience with electrical issues has been auto repair , construction and now ham radio - having to almost start over to play with electrical , relative to RF it seems .This is much like the original post , some concepts to help learn .If you are not holding onto the chain , when someones trying to pull your chain , there is no work being done .The main analogy I would put in at this point is the battery and the starter , the battery has lots going for it ready to do a lot of work , but sometimes I look at it from the load end [ starter ] of things , rather than the source [ battery ] .When the starter is being used there is not pressure in the battery , but in a sense vacuum , emanating from the starter / draw .So maybe my main point is that not thinking about pressure / voltage at the source , but vacuum / resistance at the load .To take that to an extreem , use arc-welding , the vacuum is the solid steel as a resistor , the work being done - the heat to melt the base , rod , electrode to come up with a bead of solid metal at the end .Maybe is more about entropy than anything .Another monkey-wrench thrown in the gears from a non engineer, or wana-be .Again good thread .

author: FenderBender
author_rank: Super Contributor
timestamp: January 25, 2013, 10:49:09 pm
content: Quote from: ee851 on January 25, 2013, 05:07:51 pmThanks for that. Makes enough sense thinking about it that way.

author: FenderBender
author_rank: Super Contributor
timestamp: January 26, 2013, 12:58:48 am
content: This is what I have deduced after some reading, and how I can explain that electrons do not bring energy to a load.Electrons don’t have much potential energy of their own, nor do they have much kinetic energy (like 10^-30J+). Batteries create a separation of charge. Separation of charge creates an electric field. This electric field runs within the wires of the circuit and (its energy??) pushes the electrons along.This electric field is parallel with the wires of the circuit. This field pushes along electrons. When electrons are pushed along in a circuit (current), they produce a magnetic field. This magnetic field in conjunction with the electric field creates…an electromagnetic field. An electromagnetic field will “guide” the energy from the battery to the load. If we think about a simple battery + resistor circuit, the resultant electromagnetic field “focuses” the energy on the resistor where the energy is used to help the electrons overcome the electrical resistance in the resistor.Therefore to sum up, the electrons themselves do not “contain” or have energy. They move because of a voltage which we know is related to electric field. Voltage is equal to the magnitude of the electric field * the distance between two points. This flow of electrons causes magnetic fields to form around the wires and when both fields are present you have an “electromagnetic field” which will direct energy along the wires and also into the load.So this is pretty much what I came up with. In the scheme of things, it’s still a very abstract concept to try to wrap your mind around. Still not sure if I have yet. Still not totally sure how energy, whatever energy really is, is interacting with the circuit. I also don’t really get the influence of the magnetic field. I can understand the electric field pushes the charges along. And the magnetic field contributes to the “guidance” of energy, but that sounds like bullocks to me. Any ideas?If anyone wants to proofread that and tell me where I may have flaws…This is based off a few online texts so most of the ideas I wrote are kind of my brains interpretation of the writing.

author: IanB
author_rank: Super Contributor
timestamp: January 26, 2013, 01:37:40 am
content: Quote from: FenderBender on January 26, 2013, 12:58:48 amYes, it's bullocks (or bollocks, even)I said you shouldn't read that stuffThink about it in simple terms. Electrons are pushed around the circuit, it is true, but electrons can only go where the wires go. The electrons are contained within the metal conductors and can't (normally) flow through the air or through insulators. So there is no need for electromagnetic fields to guide the energy through the wires--the wires are quite capable of doing that without invoking magic.QuoteHave a look back at my post #52 with the water circuit analogy. Imagine electrons being "pushed" around the circuit, just like water being pushed down a pipe (the pipe being already full of water, just as a wire is already full of electrons). The electrons are trapped in the wire just like the water is trapped in the pipe. If you push electrons in at one end of a wire, they are naturally going to start popping out at the other end.About energy, that's hard to explain and I'm not going to try doing it here without thinking a bit first. It took scientists a long time to figure out the concept of energy, and it was a major breakthrough when Joule and other workers demonstrated the equivalence between mechanical work and heat. Energy forms the basis of thermodynamics, and learning thermodynamics tends to make people's brains hurt. Sorry about that, but energy is not a concept that is easily explained with simple analogies...

author: FenderBender
author_rank: Super Contributor
timestamp: January 26, 2013, 01:54:12 am
content: Haha. I will read it in a second. There's been so many posts on this that it's been hard to keep track of all of the ideas...though the contributor list is not very large.Anyway, if anything I like to think of energy I guess is just a way of keeping track of different interactions that occur in the world, though I am not even confident in my own words. I guess it is better to make a personal "stopping point" where you stop questioning and accept it, because if you keep questioning, you'll never know all of the answers and you can't do anything useful...though you can't be completely uninterested in what's going on.

author: IanB
author_rank: Super Contributor
timestamp: January 26, 2013, 02:05:55 am
content: Well thermodynamics treats energy as two kinds, "heat" and "work".Heat is the obvious thing. If you add heat to something, it tends to make it hotter. If you remove heat from something, it tends to make it cooler. How hot something is is measured as temperature. Heat follows a very simple rule, in that it will tend to flow from hotter things (higher temperature) towards cooler things (lower temperature). No exceptions.Work is a less obvious thing. It requires work to move something against a resistance. For example, to push a piece of furniture across the floor requires you to do work since the furniture resists being pushed. Work can be measured by multiplying the moving force and the distance moved.The clever thing about energy is that heat and work can be related to each other. For instance, if you stir a pot of water vigorously you have to do work to turn the stirrer. But if you do this, the water gets warmer, and the amount it gets warmer by (assuming a perfectly insulated pot), is exactly equivalent to the amount of work you did stirring it.From these fundamental results is the whole of thermodynamics built. (And a few less obvious things too, but that can wait for another day.)

author: TerraHertz
author_rank: Super Contributor
timestamp: January 26, 2013, 02:13:21 am
content: Quote from: FenderBender on January 26, 2013, 12:58:48 amYou're still not quite getting that it's allinteractionof fields and charge carriers. None of the individual actors are cause, and none are purely effects. For instance you say "electric field runs within the wires" but that's not correct. E fields exist between charge carrier differences (or with varying magnetic field). Since a 'wire' is a good conductor, in steady state any E field along it will approach zero. The E fields persist where currentcan'teasily flow. Capacitors, resistors, insulators, space, etc.Consider this case: start with just the battery alone (and let's ignore the chemistry inside.) There's an electric field in the space between and around the battery terminals. The field will have a shape influenced by things in the environment around (such as a metal sheath on the battery for instance.) Charge carriers in things that are conductive will respond to the external field in such a way as to zero the E field in the 'thing'. (This happens in _any_ conductor - solid, liquid, gas, plasma.)Now add lengths of wire to the battery terminals. Nothing else yet. Initially, the wires are imposed across parts of the original E field. Electrons flow in the wires to zero the field along the wires, by distributing enough electrons to counter the field - on the wire. This causes the external field to change shape, since now the wires are effectively extensions of the battery terminals. There would be some brief transient E-M effects due to the moving charges too.If you bring the wire ends close together, you're getting a increasing E field gradient (Volts per Meter) between them, and it's going to take more electrons moved in and out of the wire ends to maintain the zero voltage field along the wires. Are we starting to be reminded of a capacitor yet?You can get an intuitive experience of this interaction of charge flow and E field, with a long, straight fluorescent tube, waved around while standing underneath a 330KV HV transmission line. The tube glows most when held parallel to the field gradient. Can only be seen in near-total darkness. You can map out the field strength around things like trees, fences, etc. Trees have a low enough impedance to zero the E field! So does your body.While doing this, think about the circuits present. You're standing inside a capacitor formed of the Earth (dirt is conductive enough, especially if a bit damp) and the wires above, with an AC high voltage signal applied.Local 'conductors' act to form zones of zero field strength. In the case of the gas in the fluro tube, enough gas atoms ionize to allow a charge separation that's enough to drop the E field along the tube to below the ionization threshold. While emitting UV photons due to ion-electron collisions and thus making the tube phosphor glow.Bear in mind that the three cables of HV lines are three phases, so the wires-to-ground field is a little complex. Best results will be a bit off to one side, where one of the the cables is the dominant field source.

author: Randall W. Lott
author_rank: Regular Contributor
timestamp: January 26, 2013, 03:37:34 am
content: Kirchhoff's current law assumes that the current density doesn't change.You need Vector Calculus to describe things accurately.This is saying that the triple integral of the divergence of a current density vector equals the rate of change of the charge.We can see that KCL is only true when charge doesn't vary with time.

author: FenderBender
author_rank: Super Contributor
timestamp: January 26, 2013, 06:55:00 pm
content: Ah indeed.Now in regards to all of these magnetic and electric fields formed. How is it that on something more complicated like a printed circuit board, we don't see much more interference between traces and components. Are the fields "too small" to interact significantly with other circuits?

author: jaqie
author_rank: Regular Contributor
timestamp: January 26, 2013, 07:34:30 pm
content: Just like a series of waterfalls from the water source to sea level, it's going to go from one level to another. the energy (voltage) is the height of the water, there is no more or less in the start than the end (ignore water vaporization and soaking for the sake of this). Lower energy as it goes through devices, but it's still got some, and there's just as much water at each stage.Put another way, it has nowhere else to go but down.

author: TerraHertz
author_rank: Super Contributor
timestamp: January 27, 2013, 01:46:43 pm
content: Quote from: FenderBender on January 26, 2013, 06:55:00 pmIt depends, on many things. At high enough frequencies, voltages or currents, interference between circuits on PCBs certainlyisa problem. You must have seen electronics where areas of PCBs are enclosed in RF shields, surely? Even in TV sets and such this is done - why do you think the RF tuner is in a metal case? Ever torn down a mobile phone? In those many circuit sections have their own shielding.And then there's instances in which 'interference' between PCB traces actuallyisthe circuit - things like stripline transformers, couplers, circulators, etc.A great deal of the art of circuit board design is knowing when interference effects could be significant, and how to eliminate them. And alas, far too much of my working career involved re-doing designs originally produced by people who had no idea about EMI, grounding problems, ESD effects, ground planes and ground bounce, etc, and so they produced systems that 'mostly worked'.

author: FenderBender
author_rank: Super Contributor
timestamp: January 27, 2013, 02:48:41 pm
content: Yes yes I am aware of the techniques used for HF VHF and beyond.Just curious as to why a very simple low frequency circuit does not induce a considerable amount of interference in other lines. Not sure how to explain it.

author: IanB
author_rank: Super Contributor
timestamp: January 27, 2013, 04:47:45 pm
content: Quote from: FenderBender on January 27, 2013, 02:48:41 pmYou have to consider what "interference" is. For these adjacent tracks to experience interference you have to change their voltages, and to change their voltages you have to inject current into them. Now even though these fields from one track are impinging on adjacent tracks, it is still hard to transfer energy through thin air. It's like trying to topple someone over by touching them with a feather. Oh, you can touch them all right, but can you move them with such a small force? This is what happens at low frequencies, the interaction is too small to matter.At high frequencies the situation is different. It's like multiplying up the feathers so instead of pushing with one feather it's like pushing with thousands or millions of feathers.For another analogy, consider the way that the energy of vibrations increases with higher frequencies. Pick up a cookie jar and move it up and down slowly and the cookies will survive. But now take this same cookie jar and shake it vigorously (i.e. move it up and down very fast) and you will end up with crumbled cookies. High frequency shaking has more energy than low frequency shaking.Don't try to match these analogies too closely with electricity as they are just pictorial in nature, but they should give you an idea.

author: FenderBender
author_rank: Super Contributor
timestamp: January 27, 2013, 05:13:54 pm
content: Excellent Ian. Thank you.

author: ee851
author_rank: Regular Contributor
timestamp: January 27, 2013, 06:03:49 pm
content: Yes, all analogies break down. No analogy is correct in the mathematical sense, except maybe in a limited range. No analogy will ever substitute for even one of Maxwell's integro-differential equatons. But some analogies are helpful in trying to grasp what is happening. Just as all conductors have a break-down voltage, every analogy has a realm of validity and a point at which it breaks down. But the analogy is a form of symbolism. It helps describe something invisible in terms of something more familiar. And I think it is just as helpful for another poster to point out the analogy's "breakdown voltage".