Inductance of the arc - Page 1
Forum section: electronics / beginners

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 03:07:36 pm
content: Does the arc have inductance? If it has then why thunder belt has DC?

author: joeqsmith
author_rank: Super Contributor
timestamp: October 03, 2020, 03:17:19 pm
content: I would need to pull out some books but would imagine when using air for a transmission line, we would have inductance, capacitance and resistance just like with wire. Interesting question. Why do you ask?

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 03:31:28 pm
content: A couple of years ago I read that an arc has no inductance. After that, I looked at oscillogram of lightning current and came too conclusion that there was some point in this statement.

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 03:39:03 pm
content: Iightning chanel is an oscillating loop with capacitance in ground and discharge area in the air. In any case it still has capacitance and with an arc chanel comprises oscillating loop provided it has inductance.

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 03:47:26 pm
content: The only reason it has dc current it has no inductance. Am I wrong?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 03, 2020, 03:54:05 pm
content: If it didn't, it would propagate faster than the speed of light.Tim

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 03:58:05 pm
content: You confuse the rate of current rise with the rate of propagation of the signal.

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 04:02:36 pm
content: The speed of lightning belt depends on processes of forming of the latter

author: IanB
author_rank: Super Contributor
timestamp: October 03, 2020, 04:03:19 pm
content: Quote from: Dlrowdna on October 03, 2020, 03:47:26 pmI am confused why you think DC current can only occur if there is no inductance? DC current occurs in many real world cases where inductance exists, for example in solenoids or DC motors.I believe inductance is that phenomenon where the presence of an electric current induces a magnetic field associated with it. Since there is an electric current in a lightning bolt there is a magnetic field produced by a lightning bolt, therefore there is inductance associated with the arc channel. We can tell this since lightning bolts emit radio waves, and the creation of radio waves requires both magnetic and electric fields to be present.

author: bdunham7
author_rank: Super Contributor
timestamp: October 03, 2020, 04:04:25 pm
content: Quote from: Dlrowdna on October 03, 2020, 03:47:26 pmI'm not sure where there is a connection between the two. First, as alluded to by T3sl4co1l, Maxwell's equations apply everywhere, so 'no inductance' is a nonstarter. But why would inductance preclude a DC current? For lightning, if there's a bunch of extra electrons that all want to go somewhere else, there will be a big, one-way burst of current. Whether you call that 'DC' is a matter of semantics. And for an arc, the current through the arc will largely depend on the potential driving it--if it is high voltage DC, the current will be DC. Plus some noise, of course.

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 04:23:22 pm
content: IanB, How can I be sure that RF waves emitted by lightning bolt? They could come from oscillations on earth too after the discharge. No? It was thoroughly investigated?

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 04:32:27 pm
content: bdunham7, after the discharge potential difference should be not only gone as in any capacitor but also inductance should recharge capacitor in reverse polarity and as a result counter current that we do not observe.

author: bdunham7
author_rank: Super Contributor
timestamp: October 03, 2020, 04:45:49 pm
content: Quote from: Dlrowdna on October 03, 2020, 04:32:27 pmSo you aren't claiming that DC can only occur if there is no inductance, but rather that since an inductor would store some energy and return it to the system as an inductive kick, if we don't observe that kick the inductance must be zero? Obviously the simplest explanation would be that if the inductance is small and the noise is large, you lose it in the noise. Or, the energy in the inductor is dissipated elsewhere. You'd have to calculate what you think the inductance should be and see how much counter current you ought to be seeing and then determine if that amount would be measurable. I have no idea about lightning, the only systems with arcs that I have any knowledge of all have a lot of inductance in the system already--think ignition coils, arc welders, discharge tubes.

author: Gyro
author_rank: Super Contributor
timestamp: October 03, 2020, 05:20:22 pm
content: Surely if the arc didn't have the magnetic field associated with inductance, magnetic blow-outs in AC and DC circuit breakers wouldn't function.

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 05:23:21 pm
content: Good argument, but not the proof.

author: Gyro
author_rank: Super Contributor
timestamp: October 03, 2020, 05:28:15 pm
content: Why not? They function every day, and for the DC ones, they stop working if you remove the permanent magnet.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 03, 2020, 05:34:01 pm
content: Quote from: Dlrowdna on October 03, 2020, 05:23:21 pmIt pretty much is. Inductance is the manefestation of storing energy in magnetic fields. The lightning has to put energy into that field and then the stored energy has to be dissipated. That is inductance.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 03, 2020, 05:38:08 pm
content: That said, inductance is a circuit parameter. Circuit theory is essentially a recasting of maxwell's equations to be more convenient for the types of electronic circuits we like to build. When you are taking about charges moving in free space circuit theory is still mostly right, but it is not necessarily the most convenient way to understand the problem.

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 05:43:40 pm
content: I would believe if the arc oriented magnet field indicator with its magnetic field. But at the same time it is necessary to exclude the influence of the conductor.

author: radiolistener
author_rank: Super Contributor
timestamp: October 03, 2020, 05:50:20 pm
content: There is no DC at arc. It has pulse. Pulse has wide bandwidth. This is why you can listen noise on the receiver from a lightning discharge.As T3sl4co1l mentioned above, missing inductance means that there will be a trouble with wave propagation speed.Any environment has distributed inductance and capacitance. Even vacuum has 1.2566 uH/m.Wave propagation speed can be calculated as:v = 1 / sqrt(L * C)in case of vacuum:L = 1.256637061 [uH/m]C = 8.854187818 [pF/m]so, the wave propagation speed in the vacuum is:v = 1000000000 / sqrt(1.256637061 * 8.854187818) = 299792458 [m/s]if you put L=0, then wave propagation speed will be:v = 1 / sqrt(0) = undefined

author: Gyro
author_rank: Super Contributor
timestamp: October 03, 2020, 05:54:03 pm
content: Quote from: radiolistener on October 03, 2020, 05:50:20 pmSurely there's enough DC to push the arc in a consistent direction? (Maybe I'm being too simplistic)

author: Siwastaja
author_rank: Super Contributor
timestamp: October 03, 2020, 05:58:45 pm
content: DC and AC are meaningless terms in physical discussion because they are based on case-by-case definition of drawing a limit at some arbitrary point in frequency spectrum.Sometimes, looking things in frequency domain makes everything more confusing, as well.You could say that light is definitely AC and the bulk component of the arc current lasting for 1 millisecond is very DC-y, but OTOH, you could also say it's not DC at all because it's so short.Do note that inductance and capacitance are also quite high-level circuit theory simplifications. Things like "free space has inductance" just have to be accepted because,by definition, it is so. Otherwise inductace couldn't be defined the way it is, and it wouldn't be very helpful in circuit design. For proof, you just reiterate the definition of inductance until happy; if still unhappy, you need to go beyond inductance. Does Maxwell satisfy you? It's still a simplified model; you can always go deeper.

author: radiolistener
author_rank: Super Contributor
timestamp: October 03, 2020, 06:01:37 pm
content: Quote from: Gyro on October 03, 2020, 05:54:03 pmlightning discharge current is a pulse. Usually it consists a series of pulses in alternate direction, but they are very short pulses, not DC.

author: Gyro
author_rank: Super Contributor
timestamp: October 03, 2020, 06:05:53 pm
content: Sorry, I was meaning the DC breaker example.

author: radiolistener
author_rank: Super Contributor
timestamp: October 03, 2020, 06:27:51 pm
content: Quote from: Gyro on October 03, 2020, 06:05:53 pmthe same thing with DC breaker, because it raise rapid resistance change in the circuit. In such condition DC current is impossible.I'm not sure about exact value of distributed inductance in the air plasma, but I think the plasma has reactive impedance because it consists of a charge particles which needs some delay between consume and emitting EM wave.

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 06:37:09 pm
content: radiolistener, I thought it will not be so difficult to understand what I meant by DC. I meant unidirectional current.

author: IanB
author_rank: Super Contributor
timestamp: October 03, 2020, 06:42:38 pm
content: Quote from: radiolistener on October 03, 2020, 05:50:20 pmStrictly speaking, this for a spark or transient discharge, not for all arcs.In the case of a carbon arc lamp you have a continuous DC arc with a steady current. The same would be true for arc welding.

author: Gyro
author_rank: Super Contributor
timestamp: October 03, 2020, 06:44:28 pm
content: ... or a Mercury arc rectifier.

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 06:48:06 pm
content: Quote from: radiolistener on October 03, 2020, 06:01:37 pmCan you prove it? I would like to see oscillogram.

author: Gyro
author_rank: Super Contributor
timestamp: October 03, 2020, 07:06:25 pm
content: I'm still confused. An arc is a conductive path of ionised plasma. All conductors generate a magnetic field when a current is passed through them, therefore they all exhibit inductance.

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 07:08:53 pm
content: From that oscillogram I can conclude that discharge current is unidirectional i.e. short DC current. I can not see here reverse current or any sign of AC that should take place in case of oscillating loop.

author: radiolistener
author_rank: Super Contributor
timestamp: October 03, 2020, 07:12:29 pm
content: Quote from: Dlrowdna on October 03, 2020, 06:48:06 pm

author: Gyro
author_rank: Super Contributor
timestamp: October 03, 2020, 07:19:00 pm
content: What question are we trying to answer here? Does an arc have inductance? ...or is a lightning stroke a series of (ok, short) DC arcs of alternating polarity?

author: radiolistener
author_rank: Super Contributor
timestamp: October 03, 2020, 07:20:19 pm
content: Quote from: Dlrowdna on October 03, 2020, 07:08:53 pmI'm not familiar with lighting discharges, but I see negative and positive pulses on your chart

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 07:23:35 pm
content: radiolistener, This is not oscillogram. The general behavior of the lightning current is not clear.

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 07:28:02 pm
content: radiolistener, On my chart??? I can see here only rise and fall of the current. Its unidirectional.

author: radiolistener
author_rank: Super Contributor
timestamp: October 03, 2020, 07:43:47 pm
content: Quote from: Dlrowdna on October 03, 2020, 07:28:02 pmOn your chart there is a short pulse below zero, about -2 kA. But anyway if you exclude that, as you mentioned your chart consist of two parts: rise and fall. That's AC behavior. It doesn't matter if it has some DC offset, but it consists of AC pulses.

author: radiolistener
author_rank: Super Contributor
timestamp: October 03, 2020, 07:50:42 pm
content: DC means flat line with no rise and no fall. Any rise or fall means AC.Regarding to bidirectional or unidirectional current flow, it depends on absolute value for zero. Just put zero value at 10 kA on your chart and you will see bidirectional AC current

author: chris_leyson
author_rank: Super Contributor
timestamp: October 03, 2020, 07:54:32 pm
content: Say the height of the cloud base is between 1km and 2km say 1.5km average and the diameter of the plasma is 1inchhttps://en.wikipedia.org/wiki/Lightning_strikeThishttp://www.consultrsr.net/resources/eis/induct5.htmgives 3.5mH.Attached pdf might be worth read. Seems return stroke is more or less a single short pulse. If the wavelength is typically 1.5km than that is why you will hear lightning strikes on LW band. There was an article in Wireless World on tracking storms, around 1970 or so.

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 08:01:08 pm
content: radiolistener, "Alternating current (AC) is an electric current which periodicallyreverses directionand changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction." The negative impulse at the very beginning has nothing to do with the oscillatory process in the loop. Reverse current should appear after full discharge and recharge of the capacitor earth- air area. There is no sign of it.

author: radiolistener
author_rank: Super Contributor
timestamp: October 03, 2020, 08:10:38 pm
content: Quote from: Dlrowdna on October 03, 2020, 08:01:08 pmyes, and your oscillogram of the lightning discharge is a good example of AC current where current flows in reverse direction.DC current means absolutely flat horizontal line. It doesn't matter at what absolute level.If current has stable value with no change, then this is DC current. If current has any change in time, then this is AC current.

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 08:12:48 pm
content: cris_leyson, You mean that inductance has a small contribution because of its small value? Then why in my oscillatory circuit consisting of a coil of 1 microHenry the oscillations are very stable as in a normal coil?

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 08:18:47 pm
content: radiolistener, negative current is below zero.

author: Gyro
author_rank: Super Contributor
timestamp: October 03, 2020, 08:20:19 pm
content: Quote from: radiolistener on October 03, 2020, 07:50:42 pmOver what timescale? Is a battery discharge curve AC?Alternating current by its very definition requires current reversal.

author: radiolistener
author_rank: Super Contributor
timestamp: October 03, 2020, 08:29:38 pm
content: Quote from: Gyro on October 03, 2020, 08:20:19 pmover measurement period of time.For example with lightning oscillogram - over period from 0.7626 S till 0.7633 S.Quote from: Gyro on October 03, 2020, 08:20:19 pmyes, current reversal are crossing mean value of AC, it happens due to rise or fall edges on the oscillogram.Zero value for AC current is a mean current value (average value) over measurement period of time.As you can see, if there is any kind of current change (rise or fall edge), then current will always cross the mean value level. So, it means that any change of current means that there is AC current.If AC mean value doesn't fit with absolute zero value, then it means that AC current also has some DC offset.So, strictly speaking, there are both types of current are present simultaneously - AC and DC.

author: chris_leyson
author_rank: Super Contributor
timestamp: October 03, 2020, 08:50:19 pm
content: My calculation was just a ball park estimate of the arc inductance assuming L=1.5km and d=2.5cm, about 3.5mH. In reality the arc length will be longer because they don't always travel in straight lines. Lightning is complicated and not fully understood and not something you can model with simple LRC circuits. There will be a small oscillatory, AC, current generated at 100s of kHz if you think of the arc as being a 1.5km line excited with a high current DC pulse, it is a very lossy and non linear line and don't forget damping. There is a frequency plot in the pdf I attached.

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 08:53:13 pm
content: radiolistener, may be you confusing AC with pulsating current? I understood your point but I meant current in which take place change in direcion of charges. You dont have that type of AC in oscillating loop at least in the first harmonic.

author: radiolistener
author_rank: Super Contributor
timestamp: October 03, 2020, 08:54:59 pm
content: Quote from: chris_leyson on October 03, 2020, 08:50:19 pmin reality we don't know plasma conductivity and permeability, and we don't know exact lighting path, so we don't know if it has inductance or capacitance. But since it has reactive impedance (due to charged particles) we can assume that it's reactance depends on the frequency.So, at some frequency lightning may have inductive reactance and at the same time, on different firequency it may have capacitive reactance. And at some resonant frequencies it works like usual resisor

author: Gyro
author_rank: Super Contributor
timestamp: October 03, 2020, 09:01:56 pm
content: Quote from: radiolistener on October 03, 2020, 08:29:38 pmWhat I see is a capacitive discharge waveform there, with some inductance thrown in, accounting for the oscillation on the fast leading edge. If the source was a 'battery' (for want of a better term), rather than a capacitor, you would not see the decay curve just a steady level. I don't think you can claim that current is positive or negative based on the mean value. That would imply a steady state 10kA DC is flowing either side of the discharge, which is clearly impossible.You seem to be thinking of this in AC (capacitively) coupled [edit: measurement] terms rather than varying levels of DC current, starting and ending at zero.

author: radiolistener
author_rank: Super Contributor
timestamp: October 03, 2020, 09:05:30 pm
content: Quote from: Dlrowdna on October 03, 2020, 08:53:13 pmFor nature there is no difference between "pulsating" and "alternating" (if both have the same waveform). From one inertial system you will get "pulsating", from another inertial system you will get "alternating". In your example with lightning oscillogram it just depends on the point against which you're performing measurement. But actually this is the same thing - this is AC. Your "pulsating" example is just a sum of DC offset with AC current, so technically this is AC+DC.This is why AC current is measured against "mean" value.

author: radiolistener
author_rank: Super Contributor
timestamp: October 03, 2020, 09:22:23 pm
content: Quote from: Gyro on October 03, 2020, 09:01:56 pmif you put that current through a high voltage capacitor (to remove DC offset component), you will get current in both directions.Quote from: Gyro on October 03, 2020, 09:01:56 pmBut all nature things are AC. The DC current is actually virtual, because there is no DC current in the nature. You can catch DC value only over a limited period of time.And if we look closely at the oscillogram, AC current is definitely present on the displayed time period, so we cannot say that this is DC only. This is AC waveform with some DC offset.

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 09:35:02 pm
content: radiolistener, I thought that officially accepted term for AC implies the movement of charges in different directions. Probably wikipedia is lying again:"Alternating current (AC) is an electric current which periodicallyreversesdirection and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction."Etymology of reverse:C14 from Old French, from Latin reversus, from revertereto turn back

author: Gyro
author_rank: Super Contributor
timestamp: October 03, 2020, 09:38:48 pm
content: Quote from: radiolistener on October 03, 2020, 09:22:23 pmSure, that's what I meant when I said you were thinking in AC coupled terms. I just don't understand where you got your high voltage capacitor from. There is clearly a Capacitive discharge going on, but you seem to have introduced another (coupling) one into the measurement, that doesn't exist.P.S. Sorry, I find myself in the following situation:

author: radiolistener
author_rank: Super Contributor
timestamp: October 03, 2020, 09:46:23 pm
content: Quote from: Dlrowdna on October 03, 2020, 09:35:02 pmyour thoughts are correct and wikipedia don't lying. But when you're talking about movement of particle, then there is some inertial system in which you're performing your measurement. And zero point of this inertial system is an average position of all particles over measurement time period.And if you perform average position of all electrons in the current flow over your oscillogram measurement time, you will get mean value. Which is not 0 kA.Just look. When you measure your battery with voltmeter, you're apply your probes between (+) and (-) terminals of the battery, but not between your mains ground and (+) or your mains phase and (-).The same thing is here. If you're performing measurement of current which is changing over the time, you're needs to get average current, to get zero value. Then you can say what is AC amplitude of this current. And then you can compare this average current with another current to say what is DC offset between these currents.

author: Dlrowdna
author_rank: Contributor
timestamp: October 03, 2020, 10:04:53 pm
content: We loosing point of the topic. I thought you can understand obvious things. They took that oscillogram in inertial system of the earth and it shows that current of lightning is unidirectional.

author: radiolistener
author_rank: Super Contributor
timestamp: October 03, 2020, 10:18:24 pm
content: Quote from: Gyro on October 03, 2020, 09:38:48 pmIf there is capacitor, then there is capacitance between capacitor plates. And when discharge happens, this capacitance don't disappears. It just turns into some kind of RLC circuit, where plasma channel plays R and L components role. So, technically we will get some kind of a large resonant antenna with a pulse of current.So, this resonant antenna has some zero point at plasma channel. And if you put your ammeter at that point, you will see that the current flows in both direction and has reverse polarity over time. Because plasma channel can be seen as a long wire. Isn't it?Quote from: Dlrowdna on October 03, 2020, 10:04:53 pmEven if you perform measurement relative to the average Earth potential, there should be some amount of reverse current polarity due to reflected wave in the plasma channel. Isn't it?

author: IanB
author_rank: Super Contributor
timestamp: October 03, 2020, 10:35:51 pm
content: Quote from: Dlrowdna on October 03, 2020, 09:35:02 pmWhile this is true, this is an idealized special case used in certain kinds of analysis, such as when dealing with periodic wave forms like what comes out of the power socket in your wall.In this case, it is frankly irrelevant, as lightning strikes do not come out of your power sockets (in ideal circumstances, anyway).What is relevant in the general case, as distinct from certain ideal, special cases, is the time-varying behavior of current, di/dt.Clearly, lighting strikes have very large values of di/dt, so they will manifest all the related phenomena, such as the generation and emission of EM waves in the RF spectrum.

author: radiolistener
author_rank: Super Contributor
timestamp: October 03, 2020, 11:03:54 pm
content: Here is current waveform of different type lightning.It looks they all have bipolar pulses of current in both direction.But different type of lightning has different waveform.And it looks that oscillogram of lightning posted earlier is a type 3b, so it just missing reverse polarity pulse, because it happens later, outside of oscillogram time period.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 04, 2020, 07:00:30 am
content: Quote from: Dlrowdna on October 03, 2020, 07:08:53 pmThe problem is, you are having one definition for terms "DC" and "AC", others may have different. In both science and engineering, it's very common to call components near 0Hz "DC" and components after some arbitrary f>0Hz point "AC".If you mean unidirectional current, why not say "unidirectional current"? We can copy-paste different definitions for AC and DC until the cows come home, but the problem remains, they can mean practically anything. It's completely fruitless to argue about the semantics. Use clear language instead.Besides, the very oscillogram you, yourself, posted, shows bidirectional current with the initial opposite spike. Also, why is the unipolarity/bipolarity of the current important regarding the inductance of the arc?

author: Dlrowdna
author_rank: Contributor
timestamp: October 04, 2020, 10:38:20 am
content: I give up

author: radiolistener
author_rank: Super Contributor
timestamp: October 04, 2020, 12:11:23 pm
content: Dlrowdna, this is pretty obvious that lightning plasma channel has inductance, the same as usual conductors such as piece of wire. The reason for that is because plasma channel consists of charged particles, so when they accelerating or decelerating, they affecting each other.

author: Dlrowdna
author_rank: Contributor
timestamp: October 04, 2020, 01:42:20 pm
content: Are you 100% sure about that? So far I see that the oscillogram says the opposite.

author: BravoV
author_rank: Super Contributor
timestamp: October 04, 2020, 02:23:35 pm
content: Quote from: Dlrowdna on October 04, 2020, 10:38:20 amGive up on what ?Are you just trying very hard to have people agree on what you're theorizing ?As you have no proof yourself, as you like to say to other people .. "where is the proof ?" .. this is DEFINITELY a PROOF that the discussion is going no where, don't you agree ? As I have real proof in front of you.

author: Dlrowdna
author_rank: Contributor
timestamp: October 04, 2020, 03:01:17 pm
content: I'm not worth even fingernail of Tesla, but now I've understood what kind of dumbs he had to deal with.

author: radiolistener
author_rank: Super Contributor
timestamp: October 04, 2020, 03:13:04 pm
content: Quote from: Dlrowdna on October 04, 2020, 01:42:20 pmYes, I'm sure. Otherwise it will break basic physics laws.Quote from: Dlrowdna on October 04, 2020, 01:42:20 pmIt doesn't says the opposite.Quote from: Dlrowdna on October 04, 2020, 03:01:17 pmNow it's more clear why you think that lightning has no inductanceIf you're a fan of a Free Energy then you're chosen wrong forum section.Free Energy is discussed here:https://www.eevblog.com/forum/dodgy-technology/

author: Dlrowdna
author_rank: Contributor
timestamp: October 04, 2020, 03:53:39 pm
content: There was not a single word about free energy in my comments. Is it so accepted now? Everything that refers to Tesla automatically classified as pseudoscience? Then why are you using alternating current? Go to use constant one.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 04, 2020, 04:56:59 pm
content: Ah yep, crossed definitions and woo for the "dumbs". I'm not surprised to see I was right to leave this thread alone... I'm rather more surprised that others have taken it to three pages.Tim

author: joeqsmith
author_rank: Super Contributor
timestamp: October 04, 2020, 04:58:27 pm
content: Quote from: Dlrowdna on October 03, 2020, 03:31:28 pmCould you provide the source? Was it in a physics book, or just something you found on the internet? I would like to read it if you are able to find it.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 04, 2020, 05:45:45 pm
content: Quote from: T3sl4co1l on October 04, 2020, 04:56:59 pmTo be fair, the possibility of him being actually serious and not a troll wasn't 100% evident from the beginning. I like to give the benefit of doubt.

author: Zero999
author_rank: Super Contributor
timestamp: October 04, 2020, 06:04:47 pm
content: I don't think the OP is a troll, just a little misguided.A couple of points:DC and AC are abstract concepts. When a current or voltage suddenly changes, it's not DC, but AC. The instant when a circuit is connected to a battery, AC properties of the circuit: inductance, capacitance and transmission liner effects play a part. Once the current has settled to a steady state, assuming the load is purely resistive, it can be treated as DC. In many applications, the AC properties don't matter, because they last for a short length of time, compared to the operation of the circuit i.e. when an LED is connected to a battery, via a resistor, the fact it takes a few tens of nanoseconds for the current stabilise is immaterial.In a way, inductance and capacitance are properties of space and time and how it interacts with electromagnetic/electric fields and charged particles. They are a side effect of not being able to travel faster than the speed of light. Everything has inductance and capacitance. There's no way of getting away from it.

author: Dlrowdna
author_rank: Contributor
timestamp: October 04, 2020, 06:45:47 pm
content: Yes i am a troll. I've made a lot of money on you and now I'm flying to Miami to get my sunbath. You better explain why the lightning current flows only in one direction. Clung to the f...g definition of AC. This was not the essence of the topic. You yourself are more like trolls trying to divert the topic from the essence.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 04, 2020, 07:07:05 pm
content: I mean, you're welcome to model the dynamics of the cloud-spark-ground system. (Good luck finding a bulk-equivalent model of charged, ionizing air?)Evidently, resistance dominates, or something approximating it, so that the inductance of the conductive channel isn't dominant.Play with values here to see:https://www.falstad.com/circuit/e-lrc.htmlThink of the act of holding the switch down, being the charging up of clouds. When the switch opens, the strike goes off. When R > sqrt(L/C), the system is well damped, and little or no reversal occurs.Mechanisms for resistance in the atmosphere include voltage drop of the arc itself, the effect of the arc propagating through the clouds (so that if zoomed in real close, the resistance moves in a stepwise fashion as sparks connect to bits of charge here and there; but averaged over millions of those sparks, the effective resistance is substantial), and radiation (the lightning bolt is a huge antenna and emits RF energy over a huge range of frequencies -- yes, AC components within a positive hump of current flow).Resistance being dominant, does not mean inductance is zero. The strike still takes time to start up. The leaders travel at some fraction of the speed of light. The strike emits huge amounts of radio waves. All of these would be impossible if inductance were zero.Tim

author: IanB
author_rank: Super Contributor
timestamp: October 04, 2020, 07:21:03 pm
content: Quote from: Zero999 on October 04, 2020, 06:04:47 pmThis use of terminology is, IHMO, a problem that will cause confusion to many learners. AC is properly applied to periodic, cyclic wave forms that are symmetric around some zero point, where certain simplified analytical tools can be applied.Trying to use "AC" in a didactic context for a situation where currents or voltages may not be periodic or symmetric is just going to lead to trouble.It is better instead to avoid such loaded terms as DC and AC, and instead talk about time domain analysis, rates of change, spectral analysis, and so forth. Then it will be possible to get into specific mathematical and theoretical discussions for each individual case.Nobody in this thread has mentioned the basic relationship that defines inductance,v=Ldi/dtand especially that in this equation there is no direct mention of frequency, periodicity or alternation. Cyclic wave forms, or reversal of current or voltage simply are not defining properties of inductance.This is evident from the question given by the OP, who seemingly wants to say that if there is inductance there is necessarily AC, and if there is no AC there is no inductance. This is wrong on many levels, but the wrongness comes from misapplying terms like AC and DC and trying to infer things that cannot be inferred.[Edit: Related to what Tim said above: Whether there is oscillation in a system containing inductance depends on the damping present in the system. If the system is over-damped then no oscillation will occur.]

author: Dlrowdna
author_rank: Contributor
timestamp: October 05, 2020, 05:24:40 pm
content: Let's use another term "periodical current" or "sinusoidal current" then if it is very important for you to understand the process. I do not believe that the reason due to fading on active component of the chanel or due to loss on radiation. The small reverse current still should be like I marked with blue on enlarged timescale. Even the form of discharge shows that it has no sign of periodical current.Even with a huge resistance, the inductance must partially recharge the capacitor. A channel heated to high temperatures will remain a good conductor for a long time for reverse current.

author: radiolistener
author_rank: Super Contributor
timestamp: October 06, 2020, 12:41:20 am
content: Quote from: Dlrowdna on October 05, 2020, 05:24:40 pm3 km 5 cm straight wire give about 7 mH inductance, so I put that into simulator and by playing with voltage and resistance got current pulse about 25 kA, the same as on your lightning waveform.Do you see any negative current? I don't see itI'm not sure that your lightning has exactly the same L=7 mH, R=744 kOhm and U=20 GV, because lightning is not straight wire. May be your lightning has a little different parameters, but at a glance it looks very similar. Isn't it?

author: Dlrowdna
author_rank: Contributor
timestamp: October 06, 2020, 01:52:15 am
content: Ok. May be that argument was not good one. Too big active resistance. I accept.