Birds on Wires

Cecil's Columns/Staff Reports
1

Oh, Karl, that was the worst description of electricity I’ve ever read from someone who should know better.

A bird on a wire doesn’t get the same kind of charge as one that sits on a Van de Graaff generator. You are confusing static charge with voltage. And to equate charge with potential in the last paragraph sets back the war against ignorance.

The wife is right. Karl’s anecdote about the squirrel proves it. The bird’s resistance to electricity doesn’t change when it touches a grounded wire. The bird has completed a circuit. Those three words are conspicuously absent from the Staff Report, and are the essence of the accurate answer.

2

Hey bug, better check your facts before you criticize somebody else’s answer.

Yes it does. There is only one kind of electric charge. You are probably thinking about the difference between charge and current. Or maybe the difference between direct current and alternating current. Or maybe you’re talking about the difference between static electricity and house current. Either way you are wrong. Charge (measured in volts) is a form of potential energy. It doesn’t matter whether it is created with a Van de Graaff generator or an outdoor power wire.

Well, although the wife is partly write, you are pretty much wrong. There is NOT a full circuit when the bird has one foot on the wire and the other on an earth ground! That would only happen if the bird managed to stretch its feet between the TWO wires. Electrons come out of the negative side of the generator (or transformer), flow down one wire, through the bird to the other wire, which leads to the positive side of the generator. THAT is a full circuit.

If the bird (or squirrel) spans one wire and ground, the electrons are NOT flowing back to the generator; there IS NO completed circuit. The reason the squirrel or bird is fried is exactly what Karl said - the earth can suck up intinite numbers of electrons (or close enough to infinite), and therefore you have a net decrease in the number of electrons in the generator circuit. Electrons are flowing OUT OF the cirucit, not through it.

The only important thing Karl didn’t mention is that the current is AC. The direction of current flow cycles 60 times per second. If it were DC, then there would be one brief burst of current flow - probably more than enough to fry a small-fry - but after a short while the generator circuit would lose enough electrons for that wire to reach earth ground potential. But it’s not DC, it’s AC. Therefore the charge on the wire will flip to positive, and electrons will flow back OUT OF the earth, through the hapless small creature, and back into the wire. With AC, you can sustain a current even though you’re not actually part of the circuit.

It may have been more helpful if Karl had brought up the concept of a circuit explicitly - his answer implied it at a few points. But I’ll give him leeway on that. He was concentrating more on the question of why a bunch of the electricity flowing through the wire doesn’t decide to take a detour and go through the bird’s body. Although Karl didn’t call it by name, it has to do with voltage drop.

Check out the other thread on this subject. There are at least FOUR effects that will cause current flow through the (un-grounded) bird that DON’T require a completed circuit.

3

(BTW, my statment that there is not a full circuit between the wire and earth ground is only true for certain kinds of power distribution wires. With a three-phase system, the circuit is isolated from ground. However, with a single-phase system, they typically do connect one side to earth ground. In that case, the squirrel does complete the circuit.)

4

Thanks for the advice, sford, but I’ve been checking those facts during my entire 17 year long career as an electrical engineer.

Yes you are right. Taken in their strict technical definitions, my words were inaccurate. But I wasn’t writing for a technical audience, and was referring to the difference between the huge static electric charge a bird sitting on a Van de Graaff generator would acquire, and the miniscule capacitive charge a bird on a wire might get due to its capacitance and the presence of a nearby ground. The first bird actually picks up additional electric charge - when it leaves the Van de Graaff generator, it carries the charge with it. That’s why it gets shocked when it lands on your sister. The second bird doesn’t pick up any electric charge, only it’s potential with respect to another wire or ground changes, and then only for as long as it is in contact with the wire. That’s why it can go land on the ground and not feel a thing.

Well right there you make it quite clear you need to check your facts. Charge is measured in Coulombs, and is not energy. It’s a fundamental physical unit. Electric potential (not potential energy) is measured in Volts. And that confusion right there is the same one that Karl made, and why I criticized him.

Again, you are dead wrong. If current flows, a circuit has been completed. It’s called Kirchoff’s Law (actually it’s just just one of them). Current only flows in loops. We call those loops circuits. When you get unintentionally shocked, it’s because you completed a circuit you didn’t know would be there.

That is why the squirrel got shocked. At the other end of the wire, the transformer is referenced to earth ground, so when Mr. squirrel spans the wire and something that’s grounded on his end, he’s completed a circuit, current flows, and squirrel gets fried.

5

sford and Carl seem VERY confused. I wrote the following before reading bughunters response so there is some repeating going on.

There are terrific differences between static electricity and kind used in power circuits and the bird on the wire question has nothing to do static.

Power circuits have a net ZERO charge on them but do have potential differences on each side of the load. The electrons that move through these wires does so through the wire itself and for every proton in the wire, there is one electron. If these lines had a net charge on them, the birds feathers would stand up on end and they would look all flustered. But they seem quite content.

Static electricity (net charge, extra electrons, whatever you want to call it) on the other hand does not reside in the middle of things but distributes along the outer surface of the thing that is charged. A person that touches a Venderthing generator is given electrons from the thing, these electrons prefer to be as far from eachother as possible so they move to the outer surfaces of the body. They even move to the hair and in the interest of getting further away from each other, they pick up the hair and make it stand on end. This person now has a net charge. No current, just charge.

The correct answer to the orginl question is that both our right, the bird has more resistance than the length of wire between his legs so the extreme majority of current passes through the wire with a very small amount of current flowing through the bird. Just not enough to bother it. The potential difference between his little feet must be smaller than 0.01mV. Probably less, I haven’t done any calculations.

If that poor little bird touches something else at a different potential (like the ground or the other wire, or the case of the transformer at the pole), he will likely be subject to a significant potential difference between his warming feet. In your neighborhood, that potential is probably 7200V or so.

Now sford, you must be an electrician’s son. You have heard of a 3-phase system but don’t understand it. In the United States, our power systems are grounded. I don’t care if you have a 4 wire 3phase 208 system without a neutral, you touch one of those phase conductors and you will see that a ground connection will complete a circuit and you will wish that it was just static.

6

Sorry, I don’t think there’s a difference. I’ve seen a 200KV VdG generator, and many high-voltage power lines are much higher than that. It doesn’t matter if the potential is caused by a belt transferring electrons from earth ground to a dome v.s. by a spinning generator and a step-up transformer. If a bird flys off a 200KV VdG generator, it will have the exact same charge as a bird that flys off a power line whose cycle happens to be at 200KV at the moment the bird releases the wire.

The only real difference is that the VdG generator is DC and the power line is AC. If a hundred birds leave an AC power line at random times, they will all have different charges (both positive and negative) depending on where in the AC cycle the wire was when each left.

Oops, you’re right. I need to learn to follow my own advice. What was I thinking of? Hmm … I think that a given charge will lead to a particular voltage that depends on the dialectric and geometry between the charged object and a neutral object. Or, to put it the opposite way, a given voltage will pump a certain amount of charge into an object, depending on the dialectric and geometry between them.

OK, I consider that a bit of a nit. Especially for a non-technical audience. A capacitive circuit doesn’t seem like a circuit to a layperson, especially when the capacitive circuit is unintentional. I.e. no wire involved. But yes, every AC circuit is actually infinitely more complex than the simple schematic indicates because there are stray capacitances and inductive currents, each of which is indeed flowing in a circuit, albeit very non-obvious ones.

Now specifically with regard to earth ground on power lines, I realize that single-phase transformers are wired with respect to earth ground. But aren’t three-phase circuits isolated from ground? If I remember correctly, you don’t have any current flowing through the ground unless there’s a fault. So, I’m assuming that a furry critter frying between hot and earth ground is the capacitive effect. I.e. he becomes the R in a very large RC circuit.

Is that correct about 3-phase?

Anyway, nits and technicalities aside, I still think that Karl answered the question that was asked pretty well. He gave a non-technical explanation of the capactive effect to show why the electrons don’t just keep streaming into the non-gounded bird (although he didn’t mention AC), and he also verified that the low resistance of the wire WRT the bird causes an insignificant (albeit non-zero) current through the non-grounded bird. Adding a lecture of circuits would have increased the breadth of his answer, but not the accuracy.

7

Only because it’s AC. Put a diode between the bird and a 500KV power line and I suspect you would see some flustered falcons. (And if you have a 500KV diode, then I wanna know where I can get one too!)

If there is a voltage potential between two reasonably conductive objects, and you add a conductor between them, electrons will flow. And during the instant of time when a 500KV line is … uh … how do you get peak from rms? When the wire is at negative 500+KV with respect to ground, there is a pretty impressive wad of electrons crowded around the surface of that wire at that point. Just as many as there would be with a VdG generator dome set at that height.

False and true respectively. :slight_smile: I looked up 3 phase and saw that they are indeed grounded, by the center taps. So they aren’t isolated. So I stand corrected - the squirrel is completing a traditional circuit, and it’s not a big capasitor as Karl seemed to imply.

And now you’ve got me curious. Consider a truely isolated case - a radio transmitter driving a balanced dipole antenna. The instantaneous voltage potentials along the transmission line would normally be measured with respect to the two conductors, not to ground. My question - IS there a changing voltage potential with respect to ground?

It’s true that since the transmitter is neither losing nor gaining electrons, the overall charge, and therefore the overall potential is constant. But along the transmission line, you have electrons bunching up and thinning out as the wave travels by. If you measure the potentials instantaneously between one side and ground, I think you would see a voltage.

So, if you touch one side of the twin-lead while standing barefoot on your basement floor, there would indeed be an AC current flow without there being a “traditional” circuit. It would be the capacitive effect with the earth being one plate and the other wire of the twin-lead (plus that wire’s half of the dipole antenna) being the other plate.

And if you’re running your linear, you’re liable to feel it.

8

No - the difference between static electricity and AC is not the same as the difference between DC and AC. billy has a proper grasp of the difference between static electricity and electric current.

The diode alone would make no difference. The bird would still only get “shocked” if it also made contact with another wire of different potential or phase, or to ground.

No, you misunderstand how electrons behave. There is no wad of electrons… just like there are no wads of water moving down a garden hose, like is shown in cartoons. Everything is full of electrons, typically dozens for each atom. Some of them are free to move (conduction band electrons) and some are quite stubborn to it (valence band electrons). In AC and DC circuits the conduction band electrons can be thought to move through conductors very much like water in a full pipe. In the case of AC, they move back and forth.

In something with a static electric charge, electrons have been added or removed from the something, often via some mechanical action like friction. But they still don’t “wad.” Electrons don’t like to be crowded together, and they will seek out any path to redistribute themselves evenly, even traveling into non-conductors. The “pressure” felt by the electrons to spread back out is mathematically the same as a voltage, and so is measured in units of volts. But once the electrons start moving, the voltage drops until they are evenly distributed again.

To get an electric current to flow in a piece of something you need three things: a current source, a current sink, and a potential between them. (What constitutes a sufficient source and sink depends on the potential, and in the case of AC, the frequency.) A bird on a wire has only a current source. There are lots and lots of potential sinks all around him, but he’s not touching one yet. As soon as he does, zot!

9

[QUOTE]
*Originally posted by bughunter *
**

When I said “flustered falcons”, I was referring to Billy’s observation that birds’ feathers don’t stand up on end when they sit on a wire. I suspect that if you had a diode between a 500KV line and a bird, it’s feathers would stand out the same as if he lit on a 500KV VdG generator.

I agree that the diode would not make any difference regarding electricution.

(I’m not even completely sure the diode is necessary, but that’s a different argument.)

As useful as the water analogy is, it does break down when you look at the physics.

Let’s run a wire from the dome of a 500KV VdG generator. I think we agree that electrons will flow into that wire until it reaches electrical equilibrium. Measure the voltage from the wire to ground and you read 500KV. The number of electrons that flowed into the wire to produce that potential depends on the dielecric of the insulator and the geometry of the system. Drape the wire closer to the ground and more electrons will flow into the wire to keep the voltage potential at 500KV. (The same as moving the plates of a capacitor closer together.)

Now put a small load on the end of the wire. It will need to be very high resistance because a VdG generator can’t maintain it’s high voltage when there’s a significant current flow. So now it’s not static electricity any more - you have a current, albeit a small one. And you still can measure 500KV with respect to ground.

How so?

Because the wire STILL has a net negative charge with respect to ground! Yes, the elecrons are flowing off the wire through the load, but they are also being replaced by the generator as quickly as they’re drained. And the voltage you measure between the wire and the ground depends on the same thing - the concentration of electrons, the dielectric of the insulator, and the geometry.

Now replace the VdG generator with a chemical battery. (Boy, I would sure like to see a 500KV battery!) Does this whole discussion change because the electrons are pumped from the earth to the wire due to chemical energy rather than mechanical??? NO! The voltage STILL depends on the number of extra electrons in the wire, the dielectric, and the geometry.

Now replace the chemical battery with a spinning generator. Again, does everything change? NO!

Well ok, it changes some. Because generators and step-up transformers generate AC.

If you have short transmission lines between the transformer and the load, then you can still treat it the same as the DC case, so long as you chop time into iddy-biddy peices. When the cycle is at a point where the voltage is -500KV with respect to ground, the wire will have the SAME OVER-ABUNDANCE OF ELECTRONS IN IT as with the DC case!!! When the voltage swings to +500KV, it will have an equiv under-abundance of electrons. Dielectric and geometry determines how many extra electrons are needed in the wire to result in a given voltage.

So, no wads, right?

Well … once the wires get very long, you have propagation delays. The wires act like AC transmission lines. And YES YOU DO HAVE WADS!!! Sine-wave shaped wads.

It’s a traveling wave, guys. At a given instant of time, you can measure the voltage between the wire and ground all along the length of the line. You will find that the voltage varies according to the distance from the transformer. It varies according to a sine wave. And JUST AS WITH THE STATIC ELECTRIC CASE, the sections of wire that are -500KV with respect to ground will have an over-abundance of electrons, and the areas that are +500KV will have an under-abundance.

So, not a wad like a cartoon fire hose, but a smooth, sine wave-shaped change in the density of free electrons, mostly along the outer skin of the wire.

Mind you, I still agree with your basic premise that birds won’t get fried until you get a completed circuit. But a small “non-frying” amount of current will flow due to capacitence between the bird and ground. The source is the power line, the sink is ground. If it were DC, you would get one brief rush, and equilibrium would be reached. Since it’s AC, the bird gets charged and discharged with each cycle. The amount of charge that flows back and forth depends on the dielectric of the insulator and the geometry. If Big Bird ™ hangs from a 500KV line and is separated from the ground by only a few inches of mica, he’s going to feel pretty hot and bothered.

10

Omigod fellas!

I’d like to see you two with tasers on a Celebrity Deathmatch!

11

bughunter

So suppose I pick up a ball that has one Coulumb of charge. I then begin walking with this ball one meter per second to the North. I have created a current of one Amp to the North, right? So you’re saying that somewhere there is some cuurent of one Amp to the South that I don’t know about? I find that rather hard to believe.

12

Ryan,

I like that question.

I’ll venture a guess and answer it yes, there will be current flowing to the south.

Remember that nature is happiest when all potential energy is at a local minimum. That is why apples fall down, rubberbands snap back, water is level accross the surface of a swimming pool, and electrons distribute themselves to minimize electric fields.

As you walk North, elctrons will be pushed away from you (assuming the ball you are carrying has a negitaive charge) and as you pass, they will fill back in behind you. This filling back in is accomplished by electrons flowing south.

Think about dragging a five gallon bucket across the bottom of the swimming pool, you can move 5 gallons of water from one end to the other but it is obvious that 5 gallons of water moved back to fill in the orginal space where the bucket was.

Not a circuit in the sense that Bughunter was talking about but if current moves one direction, other current moves the other direction.

I give the above answer with far less confidence than my previous answer. I have neglected the magnetic fields produced by walking with a ball full of charge, ignored the dielectric properties of air and used the classical model of electrons as particles.

Still, I think it is mostly correct.

13

Cool, that is a good question.

I’m going to take the opposite position however - I don’t think it is necessarily true that there would be an equiv charge moving South. I see your reasoning, and it is compelling, but I think there is another possibility.

Let’s imagine a Van de Graaff generator in space. Electrons are being pumped from the base to the dome. That counts as a current. But there is no return current … at least not right away. You build up a large potential, like charging a battery (or more accurately, a capasitor). You can then turn off the generator and the potential remains (ignoring leakage). Put a load between the dome and the base and THEN you get the return flow.

Now place the VdG generator firmlly on the ground. Disconnect the dome - that’s your charged ball - and walk North with it. As billy said, I think you would get an equiv current flow in the opposite direction in the earth as the negatively charged ball repelled free electrons in the ground.

BUT, if you instead lifted the ball higher in the air, you won’t get a return current (assuming the air is not ionized). Instead, you would get an increase in potential energy … in this case kinetic. Even ignoring gravity, the ball is attracted to the earth, so by pulling it away from the earth against that attractive force, you’re doing work and storing the energy. Let go of the ball and it will accelerate toward the base (still ignoring gravity). THAT is your return current!

14

OK, I’ve given this some more thought and have decided that me, bughunter, and sford, are all correct, and all wrong.

bughunter and I were correct that what kills the bird is current through the bird and not static electricity. The bird connected from line to line has no net charge, just a potential difference across him.

I was incorrect that for every proton in the wire there is one electron. That is true for the system, not an individual conductor. The individual conductors do carry a net charge that alternates with the the cycle.

I was also incorrect about the necessity of current flow in the opposite direction when moving a ball of charge. The sun is pouring electrons into space without a reverse flow. Positrons as well I think but not in the same numbers so there is still a one directional flow.

sford is incorrect about the example of the vendething generator in space because a vacuum will readily conduct so there will be a current flow from the ball back to the base.

sford is correct that if done in the atmosphere, you can fly around with a charged ball without opposite current flow provided the air is clean enough. On the ground, I am correct I think.

Either way, the bird is dead and both the wife and the husband were correct.

Thanks guys, I learned from all this. Nothing that will help me out in life but I still learned something.

15

Nothing that will help you out in life? You learned not to sit on electric wires, ain’t that somethin’?

16

All this mumbo jumbo and no one mentions that the bird sitting on a power line is receiving a miniscule current?

There is current flow in any power line as there are loads on all power lines (I’d be interested to be in a city where a power line wasn’t being used, unless it is in the process of being strung up).

Where would you like the circuit completed? As it stands, one power line is one “plate” of a capacitor. There is a capacitive leak current between each power line and between each line and the earth. A bird on the line changes the capacitance, but its still there.

Secondly, lets also remember that no conductor is perfect, and so between any to points on the conductor there is a resistance. When a bird straddles a power line, it forms a parallel circuit: the bird is one impedance and the power line segment is another.

If we put enough current through those lines, we could fry a bird. Anyone disagreeing with that?

17

Someone did, in fact, disagree with that. Through (mailto: noWayIWouldPostItOnAMessageBoard@nosyasshole.com), however.

You are correct, current is not the same as voltage. You are not correct in anything else you mentioned. It only takes approximately 60mA to kill a human being. Wonder why they don’t mention voltage? I don’t. Because it doesn’t matter.

Static electric discharges from your feet moving against the carpet generate up to a few thousand volts. If voltage actually killed people, we’d all die every Christmas with the Yanks up north.

18

erislover,

I was out of this topic but since you call my posts mumbojumbo and then complain that no-one mentioned that there is current flowing through the bird, I thought I’d jump back in.

Look at my first post. The one where I say that there is current flowing through the bird.

Who said 1,000,000 amps at 0.001 volts won’t be felt by the bird???
Worry about that after you find a bird that can conduct 1,000,000 amps at 0.001 volts. A 1 nanoOhm bird… We could breed those birds and use them inplace of overhead wires. You still wouldn’t want to park under them though.

19

Yeah, you sure did mention it. Sorry :slight_smile: the mumbojumbo wasn’t meant to be offensive, of course. All the information in here was good stuff.

I have no idea who sent me the email as far as posters go. Might have been a lurker. The email address didn’t ring any bells.

:stuck_out_tongue: if the birds were 1 nOhm imagine the lighting that would shoot out of their ass when they had a movement while on a wire! What a sight to see.