How is it that birds can sit on electrical wires and not get zapped? At least I have never seen it happen before.
If they’re sitting on one wire, there is no potential difference across them, no matter what the voltage of the wire with respect to earth, and so no current flow.
To expand on that answer - they are not completing a circuit. In order to get an electric shock, current needs to pass through your body. If the bird is simply sitting on one wire and not touching anything else, current isn’t going to go through the bird as it has nowhere to go.
Superman’s first appearance back in Action Comics #1 in 1938 covered the subject, on pages 12 and 13.
To further explore this topic, if the preceding statements are true…
I saw a video a few years ago showing maintenance on high-voltage lines. The technicians were in a helicopter, and as they got within 20’ or so of the lines, they held out a pole towards the line, and when they got within about 10’, lightning came off of the lines and hit the pole. They had to clip the pole onto the line to keep from being electrocuted.
I always understood that you needed a complete circuit, so how could current flow to the chopper? It wasn’t just an initial bolt; like you’d expect from a brief change in static charge; it was a constant stream of lightning 10 feet long. The chopper wasn’t anywhere near the ground - these were the really big power lines that were on towers 100’ tall or so.
Electrical energy will only flow to another point where there is a differance of voltage. The greater the voltage differance, the greater the flow.
Given several parallel paths between two points, the greatest flow will always be on the path of lowest impedance, and the lowest flow will be through the route of highest impedance.
The impedance of a wire is very low, often measured millionths of Ohms per metre, or kilometre dependant upon the normal usage ot the wire.
Overhead cables are manufactured to have very low impedance as this reduces losses over large distances.
When a bird perches on a cable, the distance between its legs is very small, often less than an inch, any possible current would have to overcome the natural impedance of the bird, which might be a few tens of Ohms.
The cable between the feet of the bird is in parallel with the circuit that the bird makes, so any possible current has two possible paths, through the wire and through the bird.
The voltage between the feet of the bird depends upon a calculation that incudes both these impedances, and also the includes amount of current travelling down the cable.
The impedance of the cable will be in the order of fractions of a millionth of an Ohm, yet the impedance of the bird is in the order of tens of Ohms, you can see that the actual impedance of the bird is so large compared to that of the cable, by factors of tens or hundreds of millions that any current flowing through the bird is practically insignificant.
Due to the total impedance of the whole cable, it would be impossible to transmit enough energy to make any voltage between the feet of the bird great enough to cause any harm.
Electrical energy can only flow through and object if there is a circular route.(physicists will point out that you can get a flow into on object for a time but once a certain electrical charge has built up that flow will then cease)
If any electrical charge flows into the bird, to actually sustain the flow there needs to way for it to return to the point of origin, we use earth as a point to tie our distribution systems down so this is our point of origin.
There is no route for electrical energy to flow from bird to ground, those cables are put up in the air to insulate them from the ground, it also insulates the bird too.
There are some strange proximity effects that can be had from very high voltage cables, the corona effect etc and these can occasionally be seen around the glass and porcelain insulators on high voltage cables on misty days - or where this is very fine drizzle. Here a charge can actually build up in the air which leaks away to the structure of a supporting pylon for example, and it can show up as a ghostly faint blue glow.
In theory it might be possible for this type of current flow to pass from the cable through the body of the bird, however this flow is extremely small, and it would have to be in very close proximity to an earthed structure. The fact that we don’t see millions of electrocuted birds would confirm this.
Another possible way to recieve an electric shock is for a body to be connected to an extremely high voltage source, but safely insulated, as I mentioned earlier, you can actually build up an excess charge of electrical energy, which then has nowhere to go.
If you had such a charge stored upon your body, and left the vicinity of the high voltage source, and then proceeded to a place of low voltage, such as earth, this excess charge would flow to earth and you could get a nasty shock it would resemble a static shock you get from wearing man-made fibres, however the storage capacity of a bird is so small that not enough current would flow to cause damage this way.
The reason that the bird would not store as much charge as a human is down to body mass, although the reality is that humans are very poor at storing electrical charge compared to specifically designed electrical components.
Anecdotal Evidence:
While waiting for the El (essentially an above ground electric ‘subway’ train) in Chicago I watched a pigeon fly down to go land on the third-rail (the rail that carries the electric power for the train). Seeing as how the rail in not insulated in any way I was thinking I was about to watch the pigeon explode. Needless to say it did not and happily walked up and down the rail with no ill effects. Of course if the pigeon had been able to stand on the rail and touch another rail (not physically possible for the pigeon) it likely would have fried.
Unfortunately the pigeon got run over by an incoming train so there are more hazards to standing on that rail than just electricity (kidjanot…dumb bird took off as the train came in and flew back across the train’s path and got sucked under the train…there was a collective “ewww” and “oh!” from waiting passengers).
Dad bird to kid bird: You’re grounded!
kid bird: PFFZZZTTTBOOOM!
I’ve also seen the video, and have often wondered the same thing. My guess is that it has something to do with either electrostatics (e.g. a large difference in static charge) or capacitance (between the helicopter and the earth).
I think there IS a potential difference between the bird’s legs.
Since it is AC, the potential at any given moment is different accross the length of the cable. But still, that difference would be extremely small. The wavelength for 60 Hz is several kilometers, while the distance between the legs is maybe 1cm.
Further to your point, I remember attending a sea survival course a few years back and being told in the event of a winch coming down towards you from a helicopter to leave it hit the water before grabbing it as you would recieve a hefty shock from the static. It would be interesting to know how high the charge on an ungrounded helicopter would be.
The spark between probe and electric line didn’t occur until about 2’.
Things that move around build up charges so the helicopter is at a greatly different potential than any part of the transmission line. The difference in potential forces charges to flow from the line to the helicoper, or vice versa, until the potential equalizes.
When you walk around you accumulate an electric charge and when you point your finger at your dog’s nose a spark jumps as the charges flow to equalize your two different potentials.
Gasoline tank trucks used to have a chain connected to the chassis that dragged along the ground to keep them from building up a charge. I’m not sure when that precaution was dropped or why. I’ve asked the question here before but got no answer. The first thing that a tanker does before discharging fuel is to connect the truck chassis to a ground.
A complete circuit is required only if a continuous flow of charges is needed. The complete circuit allows a source to maintain a constant potential difference which results in a continuous charge flow.
Despite what has been posted, the OP question still remains unsolved in my mind. I’ve heard this all before, but it fails to answer this: A bird approaching an electric wire is at zero volts, correct? I.E.: Said bird has no electric charge. So, there EXISTS a potential difference - as the bird approaches the wire. At some point, I would expect dielectric breakdown (due to the voltage gradient between wire and in-flight bird) to zap the bird…even before it lands on said wire!
Why is this not the case? I read all previous postings, but I don’t feel this has been adequately answered. Same thing with the gasoline trucks. I have also sought an answer why no chains are needed. And, if it is SOP to ground the truck before unloading…wouldn’t one expect a tremendous spark to jump in the grounding process - from truck to ground (much like the Hindenberg explosion?)
Last, along these lines, I wonder why/how is it permissible to use the chassis of a car, for example, as the terminal point for grounding wires? As I recall from physics, electrons flow from the grounding source - in an attempt to balance the charge, so to speak. (I suppose this assumes there exists some positive charge is drawing electrons from the grounding source. And, if a negative charge exists, the electrons would flow to the ground. Either way, the car chassis is left with an inbalance of electrons, right?) So, say that you are taking electrons from the car’s chassis (which appears to be insulated from the ground by the rubber tires), why doesn’t the chassis - sooner or later - develop a positive charge due to a resulting deficiency in electrons in the chassis?
Obviously, there are pieces of the puzzle missing from what I have been taught. Maybe the SDopers can fill me in to the flaws to what I’ve posted.
- Jinx
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- As noted, you need to really touch at least two different wires or one wire and the ground to get shocked.
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- As noted, you need to really touch at least two different wires or one wire and the ground to get shocked.
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- Also when I was a wee lad (25 or so years ago) I remember how in the Western USA, electrical utility companies had to go out and replace a lot of high-voltage line poles. They switched to a newer cheaper design that carried the wires narrower, and because of that it was found that many California condors (which were an endangered species) were getting electrocuted–the wires on the newer poles were close enough for the birds to contact two wires at once with their wings if they approached or took off at the wrong angle. Maybe someone from that area can elaborate, I am sketchy on the details but I remember them making a big deal about it on the news.
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But the bird doesget zapped. Just like the lineman bonding on to the wire, a very small currrent flows between it and the wire, until the potentials are equalized. In the case of the bird, the current is even smaller, because the relatively small body of the bird has a much lower capacitance than the lineman and his equipment.
I’m just as puzzled as you are about the gasoline tank trucks. My current WAG, until someone who knows comes up with an answer, is that there could be one tire made of a conducting rubber compound that continuously discharges the truck and keeps the voltage low. Such materials exist and are used in shoe soles for those who work in places where hightly combustible materials, like explosives, are handled.
As QED said, a spark does jump from bird to wire just like with your finger and the dog’s nose but the bird’s capacitance is small so the current needed to charge it to the same potential as the wire is small. I don’t have time at the moment to look up the formula for capacitance of an isolated object but for a bird my WAG is in the low picofarads.
As to the car chassis being ground. I think you are being misled by the term “ground.” The chassis is really just the common return conductor and all the the charge that flows into it is returned to the power source, i.e. the batter or alternator, “pushed back” up to a higher potential by the source, flows through the loads and back to the chassis. It thus doesn’t accumulate but just circulates around and around.
I was on a commuter train when the engine broke down. Another train pulled along side on a parallel track no more than 1.5 ft apart (2-ft, max, I WAG). The doors were purposely not aligned so no one would attempt to stretch across directly from one train to the other. They cautioned us NOT to touch both trains at the same time. (I understand, though, the pantograph(?) of one train was lowered as a precaution.)
Anyway, as it was told to me, the trains are not grounded AND (although hard for me to accept) the metal outside of the train IS at the same potential as the overhead lines (give or take). :eek: If this is true, it is hard to believe one can hold onto the metal handrail (integral with the steps and entire metal “shell” of the railcar) while boarding a train - especially from a low platform where this is absolutely necessary…and still be safe. Once again, you are coming from a zero-potential surface to the train’s potential.
If I understand the discussion in this thread, I WAG it would be argued the resistance of the human body is high enough to allow a small current to pass through you to the ground harmlessly? [It is my understanding that the resistance of a person may not be known and/or may vary - explaining why current sometimes travels across the skin’s surface (at some voltages) and other times penetrating deeper within the skin/body (at other voltages)…correct?]
BTW, the capacitance part of these answers really confuses me. This must be a piece of the puzzle I am missing. Is this to suggest people act as the dielectric of a capacitor (never gave it much thought before) when they walk across a rug, for example, until enough voltage builds up to overcome the resistance of the person OR they first touch a door knob? …Not quite sure how the same applies to the birds, though… - Jinx
Oh, hell no! Electric trains absolutely are grounded–through the wheels to the track. The metal body of the cars is kept at ground potential. In the U.S. the catenary wires are anywhere from about 3,000 to 11,000 volts, depending on the rail system. Inside the train, transformers step this down to, typically, 600 volts. Train systems which use a third rail power source operate at 600 volts, because the heavier third rail has a higher current-carrying capacity than the overhead catenary wires. If you walk along the tracks, you can see 1) jumpers connecting adjacent rail sections, and 2) heavy grounding straps connecting the rails to an earth ground every so often.
You’re acting like one of the plates in a capacitor. In actualitiy, the surface of any conductive object has some capacitance, or the ability to store charge. A familiar example of this is the dome atop a Van de Graf generator, which acts to store the charge. The larger the dome, the greater the surface area, and thus, the greater the capacitance. The capacitance of a living thing, like a bird or a person is roughly proportional to its surface area, which is why a lineman has greater capacitance than a pigeon.