Why don't birds get shocked on power lines?

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You modeled the feet and the head as separate capacitor plates, I was not expecting that, how I imagined it is since current would enter the bird at one point, pass trouigh its body, and exit somewhere when it is in a circuit, since it only has an entry when it sits on the power line the electrons would just be pumped and compressed in the body, like the whole bird would have an excess charge, not just the feet.

I also would not model the bird as two plates. To do that you would need to say that the rest of the bird is a dielectric, but birds conduct reasonably well (chicken conductivity is used reliably in slaughterhouses)

Maybe this schematic is clearer. Capacitor C2 is the air gap between feet and power line before landing. After touchdown, it is just an ohmic contact. The only importance of C2 is that its air gap is the location of the electric arc. My thought was the bird may be more sensitive to a current in the form of an electric arc during landing, than to the same current spread out over a larger contact area after landing. There is no dielectric in the model, only air and conductors.

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BTW, a convenient guinea pig model for the bird on the power line, with its capacitive current, is a person on insulating shoes, touching a neon screwdriver test light. The current that causes the neon lamp to light, is equal to the capacitive current flowing through the person's body. Without the person the current is zero. The self capacitance of a somewhat spherical person having a diameter of ~1 meter is C = 2πεD = 100 pF, corresponding at 50-60 Hz to an impedance of 30 MΩ. An experimental confirmation for this calculated impedance value is obtained by inserting a resistor between the finger and the screwdriver. A 10 MΩ resistor does not lower the brightness of the lamp, whereas a 100 MΩ resistor does.

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  • #28
Mister T
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Third, if you put a light bulb in this circuit, it should light light up, right?

If the light bulb is sensitive enough. Likewise current flows through the bird.

If the bird is modeled as a point, no current flows through it. That is the usual intent of this very common question.
 
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I am no expert on electricity, but I have seen credible reports of large birds getting killed when their wings touch 2 wires on take-off. Hawks and ospreys love to perch and to build nests on tall electric towers. Mostly they are OK, but one occasionally gets fried.
 
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However, big birds have another problem. I was at a stop light and some Canada Geese were taking off almost vertically up through a utility pole carrying probably 10kV AC distribution lines. One of the birds miscalculated and its large wing span caught two of the three conductors. It didn't end well. In this case we weren't talking about static charges, but full blown short circuit across a goose. To say, this "goose was cooked" was an understatement. It was shocking to look at (pun intended), and it did disturb me the rest of the day.
 
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  • #31
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Into the lions den! Lordie, what a contentious issue! :nb)
So, why are birds not shocked ? Errr.. they can be... with very high voltages...
Well, at the risk of starting a war here, I will offer you another slant on this matter that if I am correct, wasn't even mentioned by anyone else! And that is, that although the various models discussing capacitors and the pushing and pulling of electrons may be an aspect of the situation, considering our birds/helicopters as capacitors may be relatively minor to the question of 'being shocked'.
Yes?:rolleyes:
Errr... yes. And what is the major factor?
Athmospheric Voltage Gradient... if the term doesn't upset somebody! :biggrin:
What's that all about? Well, when we are talking about massive voltages, it's not longer a case of 'in the conductor we have a voltage, and outside of the conductor, we don't'. With a few volts on our conductor, no big deal. However, when we have tens/hundreds of thousand of volts, we may have a voltage gradient created that may be of the order of 25kV per inch! That means that even if we come close to such a conductor without even touching it, we (and birds!) may not only be exposed to a high voltage gradient, but if the air is not especially dry (ie, damp, and so conductive!) such conducting air may be capable of providing enough -current- flow to be felt to a lesser or greater level!
"But what about -thousand of volts- of Static Electricity???" I can hear someone scream!
Well, that's the thing about 'static electricity'. It's 'harmless', not because it's 'a low voltage', but because it's being created in a scenario that is not capable of providing much -current-. After all, -power- (ie, that which can actually -do- something!) is the -multiple- of Voltage with Current - and voltage without current doesn't do much at all!
So, a power line is most definitely capable of providing a high current, (maybe that's why they cell 'em -Power- lines!:biggrin:) and when we/bird are in the middle of a high voltage gradient, a current may most definitely flow, and it's that current (not just voltage) that is a measure of what can be felt, and may harm us too!
As for -where- that flow may be -to-, the answer is, the environment/earth in the form of a Coronal Discharge.
Just to elaborate... (waffle some more?:cool:) it is interesting to see the pic in another post of a guy in a hoodie holding a sparking rod... but there is no explanatory caption... :confused:
So, I'll try to fill in a little. A hoodie...? Er, not really. A Faraday Suit! Eh? Like a Faraday Cage, except personal!
What is going on? Well, the guy is approaching a live high tension line, probably 'cos he has to work on it. Why hasn't it been shut down? Probably 'cos that would mean shutting down an entire area of the countryside. And anyway, with the right kit, he doesn't have to!
So, as he approaches the line, he reaches out with a conducting rod that is connected to his suit. This will bring -the suit's- voltage (and that of the guy inside!) up to that of the power line, (without any voltage gradients across any part of his body) and as we can see, (from the spark!) -massive- current is being passed! Not just because of the guy in the suit is acting as a capacitor, but because the power is passing into the suit, and then discharged into the environment -from the suit-, and -not- through/from the guy himself.
As for 'feeling it'... (ie, feeling the voltage gradient I mentioned above) I spoke to a friend who did this kind of work. And being an adventurous kind of guy, he said yes, he'd wondered about this too... and he decided to conduct a lill' experiment. Yes? He wondered what would happen if he tried -removing- his suit while connected up to a -very- high voltage line! 😁
And what happened? Well, he said that as he began to open up his 'hoodie', (no rush!) he began to 'feel the electricity in the air'! And after he got the hood of the suit less than halfway off his head, he could stand it no more, and promptly put it back on!
Shocking stuff!
 
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  • #32
Dale
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That voltage gradient is a result of the capacitance of the line, and the current is still the charging of a capacitor. You wrote as though you think what you described is a big controversial surprise, but it is what we have been discussing.
 
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Yes, in my diagram the helicopter can be replaced by the bird just before touching the wire. The part of the bird closest to the power line, its feet, is one plate of capacitor C2, and the part that is furthest from the power line, say its head, is one plate of capacitor C2. When AC current is flowing through the bird's body, the excess charge is jumping between its head and feet. A little later, when the spark develops or when the bird is touching the wire, C2 is shorted out, and the excess charge is jumping between the bird's head and the "earth". The excess charge equals Q = CV = 150 kV · 10 pF = 1.5 μC = 10^13 electrons.

Yes, the size of the bird matters, smaller birds experience smaller currents, whereas larger birds and helicopters experience larger currents.
I know birds are small, but it's my understand that electricity takes the shortest route, which in the bird's case won't take it through the head, but from foot to foot. That much power would probably just fry the whole body, but still. When I nearly electrocuted myself I had my hand in a socket, and power flowed between just the last two fingers. I know this because you can still see entrance and exit wounds marked by the skin graft that followed. The rest of me didn't like that at all, of course. I was just a dumb preschooler, rooted to the spot, muscles frozen. A cousin tackled me to safety. My entire body was affected, but had I put both hands in travel would have been between them, likely stopping my heart along the way, but still not passing through the head.
 
  • #34
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Don't see birds on extra high voltage lines because of high density leakage and capacitive currents. EHV leakage currents would be unbearable for the birds.
I once watched a beautiful Golden Eagle land with a wing crossed over a transmission line and its other wing on the spreader. The eagle immediately combusted and caused enough damage that PG&E had to look at the tower from one of its helicopters.

It was always my understanding that birds do not exhibit enough of a path through their two legs for a current to leak off the less resisted transmission line.

I still carry the scar from leaning against a high voltage distribution line while climbing a tree when I was 10 years old. Simply being in the tree gave enough of a circuit through my arm to create a nasty burn.
 
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Electricity works in mysterious ways.
 
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  • #36
davenn
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Electricity works in mysterious ways.

that's a poor misquote

in fact electricity is pretty well understood
 
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  • #37
davenn
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. This will bring -the suit's- voltage (and that of the guy inside!) up to that of the power line

(NO)

if it were the case, there would be no point wearing the suit
 
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that's a poor misquote

in fact electricity is pretty well understood

There's a huge difference between 'pretty well' and 'perfectly'.
 
  • #39
davenn
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There's a huge difference between 'pretty well' and 'perfectly'.

it's well enough understood to totally negate your comment and has been for a long time
 
  • #40
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(NO)

if it were the case, there would be no point wearing the suit

(YES)

The point of the suit is not to somehow 'stop the occupant's voltage from becoming that of the line'... it's to prevent voltage -gradients- from being created across any part of his body. What 'does the damage' is not the voltage per se... it's the current that flows solely 'cos a gradient permits it to do so. No gradient, no current... or is that (NO) also?:cool:

PS
If (NO), you forgot to explain what IS the point of wearing the suit...
 
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  • #41
davenn
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If (NO), you forgot to explain what IS the point of wearing the suit...


Unbelievable seriously !!
It is as you stated but maybe you don't believe ... an example of a Faraday shield
The electric field DOES NOT penetrate the suit .... skin effect ensure that

There's a number of www sites that will give you that answer :smile:

you continue to contradict yourself .....

The point of the suit is not to somehow 'stop the occupant's voltage from becoming that of the line'... it's to prevent voltage -gradients- from being created across any part of his body.
 
  • #42
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Unbelievable seriously !!
It is as you stated but maybe you don't believe ... an example of a Faraday shield
The electric field DOES NOT penetrate the suit .... skin effect ensure that

There's a number of www sites that will give you that answer :smile:

you continue to contradict yourself .....
I never said it 'penetrated' anything. Believably! Seriously! Read it again - I said it prevents voltage -gradients- from being created across any part of his body.

PS - Any more examples of me 'continuing to contradict' myself?... maybe starting with one example of where I did actually contradict myself... :biggrin:
 
  • #43
davenn
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I never said it 'penetrated' anything. Believably! Seriously! Read it again - I said it prevents voltage -gradients- from being created across any part of his body.

So, as he approaches the line, he reaches out with a conducting rod that is connected to his suit. This will bring -the suit's- voltage (and that of the guy inside!)

Yes you did say it ...... there is the contradiction !

to bring the guy's body inside up to the line voltage ... it MUST penetrate the suit DOH

and as I have told you, that doesn't happen ... the guy inside is shielded
 
  • #44
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Hmmm...
The electric field DOES NOT penetrate the suit
followed by
to bring the guy's body inside up to the line voltage ... it MUST penetrate the suit DOH
Right, got it! 😄
 
  • #45
Dale
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There's a huge difference between 'pretty well' and 'perfectly'.
It is, in fact, perfectly understood. There is no known EM phenomena which cannot be explained with our current EM theory. It can correctly give the outcome of every EM experiment performed to date.
 
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  • #46
anorlunda
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The OP question has been answered over and over and over again.

Thread closed.
 
  • #47
Dale
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@davenn in this case @Sooty is correct.

A Faraday suit forms an equipotential surface. An E field is the gradient of the voltage. So the Faraday suit forces the E field inside to zero, but does not force the voltage to zero. It is equipotential, not zero potential.
 
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