It's just so insignificant that it's not worth discussing, but I mentioned it anyway so some little know-it-all doesn't pop in and try to correct it.

That doesn't seem to be much of a problem on this site, and I am grateful for that.

 

I know it all.





I just have trouble remembering most of it.

 

Let's look at this from a couple of different perspectives.

First, let's replace the bird with a hunk of metal, say a one-foot length of wire, clipped to the power line. What is the potential of the this hunk of metal relative to the ground? What is the current flowing in this hunk of metal? The answers to these are pretty easy to see, at least to first order. The potential of the hunk of metal relative to ground is one million volts and the current flowing in it is negligible.

Next, let's simplify things by talking about a HVDC line at, say, one million volts relative to the ground. Now let's image our hunk of metal moving from the ground to the wire. At what point in this journey does the voltage on the hunk of metal, relative to ground, change from 0V to 1MV?

It is NOT the instant the hunk of metal touches the wire! There is a potential field surrounding the wire that, relative to ground, is at 1MV at the wire and 0V at the ground and is at something inbetween everywhere else. For simplicity, let's say that the wire is 10m off the ground and that the voltage field is linear (it's not, but the details of the shape of it are not important to this discussion, so let's keep it simple). So that means that, at 1m off the ground, the potential field is 100kV and when the hunk of metal is 1m off the ground it will be at a potential of 100kV. As it moves up to 2m, it will be at 200kV, and when it is just a tiny distance from the wire it will be at all-but 1MV.

That this is what is going on would be easier to visualize if our hunk of metal had a net electrical charge on it, because then it would require a force to either move it in the direction of the wire or to keep it from accelerating toward the wire, depending on the polarity of the charge and the voltage on the wire. Under those conditions, we can readily see the work being done to move the hunk of metal to the wire and that its voltage must therefore be changing continuously as it moves. But nothing is different in the case when there is no net charge on the hunk of metal since it requires no work to move an uncharged object from one potential to another. Or, perhaps easier to envision, our hunk of metal is made of up lots of positive and lots of negative charges and the positive charges require a force one direction to get them to move, at constant speed, toward the wire but the negative charges require a force in the other direction that is of the same size. So while the charges that make up our hunk of metal all have work being done on them (some positive/some negative) to move from the ground to the wire, there is no net external force required to do so. It is this lack of a net external force that creates the false impression that the electric potential of the object doesn't change as it moves.

 

I just want to see the video

 

I just want to see the video

Here you go.

https://www.youtube.com/watch?v=D_Q75hCsbj8&noredirect=1

 

Here you go.

https://www.youtube.com/watch?v=D_Q75hCsbj8&noredirect=1

Over on ETO one of the members say's a transformer near his house has claimed 3 squirrels in the exact same way, Vaporized.

 

Here you go.

https://www.youtube.com/watch?v=D_Q75hCsbj8&noredirect=1

Sorry. Don't buy it. Doctored.

Notice that the bird is just sitting there, not looking around or moving or anything. Then, without doing anything, it just vanishes?

This is what the real thing tends to look like:

https://www.youtube.com/watch?v=uWB9UsQmTRA

This took place in Albuquerque, New Mexico. The bear came into town looking for her cub which, it turns out, had been found and captured some time before. When she twitches near the beginning of the video it is because she is hit with a tranquilizer dart. It took about an hour before it took effect. She hit a 7200V line on the pole pig.

She survived not only the shock but the fall with fairly minor injuries. She was treated, given time to recover, and released along with her cub. Just goes to show that large animals are very, very sturdy animals.

Here's one I remember from when it happened:

https://www.youtube.com/watch?v=P8tprAIQFCE

The guy turns himself into a human light bulb.

Here's one that particularly disturbing:

https://www.youtube.com/watch?v=aJ591hRiH50

 

Sorry. Don't buy it. Doctored.

Notice that the bird is just sitting there, not looking around or moving or anything. Then, without doing anything, it just vanishes?

Yeah, it does look fake. Of course, a six ounce bird would be more easily vaporized than a bear or a man. Still, there would be feathers in the air, I think.

 

Let's look at this from a couple of different perspectives.

First, let's replace the bird with a hunk of metal, say a one-foot length of wire, clipped to the power line. What is the potential of the this hunk of metal relative to the ground? What is the current flowing in this hunk of metal? The answers to these are pretty easy to see, at least to first order. The potential of the hunk of metal relative to ground is one million volts and the current flowing in it is negligible.

Next, let's simplify things by talking about a HVDC line at, say, one million volts relative to the ground. Now let's image our hunk of metal moving from the ground to the wire. At what point in this journey does the voltage on the hunk of metal, relative to ground, change from 0V to 1MV?

It is NOT the instant the hunk of metal touches the wire! There is a potential field surrounding the wire that, relative to ground, is at 1MV at the wire and 0V at the ground and is at something inbetween everywhere else. For simplicity, let's say that the wire is 10m off the ground and that the voltage field is linear (it's not, but the details of the shape of it are not important to this discussion, so let's keep it simple). So that means that, at 1m off the ground, the potential field is 100kV and when the hunk of metal is 1m off the ground it will be at a potential of 100kV. As it moves up to 2m, it will be at 200kV, and when it is just a tiny distance from the wire it will be at all-but 1MV.

That this is what is going on would be easier to visualize if our hunk of metal had a net electrical charge on it, because then it would require a force to either move it in the direction of the wire or to keep it from accelerating toward the wire, depending on the polarity of the charge and the voltage on the wire. Under those conditions, we can readily see the work being done to move the hunk of metal to the wire and that its voltage must therefore be changing continuously as it moves. But nothing is different in the case when there is no net charge on the hunk of metal since it requires no work to move an uncharged object from one potential to another. Or, perhaps easier to envision, our hunk of metal is made of up lots of positive and lots of negative charges and the positive charges require a force one direction to get them to move, at constant speed, toward the wire but the negative charges require a force in the other direction that is of the same size. So while the charges that make up our hunk of metal all have work being done on them (some positive/some negative) to move from the ground to the wire, there is no net external force required to do so. It is this lack of a net external force that creates the false impression that the electric potential of the object doesn't change as it moves.

@WBahn: thanks for that explanation. So you're saying that as the hunk of metal levitates (let's just imagine) up towards the power line, the voltage between it and the ground increases from a starting point of zero when in contact with the ground to a end point of 1MV when in contact with the power line? So part-way up, when the hunk of metal is not in contact with anything, there is a voltage between it and the ground?

According to my limited understanding, that seems to then imply that if you ran a wire from the levitating hunk of metal to the ground, the voltage that (I think you're saying) exists between the hunk of metal and the ground would drive a current through the wire. Intuitively, I can't believe that would happen -- just by being near a power line without even touching it, a hunk of uncharged metal could drive a current through a wire to the ground. Can that actually happen? (apart from the levitation, of course).

Or have I got it all wrong? I have read voltage described as analogous to a water pump, driving current through a wire, and described as "electromotive force" (with the caveat that it is not actually a force, but potential energy per unit charge) that provides the "push" to make a current move. From those descriptions, my understanding is that if there is a path available along which current may flow (because the resistance of that path is not prohibitive) connecting 2 points across which a voltage exists, current will flow. Is this understanding wrong?

 

I will give you an "ideal" answer, and a "real" answer.

Ideal
In an ideal world where air has infinite resistance and the bird has no capacitance, the answer to this question would be: "There is no change at all". Literally nothing happens when the bird flies up to the wire and sits on it.

But why do you measure 10kV on the bird relative to the ground, you may ask?

Simple: Your voltmeter has a smaller-than-infinite resistance, and by sticking one probe up the birds butt and the other into earth, You are closing the circuit, and the bird acts as a resistor (because its body also has a smaller-than-infinite resistance).

So this scenario:

Is basically the following:

Real
The bird has a capacitance, the air has a resistance, and a small current will flow through the bird in function with the frequency of the wire.

I can imagine a short, higher current flowing into the bird at first contact with the wires, because it would have to charge up to the new potential, but it will be far from lethal.

Also fun fact: You will probably kill the bird if you were to measure it with a voltmeter, because the current produced by closing the circuit would be enough to do so.

@TheComet: Thanks for your explanation --- and the diagrams!

So you're saying that if the bird hypothetically had infinite resistance and zero capacitance, it's body while sitting on the power line would still be at zero volts with respect to the ground (since you said literally nothing happens)?
And presumably in that case the voltage between the bird and the power line would be 10kV (or whatever the voltage of the power line is)?
(I'm ignoring the fact that it's AC. I'd just be happy to understand it as if it were DC for the time being.)

But then you're saying that in the real world, where the bird has less than infinite resistance and non-zero capacitance, a small, non-bird-distressing current enters the bird and brings it up to the same voltage with respect to the earth as the voltage between the power line and the earth? And then if I do pay attention to the fact that it is AC, you're saying that a small current will be flowing in and out of the bird in time with the frequency of the alternating current?

Is that basically what you're saying?

 

@WBahn: thanks for that explanation. So you're saying that as the hunk of metal levitates (let's just imagine) up towards the power line, the voltage between it and the ground increases from a starting point of zero when in contact with the ground to a end point of 1MV when in contact with the power line? So part-way up, when the hunk of metal is not in contact with anything, there is a voltage between it and the ground?

Yes. Perhaps the best way to look at it is that the space (the air) at a point part-way up is at a particular voltage relative to ground and when the hunk of metal is at the position, it has that voltage.

According to my limited understanding, that seems to then imply that if you ran a wire from the levitating hunk of metal to the ground, the voltage that (I think you're saying) exists between the hunk of metal and the ground would drive a current through the wire. Intuitively, I can't believe that would happen -- just by being near a power line without even touching it, a hunk of uncharged metal could drive a current through a wire to the ground. Can that actually happen? (apart from the levitation, of course).

Yes, that is what would happen. But that current would be very small and only exist for a very brief moment of time - probably measured in picoseconds. In the process, the voltage distribution in space would change and now there would be a region of 0V that extends from the ground, up along the wire, and around the hunk of metal. The remaining space between the hunk of metal would now go from 0V to 1MV in the remaining space. If the resulting electric field is strong enough, the air will ionize and you'll get an arc between the power line and the hunk of metal (and down the wire to ground).

Or have I got it all wrong? I have read voltage described as analogous to a water pump, driving current through a wire, and described as "electromotive force" (with the caveat that it is not actually a force, but potential energy per unit charge) that provides the "push" to make a current move. From those descriptions, my understanding is that if there is a path available along which current may flow (because the resistance of that path is not prohibitive) connecting 2 points across which a voltage exists, current will flow. Is this understanding wrong?

It's a useful analogy but is a simplification. "Voltage" is a property of space that describes the work that would have to be done on a charged object to move it from one point in space to another. The voltage distribution is open space (including air) is established by the charge distributions in the vicinity, including the voltages on conductors. That distribution can be changed radically by changing the charge distributions and with very little energy because there is no (or very, very little) current that actually flows as a result because of the very high resistances involved.

 

@WBahn,

Wow, thanks, that actually makes sense to me.

So there's an electric field around the power line, and the distance from any point to the power line determines the strength of the field at that point (assuming constant DC current to keep it simpler), and hence also the voltage of that point relative to the power line. Is that right?

But I got lost at the explanation in your first reply where you posited the scenario where the hunk of metal was electrically charged, so that it could be easily seen how work must be done to move it with respect to the electric field (which I understood), and then you posited the next scenario where the hunk of metal was not electrically charged, but said that work would still be done to move it with respect to the electric field -- which I didn't quite follow.

Could you run through that bit again please, if you're feeling kind?


... And uh oh, more questions are popping into my head ...

According to my understanding at least (and still just assuming DC power to keep it simpler):
* Electromagnetic field is generated by charge; and
* Current moving through the power line (in Amperes) is the charge (in
Coulombs) that is moving per second;
- but what is the charge at any moment in time? Is it the same value in Coulombs as the current in Amperes, because the current is flowing at a constant rate per second? And what is it's polarity? Is it a negative charge because it is caused by a flow of negative electrons? Would that mean that the current that would flow for a few picoseconds from the hypothetical levitating hunk of metal down a wire to the ground would be electrons flowing towards the ground (not away from the ground), repelled by the negative charge?

I hope you'll post answers, because your other answers were really well explained thanks mate.

 

@WBahn,

Wow, thanks, that actually makes sense to me.

So there's an electric field around the power line, and the distance from any point to the power line determines the strength of the field at that point (assuming constant DC current to keep it simpler), and hence also the voltage of that point relative to the power line. Is that right?

But I got lost at the explanation in your first reply where you posited the scenario where the hunk of metal was electrically charged, so that it could be easily seen how work must be done to move it with respect to the electric field (which I understood), and then you posited the next scenario where the hunk of metal was not electrically charged, but said that work would still be done to move it with respect to the electric field -- which I didn't quite follow.

Could you run through that bit again please, if you're feeling kind?

Read that second part of my post more carefully. Imagine you have two hunks of metal, one positively charged and one negatively charged by the same amount, connected by some non-conducting material (BTW, this just describes a charged capacitor). One hunk of metal will feel a net force toward the wire, but the other hunk will feel a net force away from the wire. Both of these forces are equal in magnitude, so even though the forces on each hunk could be enormous, there is no net force on the entire assembly and we can move it from one point to another within the field without any net external force beyond the tiny one to get it moving and the tiny one to bring it back to rest.

... And uh oh, more questions are popping into my head ...

According to my understanding at least (and still just assuming DC power to keep it simpler):
* Electromagnetic field is generated by charge; and
* Current moving through the power line (in Amperes) is the charge (in
Coulombs) that is moving per second;
- but what is the charge at any moment in time? Is it the same value in Coulombs as the current in Amperes, because the current is flowing at a constant rate per second? And what is it's polarity? Is it a negative charge because it is caused by a flow of negative electrons? Would that mean that the current that would flow for a few picoseconds from the hypothetical levitating hunk of metal down a wire to the ground would be electrons flowing towards the ground (not away from the ground), repelled by the negative charge?

I hope you'll post answers, because your other answers were really well explained thanks mate.

You're mixing up several concepts.

First, let's consider the electric field around a long wire that is charged with no other charged objects in the vicinity. The electric field is radially outward with a magnitude of

E = kλ/r where r is the distance from the wire and λ is the charge per unit length on the wire.

The voltage is the just the integral of the electric field, giving

V = kλ ln(ro/r) where ro is the diameter of the wire.

So if V = -1MV, ro=1cm, and r=10m, then the needed charge density on the line is

λ = V/(k ln(ro/r)) = 1MV/( 9E9 Vm/C * ln(0.01m/10m) ) = 16μC/m

So charging up a 1 km length of wire to 1MV can be done using a current of 4A in just 4ms.

You simply don't have to move much charge around in order to establish huge potential differences.

 

So you're saying that if the bird hypothetically had infinite resistance and zero capacitance, it's body while sitting on the power line would still be at zero volts with respect to the ground (since you said literally nothing happens)?

No, it's body would be equal to that of the power line. What I mean by "nothing happens" is that no flow of electrons will occur at all, but the bird will still have the same potential as the wire its touching.

 

@WBahn: Thanks again.

It's been a long time since I was at high school, and I didn't even do physics! So far I'm only familiar with the equation for electrostatic field around a point charge, and I didn't realise that there are different equations for electric field around conductors. I'll get there, though.

If you wouldn't mind, where E = kλ/r, how do you establish the magnitude and the polarity of λ (charge per unit length of wire --- and again let's just assume DC instead of AC)?

 

@WBahn: Thanks again.

It's been a long time since I was at high school, and I didn't even do physics! So far I'm only familiar with the equation for electrostatic field around a point charge, and I didn't realise that there are different equations for electric field around conductors. I'll get there, though.

If you wouldn't mind, where E = kλ/r, how do you establish the magnitude and the polarity of λ (charge per unit length of wire --- and again let's just assume DC instead of AC)?

I don't quite understand the question. Do you mean how do you establish it mathematically, or how do you establish it physically?

 

Both, I guess. If you know the current through the wire, the voltage "driving" the current to flow (DC) from the one end at, say, the power station to wherever it goes to, and you know the resistivity of the wire, is that enough to derive λ, the charge per unit length of wire, or what else do you need to know? I'm familiar with charge per unit time (current) from Ohm's law, but I don't know how you can get from that to charge per unit length. I'd like to know the mathematics, and the physical explanation behind it.

And in my understanding of the electric field, you can represent it by field lines perpendicular to equipotential zones, so it radiates out in all perpendicular directions from the wire, but I can't see how you would know if the field would attract or repel a hypothetical positively charged particle along those field lines. In other words, if there's an electric field around the wire, how do you know if it's a positive or negative field?

I got to this question by imagining that levitating hunk of metal near the power line (DC) which is suddenly wired to the ground, and trying to work out if the few picoseconds of current would be electrons moving towards the ground or away from the ground.

Thanks.

 

I know this discussion is old but since so many people commented and I ran across this video that may add some additional information.

Amazing man-on-a-500k-volt-wire video (professionally produced).

http://www.youtube.com/watch?v=U3XAIwEZpTg

Note the other videos in the right margin - "Hot washing" insulators is interesting too.