Hi - just joined, first post.

Taking my first steps in electronics, I've found several points I'm really puzzled about. Maybe you can help?

First One:

The bird sitting on the electric power line scenario - there's zero voltage differential between its feet, so it doesn't get electrocuted.

But if you put one probe of a voltmeter on the power line, and stuck the other probe up the bird's butt, would it not show a voltage differential between the power line and the internal environment of the bird?

If NOT - wouldn't that imply that the bird's body (which is presumably not electrically charged) and the power line (full of amps and amps of current being pushed around by high voltage) somehow by amazing coincidence just happen to each possess an electric field with exactly the same potential energy per unit charge (since that is what voltage is defined as)? How and why would that be the case?

And if the voltmeter would show a voltage differential, why does an electric current not flow into the bird? Wouldn't Ohm's law mean that a current, at least momentarily (until an equilibrium is reached and there is no longer any voltage differential), would flow into the bird and that current would initially be equal to the initial voltage differential between the power line and the bird, divided by the resistance of the bird?

So can anybody explain how these birds are getting away with it!?!?
Clearly they know something that I don't.

Thanks.

 

The bird has to connect across two wires to make a circuit, like pos and neg, or phase and neutral, so one wire isn't going to harm it.

 

The bird isn't grounded... and non-conductive enough so there is no viable circuit.

 

Welcome to AAC.

But if you put one probe of a voltmeter on the power line, and stuck the other probe up the bird's butt, would it not show a voltage differential between the power line and the internal environment of the bird?

Assuming you're talking about the line the bird is sitting on, the answer is no.

The bird will not see a potential difference unless he either straddles two lines or a line and a ground source.

If you take your two probes and put them on the same line, you won't see a voltage because there is no potential difference between two points on the same wire. You can think of the bird as the meter and the probes as his feet. Since there is no voltage difference between those two points, no current flows.

Additionally, the bird has some internal resistance and electricity always prefers to go through the path of least resistance (the wire), hence current is not going up one leg and out the other. If line where to suddenly split between his legs however . . .

 

Hi - just joined, first post.

But if you put one probe of a voltmeter on the power line, and stuck the other probe up the bird's butt, would it not show a voltage differential between the power line and the internal environment of the bird?

Thanks.

Just to add to what has already been said. You will not see any voltage but if you try it make sure to video it.

 

A side note, with AC the situation is a little more complicated. There will be no current from one leg into the other, but there will be some small current from both legs into the head. This is because any sonductive object has some capacitance, and this capacitance charges and discharges with the AC frequency. The capacitance is very small so the current will be small as well, but still there will be some current nontheless.

Another point are the very high voltag lines, like 400kV or similar. On these lines the corona discharge is a significant effect, where some part of the current exits into the surronding air, and it is a major factor in losses in these lines. Thus a bird sitting on such lines will not be very comfortable there as it will act as a conductor for the corona discharges passing through its body. You should be able to see that birds usually sit only on the lower voltage lines and not on the super high voltage lines, they probably have some reason for that

 

But if you put one probe of a voltmeter on the power line, and stuck the other probe up the bird's butt, would it not show a voltage differential between the power line and the internal environment of the bird?............ No..............

If NOT - wouldn't that imply that the bird's body (which is presumably not electrically charged) ...........But it is...........and the power line (full of amps and amps of current being pushed around by high voltage) somehow by amazing coincidence just happen to each possess an electric field with exactly the same potential energy per unit charge...........Yes............(since that is what voltage is defined as)? How and why would that be the case?...........No coincidence. The bird is in contact with the wire.............



Thanks.

Find a video on helicopter linemen, who are the human equivalent of birds on a wire.

These show an extreme example, of systems of such high potential, that you can see the arcing to the person, (thru discharge rod) equalizing the potential.

 

A side not, with AC the situation is a little more complicated. There will be no current from one leg into the other, but there will be some small current from both legs into the head. This is because any sonductive object has some capacitance, and this capacitance charges and discharges with the AC frequency. The capacitance is very small so the current will be small as well, but still there will be some current nontheless.

Another point are the very high voltag lines, like 400kV or similar. On these lines the corona discharge is a significant effect, where some part of the current exits into the surronding air, and it is a major factor in losses in these lines. Thus a bird sitting on such lines will not be very comfortable there as it will act as a conductor for the corona discharges passing through its body. You should be able to see that birds usually sit only on the lower voltage lines and not on the super high voltage lines, they probably have some reason for that

What happens if he sits on a DC high voltage transmission line, then?

 

I sense that the OP's question is related to confusion about the meaning of voltage, also called potential. Absolute voltage has no meaning - all voltages are relative to something else. Voltage is potential energy.

Consider the analogy of drinking a glass of water while flying in an airplane at 600mph at 30,000 feet. To you, on the plane, the only risk is spilling it in your lap. But that glass of water, relative to a person on the ground, has enough kinetic energy to kill him and also enough gravitational potential energy to kill him and anyone near him.

Same deal with voltages. Unless/until there's a way for a high voltage to fall to a low voltage, nothing happens. There's no risk per se to being at 100kV. In fact I can declare that I'm a that voltage right now. The problem with that in reality is that I'm too close to the earth, which we call 0V by convention. Some of that 100kV would find a path to ground via the air and I'd have some problems.

 

500,000 dc

http://www.youtube.com/watch?v=9tzga6qAaBA

The smaller (less surface area) you are, the less current passes into you to charge to equi-potential.

 

You might find this useful:

http://forum.allaboutcircuits.com/blog.php?b=588

The key is that, as the bird goes from one potential (say the ground) up to the wire, they are capacitively coupled to both, but the currents that have to flow to satisfy the changing potentials are so small and so short-lived as to be unnoticeable.

Consider that this bird, sitting on a 500kV AC line, is having the voltage on his body, relative to the gound, change by nearly 1.5 million volts 50 or 60 times a second.

 

On a scope with zero reference line with a + voltage ,say 30 volts....-15 volts would be

less + than 30 volts. 0 + - + -15 + - + = 30

A little bird named (shunt) - + sitting on a hot spot ,with bird friend named (demo) - +
- +

 

On a scope with zero reference line with a + voltage ,say 30 volts....-15 volts would be

less + than 30 volts. 0 + - + -15 + - + = 30

A little bird named (shunt) - + sitting on a hot spot ,with bird friend named (demo) - +
- +

______

 

Find a video on helicopter linemen, who are the human equivalent of birds on a wire.

These show an extreme example, of systems of such high potential, that you can see the arcing to the person, (thru discharge rod) equalizing the potential.

Don't know about the bird but the lineman wouldn't take too kindly to a multimeter probe stuck up his/her butt.

 

______

loosewire. Nuff said.

 

Don't know about the bird but the lineman wouldn't take too kindly to a multimeter probe stuck up his/her butt.

That might depend on just how remote and long the assignment is.

 

500,000 dc

http://www.youtube.com/watch?v=9tzga6qAaBA

The smaller (less surface area) you are, the less current passes into you to charge to equi-potential.

I highly doubt that's DC. You're not going to be able to have voltage and current that high going through such long, thin cables. Besides, nobody transmits DC anymore, not since the early 20th century. I have a feeling the person who wrote that description and said 0.5MVDC had no idea what they were talking about.

 

Anyway, to the OP: When a bird lands on a wire, it is raised to the same voltage potential as the wire does (with respect to ground/neutral, or to the other phases). If you put one probe on the bird and the other on the ground you're going to see the full potential voltage that you'd find on the wire itself (assuming your multimeter can stand that kind of volts, otherwise it'll blow up in your hands).

 

Thanks everyone who has replied.

I'll let the issue of capacitance pass over my head for now, and just concentrate on voltage.

@ inwo -- you have replied:

1) the bird and the power line DO NOT have a voltage differential between them;
2) the bird IS electrically charged;
3) the bird and the power line ARE at exactly the same state of potential energy
per unit charge; and
4) the reason why is that the bird and the wire are IN CONTACT.

Thank you very much for answering directly on point like that - very helpful.

And does everyone agree with the above, or does anyone dispute any of those points?

But my brain now wants to know:

What exactly is it about being in contact that:

1) causes there to be no voltage differential between the bird and the power line, 2) causes the bird to become electrically charged, and
3) causes the bird's body to have exactly the same state of potential energy per
unit charge as the power line ....

.....and all without ANY current flowing into or out of the bird?

Why doesn't at least a bit of current go into the bird to bring about all of this equalisation? Or since it's AC, I suppose that last question should be: why isn't at least some current flowing in and out of the bird with each cycle?

That question can be generalised like this:

How exactly does the simple fact of being in contact cause precisely the same state of potential energy per unit charge to be communicated from one object to the other? What is the medium of communication - if it is not caused by electrons moving from one object to the other?

In other words:
since contact is just physical touching at the surface of each object, by what mechanism does that cause equalisation of electrical states such that there is no voltage differential between the objects?

 

Sorry, I've alway's liked the bird on the wire, I just have to change it up a bit.

Bird number 2, weighs 200lbs and lands on a 15kv line on the very top, it's feet are spread about 2 feet apart. What happens to the bird? Is it the same as bird number 1.

The potential is the same all but 2 things (Resistance and Capacitance has changed.)

Just curious,

This is because any sonductive object has some capacitance, and this capacitance charges and discharges with the AC frequency.