AAC book:

Yes, even though they rest on two feet, both feet are touching the same wire, making them electrically common. Electrically speaking, both of the bird's feet touch the same point, hence there is no voltage between them to motivate current through the bird's body.

Even the top Google search result I get says the same thing:

http://engineering.mit.edu/ask/how-...tage-power-lines-without-getting-electrocuted

When a bird is perched on a single wire, its two feet are at the same electrical potential, so the electrons in the wires have no motivation to travel through the bird’s body.

But arguably the same explanation shows why current can't go through the wire. Bird on a wire, wire on a bird -- what's the difference? Or imagine a metal bird:

http://amasci.com/elect/elefaq1.html#o

WHY CAN BIRDS LAND ON POWER LINES WITHOUT HARM?

The charges would rather go straight through the wire, rather than taking a detour through the bird! Bird skin is a conductor, but copper has thousands of times more conductivity. If a robot bird made of metal landed on a power line, then there would be charges flowing through the metal bird.

1. Is it flawed (or at least misleading) to explain in terms of potential difference rather than resistance? Isn't resistance the better conceptual framework (which is what the amasci.com answer uses)? With my grounding thread and now with this, I'm thinking that this idealized "absolutely zero potential difference" does more harm than good for conceptual understanding in SOME cases.

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I have some more questions. I was watching that YouTube video someone gave me in my grounding thread, the one with the lineman who was working on the 500kV line. He uses a wand to equalize his voltage before hopping on the line. So there's some danger here, presumably the same danger for birds sitting on 500kV lines. This is a bit of a wrinkle in my understanding.

2. For a bird, what's the difference between sitting on a suburban wire and one of these 500kV monsters? If I have a simple circuit with a battery connected by two wires (wire A and wire B) to a fixed-value resistor, and if I pick two close points on wire A (point a1 and point a2), I know that the voltage, however small, across point a1 and point a2 will rise as the battery voltage rises (unless I've done something stupid here). Is that all that's happening on the 500kV power line -- the jump in voltage across the bird's feet (relative to the suburban power line) is driving a lethal amount of current (I haven't done the math)?

3. Given that the lineman isn't grounded, what exactly is the danger he's averting with the wand? Should I instead be looking at this in terms of electrostatics? Other physical phenomena?

Thanks for the help. I do try running Google searches before posting (as you can see above), but with all the information out there, I'm finding it hard to drill down on the answers I'm seeking.

 

Current doesn't flow - and no damage is done - if there is either 1) no path or 2) no potential difference. A bird on a wire needs to worry only about how well the air surrounding it might conduct (#1). His surface area is not great, and the conductivity of air is usually very small, so his risk is minimal. Unless the potential difference is VERY large (#2). Not sure what it takes to kill the bird. The conductivity of the wire ensures that the potential difference between the bird's feet is too small to drive a relevant current. I'm sure the path to ground via the air is a bigger concern.

I think the lineman is dissipating static. Or it's possible he is allowing enough current to flow to bring his potential up to that of the wire, but in a way that avoids having that current flow through him.

 

Yes, I think so. Think of the bird and the wire as resistors in parallel, with the resistance of the wire depending on how far apart the birds feet are. The resistance of an inch of wire is very low a couple of inches of bird is very high. So not much voltage across a high resistance.
I think the lineman is discharging static so he doesn't get surprised by it.

 

First, helicopters make a lot of static electricity. Second, you can consider the helicopter to be one plate of a capacitor between the power line and the air. At the moment of contact, the static charge is dissipated and the helicopter becomes a capacitor plate that is attached to the power line.

 

There is current flowing through the wire, for sure.
There is a potential difference between the bird's two feet but too low to electrocute the bird.

The helicopter is a sizable body that has capacitance. The lineman has to raise the potential of the helicopter to match that of the power line. The sparks you see is the charge flow required to charge the helicopter.