why doesn't a bird get elctrocuted when sitting ona transmission cable?

Question

is it because both its feet are at the same potential, but since every wire has resistance there has to be some potential difference between its feet .

Answer 1

Because the electricty is flowing through the bird. The bird would have to be touching the ground to actually become electrocuted. But instead the bird is only touching the wire. The same thing would happen to you if you were able to hang from the wire and not touch anything else. But if you touch the ground and the wire then zap you have created a ground for the electricity.

Answer 2

Yes, apra d, you are right.
The bird is not harmed because its feet are at almost the same potential. The voltage difference between its left foot and its right foot is very small. There's not enough of a difference to hurt the little creature.

Yes, it's true that the wire has some small resistance. However that resistance is very small. The bird's feet are only about an inch apart, and there is not much resistance in that short stretch of wire.

In order to get shocked, the bird would have to touch two things at the same time. (for example: wire and tower, or else wire and another wire, or else wire and earth) The bird reaches the wire by flying, so it never touches two things at once.

Squirrels sometimes walk along wires, too. They do it by leaping from the pole to the wire. Occasionally, an unlucky squirrel will step across that gap instead of leaping across it. When a squirrel does that, it does get electrocuted.

Answer 3

I like Mike N's answer. :o)
It doesn't get electrocuted because it is not completing a circuit to ground. For electricity to flow there has to be a difference in potential and if the bird does not complete the circuit from the high tension or transmission lines current will not flow through him. Now if he were to touch the crossarm or a fuse holder or the insulator he would then be a short circuit on the system, allowing current to flow.
Some high tension line workers actually use helicopters and line clamps to charge the helicopter and work the lines hot - these are upwards of 500kv lines!

EDIT: Just for the record transmission lines are not covered in rubber insulation (except when they are being worked on and they are temporary rubber sleeves), they are insulated from ground by pole top insulators and other styles of insulators that prevent their potential from going to ground. Incidentally, you can tell basically what voltage the line is by how many insulators there are. The higher the voltage the more resistance is required to keep the circuit from going to ground.

Answer 4

Your own answer is about the closest to the most accurate, except that the resistance of the wire is so low that there will be less than a few microvolts of voltage difference between tweety's feet. Therefore he is safe, as long as he doesn't try to straddle 2 wires - very dangerous even with insulated wire, as at high voltages (say 4 to 32kV on the top wires above the transformer), some current will flow thru the insulation, thru the much-lower resistance of a mostly-water being, and the the insulation of the other wire, enough to be lethal. You can estimate the voltage of a set of wires just by the distance they are kept separated as they travel along the poles, the more distance, the higher the voltage. Same for the length of the insulators used to hang them with.

Answer 5

It happens all the time to all kinds of animals. Especially if two birds touch and their feet are at different potentials. And yes it is always because of the difference in potentials across the bird.

I worked as an electrician and the first thing we had to do in the morning was inspect the substation for animal parts. This would give us a clue that there might be a problem. Often we found squirrel heads or bird claws etc.

Answer 6

I've never seen a bird sitting on the live wires, they all seem to prefer the earth wire at the top, presumably a bird at ground potential alighting on a live wire would receive some sort of tingle as it's body becomes charged to 66,000 volts or whatever the voltage is, and on alighting on the ground afterwards would get another tingle as the voltage discharges.

Answer 7

It's actually a parallel circuit -- the inch of wire between the bird's feet has one resistance, and the inch of bird has another. The wire resistance is nearly zero but the bird resistance is several kiloohms. But the voltage drop across the bird is the same as across the wire -- about 4 thousandths of a volt, for a half inch of aluminum steel reinforced wire at 300 amps or so.

Answer 8

You're right about the resistance of the wire, but it would be too small to matter. Virtually all the current will flow through the wire.

Another point is this: in the case of high-voltage AC transmission lines, hardly any current flows anyway (high voltage = low current - Ohm's law). That is relevant to your point about the bird's two feet.

The high voltage is delivered to a step-down transformer, when it can be used by the consumer. A high current will only flow if you connect them to something with a vastly different potential, like the earth.

Answer 9

you're almost on it.
For a current to flow you have to have two points of different potential. (A potential difference)
The total potential difference across the transmission wire (eg 240 Volts here in Australia) is across the ENTIRE LENGTH from power station to homes and back and most of the potential difference is found at the relatively high resistances of household appliances etc.
The result of this is that, although there IS a potential difference between the bird's feet it would only be a fraction of a Volt and is therefore unable to cause a lethal current through the bird.
Interestingly, we have high Voltage transmission cables near here (22 kV)(22,000 Volts) and the birds avoid those.

Answer 10

There is an interesting story to go with this one.
A lady asked a tram conductor if she would be electrocuted if she touched the rail the tram ran on.
He replied, 'No, Madam, not unless you put your foot on the wire overhead.' She complained and he was fired.
It's still true. You need a live wire and a return to make the circuit. NOT NECESSARILY an earth/ground connection.
RoyS