Why can a bird survive on a high voltage power line without getting fried?

[Hereformore]

Homework Statement

Birds can be seen perched on a high voltage power line yet seem unaffected by the high voltage. This is due to...

A) Low resistance of the birds compared to the wire, minimizing current through the birds
B) birds keeping only one foot on the wire, thus not completing a circuit
C) low capacitance of the birds (?)
D) Minimal potential difference across the birds

Homework Equations

V=IR

R = p(L/A)

The Attempt at a Solution

Assuming that current is constant across the entire transmisison line (it is right?), since the distance between the birds feet is very small, and since resistance is determined by length, the resulting resistance across that portion of wire would be very small.

Since current is constant across the wire, and the resistance is very small, then the resulting Voltage across those two points must be very small as well. The bird is effectively in parallel with the wire portion so it feels the same voltage,. Since the voltage is so small, it is essentially unaffected.

So that is why D is the correct answer.

What i don't understand is why a bird keeping only one foot on the wire wouldn't be a legitate reason either. There is no complete circuit so charge can't flow.

I was thrown off by the "low capacitance of the birds" statement, and since q=CV , I knew that would be false. But does capacitance even apply here? I thought it only applied when you had two parallel plates?

Finally, in this scenario, we know the voltage difference between the beginning and end of the wire (thats what we refer to when we say it's "high voltage" right?), current is constant throughout the wire right? I ask because it seemed odd to base my calculations of V=IR upon the constant current. Usually you use voltage to find the curent.

Thanks!

 

[Borek]

What i don't understand is why a bird keeping only one foot on the wire wouldn't be a legitate reason either.

Because most birds don't stand on the wire on one leg.

However, it is not true that birds survive on a high voltage power line without getting fried. Sometimes they don't. There is a 15 kV three phases line not far from here, often used by stock doves. Once when they started I saw a flash and heard an arc. Apparently one unlucky dove short circuited two phases.

 

[KL7AJ]

Because most birds don't stand on the wire on one leg.

However, it is not true that birds survive on a high voltage power line without getting fried. Sometimes they don't. There is a 15 kV three phases line not far from here, often used by stock doves. Once when they started I saw a flash and heard an arc. Apparently one unlucky dove short circuited two phases.

In Alaska we have ravens with enough wingspan to bridge between the two legs of a 250Kv liine. Every once in a while one manages to do it, and there is a rather impressive mushroom cloud of feathers and beaks. ")

 

[cwasdqwe]

I do agree on D as the right answer. As you say, across the wire the potential is really small in the wire portion a bird can occupy, so the electrons will rather travel through the metal wire (where conducting electrons move freely) than through the bird's body. If this bird was to touch another wire which had a different potential, then the electrons would prefer, this time, to take the shortcut that's the bird... And the electrons move, and our friend... would shine electric. So it has to do with potential and resistance (exactly the opposite of answer A).

 

[mfb]

Low capacitance is an important point as well. You can view the birds as one capacitor "plate", with the whole space around them as "gap" (there is no need for a second plate, but the other lines, the ground and so on work as one). As (nearly all) HV transmission lines operate with AC, each cycle charges and discharges the birds a bit. As their capacitance is tiny, this is not relevant for birds. In some countries, there are workers on live wires, where you can see this effect - the approaching helicopter does have a significant capacitance (example, starting at 1:00).

 

[Orodruin]

we know the voltage difference between the beginning and end of the wire (thats what we refer to when we say it's "high voltage" right?)

Just to clarify. The "high-voltage" refers to the potential difference between the wires, not between the beginning and end of a wire. Generally you want to drive a load which takes some given power and you do not want significant losses in the wires. Power lost in the wires is $P = R I^2$ and so the less current you need to use the less power you will lose in the wires. Thus you get more power left for the load if you can have a high voltage difference in the load (load power is UI where U is the potential across it - more U, less current). Typically the "load" would be a transformer station which then distributes electricity for local consumption at a lower voltage.

 

[rcgldr]

The "high-voltage" refers to the potential difference between the wires, not between the beginning and end of a wire.

There's also a voltage differential along a high voltage wire if the length doesn't correspond to the effective wave length of the AC voltage wave. There are line "inspector" vehicles that hang from a transmission line via two wheels about 1 meter apart and use transformer effect to extract energy from a single line.

I also had the impression that birds could get zapped (arc or antenna effect) by the really high voltage wires, 500 kv to 750kv used in power tranmssion lines. These usually have a ground wire positioned at the highest point (this wire is grounded locally, and not connected to the ground of the generating source), and hopefully birds will choose to land on that ground wire. The humming and or sensation of the voltage on the transmission lines may scare off the birds. The guys that service these lines wear a faraday cage suit, approach the lines from helicopter based platform, and during transition on and off the line, they have to connect a wire to synchronize the helicopter with the voltage potential of the transmission wire. Youtube video: