Physicists - electrocution?

Question

When a bird sits on a telephone wire it doesn't get electrocuted. My understanding of this is that it is not earthed and so provides greater resistance than the wire. The electricity just goes down the wire. But what is confusing me is.. how does the electricity 'know'? Surely it has to jump into the bird to find it doesn't lead to earth? In which case the bird would be electrocuted. How does the resistance at the point of contact with the wire change if the bird is earthed?

I know the electricity is not aware. I was trying to put my question simply. Thanks to all the people who actually answered with a scientific explanation of why the bird touching the ground makes a difference (instead of just telling me that it does). Also, I was just using the bird/wire as an example, I just wanted to know what happened in that general situation.

Answer 1

You're right, the bird doesn't get electrocuted because there is not a continuous path from the bird to 'ground' making a complete circuit.

The electricity doesn't have a 'being'.......it doesn't have knowledge so it doesn't know or not know anything about the bird. It's like pouring a glass of water out onto the table and watching if fall over the edge onto the floor and asking; "how does the water know to hit the floor".

It's reacting to that which is natural as opposed to making a decision.

The fact that the necessary conditions are not present for the electricity to travel thru the bird is reason why it does't do so.

The electricity follow the path of least resistance. So it can either flow along nice metallic conductors (the wires) or it can try and jump the enormous gap from the bird to the ground.....tremendous resistance to overcome to do that. So, if it doesn't not 'see' another viable path, it doesn't not go thru the bird.

Make any sense?

Answer 2

You are confused. The resistant of the wire is intrinsic and is not related to external conditions other than heat. The reason the bird on the wire does not get electrocuted is because it is not grounded. Electricity knows to go to ground the same way water knows to run down hill. Water wants to be at its own level and because of gravity higher water will seek the lowest level it can find. Electricity is the flow (DC) or vibration (AC) of electron, this flow or vibration is called Current, the force that puts it into motion is called Voltage. Before you apply a voltage to a wire the electrons are at Ground potential.

The voltage source can be looked upon as a dam of water, the higher the water is raised from ground lever the greater the force it will generate getting back to ground level. The water flowing to ground level is the current; the water in the dam and the size of the opening in the dam controls the current. The lager the hole the weaker the current, the lower the dam the weaker the current. The same with electricity, you build up a voltage by magnetic or chemical means and the force of the current is controlled by how strong a voltage you generate and the amount of resistance in the path the current is given to ground, the less the resistance the greater the current. The voltage is measured from ground level, or potential, and the higher the voltage the greater the potential different will be from ground.

Now back to your bird on the wire, the electricity will not see the bird as a path to ground unless it touched the wire and another point of a lower potential. Since electricity (like water) is always looking for the quickest way to ground it will take the short cut that the bird provides, and in so doing electrocute the bird. It would not even have to be the ground that the bird touched, it touched another wire that happen to be of a different potential then the wire he was sitting on he would get shocked, if he got electrocuted or not would depend upon the size of the potential different.

Answer 3

This is a good question. The bird doesn't get electrocuted because first the bird is insulated from the wire by a non-conductive coating. However, the most significant reason the bird doesn't get electrocuted is that there is no potential across the bird. In the wires, electrons are continously flowing from an area of high potential to an area of low potential. When the bird is sitting at the wire, the bird and the wire are at the same potential therefore their is no flow. However, if the bird was able to complete the circuit, the bird and the wire would have a potential. It can do this by shorting across at least two wires or the ground and the wire. The ground actually is a conductor of electricity (scary as it seems) and is quite efficent at carrying a charge back to the generating station. Power companies actually use the earth as a conductor. However, in most situations the electricity would much rather travel a string of copper to the power station then arc through a long distance of wide open air to the ground. Electricity 'knows' which way to go as there is a force being exerted on each electron from the electric field. Therefore, electrons are pushed in a direction of the more positive charge and repeled by the negative charge. Also, a fundamental principle of nature picks the path that electrons flow. Electricity, water streams, animals and even flows of traffic all try to pick the path of least resistance. In the case of electricity, electrons will attempt to go into the path of higher resistance but eventually get deflected into the path of lowest resistance.

Answer 4

Imagine the bird sitting on the wire.

1. The bit of wire between the birds feet will have a very low resistance (it is a conductor), of say 0.000001 Ohm. This means there will be a very, very small voltage drop across this part of the wire.

2. The electrical path from one claw, up through the leg, bird and out the other side will have a relatively high resistance of say 5000 Ohm.

3. The current flowing through the bird and wire is inversely proportional to the resistance (you can do the parallel resistance bit if you want 1/Rtotal = 1/Rwire + 1/Rbird etc). This means that a current does flow through the bird, but it is extremely small and of course the voltage drop is also very small. For practicality you can say all of the current flows through the wire.

4. If the bird touches the wire and earth at the same time, then it now has the whole volltage of the wire (several thousands of volts) across it, which is why it would disappear in a flame and flash of smoke.

Answer 5

As the bird is approaching the line, he is being charged by the electric field around the wire. It happens pretty quickly and so when he actually touches the wire, it is already close to the potential of the wire. Otherwise, a spark would occur and scare the bird off.

Now, when he is in contact with the wire, the current going under his feet has two choices when it gets to his left foot,
does it stay in the wire for the next 2 inches of wire and go under his right foot as well, or should some of it go up the left leg and down the right.

The current will divide proportional to the resistance of the two paths. The wire path would be extremely small for such a short distance while the bird path would probably be pretty high (greater than 10,000 ohms). Therefore, only a small amount of current would go throught the bird. It would probably be in the range of microamps and not be felt.

Now, if the bird could reach down and touch the ground, then the current has a different choice, go through the bird to the ground, or continue down the wire. The difference now is that the bird's resistance path probably hasn't changed much unless it in now standing with one leg in water, but the resistance of the wire is no longer the small amount of wire that was between his legs, it is now the resistance of the electrical system looking down the line. Again, I don't know exactly what that number would be but it would be at least a couple of ohms. Now the current division will at least be in the range of several hundred milliamps or more. For our example of 10,000 ohms contact resistance, if we assume 14,000 volts to ground, the current will flow at 1.4 amps. The problem is that now you have established a current flow inside an electric field with a voltage of 14,000 volts. This generally leads to a breakdown where the path becomes ionized and bam, suddenly you can easily have 5k - 10k amps flowing through the arc. So even if the 1.4 amp initial flow through the bird doesn't kill it, the flashover may.

Hopefully, that makes sense, and perhaps even more hopefully, I believe it to be a correct explaination.

Answer 6

The bird and the wire are at the same potential (voltage). If a wire was tossed from a person on the ground up to the bird, touching the bird, he would be electrocuted. Voltage is just a measure of electrical potential energy: how much current can potentially pass through some conductor. If the bird is sittin on a wire with a potential of 50,000 volts, the bird is neutral in reference to the wire, but +50,000 volts in reference to the earth (ground).

Answer 7

I think that the answer to this is in your question!!
"the bird is not earthed (grounded)!!"
if the bird touched a wire/wet tree limb thus making it grounded, and a bare spot on the wire while electric pass through the wire, sorry for the bird he would then be fried by electrocution...

Answer 8

For me, the key to understanding electricity is to think about water going through a pipe. Let's say you have a big water pipe with water going through it. That's the power line with electricity going through it. Now, let's say there's a split where a faucet goes off. Water can keep going this way because there's nothing to stop it. Now, let's say you turn the faucet off. The water in the faucet will hit the inside of the faucet and have nowhere to go. Literally speaking, the electrons in the faucet repel the electrons in the water atoms nearby which in turn repel the electrons in farther away molecules and so on all the way back to the main pipe.
Now, let's see what happens when a bird lands on a power line. I'll simplify a little here. As soon as the bird touches the power line, exactly one electron tries to flow into the bird. That electron pushes its way all the way through the bird to the far side of the bird. There's no place for the electron to go so that electron is just stuck there. The bird now has a static charge because it has on extra electron. That charge is pushing back against the electrons in the wire. A second electron goes into the bird, but this electron has to work harder since the bird has a second charge. A third electron goes into the bird, but this electron has to work even harder since the bird has twice the static charge as before. Eventually, the bird will have so many electrons that no more electrons can enter the bird. I can't tell exactly how many electrons really flow into the bird. This process is happening close to the speed of light so it's not like you could shoot a video of it happening. Also, power lines have AC so electrons will actually keep flowing into and out of the bird 60 times a second, but just a tiny number of them.

Answer 9

Electricity follows the path of least resistance, the wire conductor has a relatively low resistance the bird has higher resistance and no path for the current to flow. If there are 2 wires and the bird spreads his wings and is able to make contact the the current will flow through the bird

Answer 10

When the bird is only touching one wire at a time, he does not furnish the complete circuit required for current flow. A large bird touching both wires at once will electrocute him. It has happened to many eagles in California, so now the people who operate the power grid put roost boards above the power lines to keep the eagles off the wires.