My second grade daughter's science project is "why don't birds get electrocuted when they land on power lines?"
Can someone help? Is it because they are not grounded or because they have strange non-conductive feet? I haven't a clue. If someone can help explain the answer, I might be able to find an experiment she can do to demonstrate the basic principles.
Does a teacher really expect a second grader to explain this on her own? To fully explain and demonstrate it you need to understand electron flow, what constitutes a complete circuit, the fact that current will flow through the bird's legs and body (parallel path), but will be very low due to the high resistance compared to the conductor, and the bird's sensitivity to current. Was this intended to be a student/parent/any other more knowledgable person project? Is today's second grade reading level up to understanding the research material she would need to read for this project?
Keep in mind that some wires are insulated anyway. But that is probably not the answer that they are after. Bennie's answer is probably the best to describe why. If the bird is only sitting on the one wire and is too small to physically touch anything else (another wire (at a different potential) or grounded object) at the same time he's fine. If a bird on an adjacent (different) wire decided to (and could) lean over and touch him/her they've both got a problem. But the wires are usually too far apart for that to happen. (Except maybe in Jurassic Park)
As a side note, if the bird was on a cable, which is several wires twisted together it could touch two at the same time but they are insulated from each other - only 1 would be bare.
Mr. Kelley: I agree with Gerald Powell, this seems to be a bit complex for the second grade level, unless she is in a Cal Tech. prep school. My grandchildren are attending a private school that progresses in curriculum as fast as the student can absorb the information. My grandson, in the first grade, is reading at the third grade level, so I will believe anything now days.
I must apologize for my flippant answers. You asked a very good question, and it deserves a proper response. The insulation on the pads of their feet will help prevent a circuit from one foot to the other. The voltage drop on a one inch length of wire, is probably the most that would be applied between the feet. The line voltage, and current flow would determine the amount of this voltage. It does not appear to be a problem. A bird making contact with two conductors, is entirely different, it will produce instant smoke, and bad odor. There have been some incidents, when birds have dumped substations by trying to fly between the high voltage bushings at transformers.
Dave: Thanks for the Email. Your question; " Why does a person, or animal, making contact with only one conductor not get electrocuted?" My answer is; " There has to be a complete closed circuit from a voltage source through the person, animal, or object". There is an old story; A new apprentice was working for an old timer journeyman. The journeyman informed the apprentice that he was splicing some wires while they were energized. He also said "don't ever do this the way I am doing, stand back and wait". The apprentice noticed the journeyman would lift his left foot, and touch the live wires while he twisted them. The apprentice thought, this is easy, I can do that. He went over to an outlet, lifted his left foot, grabbed the live wire, and was knocked to the floor. The journeyman picked him up, and showed him his plastic artificial right foot.
I'm not sure if you got the short legs reference. As you may know there is a certain resistance in a wire. Usually it is very low. The Body, ours, or a birds' under normal conditions has a much higher resistance. By touching the same wire in 2 places you (or the bird) would be putting yourself in parallel with the wire. Current will have 2 paths to follow:
#1 (the wire) with low resistance will contain most of the current flow.
#2 (the Body) with high resistance will have very small amount of current flow.
The bird's short legs mean that the piece of wire it touches will be very short and therefore of low, low resistance and practically all of the current would flow on the wire.
Note that you, however can touch a much larger span of wire (so don't try this at home!) Also note that the higher the voltage, the more current will flow. And anything that lowers the resistance of the body (ie: being wet) would make it a more attractive path for the current to flow on.