Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 30/10/2013 20:54:44
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Why can birds sit on a power line and not be electrocuted ?
Asked by Mary, Cottenham
Visit the webpage for the podcast in which this question is answered. (http://www.thenakedscientists.com/HTML/podcasts/naked-scientists/show/2002.07.07/)
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We answered this question on the show...
The reason that birds can sit unharmed on power lines is that no current flows through them because, [img float=right]http://www.thenakedscientists.com/forum/copies/RTEmagicC_Pylon_ds_01.jpg.jpg[/img]by sitting on one line only, they are not completing the electrical circuit.
Rather like water flowing downhill, electricity needs to flow down an electrical hill, otherwise known as a potential difference. So, if you connect the plus side of a battery to the minus side, current flows because there are lots of positive charges at the plus pole and very few at the negative, so the current flows to try to equal things out.
Returning to the bird sitting on a power line, no current flows through the bird because it is at the same potential as the wire it is sitting on, so no current flows.
But, if it were to straddle a positive and a negative line, it would certainly be curtains!
Ditto if it put one leg on the ground and another leg on the live wire. Under these circumstances, there is a big potential difference between the wire and the the ground, so electricity would flow through the bird in an attempt to balance things out. ZAP!
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In some locations, officials make an effort to discourage large birds from putting their nests on top of telephone poles where they could risk endangering the birds, as well as the circuit.
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I have seen many an unfortunate bat, barbecued between two wires. Perhaps because their wings are extensions of arms and claws, bats and flying foxes are put at greater risk of closing the circuit with their wingspan.
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I often see birds sitting on "low voltage"* street wires.
I don't recall seeing birds sitting on high-voltage wires that run at 33,000-250,000 V (such as the tower whose photograph appears above). This is probably because corona discharge would occur from their bodies, leaking a small amount of electricity into the air. I imagine this would make their feathers tingle!
*For historical reasons, the 110V-230V AC wiring is called "low voltage", even though it can kill you... It seems strange to us today, with so many USB-powered appliances that run off just 5V!
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hey