Physics Puzzle: Birds on a Wire
Level of Difficulty: Highschool
And the birds up on the wires and the telegraph poles
They can always fly away from this rain and this cold
You can hear them singing out their telegraph code
All the way down the telegraph road
I used to listen to that song growing up, and it still remains a personal favorite of mine. Birds have a habit of perching wherever there’s a line to support them. Thinking back, when you were little your parents probably used to tell you never to go near high-power lines, that they’re dangerous; yet it seems that birds can rest on them effortlessly, without being hurt at all. How can that be?
P.S.
To set the record straight, telegraph wires - at least the early ones, as I’m not a telegraphy historian - were DC and relatively safe, so I cheated a bit when quoting “The Telegraph Road”!
EDIT, 6/May/2009: A solution has been posted here.
To get electrocuted a body has to come between enough potential difference across it.
Easiest way would be to be earthed or in contact of the neutral and then coming in contact with a live wire, but that is not the only way. A person can get electrocuted even if he/she in not earthed by coming in contact at two different point of the same wire when high enough potential difference is present between the two points.
In case of birds,
They ‘Always’ perch on one single wire.
The span on the wire between its feet is so small that the potential drop across it doesn’t cause electricity to flow through it.
They have a much lower water and electrolyte content in their body in comparison to us (especially their legs, which are mostly bone and cartilage) which makes them a bad conductor.
Still if you have observed closely, once in a while birds do get electrocuted, especially the bigger ones. This is because when they are about to perch they come in contact with more than one wire simultaneously.
A complete, full and undeniably correct solution :). I’ve posted a solution with a small diagram offering a more visual explanation, for the sake of completeness.