Birds on pylons

[giraffe_02]

How can birds perch on electricity cables without getting electricuted? if humans were to touch them we would be electricuted

[KebabMeister]

I assume it's because they're not earthed? (i.e. touching the pylon frame, other cables, or the ground)

[gizmo338]

It is all a question of potential difference. If the bird's feet were a long way apart there would be a potential difference between its feet due to the impedance of the wire and the resultant volt drop. The bird would then fry.

By the same reasoning, if we touch the cable with one hand and our feet are at earth potential, say, then wew fry.

[Buenchico]

Electricity flows from a higher potential to a lower one. If you connect your hand to the live terminal of a domestic mains circuit while standing with bare feet on damp ground, your hand will be at a potential of 230V while your feet will be 'earthed' (i.e. at a potential of 0V). So electricity will flow from your hand to your foot. If however you were to connect both your hands and feet to live mains terminals, while standing on an insulated surface, no electricity would flow because there would be the same potential (230V) present at the ends of both your upper and lower limbs (and, indeed, throughout your body)..

If a bird sits on an electricity cable, there will be a potential of, say, 25000V at its feet (and throughout its body) but since no part of its body is earthed (or otherwise at a lower potential), no electricity can flow.

A tight-rope walker (using an insulated pole for balancing) could walk along the cable with the same immunity from electrocution. (Once again, the whole of his body would be at a potential of 25000V). If, however, he chose to swing from the cables he might have a problem, especially if the air was damp. This is because his entire body would be at a potential of 25000V while the ground below him would be at a potential of 0V. This is fine as long as the air between his feet and the ground (or between his feet and the earthed metal structure of the pylon) doesn't conduct electricity. If it does do so (e.g. under damp conditions) electricity will flow.

Chris

[qapmoc]

Hi gizmo338, you are wrong on the thing about the potential difference giving a shock on power lines but this is a reason why lightning strikes can kill you even if it does not hit you directly. the voltage on the ground at the strike point might be a million volts or whatever but at some point distant from the strike it will be zero, therefore there is a potential difference in the ground which can reach twenty thousand volts per meter, as you legs could be half a meter apart you would get a difference of ten thousand volts between each leg, and this would definately fry anything in the current path directly between your two legs!!!

[4lec]

I understand all this about potential difference etc and not being earthed, so electricity doesnt flow.
BUT.... how come when u hit a golf ball at a power cable, it comes back to earth burnt, and when balloons hit power cables the electicity "jumps" and bursts them, sometimes creating a huge spark, surely these two occasions the objects are not earthed, but then again they are both plastic/rubber if that makes a diff.
sorry for going on but i was just wondering about those two times ive seen big sparks at power cables
alec