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Hey guys, please someone help me with this exercise :)

For our exercise, assume that the bird’s body resistance (from leg to leg) is 50kΩ. Given is a high voltage power line with a current of 100A where the bird takes a rest. The high voltage power line is made out of aluminium, which has a conductivity of $3.79\times10^7 \,\rm S/m$ and a cross section of $100\rm\, mm^2$. 
Determine the theoretical distance between the two bird legs.

I know, the formula is V = R x I

I = 100 A, right?

R = ?

Please someone explain step by step?

Thanks.

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  • $\begingroup$ I thought if a bird was on a power line then .... nothing will happen? It's only if the bird has one leg on the power line and the other on the ground, so that then there is potential difference which will cause a current to flow. $\endgroup$ – jim Apr 12 '16 at 21:43
  • $\begingroup$ @jim, in general, you are correct. But power lines are not ideal conductors, so there must be some voltage drop along their length. For some distance then (presumably less than the stance of a bird), there will be a significant voltage difference. $\endgroup$ – BowlOfRed Apr 12 '16 at 23:41
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The way you have asked the question there is not enough information...

To work out R of the wire you have the conductivity and cross section of the wire..., but you also need the length of wire, which might be the unknown (the gap between the birds legs).

If you have the voltage between the two legs you can go forward with this by working out the resistance of the wire, R, and then you can use conductivity and cross section to work out your length....- is the voltage between the legs in your question?

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  • $\begingroup$ No actually not :/ this was the only information given and I don't understand how to calculate this exercise without knowing R $\endgroup$ – Krissy Apr 13 '16 at 20:21
  • $\begingroup$ ok - so the problem is with your question... i'm afraid it is not possible to work out the distance between the feed of the bird without any more information. $\endgroup$ – tom Apr 14 '16 at 19:36
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I think you might be missing information, as tom pointed out. For example you can look at this setup as a resistor (the bird) in parallel with the power line. You do need to specify something more about the bird, perhaps the power transferred...

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  • $\begingroup$ I guess these are the 50kOhm. Sorry I'm from Germany maybe it's a bit confusing $\endgroup$ – Krissy Apr 13 '16 at 20:44
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Have a look at Birds sitting on electric wires: potential difference between the legs this gives some very good answers, which suggest any pd across and current through is entirely negligible (assuming a standard bird as defined by European law). Essentially, the pd drop over significant distances is small by design so that it is correspondingly smaller over smaller distance.enter image description here

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  • $\begingroup$ As opposed to standard birds as defined by African law? $\endgroup$ – JAB Apr 30 '16 at 21:19
  • $\begingroup$ It is well known that there is a difference, for example, between African and European swallows. $\endgroup$ – jim Apr 30 '16 at 21:24

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