Birds will often sit on high voltage power lines but not be injured. Suppose a b
ID: 585925 • Letter: B
Question
Birds will often sit on high voltage power lines but not be injured. Suppose a bird is sitting on a 1km long power line which is carrying 10 MW of power. The entire line has a resistance of 30 .
a.
What is the current in the power line? You should get a large number.
b.
The bird’s feet at 15cm apart. What is the electrical resistance of the length of wire between the
bird’s feet? No, you don’t need to know what the wire is made of or its area. Just remember how the resistance depends on the length.
c.
What is the difference in voltage between the bird’s feet? Does this seem like a large enough voltage to be dangerous to the bird?
d.
The difference in voltage between the wire the bird is on and a neighboring wire is 250 kV. If the bird puts one foot on each wire, what is the difference in voltage between the bird’s feet? Does this seem like a large enough voltage to be dangerous to the bird?
Explanation / Answer
a)
Power P = 10 MW and the resistance R = 30 ohm
hence the current = sqrt(P/R) = sqrt(10/30)*10^3 A = 577.35 A
b) Resistance is proportional to the length.
Hence resistance of the length of wire between the bird’s feet = (0.15/1000)*30 = 4.5 m.ohm
c) voltage difference v = I*R = 577.35*4.5/1000 = 2.598 V. No its not a very large voltage.
d) In this case the voltage will be 250 kV and it is very large and dangerous.
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