Migratory birds are able to use the earth\'s magnetic field to navigate even whe
ID: 1568465 • Letter: M
Question
Migratory birds are able to use the earth's magnetic field to navigate even when clouds and darkness prevent them from having visual references for their flight. The range of sensitivity of such birds extends to magnetic fields as small as about a third of the earth's natural field. If such a bird is flying past a power line which carries 105 amps, and if we assume that the minimum field detectable at 60 Hz is the same as the minimum field detected at DC, at what distance could the bird detect the presence of the power line? Why will the real answer be much smaller? (think about how power lines are arranged)
Explanation / Answer
Earth's Natural Magnetic Field, Bearth = 0.51 Gauss Approx.
One-third of the Earth's Magnetic Field = 0.17 Gauss -> Least detectable Magnetic Field by the Migratory Birds
Magnetic Field Caused due to a Current-Carrying wire, B = (µ0*I)/( 2R) , where
µ0 = 4 * 10-7 T.m/A, where 1 T = 104Gauss
I = Current in the Wire
R = Distance from the Wire
0.17 Gauss = (4 * 10-3 Gauss.m/A * 105A)/ ( 2**R)
-> 0.17 = 0.210/R
-> R = 0.210/0.17 = 1.23m
Hence, the Min Distance Approximately, from which the Magnetic Field can be detected by Migratory Birds is 1.23m.
The Real Answer will be much smaller beacuse there will be numerous number of Current-carrying Wires in the Power Grid, thereby adding to the value of B. As B is inversely proportional to R, the Real Answer will be much smaller.