A biology experiment wants to cancel Earth\'s magnetic field, using a 3-turn coi

Question

A biology experiment wants to cancel Earth's magnetic field, using a 3-turn coil made from 18.29 meters (60ft) of wire. . . . a) show that this coil with diameter 1.94 meters , would require more current than the 20 Amp allowed in 14 AWG wire. . . . b) to use the same length of wire (already purchased) but wrapped 6 times instead of 3, what current would be needed? . . . c) it is better to wrap the 6 turns into a pair of 3-turn coils, and separate those coils along their axis by 1 Radius ("Helmholtz" geometry optimizes field uniformity in the center between the coils) ... would this geometry require more current? ... use the diagram to figure out how many times as much.

Explanation / Answer

a)

B = N u I /2piR

==> I = 2piRB/Nu = 2*3.1416*1.94/2*2e-5/3/4/3.1416e-7 = 32.3 A

32.3 > 20

b)

6*2piR= 18.29

R = 18.29/2/3.1416/6 = 0.485156 m

I = 2piRB/Nu = 2*3.1416*0.485156*2e-5/6/4/3.1416e-7 = 8.09 A

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