Question

# Two parallel long wires carry currents i1 and i2 (i1 > i2). When the currents are in the same direction, the magnetic induction at a point midway between the wires is 10 μT. If the direction of i2 is reversed, the induction becomes 30 μT. The ratio of i1i2is :

A
4
B
3
C
2
D
1
Solution
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#### 1st case: B=B1−B210×10−3=μ0i12πr−μ0i22πrμ02πr(i1−i2)=0.012nd case:B=B1+B230×10−3=μ0i12πr+μ0i22πrμ02πr(i1+i2)=0.03∴i1−i2i1+i2=3i1−i2=3i1+3i2i1i2=2

1
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