A long vertical wire in which a current is flowing produces a neutral point with the earth's magnetic field at a distance of `5 cm` from the wire. If the horizontal component of the earth's magnetic induction is `0.18` gauss, then of the wire is

Options

A. `0.45` amp
B. `4.5` amp
C. `0.9` amp
D. `0.036` amp

Answer

B

Explanation

The magnetic induction `B` due to a current `I` ampere a long straight conductor at a distance `r` meter from is given by:
`B=(mu_(0)I)/(4pi)xx((2I)/r)`
Here `r=5cm=5xx10^(-2)m`
` :. B=10^(-7)xx((2xxI)/(5xx10^(-2)))=41xx10^(-2) gauss`
At the neutral point, the magnetic induction due to earth's horizontal component is equal and opposite to that due to current in the conductor.
`:. 4Ixx10^(-2)=0.18` or `I=4.5 gauss`