Question

A current of $30A$ is flowing in a vertical straight wire. If the horizontal component of earth's magnetic field is $2\times {10}^{-5}T$, then the position of null point will be

• A. $0.9m$
• B. $0.3mm$
• C. $0.3cm$
• D. $0.3m$

Answer 1

The correct option is D $0.3m$

null point will happen when magnetic field produced cancels earths magnetic field
$=\frac{U_{0}i}{2Ar}=2\times {10}^{-5}T=\frac{2\times {10}^{-7}\times 30}{r}=2\times {10}^{-5}$
$r=30\times {10}^{-2}m$
$r=0.3m$