Question

The magnetic field at a point $P$ which is at the distance of $4$ $cm$ from a long current carrying wire is ${10}^{-3}T$, the magnetic field at a distance $12$ $cm$ from the current carrying wire will be

• A. $3.33\times {10}^{-4}T$
• B. $1.11\times {10}^{-4}T$
• C. $33\times {10}^{-3}T$
• D. $9\times {10}^{-3}T$

Answer 1

The correct option is A $3.33\times {10}^{-4}T$

$B\propto \frac{1}{r}$(By Ampers's law)
As the distance is increased to three times, the magnetic induction reduces to one third.
Hence, $B=\frac{1}{3}\times {10}^{-3}T=3.33\times {10}^{-4}T$