Question

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

Answer: (A)

$B \propto 1 / r$. By Ampère's law.
As the distance is increased to three times, the magnetic induction reduces to one third.
Hence $B=\frac{1}{3} \times 10^{-3}$ tesla $=3.33 \times 10^{-4}$ tesla