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Physics Question on Magnetic Field

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

A

3.33×10−4 T3.33 \times {10}^{-4} \,T

B

1.11×10−4 T1.11 \times {10}^{-4} \,T

C

33×10−3 T33 \times {10}^{-3}\, T

D

9×10−3 T9 \times {10}^{-3}\, T

Answer

3.33×10−4 T3.33 \times {10}^{-4} \,T

Explanation

Solution

B∝1/r.B \propto 1/r . By Ampere's law.
As the distance is increased to three times, the
magnetic induction reduces to one third.
Hence B=13×10−3B =\frac{1}{3}\times {10}^{-3} tesla =3.33×10−4=3.33 \times{10}^{-4} tesla