Question

A straight wire of diameter 1 mm carrying a current of 2 amperes is replaced by another wire of 2 mm diameter. The strength of the magnetic field far away is :

• A. twice the earlier value
• B. one half the earlier value
• C. one quarter the earlier value
• D. same as the earlier value

Answer 1

Answer: (D)

same as the earlier value

Apply Ampere's Law to the loop outside the wire at the given distance,

${\mu }_{0}I=B\left(2\pi r\right)$

$⟹B=\frac{{\mu }_{0}I}{2\pi r}$

Hence the field due to current carrying wire outside is independent of the diameter of the wire.