CBSE Class 12 · 2023 · Magnetic Effects of CurrentMultiple Choice

A long straight wire of radius

a

carries a steady current

I

distributed uniformly across its cross-section. The ratio of the magnitudes of the magnetic field

B_{1}

\frac{a}{2}

and

B_{2}

at a distance

2 a

from the axis is

Answer: (B)

Solution

Inside (

r < a

B_{1} = \frac{μ _{0} I r}{2 π a ^{2}}

r = \frac{a}{2}

gives

B_{1} = \frac{μ _{0} I}{4 π a}

. Outside (

r > a

B_{2} = \frac{μ _{0} I}{2 π r}

r = 2 a

gives

B_{2} = \frac{μ _{0} I}{4 π a}

. Hence

\frac{B _{1}}{B _{2}} = 1

CBSE Class 12 Board · 2023 · Single correct option