JEE Main · 2008 · 27 Apr 2008 · Offline · Magnetic Effects of Current

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A horizontal overhead power line is at a height of 4 m from the ground and carries a current of 100 A from east to west. The magnetic field directly below it on the ground is (

μ_{0} = 4 π \times 1 0^{- 7} T m A^{- 1}

)

(A) $2.5 \times 1 0^{- 7} T$ southward
(B) $5 \times 1 0^{- 6} T$ northward
(C) $5 \times 1 0^{- 6} T$ southward✓ Correct
(D) $2.5 \times 1 0^{- 7}$ northward

Answer: (C)

Solution

The field of a long straight wire at perpendicular distance

R

B = \frac{μ _{0} I}{2 π R}

. With

I = 100 A

and

R = 4 m

B = \frac{4 π \times 1 0 ^{- 7}}{2 π} \cdot \frac{100}{4} = 2 \times 1 0^{- 7} \times 25 = 5 \times 1 0^{- 6} T

. For current flowing from east to west, the right-hand rule places the field at a point directly beneath the wire pointing towards the south, so the field there is

5 \times 1 0^{- 6} T

directed southward.

JEE Main · 2008 · 27 Apr 2008 · Offline · Single correct option

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JEE Main · 2008 · 27 Apr 2008 · Offline · Magnetic Effects of Current

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