Question

A long straight wire in the horizontal plane carries a current of 75 A in north to south direction, magnitude and direction of field B at a point 3 m east of the wire is :

• A. 4 x ${10}^6$ T, vertical up
• B. 5 x ${10}^6$T, vertical down
• C. 5 x ${10}^{-6}$T, vertical up
• D. 4 x ${10}^6$ T, vertical down

Answer 1

The correct option is C 5 x ${10}^{-6}$T, vertical up

From Ampere circuital law
$\varphi \overline{B}.\overline{dl}$ = ${\mu }_0{I}_{enc}$
B x $2\pi R$ = ${\mu }_0{I}_{enc}$
B = $\frac{{\mu }_0{I}_{enc}}{2\pi R}$ = 2 x ${10}^{-7}$ x $\frac{75}{3}$ = 5 x ${10}^{-6}$ T
The direction of the field at the given point will be vertical up determined by the screw rule or right-hand rule.