Question

Figure shows two long parallel wires separated by a distance of $18 \,cm$. There is current of $8 \,A$ in wire $ 1 $ and $12\, A$ in wire $2 .$ At what point on line joining of the wires, the magnetic field is zero?

• A. 5.2 cm from wire 1
• B. 9.4 cm from wire 2
• C. 7.2 cm from wire 1
• D. 10.3 cm from wire 2

Answer 1

Answer: (C)

The field is zero between the wires then

$\frac{I_{1}}{I_{2}}=\frac{r_{1}}{r_{2}}$
$\frac{I_{2}}{I_{1}}=\frac{r_{2}}{r_{1}}$
$=\frac{12}{8}=1.5$
$r_{2}=\frac{3}{2} r_{1}$
$r_{1}+r_{2}=18$
$r_{1}+1.5 r_{1}=18$
$\therefore r_{1}=\frac{18}{2.5}=7.2 \,cm$ from $1$