Question

A wire of length $L$ , mass $m$ and carrying current $i$ is suspended from point ${}^{\prime }{O}^{\prime }$ as shown. An another infinitely long wire carrying the same current $i$ is at a distance $L$ below the lower end of the wire. To keep the suspended wire stationary by placing a third infinitely long wire carrying an upward current, third wire should be placed. (Given $i=2\text{A},L=1\text{m}$ and $m=0.1\text{kg}$. $[\ln 2=0.693]$)

• A. to the left of suspended wire.
• B. to the right of suspended wire.
• C. we cant keep suspended wire stationary by placing a third wire to the right or to the left of it.
• D. we can keep it either to the right or to the left . It will depend on the magnitude of the current in the third wire.

Answer 1

The correct option is A to the left of suspended wire.

To keep the suspended wire stationary, the net torque acting on the wire about point $O$ should be zero.

Magnetic field due to first infinite long wire on the suspended wire is perpendicular to the plane of paper outwards.

So, it is clear that, magnetic field due to the second infinite long wire on the suspended wire is perpendicular to the plane of paper inwards.

Hence, option (a) is the correct answer.