Question

A circular coil of wire carries a current. $PQ$ is a part of a very long wire carrying a current and passing close to the circular coil. If the direction of currents are those as shown in the Figure, what is the direction of force acting on $PQ$?

• A. Parallel to $PQ,$ towards $P$
• B. Parallel to $PQ,$ towards $Q$
• C. At right angles to $PQ,$ towards right
• D. At right angles to $PQ,$ towards left

Answer 1

The correct option is D At right angles to $PQ,$ towards left

There is a circular coil which is a dipole placed in a non uniform magnetic field due to PQ. Now magnetic field due to circular coil will be out of the plane and not uniform. Therefore, force which will act on PQ will have direction,
$F=\overrightarrow{dl}\times \overrightarrow{B}$
since $\overrightarrow{dl}$ is vertically downwards and $\overrightarrow{B}$ is out of the plane therefore, direction of $\overrightarrow{F}$ will be towards left that is perpendicular to the PQ. or right angled with PQ.
Therefore, option(D)