Magnetic lines of force due to a current in a circular conductor are

An equilateral triangle is made by uniform wires $AB, BC, CA$. A current $I$ enters at $A$ and leaves from the midpoint of $BC$. If the length of each side of the triangle is $L$, the magnetic field $B$ at the centroid $O$ of the triangle is:

$A$ circular loop is kept in the vertical plane that contains the north-south direction. It carries a current that is towards the south at the topmost point. Let $A$ be a point on the axis of the circle to the east of it and $B$ be a point on this axis to the west of it. The magnetic field due to the loop is:

An infinitely long straight conductor is bent into the shape as shown in the figure. It carries a current $I$ and the radius of the circular loop is $r$. The magnetic induction at the center $O$ of the circular loop is:

$A$ wire carrying current $I$ has the shape as shown in the adjoining figure. The linear parts of the wire are very long and parallel to the $X$-axis,while the semicircular portion of radius $R$ lies in the $Y-Z$ plane. The magnetic field at point $O$ is:

When equal current is passed through two coils,equal magnetic field is produced at their centres. If the ratio of the number of turns in the coils is $8: 15$,then the ratio of their radii will be