Connecting wires carrying currents in opposite directions are twisted together in electrical appliances in order to reduce:

In the figure,what is the magnetic field at the point $O$?

$A$ circular coil is in the $y-z$ plane with its centre at the origin. The coil carries a constant current. Assuming the direction of the magnetic field at $x = -25 \, cm$ to be the positive direction of the magnetic field,which of the following graphs shows the variation of the magnetic field along the $x-$ axis?

When a helium nucleus covers a circle of radius $0.8 \,m$ in $2 \,s$, find the value of magnetic field $B$ at the centre of the circle.

Consider a tightly wound $100$ turn coil of radius $10 \; cm$,carrying a current of $1 \; A$. What is the magnitude of the magnetic field at the centre of the coil?

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: