A wire of resistance $2 R$ is stretched such that its length is doubled. Then the increase in its resistance is

It is found that a non-zero current element is unable to produce any magnetic field at a particular point. Then the angle between the current element and the position vector of that point with respect to the current element is

Three long, straight, parallel wires carrying different currents are arranged as shown in the diagram. In the given arrangement, let the net force per unit length on the wire $C$ be $\mathbf{F}$. If the wire $B$ is oved without disturbing the other two wires, then the force per unit length on wire $A$ is

If $\chi$ is the susceptibility and $\mu_r$ is the relative permeability of a ferromagnetic substance, then