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

A current $i$ flows in an infinitely long, straight and thin walled pipe, then

A closely wound solenoid of 80 cm long has 5 layers of windings of 400 turns each. The diameter of the solenoid is 1.8 cm . If the current carried is 8 A , then the magnitude of the magnetic field inside the solenoid near its centre is approximately