The effective capacitance between points $A$ and $B$ shown in the circuit is

The Wheatstone bridge shown in the diagram is balanced. If $P_3$ is the power dissipated by $R_3$ and $P_1$ is the power dissipated by $R_1$, then the ratio $P_3 / P_1$ is

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