A condenser of capacity $C_1$ is charged to potential $V_1$ and then disconnected. Uncharged capacitor of capacity $C_2$ is connected in parallel with $C_1$. The resultant potential $V_2$ is

A potentiometer wire is 4 m long and potential difference of 3V is maintained between the ends. The e.m.f. of the cell which balances against a length of 100 cm of the poteniometer wire is

Two wires A and B of equal lengths are connected in left and right gap respectively of a metre bridge, null point is obtained at 40 cm from left end. Diameters of the wires A and B are in the ratio 3:1 respectively, the ratio of specific resistance of A to that of B is

An iron rod is placed parallel to magnetic field intensity 2000 A/m. The magnetic flux through rod is $6 \times 10^{-4}$ wb and its cross sectional area is 3 cm$^2$. The magnetic permeability of the rod in wb/A-m is