Two parallel plate air capacitors are connected in parallel. Each capacitor has plate area $A/2$ and separation between the plates is $d$ and $2d$ respectively. The equivalent capacity of the combination is

( $\epsilon_0$ = absolute permittivity of free space)

In metre bridge experiment, the resistance in the left gap is 15 $\Omega$ and in the right gap is 45 $\Omega$. The bridge is balanced. The distance of the null point from the centre of the wire is

A potentiometer wire is 4m long and potential difference of 2V is maintained between the ends. The e.m.f. of the cell which balances against a length of 80 cm of the potentiometer wire is

An iron rod is placed parallel to magnetic field intensity 1000 A/m. The magnetic flux through the rod is $3 \times 10^{-4}$ Wb and its cross-sectional area is $1.5\ \text{cm}^2$. The magnetic permeability of rod in $\text{Wb/}_{\text{A-m}}$ is