A magnetizing field of $5000 \mathrm{~A} / \mathrm{m}$ produces a magnetic flux of $4 \times 10^{-5} \mathrm{~Wb}$ in an iron rod of cross-sectional area $0.4 \mathrm{~cm}^2$. The permeabilit of the rod in Wb/A-m, is

When a large bubble rises from bottom of a water lake to its surface, then its radius doubles. If the atmospheric pressure is equal to the pressure of height $H$ of a certain water column, then the depth of lake will be

A block of mass $M$ is pulled along a smooth horizontal surface with a rope of mass $m$ by force $F$. The acceleration of the block will be

In the network shown cell E has internal resistance $r$ and the galvanometer shows zero deflection. If the cell is replaced by a new cell of emf $2 E$ and internal resistance $3 r$ keeping everything else identical, then