In an experiment to find emf of a cell using potentiometer, the length of null point for a cell of emf $1.5$ V is found to be $60$ cm. If this cell is replaced by another cell of emf $E$, the length of null point increases by $40$ cm. The value of $E$ is $x/10$ V.
The value of $x$ is

A charged particle $q$ is accelerated by a potential difference of $V$ enters a region of uniform magnetic field of induction $B$ at right angles to the direction of the field. The charged particle completes semi circle of the radius $r$ inside the magnetic field. The mass of the charged particle is (all quantities are in SI units)

Two long straight wires are arranged parallel to each other and are kept $20$ cm apart in vacuum. They carry currents of $2$ A and $4$ A respectively in the same direction. What will be the magnetic force on a length of $10$ cm of either wire?
($\mu_0 = 4\pi \times 10^{-7}$ SI units)

A rod with a circular cross-section area $2$ cm$^2$ and length $40$ cm is wound uniformly with $400$ turns of an insulated wire. If a current $0.4$ A flows in the wire winding, the total magnetic flux produced inside the winding is $4\pi \times 10^{-6}$ Wb. The relative permeability of the rod is
(Given permeability of vacuum $\mu_0 = 4\pi \times 10^{-7}$ N m A$^{-2}$)