Physics Current Electricity

Two resistors $400\mathrm{\Omega }$  and   $800\mathrm{\Omega }$ are connected in series across a  $6\vee$ battery. The potential difference measured by a voltmeter of $10\mathrm{k}\mathrm{\Omega }$ across $400\mathrm{\Omega }$  resistor is close to:

Hydrogen ion and singly ionized helium atom are accelerated, from rest, through the same potential difference. The ratio of final speeds of hydrogen and helium ions is close to:

An ideal cell of emf 10 V is connected in circuit shown in figure. Each resistance is 2Ω. The potential difference (in V) across the capacitor when it is fully charged is

A potentiometer wire PQ of 1 m length is connected to a standard cell E₁. Another cell E₂ of emf 1.02 V is connected with a resistance 'r' and switch S (as shown in figure). With switch S open, the null position is obtained at a distance of 49 cm from Q. The potential gradient in the potentiometer wire is: