If $E_1=4 \mathrm{~V}$ and $E_2=12 \mathrm{~V}$, the current in the circuit and potential difference between the points $P$ and $Q$ respectively are

Two identical cells gave the same current through an external resistance of $2 \Omega$ regardless whether the cells are grouped in series or parallel. The internal resistance of the cells is

Two toroids with number of turns 400 and 200 have average radii respectively 30 cm and 60 cm . If they carry the same current, the ratio of magnetic fields in these two toroids is

Three rings, each with equal radius $r$ are placed mutually perpendicular to each other and each having centre at the origin of coordinate system. $I$ is current passing through each ring. The magnetic field value at the common centre is