The figure shows a tetrahedron,each side of which has a resistance $r$. Choose the correct diagram$(s)$,which show a two-dimensional equivalent of the tetrahedron.

When an electric heater is switched on, the current flowing through it $(I)$ is plotted against time $(t)$. Taking into account the variation of resistance with temperature, which of the following best represents the resulting curve?

First,a set of $n$ equal resistors of $10 \; \Omega$ each are connected in series to a battery of emf $20 \; V$ and internal resistance $10 \; \Omega$. $A$ current $I$ is observed to flow. Then,the $n$ resistors are connected in parallel to the same battery. It is observed that the current is increased $20$ times. The value of $n$ is .... .

In the circuit shown here,$E_1 = E_2 = E_3 = 2 \, V$ and $R_1 = R_2 = 4 \, \Omega$. The current flowing between points $A$ and $B$ through battery $E_2$ is

In the circuit shown,$E, F, G$,and $H$ are cells of e.m.f. $2\,V, 1\,V, 3\,V$,and $1\,V$ respectively,and their internal resistances are $2\,\Omega, 1\,\Omega, 3\,\Omega$,and $1\,\Omega$ respectively.

$A$ battery of $6$ cells,each with an $e.m.f.$ of $2\,V$ and an internal resistance of $0.5\,\Omega$,is being charged by a $220\,V$ $D.C.$ mains using an external resistor of $10\,\Omega$. What is the charging current in $A$?