36 cells each of internal resistance $0.5 \Omega$ and emf 1.5 V each are used to send current through an external circuit of $2 \Omega$ resistor. For getting maximum current, $n$ cells are combined in series in each row. If total number of such rows are $m$, then the value of $(n / m)$ will be

The maximum power rating of a $20 \Omega$ resistor is 2 kW . Then, it cannot be used with 300 V DC source because the

The magnitude of resistance $X$ in the circuit shown below in the figure, when no current flows through the $5 \Omega$ resistor, is

A constant voltage is applied between the two ends of a uniform metallic wire, Some heat is developed in it. The heat developed is halved if