Cells In Series

When identical cells are joined in series, their emfs add up and their internal resistances also add up, since the same current passes through each cell in turn, a rule explained in NCERT Class 12, Chapter 3 (Current Electricity). The net emf is $n\epsilon =4\times 1.5=6$ V, and the total internal resistance is $nr=4\times 0.2=0.8$ ohm. Series stacking is chosen when a higher driving voltage is needed. The option 1.5 V and 0.05 ohm instead describes a parallel connection of identical cells, where the emf stays that of one cell and the internal resistance drops to $r/n$. The option 6 V and 0.05 ohm incorrectly mixes the series emf with the parallel internal resistance, combining rules from two different arrangements. The option 1.5 V and 0.8 ohm wrongly keeps a single cell's emf while correctly adding the resistances, an inconsistent pairing. A consistency check confirms the result: four 1.5 V cells stacked in a torch sensibly give 6 V, and internal resistances placed in series add just like ordinary series resistors, so 0.8 ohm is the correct total and the emf and resistance both scale by the same factor of four.