Four identical cells, each having an emf of 1.8 V, are linked in parallel and connected to an external network made of two resistors of 10 Ω each arranged in parallel. If the potential difference across the output terminals is measured to be 1.5 V by an ideal voltmeter, what is the internal resistance of each cell?

When multiple identical cells are joined in parallel, the net emf remains the same as that of a single cell. Hence,
Total emf (E) = 1.8 V Let r be the internal resistance of each cell. Since the cells are in parallel, their effective internal resistance is reduced: Now, calculate the combined resistance of the external circuit: Two 10 Ω resistors in parallel: Using Ohm’s law: The potential drop inside the battery system due to internal resistance is: