Question

Four identical cells, each of emf 2V, are joined in parallel providing supply of current to external circuit consisting of two 15 Ω resistors joined in parallel. The terminal voltage of the cells as read by an ideal voltmeter is 1.6 V. Calculate the internal resistance of each cell.

Answer 1

 For parallel connection of n identical cells, the net emf is equal to that of one cell and the net internal resistance is the parallel combination of resistances of all the cells r eq = r/n.

Four cells are connected in parallel to the parallel combination of two 15 Ω resistors as shown in figure.

Let r be internal resistance of each cell and I be the current I in the circuit. Since the cells are connected in parallel.

Total e.m.f in the circuit – e.m.f. one cell -2V

Further, total internal resistance of the cells is gives by 

Let R be resistance of the parallel combination of two 15 Ω resistors. Then, total external resistance,

Now, internal resistance of the parallel combination of cells is given by