In the given circuit, an ideal voltmeter connected across 6 reads 5V. The internal resistance r of each cell is:

Step-by-step solution

The equivalent resistance of the upper branch is 9 + 3 = 12 . The equivalent resistance of the parallel combination is R_p = 12 6 12 + 6 = 4 . The voltage across the parallel combination is given as V_p = 5 V . The main circuit current is I = V_p R_p = 5 4 = 1.25 A . The total external resistance is R_ ext = 2 + 4 = 6 . The total EMF of the circuit is E = 4 + 4 = 8 V and total internal resistance is 2r . Applying Ohm's law for the complete circuit: I = E R_ ext + 2r 1.25 = 8 6 + 2r 6 + 2r = 8 1.25 = 6.4 2r = 0.4 r