Determine the internal resistance with an EMF equal to 12 V, if with an external resistance of 8 ohms, the current is 0.4 A

An imperfect power supply can be thought of as an ideal power supply connected in series with an EMF of 12 V and a resistance r equal to the internal resistance of the source.

We obtain a circuit in which two resistances r and R = 8 Ohm are connected in series to an ideal source with an EMF equal to 12 V.

Thus, the total resistance is r + 8, from Ohm’s law equal to U / I = 12 V / 0.4 A = 30 Ohm.

From which it follows that r = 30 Ohm – R = 30 Ohm – 8 Ohm = 22 Ohm.

Answer: internal resistance 22 ohms.