A battery of emf 12 V and internal resistance 0.5 ohms is connected to an external resistor of 5.5 ohms. What is the potential difference across the terminals of the battery?

1. The internal resistance behaves like a resistor in series with a perfect source of emf. 2. The total resistance of the circuit is therefore 5.5 ohms plus 0.5 ohms, which is 6.0 ohms. 3. The current is the emf divided by the total resistance, so it is 12 volts divided by 6.0 ohms. 4. That gives a current of 2.0 amperes in the circuit. 5. The potential difference lost inside the battery is the current multiplied by the internal resistance. 6. That is 2.0 amperes multiplied by 0.5 ohms, which is 1.0 volt. 7. The terminal potential difference is the emf minus that loss, so it is 12 volts minus 1.0 volt, which is 11 volts. 8. The 12 volt answer is the trap for treating the battery as ideal, the 1 volt answer quotes only the loss inside the battery, and the 10 volt answer doubles that loss by using 1.0 ohm as the internal resistance. _Source: Siyavula Physical Sciences Grade 12 (Everything Science, CC BY 4.0), Chapter 10: Electric circuits_