A 12.0 V battery has an internal resistance r. The measured voltage (emf of the battery) is 12.0 V. When connected to a resistor R the terminal voltage is 11.7 V and the current is 0.10 Amperes. What is the value of the external resistor R in ohm? Ok I thought the problem was simple but for some reason I keep getting it wrong. (E denotes electromotive force). I set up conservation of energy like this: E - V(resistor) = 11.7 E = 12 in this case and V = i*R = 0.1*R so.. 12 - 0.1*R = 11.7 R equals 3 from my equation but it's wrong. Dead wrong. Any ideas? It's the language that seems most unclear to me.