Hello all! I'm confused about an explanation that my textbook gives, and I'm wondering if someone can help me understand it. The passage is as follows: A circuit consists of a 12 V battery with negligible internal resistance, in parallel with a charging unit with an emf of 15 V and internal resistance of 0.5 ohms. These two components provide power for all the car's electrical equipment. The car's battery maintains an emf of 12 V across it, because of its negligible internal resistance. Hence, the terminal p.d. of the charging unit must also be 12 V. This implies that 3 V must be "lost volts" across the internal resistance of the charging unit. Since its internal resistance is 0.5 V, there must be a current of 6 A supplied by the charging unit. I'm confused specifically with the underlined statement - why does the p.d. of the charging unit have to be 12 V as well? Is it because the battery is actually powering the charging unit? Why then does it have an emf of 15 V? Any insight would be appreciated. Miss E.