1. The problem statement, all variables and given/known data Why is the voltage of the right source negative? 2. Relevant equations 3. The attempt at a solution I learned that a circuit element is positive or negative, because of the terminal with respectively the highest or lowest voltage. So, if the positive terminal has a lower voltage/ potential than the negative terminal, then the element has a negative voltage. But this would mean that the polarity of the voltage source has changed. I guess I found a rare situation where the polarity of a source will change: "if you have a wet cell battery are filling it for the first time, and are using an old style, non smart charger, and short the terminals while you are filling it, yes it is possible to hook up the charger backward and reverse charge it." But I don't think that the practice problem meant this and therefore I'm wrong for a reason I can't figure out.