Electric potential, voltage (article) | Khan Academy

Go back to the equation for Electric Potential Energy Difference (AB) in the middle of the section on Electric Potential Energy. That equation tells you how electric potential energy changes when you move a test charge from point A to point B. For example, you could be moving your test charge towards or away from some charged object. To use this equation you have to put in two locations, A and B. There are just a few oddball situations that give us some trouble… What if I told you where B was but did not mention A? I might say it this way: “What is the potential energy of a test charge when you place it at B”? Well, you need an A to answer that question. If I don’t give it to you, you have to make one up. A common choice that lots of engineers and scientists make is “A is infinity away from the charged object.” When we make that choice, we say we are determining the absolute potential energy, or the absolute voltage. It’s the same voltage as usual, but with the assumption that the starting point is infinity away. In almost all circuits, the second point is provided and this absolute idea isn’t needed.

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