Hello, I'm struggling to understand how the electric potential difference is measure especially when a distance is not given. For instance in Serway, on the explanation of the Van de Graaff Generator the authora write, "Van de Graaff generators can produce potential differences as large as 20 million volts." But what does that exactly mean? Where is this potential difference considered? I mean, the distance between two the electric potentials could be very far away or very close....so I'm not seeing the purpose of the statement. It seems too arbitrary to make sense.