Review the definition of voltage as energy/charge. Using the transfer of heat from higher to lower, and the movement of water from a higher lever to lower, lead into the fact that charge also flows from higher charge to lower charge. The difference in charge, from high to low is the voltage difference, or potential difference. Just as in heat transfer and water motion, the flow of charge will continue until both reach a common potential. If there is no potential difference there is no flow of charge. Define electric current as the flow of electric charge in a conductor, with electrons carrying the charge through the circuit.