The calculation method of the potential at a certain point in the circuit is: (1) confirm the position of the potential reference point; (2) determining the current direction in the circuit and the positive and negative polarities of the voltages at both ends of each element; (3) Starting from the sought point, winding to a potential reference point through a certain path, and the potential of the point is equal to the algebraic sum of all voltage drops on the path: the voltage drop of the resistance element is written in the form of RI, and when the reference direction of the current I is consistent with the winding direction of the path, the "+"sign is selected; Otherwise, select the "-"sign. The electromotive force of power supply is written in the form of+/-E. When the direction of electromotive force is consistent with the circuitous direction of path, select "-"; Otherwise, select the "+"sign.