In the water pump controller, the assumption is that the positive and negative plates gathered a certain amount of positive and negative charges respectively, after the open circuit connected, because the charge under the action of electric field force, mobile, positive and negative charge on the happening of neutralization, the voltage between the poles soon reduced to zero, current is reduced to zero. Constantly, in order to maintain current in the power supply must have a kind of internal power force (non electrostatic force), it can move from power negative electric charge by the power within to the positive, in order to maintain the poles gather a certain amount of positive and negative charges respectively. Power of the force is to complete this task. In the battery, the power force is the electrode and the electrolyte chemical force produced by chemical reaction. Power force move positive charge from the cathode by the power within to the anode, electric field force to overcome the work, in order to measure the power capability of power supply power to charge, electromotive force, a physical quantity is introduced. The power force will charge from the cathode by the power within to the positive work, known as the power source electromotive force. Electromotive force and voltage with the same dimension. Although it is important to note that the voltage and the electromotive force have the same unit, but there are different physical concept. So we provisions of electromotive force by the power supply is the actual direction of the positive and negative points to the power supply by low potential point to namely high potential.
