In accordance with the principles of energetics, any change which involves a decrease in the total available energy of the system will tend to occur, and thus the necessary and sufficient condition for the production of electromotive force is that the available energy of the system should decrease when the current flows.
This process involves a decrease in the available energy of the system, for the dissolution of zinc gives out more energy than the separation of copper absorbs.
It is necessary to observe that the condition for change in a system is that the total available energy of the whole system should be decreased by the change.
Thus in the Daniell cell the dissolution of copper as well as of zinc would increase the loss in available energy.
In order that positively electrified ions may enter a solution, an equivalent amount of other positive ions must be removed or negative ions be added, and, for the process to occur spontaneously, the possible action at the two electrodes must involve a decrease in the total available energy of the system.