For moving object need power, the same cases on atomic event, to move the electron in order move to other position need power or need energy. The power to move electrons in conductor called as electromotive force (emf). Electromotive force often called as Electric Force. Appliance which can yield electric force is called electric current source or electric source. The principle of this appliance is similar with a pump work, which pushing electrons to move regularly in the electric conductor.
This term of electric source should be understand not the source of electric that yield electric current, but providing energy to move electrons. So other term for electric source is defined as Electric Generator. As electric generator available in many ways:
Electric generate because of chemical reactions, such as: Accumulator or Battery.
Electric generate because of magnetic induction, such as: Dynamo and Generator.
Electric generate because of heat, such as: Thermocouple.
Electric generate because of light effect, such as: Quartz, Crystals, Photoelectric Cell. Each of those appliances works with a certain condition and have special rule to generate electric.
This condition should be fulfilling as to result the electric with a suit result as we want. If the condition then changes doe to other factor like age or environment, yield of electric current will digressing and need reconditioning.