What causes the flow of charges in a voltaic pile?
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Think of a river flowing downhill from one lake into another. The water flows from a higher to lower altitude. Electrical potential difference is like the change in height of the water on the two ends of the river. Electric current, the flow of charge, is like the water current, the flow of water over the falls. If there is no difference in altitude of the two lakes, then there will be no flow of water. If there is no difference in voltage, there will be no flow of charges.
Volta invented the term “electromotive force” (emf) to describe what causes the separation of charges. The more disks there were in the voltaic pile, the greater the emf. Unfortunately, the word “force” is an incorrect use of that term, because there is no mechanical push, measured in newtons, on the charge. The correct term is potential difference or voltage, the energy change per unit charge separated. Both the term voltage and the unit in which it is measured, the volt (V), are named after Volta.