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Electric current is the rate of charge flow past a given point in an electric circuit, measured in Coulombs/second or Amperes. In direct current circuits, it can be assumed that the resistance to current flow is constant so that the current in the circuit is related to voltage and resistance by Ohm’s law given as:
Electric current = Voltage/ Resistance => I = V/R
Current that flows in a single direction is called Direct Current (DC), and current that alternates back and forth through the circuit is known as Alternating Current (AC). Whether ac or dc, current only flows through a circuit when a voltage source is connected to it with its "flow" being limited to both the resistance of the circuit and the voltage source pushing it. Current sources are the opposite to voltage sources and they like short or closed circuit conditions but hate open circuit conditions as no current will flow.
Electromotive Force:
A constant current can be maintained in a closed circuit through the use of a source of electromotive force (emf), which is a device that produces an electric field and thus may cause charges to move around a circuit. The example of such a source is battery or generator. The source of emf acts like a charge pump. When an electric potential difference exists between two points, the source moves charges from the lower potential to the higher potential. Thus, the emf (ε) describes the work done per unit charge. The SI unit of emf is volt.
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Electromagnetism